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The chassis of a motor vehicle includes:

● Wheel suspension ● Steering● Suspension ● Brakes● Wheels and tyres

They are responsible for the dynamics of vehicularoperation and for the road safety of the vehicle.

18.1 Dynamics of vehicularoperation

The movements occur about the longitudinal axis,transversal axis and vertical axis (Fig. 1).

A distinction can be made between:● Forces acting along the longitudinal axis: motive

force, braking force, friction force● Forces acting along the transversal axis: centrifugal

force, wind force, lateral force● Forces acting along the vertical axis: wheel load,

forces created by jolts from a rough road surface

The movements resulting from all forces acting togeth-er express themselves in the drivability of the vehicle.

Factors influencing the drivability are:

● The location of the centre of gravity, roll centre, rollaxis, driving axis

● The type of drive and the mounting location of thepower plant

● The wheel suspension and the wheel positions● The suspension and the oscillation damping● The wheel control systems, such as ABS, TCS, ESP

Roll centre (instantaneous centre, Fig. 2). This is thepoint (W) on an imaginary perpendicular to the cen-tre of the axle, about which the vehicle body rotatesdue to the action of lateral forces FS.The roll centre of a vehicle axle is located in the cen-tre of the vehicle when viewed from the front. Itsheight depends on the wheel suspension.

Roll axis. This is formed by connecting the roll cen-tres of front axle WF and rear axle WR (Fig. 3). It usual-ly slopes down towards the front of the vehicle, sincethe roll centre is lower at the front wheel suspensionsthan at the rear. The closer the centre of gravity S lies to the roll axis,the less the vehicle tilts when cornering.

Axis of symmetry. This runs in vehicle longitudinaldirection through the centre of the front and rearaxles (Fig. 4).

Geometrical driving axis. This is formed by the posi-tion of the rear wheels and is the bisector of the toe-in angle of the rear wheels (Fig. 4).The wheel offset is the angle by which the two rearwheels are offset against each other towards thefront (+) or towards the rear (–) for example (Fig. 4).

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The dynamics of vehicular operation deal withthe action of the forces affecting the vehiclewhile it is being driven and the resulting move-ments of the vehicle.

Rear-wheel drive Vertical axis (yaw axis)

Longitudinal axis

Transversal axis

FB Braking forceFA Motive forceFS Lateral forceFN Vertical force

FB

FA

FS

FN

FB

FB

FB

FN

FN

FN

FA

FS

FS

FS

Fig. 1: Forces and axes on the vehicle

W

Fs Heig

ht

Fig. 2: Roll centre

WF

WR

S

Fig. 3: Roll axis

Axis ofsymmetry

Driving axis+

–

Wheeloffset

Fig. 4: Axis of symmetry, driving axis

Wheel-slip angle. If a vehicle is hit by a lateral in-terference factor while it is in motion (e.g. windforce, centrifugal force), lateral forces FS act in thetyre contact patches of all four tyres. If the steeringis corrected, the direction of travel of the wheelschanges, they run at an angle to the original direc-tion of travel by an angle of α (Fig. 1).

Attitude angle. This relates to the whole vehicle(Fig. 1).

Self-steering effectTo assess drivability, standard driving manoeuvresare performed, e.g. steady-state turn, and the self-steering effect of a motor vehicle is determined.

Up to the cornering limit speed, the adhesion be-tween tyres and road surface is adequate for es-tablishing the lateral forces required.

If the corner is taken at a higher speed, lateral slipoccurs at the front or rear wheels or at all wheels.

A distinction is drawn between:

● Understeer (Fig. 2). Wheel-slip angles αF of thefront wheels are greater than those of the rearwheels αR. The vehicle wants to steer a larger ra-dius of bend than that corresponding to the lockon the front wheels and drifts outwards over thefront wheels.

● Oversteer (Fig. 3). The wheel-slip angles of therear wheels αR are greater than those of thefront wheels αF. The vehicle wants to steer asmaller radius of bend than that correspondingto the lock on the front wheels and the vehiclestarts to break away at the rear.

● Neutral drivability. The wheel-slip angle of thefront and rear wheels is the same. The vehicledrifts evenly on all the wheels.

Vehicles with

● front wheel drive tend to understeer● rear engines and rear-wheel drive tend to over-

steer● all-wheel drive tends towards neutral drivability

The aim is for neutral or slightly understeered dri-vability (with the exception of sports vehicles).

Yawing is the rotational motion of the vehicleabout its vertical axis (yaw axis) (Fig. 1, Page 414).The yaw velocity is measured by yaw sensors onvehicles with ESP.

Rolling is the tipping movement about the roll axis(Fig. 3, Page 414).

Pitching is the rotational motion of a vehicle aboutits transversal axis (Fig. 1, Page 414).

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Wheel-slip angle α is the angle between thewheel plane and the actual direction of wheelmotion.

Wheel-slipangle

Direction ofwheel motion

Wheelplane

Wheelangle

Attitude angle

Centreof gravity

To centreof curve/bend

Vehiclelongitudinalaxis

a

Directionof travel

Fig. 1:Wheel-slip angle and attitude angle

The attitude angle is the angle between the di-rection of travel (direction of motion of the ve-hicle) and the vehicle longitudinal axis.

αF > αR

αR

αF

FS

Fig. 2: Understeer

αR

α F

FS

αR > αF

Fig. 3: Oversteer

REVIEW QUESTIONS

1 What are the 3 spatial axes of a vehicle and whatare the movements about them called?

2 What is meant by roll centre (instantaneous cen-tre)?

3 How is the roll axis of a vehicle formed?

4 What is the wheel-slip angle?

5 Explain the terms understeer, oversteer and neu-tral drivability.

18.2 Basics principles of steering

The main steering components in the motor vehi-cle are (Fig. 1):● Steering wheel ● Steering spindle● Steering gear ● Tie rod● Tie-rod arm

Functions:● Turning (swivelling) the front wheels.● Enabling different steering angles.● Strengthening (gearing up) the torque generat-

ed manually at the steering wheel.

Designs:● Swinging beam steering ● Ackermann steering

18.2.1 Swinging beam steering

When the wheels of the steering axle are turned,they are pivoted about a common rotational axis(steering axis). The tendency to tilt increases dueto the reduction in the size of the standing area.Swinging beam steering is used on twin-axle trail-ers. It offers good manoeuvrability.

18.2.2 Ackermann steering

Each wheel is pivoted about its own axis, the steer-ing axis. It is formed by the connection of the up-per and lower mounting points of the wheel sus-pension (Fig. 2, Page 418) or by the longitudinal

connection of the kingpins. Ackermann steering isused on all dual-track motor vehicles. When thewheels are turned about the steering axis, thestanding area remains almost the same size.

Behaviour of the wheels when corneringIn order for the wheels to be able to roll faultlesslywhen cornering, each steered wheel must beturned to an angle appropriate to the radius ofbend. A greater wheel angle is required for a smallradius bend than for a larger one.

Since on dual-track vehicles, the wheels on the in-side of a curve follow a smaller radius of bendthan those on the outside of a curve, they must beturned to a greater angle than the wheels on theoutside of a curve.

The different steering locks are achieved by thesteering trapezoid.

Ackermann principle. The wheels must be turnedsuch that the projected centre lines of the steeringknuckle of the wheels on the inside and the outsideof the bend meet the projected centre line of therear axle. The circular trajectories covered by thefront and rear wheels then have a common centrepoint (Fig. 3).

Steering trapezoidThis is formed by the tie rod, the two tie-rod armsand the line through the two steering axles (Fig. 4)when the front wheels are set to the straight-aheadposition.

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Steering wheel

Steeringspindle

Tie-rod arm

Tie rod

Steering gear

Tie-rod end

Tie rod

Fig. 1: Main steering components

Standing area

Steering axle Steering axle

Fig. 2: Swinging beam steering, Ackermann steering

bd

a

Steeringknuckle

Wh

ee

lba

se

d Toe differencea, b Wheel angle

Fig. 3: Ackermann steering, toe-difference angle

The steering trapezoid allows the front wheelsto turn at different angles, the inside wheel be-ing turned further than the outside wheel.

Tie-rod arm Tie rod Steering knuckle

Steeringtrapezoid

Steering axle

Fig. 4: Steering trapezoid

18.2.3 Steering linkage

Functions:● Transfer of the steering movement produced by

the steering gear to the front wheels.● Guidance of the wheels in a particular toe-in angle.

Main componentsTie rod(s), tie-rod joint, tie-rod arm, possibly inter-mediate lever and steering rod.

Rigid front axle. Recirculating-ball steering gear isusually used as the steering gear on commercialvehicles. The movement is transferred by the steer-ing-gear pitman arm via the steering rod to the in-termediate lever and track arm (tie-rod arm). Thelatter is connected to the one-piece tie rod and thetrack arm of the other side of the axle by a tie-rodlinkage (Fig. 1).

18.3 Wheel adjustment

Wheelbase

Track width

Wheelbase times track width gives the wheel con-tact area.

Toe-in

The toe-in is measured at the hub height from rimflange to rim flange and may be given as the toe-inangle (for both wheels) either in millimetres or indegrees (°).

A distinction is drawn between: ● Toe-in● Zero toe● Toe-out

Toe-in (l2 – l1) > 0 (Fig. 4)This is used with rear-wheel drive and positivekingpin offset. The wheels are pivoted outwards bythe rolling resistance at the front.

Zero toe (l2 – l1) = 0

Toe-out (l2 – l1) < 0 (Fig. 5)This is used with front-wheel drive and positivekingpin offset. The wheels are turned inwards bythe motive force acting on the tyre contact patch.

Toe, camber, kingpin inclination, kingpin offset andcastor are determined such that the following ob-jectives are attained:

● Small and favourable self-steering effect

● Good straight-running stability

● Low tyre wear

● Compensation for play in the wheel location

● Little or no tendency of the wheels to wobble

Toe-difference angle

The toe-difference angle is determined at a steer-ing angle of 20° on the wheel on the inside of abend.It is needed when checking the steering trapezoidfor faults, e.g. if the track arms or tie rods are de-formed.

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Pitman arm

Recirculating-ballsteering gear

Steering rod

Reversing lever

Tie-rod arm

Tie rod

Connection for steering spindle

Fig. 1: Rigid axle with single-piece tie rod

The wheelbase is the distance between the cen-tre of the front wheels and the centre of the rearwheels (Fig. 2).

Fig. 2:Wheelbase

EUROPE

Fig. 3:Track width

l2

e2

e2

l1

Fig. 4:Toe-in

e2

e2

l1

l2

Fig. 5:Toe-out

The track width is the distance between thewheels on one axle, from the centre of one tyreto the centre of the other, measured whenstraight (Fig. 3).

The toe is the difference in length l2 – l1 be-tween the front of the two wheels and the rearof the two wheels when set straight ahead.

The toe-difference angle δ is the angle by whichthe wheel on the inside of the bend is turnedbeyond the angle turned by the wheel on theoutside of the bend (Fig. 3, Page 416).

To optimise a vehicle's handling characteristics inrespect of the self-steering effect, straight-runningstability, directional stability and of the tendencyof the wheels to wobble, the various wheel set-tings, such as camber, kingpin inclination, kingpinoffset, castor and toe-in, are co-ordinated. The aimof this is the least possible tyre wear.

Camber

Camber angle γ is given in degrees and minutes. Adistinction can be made between:

● Positive camber ● Negative camber

Positive camber. The wheel plane tilts outward atthe top. Positive camber produces a cone effect.The wheel thereby tends to turn (pivot) outward.The greater the positive camber, the lower the lat-eral force when cornering.

Negative camber. The wheel plane is tilted inwardat the top. The cone effect causes the wheel to tendto turn inwards.Negative camber improves the lateral guidancewhen cornering, however it produces increasedtyre wear on the inside of the tread.Most vehicles have a camber of – 60’ to + 30’ at thesteered front wheels when the wheels are in thestraight-ahead position. Deviations of ± 30’ are per-mitted.Generally, a negative camber of – 30’ to – 2° is usedat the rear wheels.

Kingpin inclination

The steering axis runs through the upper and low-er wheel suspension points, for example.

The kingpin inclination γ is given in degrees andminutes. Kingpin inclinations of 5° to 10° are usual.

Kingpin inclination and camber together form anangle, the size of which remains the same duringcompression and rebound. If the kingpin inclina-tion becomes smaller γ, the camber angle be-comes larger and vice-versa.

With positive kingpin offset, the kingpin inclinationcauses the vehicle to be raised at the front whenthe wheels are turned.

The weight of the vehicle creates a torque, whichcauses the wheels to return automatically to thestraight-ahead driving position.

Kingpin offset

The kingpin inclination and camber together influ-ence the kingpin offset. A distinction can be madebetween:

● Positive kingpin offset

● Zero kingpin offset

● Negative kingpin offset

Positive kingpin offset (Fig. 3)

If a braking force acts on the tyre, the wheel pivotsoutward. If the grip of the wheels is different, thewheel with better grip is pivoted further outwardand the vehicle pulls to one side. The aim is a smallkingpin offset, to keep the effect of outside forceson the steering to a minimum.

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Camber is the angle of the wheel plane in rela-tion to a vertical line at the wheel contact pointat right angles to the vehicle longitudinal axis (Fig. 1).

γ γ

Positive Negative

+ –

Fig. 1: Positive and negative camber

The kingpin inclination is the angle of the steer-ing axis or kingpin at right angles to the vehiclelongitudinal axis in relation to the vertical fromthe road surface (Fig. 2).

d

Steeringaxis

Fig. 2: Kingpin inclination

g

Steeringaxis

R0

d

Fig. 3: Positive kingpin offset

The kingpin offset R0 is the lever arm on whichthe frictional forces which occur between thetyres and the road act (Fig. 3). It is measuredbetween the centre of the tyre contact patchand the intersection of the extended steeringaxis with the road surface.

The extended steering axis intersects with theroad surface beyond the centre of the tyre con-tact patch towards the inside of the tyre.

Negative kingpin offset

Negative kingpin offset is enabled by using dishedwheels and floating-calliper disc brakes, for exam-ple.

The braking forces acting on a wheel produce atorque which pivots the wheel inwards at the front,since the pivot is located on the outer side of thewheel. If differing adhesion characteristics occur,e.g. during braking (one wheel on a dry road sur-face, the other on an icy one, or in the event of apuncture), the wheel with the greater grip is pivot-ed inward more. This creates an automatic coun-tersteer, which counteracts the tendency of a vehi-cle to pull towards the side of the more heavilybraked wheel (Fig. 3).

Zero kingpin offset

Characteristics:● Low action on the steering by interference fac-

tors while driving.● The wheel moves when the steering lock is ap-

plied while the vehicle is stationary.

Castor

Castor is usually expressed as an angle ε in de-grees and minutes. Castor may also be given as adistance na in mm.

The wheels are pulled by positive castor. This isused with rear-wheel drive and helps to stabilisethe steered wheels.

If the castor angle is positive, the wheel on the in-side of a bend is lowered and the wheel on theoutside of a bend is raised when the wheels areturned. This gives a steering aligning torque aftercornering. A negative camber is also produced onthe wheel on the outside of a bend.

On vehicles with front-wheel drive, zero castor orsmall negative castor can be used. This causes areduction in the return forces and prevents thewheels from being turned back to the straight-ahead position too quickly after cornering.

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The extended steering axis intersects with theroad surface beyond the centre of the tyre con-tact patch towards the outside of the tyre (Fig. 1).

R0 < 0

Negative

Steeringaxis

Fig. 1: Negative kingpinoffset

R0 = 0

Zero

±0

Steeringaxis

Fig. 2: Zero kingpin offset

Wheel contact points

Vertical axis

M

Pivot

R0

One-sidedbraking force

MWheel

Negativekingpin offsetCountersteer moment

Moment fromone-sidedbraking force

Fig. 3: Effect of negative kingpin offset

The extended steering axis intersects with theroad surface exactly in the centre of the tyrecontact patch (Fig. 2).

Castor is the result of the steering axis or king-pin being angled along the vehicle longitudinalaxis so that it is not perpendicular to the roadsurface (Fig. 4).

Steering point

Steering axis

Steeringpoint

Directionof travel

(+) na na (–) in mm

e

Castor offset

Negativecastor angle rearward rake

Positivecastor angle

Fig. 4: Castor

Positive castor. The wheel contact point is be-hind the steering axis intersection with the roadsurface.

Negative castor. The wheel contact point is infront of the steering axis intersection with theroad surface.

Castor, kingpin inclination and kingpin offsetjointly influence the return forces on the turnedwheels. They have a stabilising effect on thesteering.

18.4 Computerised axle alignment

The measurement is made, e.g. by 8 pickups(Fig. 1), which relay the signals to the computer.The computer processes the data received intodigital display values which are output on the dis-play screen or on the computer. The individualmeasured variables can be given to an accuracy of± 5’ to ± 10’.

Measurement process

● Position the vehicle on a horizontal surface, e.g.a measuring platform.

● Check the tyre wear profile, tyre and rim size,tyre pressure, condition of the tie-rod joints,wheel bearings and steering pins.

● Position the front wheels on rotating under-plates, the rear wheels on sliding underplates.

● Compress the vehicle springs.

● Secure the angle sensor to the wheels usingclamping fixtures.

● Establish communication between the anglesensor and the computer.

● Enter the vehicle data into the computer.

● Run rim run-out compensation, if necessary, byturning the angle sensor.

● Perform the measuring process for the individ-ual wheel setting values and adjust if necessary.

● After adjustment work, perform a reference di-mension check.

● Print out the result report.

The geometrical driving axis is formed by the posi-tion of the rear wheels (Fig. 1).

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For computer axle alignment, the wheel posi-tion dimensions of the motor vehicle are elec-tronically detected and processed by a comput-er using measurement software.

The geometrical driving axis is automaticallyused by the system as the reference axis forcomputer axle alignment.

21

Rotatingunderplate

Axis of symmetry

Sliding underplate

Geometricaldriving axes

– +

34

Rotatingunderplate

Pickup Pickup

5

7 8

6

Display screen

Computer

Keyboard

Printer

Single wheel toe,front axle

Remote control

+0°03’

Pickup Pickup

–+

–+

–+

–+

– + –+

– +–+

Fig. 1: Computer axle alignment

REVIEW QUESTIONS

1 What functions does the steering have?

2 Describe the trajectory of the front wheels on a ve-hicle with Ackermann steering when cornering.

3 What makes up the steering trapezoid?

4 What functions does the steering trapezoid have?

5 What functions does the steering linkage have?

6 Explain the terms toe-in and camber.

7 What does the term toe-difference angle mean?

8 Where is the steering axis of a wheel?

9 Which different wheel settings are there?

10 What do positive and negative camber mean?

11 Explain the term kingpin inclination.

12 What effect does kingpin inclination have on thevehicle when the front wheels are turned?

13 Explain the term kingpin offset.

14 What effect does negative kingpin offset have onthe front wheels under braking with a one-sidedbraking force?

15 How is the toe-difference angle measured?

16 Describe an alignment process.

18.5 Steering gear

Functions:● Conversion of the rotary motion of the steering

wheel into displacement of the rack and/or mov-ing the pitman arm.

● Amplification (gearing up) of the torque generat-ed by hand at the steering wheel.

The transmission ratio in the steering gear mustbe designed such that the maximum force at thesteering wheel, e.g. 200 N for vehicle category M3,is not exceeded.

The transmission ratio is up to i = 19 on passengercars, up to around i = 36 on commercial vehicles.

Nowadays, rack-and-pinion steering gear (Fig. 1) isused on almost all passenger cars, whereas com-mercial vehicles generally use recirculating-ballsteering gear.

Rack-and-pinion steering gear (mechanical)Structure. A pinion fitted in the steering-gear hous-ing sits on the steering spindle and engages withthe rack by way of helical teeth. The rack is guidedin bushes and continuously pressed against thepinion by a thrust member and disc springs toeliminate play (Fig. 1).

Operating principle. When the steering wheel isturned, the rack is displaced axially by the rotarymotion of the pinion and pivots the wheels via thetie rods, tie-rod arms and steering knuckles.

Rack-and-pinion steering gear features directtransmission ratio, easy return and flat design.

Constant transmission ratio. The tooth pitch is thesame over the whole rack.

Variable transmission ratio. On mechanical steer-ing gear without hydraulic assistance, the trans-mission ratio is designed such that the steering inthe range of smaller deflections (central range) hasa more direct effect than with larger deflections inthe outer range (Fig. 2).

Advantages of the variable transmission ratio:● More direct steering for fast straight-ahead driving.● Low amount of effort required for large steering an-

gles, e.g. when manoeuvring into a parking space.

18.6 Steering systems

A distinction is drawn between steering gear with

● hydraulic assistance, e.g. rack-and-pinion steer-ing and recirculating-ball power steering,

● electro-hydraulic assistance (servo effect), e.g.Servotronic and active steering, and

● electrical assistance, e.g. Servolectric and activesteering.

18.6.1 Hydraulic rack-and-pinion steering

Structure (Fig. 1, Page 422).This consists of:

● Mechanical rack-and-pinion steering gear● Hydraulic working cylinders with working

plungers● Rotary slide as control valve● Oil pump, pressure-limiting valve, oil reservoir

The rack is driven by the pinion, the drive applied tothe tie rods is designed on both ends as a side output.

The housing for the rack constitutes the workingcylinder, which is divided into two working cham-bers by a plunger.

Rotary slide valves (Fig. 1, Page 422) or rotatingplunger valves are used as control valves.

The torsion bar is connected by 2 pins on one endwith the control bushing and the drive pinion, atthe other end it has a rigid connection to the steer-ing spindle and the rotary slide valve.

The rotary slide valve is composed of the rotatingslide and the control bushing. They have controlgrooves on their lateral surfaces. The grooves onthe control bushing open into housing flutes whichlead to the two ram chambers, to the vane pumpand to the oil reservoir.

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Rubber bellows

Tie rod

Rack Thrust member

PinionConnection,steering spindle

Fig. 1: Mechanical rack-and-pinion steering gear

Outer rangeindirect

Middle rangedirect

d1

P1 P2

d2

Outer rangeindirect

Fig. 2:Variable transmission ratio on mechanical rack-and-pinion steering gear

Operating principle. When the steering wheel isturned, the steering force applied manually istransferred via the torsion bar to the drive pinion.At the same time, the torsion bar is stressed inproportion to the counterforce and twisted slightly.This causes the rotary slide to turn in relation tothe control bushing surrounding it. This changesthe positions of the control grooves in relation toone another. The inlet slots for the pressure oilsupply are opened. The pressure oil coming fromthe oil pump flows through the inlet slots into thelower radial groove of the control bushing and ischannelled into the relevant ram chamber.

The fluid pressure acts on either the right-hand orthe left-hand side of the working plunger and gen-erates the hydraulic assisting force here. It acts inaddition to the steering force transferred mechani-cally from the pinion to the rack.

If the steering wheel is not turned any further, thetorsion bar and rotary slide valve return to theneutral position. The ports to the ram chambersare closed, the ports for the return flow areopened.

The oil flows from the oil pump via the controlvalve back to the supply reservoir.

18.6.2 Servotronic electro-hydraulicpower steering

At low driving speeds, the full assisting force ofthe hydraulic rack-and-pinion steering takes effect.The hydraulic assisting force is reduced as drivingspeed increases.

Structure (Fig. 2). Servotronic consists of:

● Electronic speedometer ● ECU

● Hydraulic rack-and-pinion steering ● Oil reservoir

● Electro-hydraulic converter ● Oil pump

Operating principle. At speeds below 20 km/h thesolenoid valve controlled by the ECU remainsclosed.As the speed increases, the solenoid valve is grad-ually opened.

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Rack

Drive pinion

Working cylinderWorking plunger

Pin

Oil pumpTorsion bar

Pin

Oil reservoirInlet slot

Rotary slide

Control bushing

Return flow

Port

Supply

Torsion bar

Controlbushing

Rotary slide

Radial grooves

Axial groove

Fig. 1: Hydraulic rack-and-pinion steering with rotary slide valve steering right

Servotronic is an electronically controlled rack-and-pinion steering system in which the hy-draulic assisting forces are influenced by thedriving speed.

Speedometer

Oil reservoir

ECU

Electrohydraulic converter

Pressure-oilpump

Hydraulic rack-and-pinion steering

+--

Fig. 2: Servotronic with hydraulic rack-and-pinion steering

Steering right at low speed.If the steering spindle is turned clockwise, rightvalve plunger (6) is pushed down by the torsionbar and the lever fitted to it. The pressure oil flowsinto ram chamber (12), acts on the workingplunger, thereby assisting the steering force.At the same time, the oil flows through open non-return valve (8) into chambers (4) and (5).

Steering right at high speed. The solenoid valve isfully open. The pressure oil flows from ram cham-ber (12) via open non-return valve (8), throttle (10)and the open solenoid valve to the return flow.As a result of the oil flowing in through non-returnvalve (8) and the throttle effect of throttle (10), thepressure in chamber (4) is greater than in chamber(5). This pushes the lever of plunger (6) upwardsand produces a reaction torque on the torsion barand steering spindle.The steering power assistance thus decreases, thedriver must apply more steering force to the steer-ing wheel, the steering is more direct.

18.6.3 Electric power steering (Servolectric)

Operating principle.The steering torque applied bythe driver is measured via a torsion rod with atorque sensor and in addition the speed is mea-

sured via a speed sensor. The two signals are fedto the ECU. The ECU calculates the torque requiredand its force-transfer direction using stored pro-gram maps and sends the relevant output signalsto the electric motor. The latter generates an assist-ing torque which is transformed by a worm-gearpair and transmitted via the steering spindle to therack-and-pinion steering gear.

18.6.4 Active steering

This system primarily consists of:

● Hydraulic rack-and-pinion steering

● Electric motor ● Planetary gearbox

● ECU ● Sensors

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ECU

km/h

Speedometer

Torsion bar Pressure oil

Solenoidvalve

Workingplunger

9 11 10 8

45

12

67

Lever

Plungerrod

Fig. 1: Servotronic hydraulic system steering right and v < 20 km/h

With Servolectric (Fig. 2), the assisting force isgenerated by an electronically controlled elec-tric motor. The electric motor is only switchedon when required.

ECU

Electricmotor

Worm-gearpair

Torque sensor

ECU

Torque sensor

Rack-and-pinion steering gear

Speedometer

Rack Pinion

Fig. 2: Servolectric electric power steering

Active steering allows a steering movement tobe made without any driver input.

REVIEW QUESTIONS

1 Name the functions of a steering gear.

2 What is a variable transmission ratio on a rack-and-pinion steering gear?

3 What are the different types of steering systems?

4 How is a hydraulic rack-and-pinion steering unitconstructed?

5 Describe the operating principle of hydraulic rack-and-pinion steering.

6 How is Servotronic electro-hydraulic power steer-ing constructed?

7 How is Servotronic electro-hydraulic power steer-ing distinguished from Servolectric electric pow-er steering?

18.7 Wheel suspension

The wheel geometry should change little or in thedesired manner when the springs on the axles arecompressed, to achieve a high degree of drivingsafety and comfort with low tyre wear. A distinc-tion is drawn between

● rigid axles ● independent suspension● semi-rigid axles

Rigid axlesThe two wheels are connected to each other by arigid axle and sprung against the body.

When the vehicle is driven over an obstacle on oneside, the whole axle is tilted and the camber of thewheels is changed.

Rigid axle with integrated drive.The axle is usuallydesigned as housing for the final-drive unit withdifferential and the axle shafts. Since the housingis generally made from cast steel, this results inrelatively large unsprung masses, which reducedriving smoothness and driving safety. On com-mercial vehicles, it is easiest to secure to the frameor to the body using the leaf springs. In addition tothe suspension, these can also take over wheelguidance in a longitudinal or lateral direction.When helical springs or air springs are used

● trailing arms transfer the wheel forces in a longi-tudinal direction.

● transverse struts (panhard rod) transfer the lateralforces (Fig. 1).

The use of several trailing arms can reduce divingunder braking and rear squatting under accelera-tion.

Rigid axle with separate drive (De Dion axle).To re-duce the large unsprung masses of the drivenaxle, the final-drive unit is separated from the axleand is fitted to the bodywork. Power is transmittedvia propeller shafts, each having two homokineticjoints with additional length compensation. Therigid rear-axle tube twisted into a U-shape can belaterally guided by:

● Two transverse struts (Fig. 2)● A Watt linkage

● A Panhard rod

Rigid axle as a steering axle. This usually consistsof quenched forging with a I-shaped cross section.To ensure that the engine has sufficient room, theaxle is bent downward (Fig. 3). As a mounting forthe steering knuckle, a stub – stub axle – or a fork –fork axle – is forged on (Fig. 4).

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Wheel suspensions have the task of forming aconnection between the vehicle body and thewheels. They must absorb high static forces(load) and dynamic forces (motive, braking andlateral forces).

On rigid axles, there is no change in the toe-inor camber during the compression and re-bound of a wheel, which reduces tyre friction.

Upper trailing arm Panhard rod

Lower trailing arm

Fig. 1: Rigid axle with integrated drive

Rear-axle tube Body mounting

Final drive

Transverse strut Transverse strut

Fig. 2: De Dion axle

Stub Fork

Fig. 4: Stub axle, fork axle

Leaf spring Front axle

Stubaxle

Fig. 3: Rigid axle as a steering axle

Semi-rigid axles

On vehicles with front-wheel drive, the use of semi-rigid axles is preferred, the rear axle can be of simpledesign so that the unsprung masses remain light.

Torsion-beam axle.The rear wheels are suspendedfrom the trailing arms, which are welded to across-member made of spring steel (Fig. 1). Thecross-member itself is screwed on to the bodywith rubber-metal bearings. If both wheels com-press to the same extent, e.g. under load, thewhole axle housing is pivoted evenly in the rub-ber-metal bearings. If only one wheel spring iscompressed, the cross-member becomes twistedin itself and acts like an anti-roll bar. Only small toeand camber changes occur.

Independent suspension

The following are used for the front-wheel suspen-sion:● Double-wishbone axles● Multiple suspension arms● McPherson suspension strut with control arm

The rear wheels are predominantly suspended on:● Trailing arms● Semi-trailing arms● Multiple suspension arms

Wheel suspension on double-wishbone axles.Twocontrol arms, one on top of the other are each con-nected via a ball joint to the steering knuckle. Cam-ber and toe changes can be controlled during op-eration by the length of the individual suspensionarms.

Control arms are usually of the wishbone type, toincrease rigidity in the direction of travel. They aresecured to the chassis by two bearings.

Wheel suspension on unequal-length control arms(trapezium shape, Fig. 2).The upper control arm isalways shorter than the lower one. This results in anegative camber and little toe change during com-pression and rebound, which improves stabilitywhen cornering.

Wheel suspension on equal-length control arms(parallelogram shape). The camber does notchange during compression and rebound, howev-er there is a toe change.

Wheel suspension with suspension strut and con-trol arm (McPherson axle). The McPherson axle(Fig. 3) developed from the double wishbone axle.The upper control arm was replaced by a vibra-tion-damper pipe, to which a steering knuckle is at-tached. The plunger rod of the damper is securedto the vehicle body in an elastic rubber bearing.There is a helical spring between this attachmentpoint and the spring seat on the damper pipe. Dueto the large braking, acceleration and lateralforces, the plunger rod and plunger rod guide are

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On semi-rigid axles, the wheels are fixed rigidlyto each other by axle supports. The wheels canmove independently of each other to a certainextent due to the elasticity of the axle supports.

A semi-rigid axle acts like a rigid axle if bothwheels are compressed at the same time andlike independent wheel suspension if com-pressed at different times.

Independent suspension allows the mass of theunsprung parts to be kept small. The compres-sion and rebound of a wheel has no influenceon the other wheels.

Trailing arm

Cross-member

Bearing block

Fig. 1:Torsion-beam axle

Subframe

Anti-roll bar

Triangular control arm

Fig. 2:Wheel suspension on double-wishbone axles

McPherson suspension strut

Anti-roll bar

Control arm

Fig. 3: McPherson axle

of a particularly sturdy design. The rubber bearingmust absorb large axial forces and allow large an-gles of twist at the steering axles. The wheel hous-ing is strengthened at the upper attachment point.

Wheel suspension on trailing arms.This is particu-larly suitable for vehicles with front-wheel drive,since the boot floor between the rear wheels canbe lower. If the suspension rotational axis is lyinghorizontal, the track width, toe-in and camber donot change during compression and rebound.

Subframe (Fig. 1). To keep noises and vibrationsaway from the body more adequately, suspensionarms are not attached directly to the body, but areattached to a subframe. This consists of 2 retainerarms which are connected to a horizontal tube. It isbolted to the body at 4 rubber bearings, with thefront rubber bearings designed as hydro mounts.The two trailing arms are attached to the subframeby taper roller bearings. To minimise toe changescaused by the lateral forces created during corner-ing, the trailing arm has a tension bolt. These twotogether form a four-bar linkage.

Wheel suspension on semi-trailing arms. Semi-trailing link axles (Fig. 2 and Fig. 3) consist of twowishbones, on which the rotational axis of the twomounting bearings runs diagonally to the trans-versal axis of the vehicle (α = 10° to 20°) and hori-zontally or slightly tilted towards the centre of thevehicle (�).

The toe and camber changes during compressionand rebound are dependent on the inclined posi-tion and slope of the semi-trailing arm. If angles αand � are increased, the wheels adopt greater neg-ative camber during compression, which increasesthe lateral force when cornering.

With this type of wheel suspension, the driveshafts change length during compression and re-bound, which necessitates 2 slip joints on eachside each with length compensation.

Multiple suspension arm axles. All existing wheelsuspensions permit undesirable steering move-ments while the vehicle is in motion due to theelastic suspension mounting on the body, sub-frame or wheel carrier. Steering movements occurwhen forces act on the wheel and move it out ofthe direction of travel by a steering angle towardstoe-in or toe-out. This can cause the vehicle tochange course significantly, e.g. if there is a cross-wind.

Type and effect of forces on the wheels:

● Motive forces act in the centre of the wheelalong the longitudinal axis of the vehicle andturn the wheel in the toe-in direction.

● Braking forces act in the centre of the tyre con-tact area along the longitudinal axis of the vehi-cle and turn the wheel in the toe-out direction.

● Lateral forces act just behind the centre of thetyre contact area at right angles to the vehiclelongitudinal axis. When cornering, the wheel onthe outside of a bend is steered into the toe-outdirection, which reduces cornering safety. Whencornering sharply, the tread of the tyre is de-formed by the rolling movement of the bodyand by the lateral force which reduces the tyre'sreserves of adhesion.

● Vertical forces act in the direction of the vehiclevertical axis. These occur if the road surface isuneven or if the vehicle is loaded and causesmall toe and camber changes.

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Anti-roll bar

Hydro mount

Subframe

Trailing arm

Four-bar linkage

Tension bolt

Tension bolt

Fig. 1:Wheel suspension on trailing arm

Top view View from rear

a

b

Fig. 2:Tilt angle on semi-trailing arms

Subframe

Semi-trailing arm

Anti-roll bar

Miniblock spring

Fig. 3: Rear-wheel suspension on semi-trailing arms

Elastic steering faults. Fig. 1 shows the steeringangle created by the motive force. While the rearrod control arm is tensioned and elongates slightlydue to the elastic suspension, the front rod controlarm is placed under pressure, leading to a slightcontraction. The wheel is turned out of the direc-tion of travel.

Multilink rear suspension. This compensates forelastic steering faults. It was developed from thetwin-control-arm axle with anti-roll bar. The suspen-sion arms which were originally rigidly coupled werebroken down into 5 individual beam suspensionarms, which lie in exactly fixed position in relation toeach other in space and guide the wheel (Fig. 2).

The intersection point of the suspension-arm cen-tre line lies outside the wheel midplane, so that thewheel, for example by the action of motive forces,steers exactly as far outwards (M2) as is steered in-wards by the elastic fault (M1).

Kinematics of the multilink rear suspension. Thecritical factors for drivability are primarily the toe-inand camber changes, since the self-steering effectof the vehicle is determined by these. If there arechanges in the toe angle, a lateral force is createdwhich disrupts the straight-running stability. InFig. 3 it is possible to see that the toe-angle changeduring compression or rebound is almost zero.Camber changes in the middle zone of the corner(straight-ahead travel) should be as small as possi-ble, in order not to create large lateral forces. Anegative camber arises from compression duringcornering, which improves lateral guidance.

Roll centre (instantaneous centre).This is the pointabout which the body, connected to the chassis bysprings, tilts under the action of a lateral force. In-stantaneous means that this point is only locatedin this position for a moment.

The higher the roll centre, the less distance to thecentre of gravity of the vehicle, i.e. the lever armon which the centrifugal force acts becomes small-er, the lateral tilt is reduced. However, the greatertoe-width changes are a disadvantage as theycause uneven straight-running stability. The con-necting line through the roll centres of the frontand rear axles gives the roll axis. Its distance to thecentre of gravity determines the lateral tilt of thebody.

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Motiveforce

Com-pression

Tension

a = Movement inresilient bearing

Front rodcontrol arm

Rear rodcontrol arm

Resilientbearing

Steeringangle

a

a

a

a

Direction of travel

Fig.1: Generation of a steering angle

Wheel axisM2

M1

Fig. 2: Multilink rear suspension

REVIEW QUESTIONS

1 What are the advantages and disadvantages ofrigid axles?

2 What is a semi-rigid axle?

3 Name the most important types of independentsuspension.

4 What advantage does the wheel suspension on adouble-wishbone axle have?

5 What is an elastic steering fault?

6 What is a subframe?

7 How is a McPherson axle constructed?

8 Which forces act on the wheel when the vehicleis in motion and how does it react to them?

9 What are semi-trailing arms?

10 How is a multilink rear suspension constructedand what advantages does it have?

11 What is the roll centre?

12 How does a high roll centre affect the tilt of thebody?

80

60

40

20

Toe-out Toe-in

Com-pression

20

40

60

80

Rebound

0.8 0.6 0.4 0.2

mm

0.2 0.4 (°) 0.8

Toe-anglechange

Camberchange

Positive

80

60

40

mm

20

20

40

60

80

–4 2–2 4(°)

Negative

Fig. 3: Changes in toe angle and camber

18.8 Suspension

18.8.1 Function of the suspension

Due to the unevenness of a road surface, thewheels of a vehicle must perform movements upand down in addition to their rotational motion.When the vehicle is being driven fast, these move-ments occur within a very short space of time,generating accelerations and decelerations per-pendicular to the road surface which are a multipleof acceleration due to gravity. This causes signifi-cant, impulsive forces to act on the vehicle, whichare greater if the inertia is greater.

Suspension and damping are decisive for

● driving smoothness. The vibration of the bodymoderates the uncomfortable impacts whichcould cause injury to the occupants and fragileloads are protected.

● driving safety. If the road surface is very un-even, contact with the surface may be lost; ifwheels are up in the air, they cannot transmitany forces, e.g. motive forces, braking forces.

● cornering ability. When the vehicle is corneringat high speeds, the low wheel grip on thewheels on the inside of a bend causes a reduc-tion in the lateral force. To prevent the vehiclesliding out of a bend, the suspension must haveshock absorbers and an anti-roll bar to ensureconstant grip of the wheels.

The springs are fitted between the wheel suspen-sion and the body. The action of the springs is sup-ported by the tyres. An additional suspension,which is only of benefit to the occupants, however,is the seat suspension (Fig. 1).

Lateral suspension. In addition to the vertical joltsfrom rough road surface, slight lateral jolts alsooccur. The suspension must therefore also be ef-fective in this direction. In addition, lateral suspen-sion can be provided in part by the tyres and bythe rubber bearings, which serve to secure andguide the wheel suspension components.

18.8.2 Operating principle of the suspension

Due to the suspension, the motor vehicle becomesa vibratory structure with its own vibration fre-quency defined by the vehicle weight and by thespring (body-vibration frequency).

In addition to the jolts from rough road surface,other forces also act on the vehicle (motive forces,braking forces, centrifugal forces). Movements andvibrations can thus occur along the 3 spatial axes(Fig. 2).

VibrationsIf the wheel of a motor vehicle travels over an ob-stacle, both the body and the wheel start to vi-brate. The upwards movement of the wheel causesthe helical spring to be compressed, the springforce accelerates the body upwards. The springforce generated when the spring expands slowsthe body down again, the upper reversing point isreached. The body is accelerated downwards bythe weight, beyond the rest position. The spring iscompressed (tightened), the resulting spring forceslows the movement of the body down to the low-er reversing point.

This motion sequence is repeated until the kineticenergy is converted into heat by spring and air fric-tion (Fig. 3).

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The suspension works together with the damp-ing to absorb jolts from the road and to convertthem into vibrations.

Fig. 1: Suspension in a passenger car

Vertical axis

Transversal axis

Longitudinal axis

Wobbling

Pitching

SkiddingYawing

Motive force

Braking forceLateral force

TiltingRolling

Lifting

Lowering

Drifting Jerking

Fig. 2:Types of vibration acting on the motor vehicle

The travel from the upper to the lower revers-ing point of a vibration is known as the ampli-tude of oscillation.

0

Am

plitu

de o

f o

scilla

tio

n

Time

Fig. 3: Damped vibration

Resonance. The vibration is pitched if the body isjolted at the frequency of natural oscillation, e.g.when driving over rough roads, where the obsta-cles are equal distances apart one after the other(Fig. 1).

Frequency. This is the number of vibrations persecond. Since a body does not vibrate very quickly,the number of vibrations is also given per minute(vibration frequency, body-vibration frequency).

Spring rate. This indicates the properties of thespring (hard, soft). To check or compare springs, aload is applied to them and the resulting compres-sion is measured. The ratio of force F to travel l isreferred to as the spring rate c in N/m.

Spring characteristics. If the spring rate is thesame over the whole range of spring (constant), asfor a normal helical spring, for example, the springhas a linear characteristic (Fig. 2).

If the spring rate increases as the range of springincreases, e.g. with multi-leaf springs or conicalhelical springs, the characteristic is plotted as acurve. The spring has a progressive characteristic(Fig. 3).

Sprung masses, unsprung massesOn motor vehicles, a distinction is drawn betweensprung masses (body with load) and unsprungmasses (wheels with drum or disc brakes, parts of

the wheel suspension). These different masses areconnected (coupled) to one another by thesprings. This causes feedback to one another, sothat the two masses vibrate in different frequencyranges independently of each other (Fig. 4). If a vi-bration damper (shock absorber) is fitted betweenthe two masses, the amplitude of oscillation be-comes smaller, the vibration dies out more quickly.

If a vehicle is driven over a hump at high speed,the body initially remains balanced due to thelarge mass. The wheel, with its small mass in rela-tion to the body, is accelerated upwards veryrapidly, and in doing so it compresses the spring.Only the force corresponding to this spring travelis acting on the body.

On the other side of the hump, the wheel is accel-erated downwards by the preloaded spring. Onlythe load relief of the spring corresponding to thebump acts on the body.

If the force from the wheel is greater than the ini-tial tension of the spring, the wheel loses adhesionon the road surface for a short time, as the initialspring tension is insufficient to move the wheeldownwards quickly enough.

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Am

plit

ud

e o

fo

scill

atio

n

0t

Fig. 1: Pitched vibration

A large mass and soft springs result in a lowfrequency (vibration frequency) and a largespring travel.

2,500

2,000

1,500

1,000

500

00 0.05 0.10 0.15 0.20m

N

s

F

Soft c = 7,500 N/m

Hard

c=

25,0

00N

/m

F

Fig. 2: Linear spring characteristics

0.05 0.10 0.15 0.200 m

2,500N

1,500

1,000

500

0

F

sF

2,000

Fig. 3: Progressive spring characteristic

Sprung mass

Unsprung mass

Fig. 4: Motion sequence when driving over an unevenroad surface

To achieve good driving safety and the bestpossible comfort, the unsprung mass should beas small as possible.

Body vibration frequenciesThese can be determined by the vibration at thefront or rear of the vehicle. A complete vibrationconsists of the spring compression and reboundprocess. The number of vibrations per minute thengives the body-vibration frequency. Vibrationdampers do not control the vibration frequency, theamplitude of oscillation is downrated by the greaterresistance. In contrast, the mass plays a large part.The heavier the vehicle or the larger the payload,the lower the vibration frequencies become.

Soft suspension: 60 vibrations per minute or lowercan cause nausea. This can be rectified by strongerdamping.

Hard suspension: 90 vibrations per minute or low-er jar the spine. Hard springs are often required forhigh payloads on the rear axle, however, wherebya more moderate driving smoothness when un-laden is achieved. This applies particularly to smallvehicles, which must be equipped with sustain-able, i.e. hard, springs due to the unfavourable ra-tio of net weight to maximum load.

18.8.3 Types of springs

18.8.3.1 Steel springsMost motor vehicles are fitted with steel springs.These may be:

● Leaf springs ● Helical springs● Torsion-bar springs ● Anti-roll bars

The spring effect is caused by the elastic deforma-tion of spring steel (e.g. chrome-vanadium springsteel) up to the limit of elasticity. The spring char-acteristic is linear, but the design of the spring cancause it to be progressive.

Leaf springsThese have a minor role in passenger cars. How-ever, in heavy vehicles, they are the most com-monly used type of spring (see Chapter Commer-cial vehicle technology).

Helical springsThese are primarily used as compression springsin passenger cars.

Advantages: Low weight, low space requirementsDisadvantages: Almost no damping, no transmis-

sion of wheel forces (longitudinaland transverse forces).

Helical springs usually have a linear spring charac-teristic. Soft helical springs differ from hard helicalsprings in that they have a:

● Smaller wire diameter● Larger spring internal diameter● More loosely wound coil

Helical springs with a progressive characteristicmust be fitted to allow a greater payload and ade-quate comfort when the vehicle is unladen. Thiscan be achieved with the following:

● Different sizes of the internal diameter, e.g. tapershape, barrel shape, waist shape

● Different wire diameters (Fig. 1)

The barrel-shaped miniblock spring has the advan-tage over the cylindrical helical spring that thespring coils cannot touch when the spring is com-pressed while the vehicle is in motion becauseeach coil lies inside the larger ones forming a spi-ral (Fig. 2). This means that the spring can beshorter without sacrificing a long spring range fora high load-carrying capacity. The miniblock springincorporates all the options for a progressivespring.

They are therefore only used in axle designs inwhich the motive, braking and lateral forces aretransferred by other elements (control arm, trailingarm, McPherson-suspension strut). Vibrationdampers are nowadays only rarely used inside thehelical spring (Fig. 2) because fitting and removalare very time consuming.

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Normal spring Barrel shape Taper shape Waist shape

Different wire diameterat either end

Barrel shape with differentwire diameter

Fig. 1:Types of helical springs

Fig. 2: Miniblock spring

Helical springs cannot transfer wheel guidanceforces.

Torsion-bar springA torsion-bar spring is a rod made of spring steel(Fig. 1) which is caused to twist by a lever on whichthe wheel is mounted.

Torsion bars are mostly round rods, square barsand packages of flat bars. They may be arrangedlongitudinally or transversely. A longitudinalarrangement allows greater length and therefore agreater torsion angle. These springs are softer andhave a longer travel.

Torsion bars cannot be subjected to bending. Theyare therefore often fitted in a tube which providessupport against bending and which also providesprotection.

The heads are usually interlocked. This toothed in-terlocking allows the initial tension to be changedand to be adjusted evenly on all wheels.

Anti-roll barThis is a suspension element which helps to im-prove the roadholding. U-shape torsion bars areusually used (Fig. 2).

The centre section of the anti-roll bar is able to ro-tate in its mounting on the body and the two linksare attached to the wheel suspension, e.g. controlarms, via rubber elements.

When a wheel is lifted (compression), the twistingaction of the anti-roll bar also raises the otherwheel and lowers it when the wheel is lowered.

This counteracts the excessive rolling action (lean-ing to one side) of the body when the vehicle iscornering. The anti-roll bar has no effect if bothwheels are compressed at the same time.

18.8.3.2 Rubber springNatural and synthetic rubber are very elastic andhave high internal damping characteristics. Manydifferent types of rubber springs are manufactured(Fig. 3) but are not actually used as vehicle springs.The high internal damping and elasticity of therubber is used to intercept high-frequency vibra-tions and as noise insulation. To this end, the actu-al vehicle springs or mountings, e.g. the controlarm, are mounted in rubber cushions. This also im-proves the transverse suspension.

Hydraulically damped elastomer mountings (hy-dro mount, Fig. 4) are used instead of simple rub-ber springs to prevent vibrations of various fre-quencies from being transferred from the engineto the body. These consist of an elastic bearingspring made from natural rubber, which forms themechanical connection between the engine andthe body, and a hydraulic section, which consistsof a working chamber and a compensating cham-ber and which is filled with hydraulic fluid. A perfo-rated plate between the two chambers impedesthe flow of fluid into the compensating chamberand damps any vibrations that have been trans-ferred here (see also Chapter Mechanical EngineComponents and Engine suspension).

18.8.3.3 Gas-filled springA gas-filled spring exploits the elastic properties ofan enclosed volume of gas (air or nitrogen) for thepurposes of suspension.

Air springThese are the most commonly used, but they re-quire a pressure generating system and are there-fore primarily used in buses and commercial vehi-cles which already have one of these for the brakes(see Chapter Commercial vehicle technology).

The air spring has a progressive characteristic andhas the advantage that the travel of the spring can

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Torsion bar Frame

Support tube

Spring strut(lever)

Fig. 1:Torsion-bar suspension

Shock-absorberstrut

Helical spring

Triangular control arm

Anti-roll bar

Fig. 2: Anti-roll bar

Workingchamber

Perforated plate

Bearing spring

Natural rubber

Compensatingchamber

a

Fig. 3: Rubber spring Fig. 4: Hydro mount

be adjusted to the load by altering the air pressure.The height of the load area or entrance can also beset or maintained using level control.

On passenger cars, the body can be raised andlowered according to the speed of the vehicle. Theangle of the body when cornering can be consider-ably reduced by control interventions.

To prevent pressure loss, the enclosed volume ofair is sealed in fixed rubber bellows. This may beroll bellows or a gaiter seal (Fig. 1).

Air only has a low level of internal damping. Thismeans that vibration dampers must also be fittedor a suspension strut used which consists of acombination of rubber bellows and a gas-pressureshock absorber.

Air springs cannot transfer wheel forces and theyare therefore fitted between suspension arms oraxles, e.g. torsion-beam rear axles (Fig. 2), and thebody.

Hydro-pneumatic springIn principle, a hydro-pneumatic spring (Fig. 3) is agas-filled spring combined with a working cylin-der. It has the effect of both suspension and ashock absorber. A constant volume of gas (usuallynitrogen) in a spring ball is compressed to agreater or lesser extent by pumping in or releasinghydraulic fluid. The gas and fluid are separated bya diaphragm. Gas and fluid are pressurised equal-ly. The pressure is generated by a high-pressurepump and is approximately 180 bar.

Depending on the space available, the spring ballmay be on the side next to the working cylinder orit may be completely separate from it.

The valves between the working cylinder and thespring ball throttle the flow of fluid in both direc-tions and act like a vibration damper.

All the suspension elements are interconnected bya network of lines. The hydraulic cylinder plungerrod is fixed to the trailing arm or the wheel-sus-pension control arm.

Level control. The ground clearance of the bodycan be adjusted, e.g. for travelling over roughground or for changing a wheel, using a manuallyoperated level-control valve. The level can be con-trolled automatically for all load conditions by alinkage which is fixed to the trailing arm and whichacts on the ride-height controller plunger (Fig. 4). Ifthe vehicle is more heavily laden, the rear sinksand the plunger rod in the cylinder moves in. Atthe same time, the plunger in the ride-height con-troller is moved by the trailing arm and linkage,thereby allowing the pressure oil to flow in. Theplunger rod in the cylinder moves out until the oldlevel is reached and the flow of fluid into the ride-height controller is shut off.

The increase in load causes an increase in the hy-draulic pressure in the cylinder and a simultane-ous pressure increase in the nitrogen. The springsbecome harder and, because the vibration fre-quency of the body also increases, the suspensioncharacteristics become more uncomfortable.

Fitting a third spring ball per axle increases thevolume of gas and therefore the volume of thespring; this improves the comfort characteristics ofthe chassis when driving in a straight line.

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Fig. 1: Roll bellows and gaiter seal

Cross-member

Roll bellows Trailing arm

Fig. 2:Torsion-beam rear axle with roll bellows

Gas

Oil

Pressure-oil port

Diaphragm

Plungerrod

Gas chamber

Diaphragm

Oilchamber

Cylinder

Plunger

Valves

Fig. 3: Hydro-pneumatic suspension elements

Unloaded Loaded

Pressure-oil supply

Return flow Ride-height controller

G

Trailingarm

h

Fig. 4: Hydro-pneumatic suspension

Hydractive chassisStructure. Additional components connect the hydro-pneumatic suspension system to a chassis which isable to ...

● … reduce the lateral roll of the body when cornering.

● … counteract diving of the front of the vehicle un-der braking and squatting of the rear of the vehi-cle under acceleration.

● … change the ride comfort between soft and hard,regardless of whether comfort or sports tuningis selected.

The following additional components are required(Fig. 1):● 2 anti-roll bars, each with 2 spring cylinders, for the

front and rear axles

● A centre spring ball with hardness controller forboth the front and rear axle

● Hydraulic block

● Height sensors for the front and rear axle

● Steering angle, accelerator-pedal and brake-pedalsensors

Problematic driving situations can occur when the vehi-cle corners or swerves suddenly. The angle of the bodymeans that the load on the wheels on the inside of thebend is reduced so that smaller forces are transferred tothe road surface. This can result in the vehicle breakingaway at the rear or rolling. The cornering speed and thedistance of the rolling axis from the vehicle's centre ofgravity determine the lateral roll of the body.

The angle can be reduced by fitting anti-roll bars. Ifthe wheels are compressed to a differing extent, the

anti-roll bars are twisted, they act as additional tor-sion-bar springs and the suspension becomes gener-ally harder and more uncomfortable.

Structure. On a hydractive chassis, the front springcylinders are mounted vertically and attached to theanti-roll bar via coupling rods, the rear spring cylin-ders are mounted horizontally. Depending on the hy-draulic pressure supplied in the spring cylinders, ad-ditional forces may act on the anti-roll bar and causethe spring action to be too hard.

There is a third spring ball and a height sensor be-tween the spring cylinders on each axle. Springmovements cause the anti-roll bar to twist, which theheight sensor reports to the ECU as a change of bodyattitude.

All the spring cylinders and spring balls are intercon-nected via the hydraulic block.

Hydraulic block. This consists of the hydraulic pumpwith electric motor, 4 solenoid valves and the ECU.The spring balls are supplied with hydraulic fluid bythe hydraulic pump. The operating pressure of thesystem is between 80 bar and 140 bar. Two solenoidvalves on both the front and rear axle control thesupply and return flow of the hydraulic fluid. Thismeans that the front end of the vehicle can be raisedor lowered independently of the rear end and viceversa.

Operating principle of the hydractive chassis. Byselecting the “Comfort” or “Sport” drive program,the driver can choose between a soft or hard sus-pension setting.

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Hydraulic lines Electric cables

Hydraulicblock

Front spring cylinderwith coupling rods

Rear spring cylinder

Front centre spring ball with hardness controller

Front ride-heightsensor

Rear centre spring ballwith hardness controller

Rear ride-height sensor

ECU

Steering-angle sensor

Accelerator-pedal andbrake-pedal sensors

Fig. 1: Hydractive chassis components and system

Depending on the driving conditions and drivingstyle (e.g. rapid cornering), the ECU may also makethe suspension hard in the “Comfort” drive program.

“Comfort” drive program. The three spring ballson each axle are interconnected. When the suspen-sion is compressed, the spring-cylinder plungerrod moves in and pushes out the hydraulic oil,which can flow into the spring balls, press againstthe diaphragms and compress the nitrogen cush-ion. The 3rd spring ball provides an additional gascushion which allows a softer spring action (Fig. 1).

“Sport” drive program. If the hardness controllersolenoid valve on the centre spring ball is activat-ed by the ECU, the flow to these spring cylinders isblocked. This means that only the volume of gas intwo spring cylinders is available and the suspen-sion becomes harder.

Function of the hardness controller in the “Sport”drive program (Fig. 2). The solenoid valve is sup-plied with power, the return flow to the hydraulicfluid reservoir is opened, the bottom of the valvespool is at zero pressure. Because the top is stillsubject to the pressure of the suspension, the valvespool is pushed downwards, thereby breaking theconnection between the suspension elements andfrom the suspension elements to the spring ball.

Processes during cornering, acceleration, braking.When the vehicle corners, the ECU receives infor-mation about the speed and angle of the steeringwheel from the steering-angle sensor. To counter-act any rolling movement by the body, the sole-noid valve is supplied with power and the connec-tion to the spring balls is established.

The suspension becomes harder and the tilt angleof the body is reduced. If this measure were not inplace, the body would dive towards the wheels onthe outside of the bend and the hydraulic fluidwould flow into the suspension elements on theother side. To counteract the squatting of the rearof the vehicle under acceleration, the ECU uses in-formation from the accelerator pedal sensor to dis-connect the centre spring ball on the rear axle.

18.8.4 Vibration dampers

These are fitted between the wheel suspensionand the body. Vibrations of the wheels and bodyhave different frequencies. A good damper mustbe set up so that it is effective for the two differentvibrations.

Nowadays, hydraulic vibration dampers are al-most exclusively used. These consist of a plungerwhich moves in a cylinder, pushing fluid throughsmall holes or valves (throttle points).

Rebound. The wheel moves downwards and pullsthe vibration damper apart telescopically (tele-scopic shock absorber).

Compression. The wheel moves upwards. As itdoes so, the vibration damper is pushed back to-gether.

By changing the flow resistance for the fluid as theplunger moves backwards and forwards, it is pos-sible to adjust the vehicle characteristics.

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Centre spring ball

Springcylinder

Hydraulic block

Supply reservoir

Returnflow

Electrics

Hardnesscontroller

Spring ball

Springball

Fig. 1: Hydractive chassis in comfort setting

Centre spring ball

Valvespool

Solenoidvalve

To leftspringcylinder

Return flow

Supply fromhydraulicblock

To rightspringcylinder

Fig. 2: Hardness controller in sports setting

Vibration dampers (shock absorbers) allow vi-brations from body and wheels to subsidemore quickly and therefore increase safety anddriving comfort.

Kinetic energy is converted into thermal energyby vibration dampers.

18.8.4.1 Twin-tube vibration damper

In a twin-tube vibration damper (Fig. 1), the plungerrod and protective tube are fixed to the body andthe cylinder is fixed to the wheel suspension.

The cylinder consists of an inner and an outertube. The inner tube contains the working chamberin which the plunger moves. This is completelyfilled with oil.

Between the inner and the outer tube, there is thecompensating chamber. This is only partially filledwith oil and is designed to take the oil which ispushed out of the working chamber when theplunger rod moves in.

Valves are fitted in the plunger and the workingchamber which throttle the oil flow at differing rates.

During the rebound stage, damping is stronger. Asthe plunger moves upwards, the oil has to bepressed through fine openings in the plate valvesin the plunger. At the same time, oil is sucked backout of the compensating chamber through thebase valve.

18.8.4.2 Single-tube gas-pressure shockabsorber

The single-tube gas-pressure shock absorber(Fig. 2) behaves exactly the same as the twin-tubevibration damper on the upwards and downwardsstrokes. However, a special compensating cham-ber is not required to compensate for the plungerrod volume and so there is no outer tube.

Compensation is achieved with a gas cushion ofnitrogen which is usually separated from the oilchamber by a moveable plunger. The gas cushion,which is at a pressure of 20 bar to 30 bar, issqueezed and further compressed by the oil forcedout by the plunger rod when the working plungermoves down. The gas cushion and oil are alwayspressurised, which prevents the oil from foamingand causing a reduction in the damping effect.

18.8.4.3 Twin-tube gas-pressure shock absorberThe twin-tube gas-pressure shock absorber (Fig. 3)has a similar structure to the twin-tube vibrationdamper. A ring-shaped compensating chambercontains nitrogen at an initial pressure of 3 bar to8 bar. This prevents the formation of gas bubbleswhen the vibration damper moves quickly. Thedamping forces are improved in nearly all vibra-tion ranges.

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Hydraulic vibration dampers basically consistof a cylinder in which a plunger with plungerrod can move up and down.

Rubber bearing

Plunger rod

Protective tube

Seal

Air chamber

Working chamber

Outer tube

Inner tube

Compensating chamber

Plate valves

Plunger

Base valve

Cylinder

Fig. 1:Twin-tube vibration damper

Installation only with the plunger rod at the top,as otherwise air would be drawn out of thecompensating chamber, which would cause theoil to foam and the damping to fail.

Gas cushion

Compensating chamber

Working plunger

Separating plunger

Gas cushion

Base valve

Oil chamber

Fig. 2: Single-tube gas-pres- Fig. 3: Twin-tube gas-pres-sure shock absorber sure shock absorber

Single-tube gas-pressure shock absorbers witha separating plunger can be fitted in any loca-tion. For versions with an impact plate, theplunger rod must always be at the bottom.

Twin-tube gas-pressure shock absorber withvariable dampingIn the past, it was practically impossible to adjust ashock absorber to the different load conditions of avehicle. Vehicles with heavy loads (e.g. heavygoods vehicles with trailers) require strong damp-ing, but this results in unpleasant shaking andbouncing when the vehicle is unladen and drivenon rough roads.

One or more grooves in the cylinder wall of a twin-tube gas-pressure shock absorber (Fig. 1) can pro-vide the desired variable damping characteristics.

Light load.The working plunger moves in the areabetween the two grooves. The oil can flow throughboth the plunger valves and the grooves. This ad-ditional bypass reduces the damping force, there-by increasing comfort.

Heavy load. The working plunger moves under-neath the area with the grooves where there is noadditional throughflow cross-section. There aremaximum damping forces.

The number and length of the grooves, as well astheir height offset, allow the damping forces to beadjusted not only to the load, but also to all thesuspension systems used.

18.8.4.4 Test graphsVibration damper removed. In order to obtain thecharacteristic curves of a vibration damper, thedamper must be tensioned in a testing device. Thedamper is moved by a cranked drive. The dampingforces as shown by the plunger travel are mea-sured and plotted on a graph. A constant reboundand compression results in closed curves (Fig. 2).An increase in the radius of the crankshaft on thetesting device also increases the rebound andcompression of the damper, resulting in further

closed curves. The damping force increases be-cause plunger speed in the damper increases if thecrankshaft drive is rotating at a constant speed.

Fitting valves with various throughflow cross-sec-tions in the plunger results in varying dampingforces in the rebound and compression stages. Theratio of the damping forces in the compressionstage to those in the rebound stage is between 2and 5.

Vibration damper fitted. All the dampers on oneaxle are tested on a shock tester at the same time.The wheels rest on a plate and are each caused tovibrate by an electric motor via an eccentric ele-ment and a compression spring. Once the motor isswitched off, the vibration is allowed to continuethrough its entire frequency range until it comes toa standstill and a measuring instrument recordsthis on a disc (Fig. 3).

The greatest amplitude is displayed at the reso-nance point. This indicates the damping capabili-ties of the damper concerned. If the measured res-onance amplitude is greater than or equal to thelimit value given, the damper is faulty. A discgraph can be used to show the damper vibrationson one side of the vehicle.

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Gas cushion

Groove(bypass)

Reduced damping,partial load

Strong damping,full load

Closed

Fig. 1:Twin-tube gas-pressure shock absorber withvariable damping

Compression

Rebound

Plungertravel

Dampingforce

–10–20 10 mm 20

4,000N

3,000

2,000

–1,000

Fig. 2:Test graph of a gas-pressure shock absorber

Limit valueActual value

42 MM35 MM

Rear No Yes REPLACEMENT FrontNo Yes

35 mm

49 mm

42 MM49 MM

Fig. 3:Vibration patterns of 2 dampers

18.8.4.5 Vibration dampers in the compoundsuspension system

Suspension strut

Suspension struts can also be used as wheel sus-pension if they have an additional steering knuckle(Fig. 1). Vibration-damper cartridges are used sothat the entire suspension strut does not have tobe replaced if the vibration damper is faulty. Ifthere is a reduction in damping forces, the car-tridge can be changed by opening a threaded con-nection at the top of the container tube.

Vibration dampers with a level control systemThe suspension in a passenger car is usually de-signed so that the optimum roadholding isachieved when the vehicle has an average load. Ifthe vehicle is fully laden, the rear of the vehiclesquats significantly, the ground clearance andspring range are reduced and roadholding is im-paired. This often also results in uncontrolledsteering characteristics, cross wind sensitivity andglare for oncoming traffic when driving at night.Driving comfort deteriorates because the in-creased load changes the vibration frequency ofsteel springs. A constant natural frequency of 1Hertz (corresponds to a vibration frequency of 60)in all load conditions is only possible with a level-controlled gas spring. This allows the height of thevehicle to be automatically maintained, even whentowing a trailer. A distinction is made betweenpneumatic and hydro-pneumatic systems.

Pneumatic level control system. The system con-sists of a compressor, an ECU and two air-springdampers, each with an induction sensor. The air-spring dampers consist of a combination of single-tube gas-pressure shock absorber with air springs(Fig. 2).These bear the entire axle load.

The air spring, which is attached via the gas-pressureshock absorber, consists of an air bell and roll bel-lows. If the load increases, the shock-absorber tubesinks further into the sensor coil integrated in the airbell and generates an induction voltage which is for-warded to the ECU as a signal. The ECU allows air toflow in via the compressor until the specified vehicleheight is reached. The pressure in the suspension airbag is between 5 and 11bar, depending on the load.

Hydro-pneumatic level control system.The systemconsists of:

● Suspension struts and spring-type actuators(Fig. 3)

● Compressed-oil system with radial-piston pumpand oil reservoir

● Control device with level controller and actua-tion linkage

The spring-type actuators work like a hydro-pneu-matic auxiliary spring. If the rear of the vehicle hassquatted, the spring element is supplied with pres-sure oil via the level-control valve until the normallevel is reached. The oil is then returned to the con-tainer by the pump at nearly zero pressure.

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The combination of a vibration damper in a re-inforced construction with a spring, usually ahelical spring, is known as a suspension strut.

Helicalspring S

troke

Threadedconnection

Vibration-dampercartridge

Steeringknuckle

Control arm

Vibrationdamper

Fig. 1: Suspension strut

Vehicle loaded Vehicle unloaded

Sensor cable,induction signal

Air line

Air bell

Sensor coil

Roll bellows

Gas-pressurevibration damper

Fig. 2: Air-spring damper

Connection forhigh-pressureoil pump

DiaphragmOil

Body

Suspensionstrut

Gas cushion

Spring-typeactuator

Wheel suspension

Fig. 3: Suspension strut with spring-type actuator

18.8.5 Active Body Control (ABC)

StructureEach wheel is mounted to a suspension strut consisting of a vibrationdamper and a helical spring.The plunger is a dynamically adjustable hydraulic cylinder which is ableto generate forces which counteract wheel or body movements. To dothis, the plunger moves the base of the helical spring and changes thetension. This reduces body movements in the direction of the 3 vehicleaxles.

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Hydraulic cylinder (plunger)

Vibrationdamper

Helicalspring

Fig. 1: Suspension strut withplunger

Active Body Control (ABC) is an electro-hydraulic active chassis sys-tem which, in addition to its suspension and damping functions, en-ables automatic level control while the vehicle is in motion. This main-tains the vehicle body at practically the same level at the front andrear axles when the vehicle brakes, accelerates, drives over unevenroad surfaces and bends.

Front suspen-sion strut (40)

Front bleedscrew (56)

Oil cooler(9)

Valve unit ABC,front axle (Y36/1)

Pressure accumulator,front axle (14) Front bleed screw (56)

Front suspen-sion strut (40)

Oil reser-voir (2)

Radial-pistonpump (1)

Valve unit,pressure supply (52)

Pulsationdamper (52a)

Pressureaccumulator,return (53)

Rear bleedscrew (57)

Rear suspen-sion strut (41)

Pressure accumulator,rear axle (4)

Rear bleedscrew (57)

Rear suspen-sion strut (41)

Valve unit ABC,rear axle (Y36/2)

Fig. 2: Active Body Control (layout)

Legend for the ABC diagrams

a Suction line 53 Pressure accumulator, return B24/12 Lateral-acceleration sensorb Operating pressure 56 Front bleed screw B24/14 Longitudinal-acceleration sensorc Control pressure 57 Rear bleed screw B24/3 Body acceleration sensor, front leftd Return line F1 Fuse 1 B24/4 Body acceleration sensor, front right

F2 Fuse 2 B24/6 Body acceleration sensor, rear1 Radial-piston pump N51/2 ABC ECU Y36/1 Valve unit ABC, front axle2 Oil reservoir N10/6 SAM ECU y1 Suspension strut control valve, front left 2a Oil filter B4/5 ABC pressure sensor y2 Suspension strut check valve, front left9 Oil cooler B22/1 Plunger travel sensor, rear left y3 Suspension strut control valve, front right4 Pressure accumulator, rear axle B22/4 Plunger travel sensor, front left y4 Suspension strut check valve, front right14 Pressure accumulator, front axle B22/5 Plunger travel sensor, front right y36/2 Valve unit ABC, rear axle40 Front suspension strut B22/6 Plunger travel sensor, rear right y1 Suspension strut control valve, rear left41 Rear suspension strut B22/7 Level sensor, rear left y2 Suspension strut check valve, rear left52 Valve unit, pressure supply B22/8 Level sensor, front left y3 Suspension strut control valve, rear right52a Pulsation damper B22/9 Level sensor, front right y4 Suspension strut check valve, rear right52b Pressure-limiting valve B22/10 Level sensor, rear right y86/1 ABC vacuum valve

B40/1 ABC oil temperature sensor

Task and function of the sensorsPressure sensor B4/5 reports the hydraulic pres-sure to the ECU via pin 36, pin 37 plug 2. This isregulated to 180 to 200 bar by the vacuum valvey86/1.

Oil temperature sensor B40/1 measures the hy-draulic-oil temperature in the return flow pin 26,pin 2 (plug 2).

Travel sensors in the hydraulic cylinder (plunger)B22/6; B22/1; B22/4; B22/5 transmit the actual po-sition of the positioning cylinder in the suspension

strut to the ECU pin 20; pin 17 (plug 1), pin 18,pin 16 (plug 2).

Level sensors B22/7, B22/10, B22/8, B22/9 detect thelevel of the vehicle body using the relevant controlarm pin2; pin5 (plug 1), pin20; pin42 (plug 2).

Body acceleration sensors B24/3, B24/4, B24/6measure the vertical acceleration of the vehiclebody. They consist of electronic vibration moduleswhich send their signals to the ECU via pin 6, pin 8(plug 2), pin 29 (plug 1). They are required to beable to record the lifting movements of the body.

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Y36/1

40

B22/5

56

y4

y3

y1

y2

56

B22/4

40

Y36/2

41

B22/6

57

y4

y3

y1

y2

57

B22/1

41

PU

TU

2

2a

9

14

B40/1

1Y86/1

53

52b

52a

B4/5

52

a

b

c

d

4

Fig. 1: ABC hydraulic-circuit diagram

B22/5

1610

15

B22/9

4230

41

B22/4

1819

17

B22/8

2034

44

B24/4

840

B24/3

396

B40/1

26 2

B24/6

2910

12

B24/12

278

B24/14

2325

B22/1

1716

18

B22/7

214

3

B22/6

2019

21

B22/10

54

6

B4/5

3637

38

5 29

y2

7 31

y4

1 25

y1

3 27

y3

35 11

y86/1

26 11

y2

24 9

y4

30 15

y1

28 13

y3

F1 F2

4846

Y36/1 Y36/2

1 H LPlug 3ECU plug 1ECU plug 2 N51/2

30

15

CAN CCANSAM

N10/6

2321

Diagnosis31

Fig. 2: ABC schematic diagram

Lateral and longitudinal acceleration sensorsB24/12, B24/14 determine the lateral and longitudi-nal dynamics of the vehicle pin 27, pin 25 (plug 1)and are required to compensate for rolling andpitching movements.

Signal acquisition and actuation module SAM ac-tivates the ECU via pin 23 (plug 2) via the remotecontrol, door contact switch or luggage compart-ment lighting. The ECU checks the vehicle level inorder to lower it to the preselected level if neces-sary.

ABC ECU N51/2 compares stored and preselectedprogram maps (sport/comfort) in order to controlthe actuators using incoming sensor signals andinformation that is transmitted from other systemsvia the CAN bus.

Task and function of the actuators

The vacuum valve y86/1 regulates the quantity ofoil sucked in by the oil pump so that an oil pres-sure of 180 to 200 bar can be established andmaintained in the ABC system. When it is not ener-gised, the valve is closed in order to maintain thepressure in the system.

Control valves y1, y3.The positioning cylinders aremoved when the control valves are actuated. Thiscauses the body to sink or rise at the correspond-ing wheel. The downforce of the wheels may bebriefly increased by this.

Check valves y2, y4 are closed when the engine isoff, the vehicle is stationary and if faults occur toprevent pressure loss. This also prevents the posi-tioning cylinders from being pulled apart if thewheel is changed or the vehicle is placed on a lift-ing platform, for example.

Control procedures

Starting the engine. When the vehicle door isopened, the ABC ECU is activated by the signal ac-quisition and actuation module pin 23 (plug 2). Thelevel sensors B22/7 ... 22/10 are used to comparethe actual level with the target level. If the actuallevel is higher than the target level, the controlvalves y1, y3 are actuated and the vehicle is low-ered to the target level. The ECU is powered withbattery + via pin 48 and with battery – via pin 21 inorder to carry out this control procedure. Once theignition has been switched on, there is an addition-al power supply via pin 46 plug 2.

Cornering. When the vehicle is corning, the lateral-acceleration sensor B24/12 registers centrifugalforces. The relevant signal is transferred to theECU via pin 27 plug 1. The ECU uses the speed ofthe front right and front left wheels from the CAN Cto determine whether it is a left-hand or right-handbend. If it is a left-hand bend, the ECU N51/2 actu-ates control valves y3 via pin 3, pin 27 (plug 2) andpin 28, pin 13 (plug 1), so that the plunger movesout and the side of the vehicle on the outside ofthe bend is raised. At the same time, the controlvalves y1 are switched via pin 1, pin 25 (plug 2) andpin 30, pin 15 (plug 1) so that the load on theplunger on the side of the vehicle on the inside ofthe bend is relieved. The side of the vehicle on theinside of the bend is lowered. The level sensorsB22/22/7 ... 22/10 are used to compare the actuallevel with the target level.

Acceleration. When the vehicle accelerates, thelongitudinal-acceleration sensor B24/14 registersacceleration forces on the longitudinal axis of thevehicle. The signal is transferred to the ECU atpin 25 plug 1 which actuates the control valves sothat the vehicle body sinks at the front axle and israised at the rear axle.

Braking. When the vehicle brakes, the ECU re-ceives information that a braking procedure hasbeen commenced from the closed brake-lightswitch via the CAN C. The longitudinal-accelerationsensor supplies the ECU with information aboutthe deceleration rate via pin 25 plug 1. The ECU ac-tuates the control valves so that the vehicle bodyis raised at the front axle and lowered at the rearaxle.

Driving straight ahead. When the vehicle is drivingstraight ahead, the ECU receives information aboutthe vehicle speed via the CAN C. The ECU actuatesthe control valves to automatically lower the vehi-cle according to the preselected program map. Ifthe driver wishes, the vehicle can be raised by 25or 50 mm (by pressing the level switch (CAN C)).

Vertical vibrations. If the vehicle vibrates in the di-rection of the vertical axis due to an uneven roadsurface, these movements are transferred to theECU from the body acceleration sensors B24/3,B24/4, B24/6 via pin 6, pin 8 (plug 2) and pin 29(plug 1). The level sensors B22/7, B22/8, B22/9pin 42, B22/10 report the amplitude via pin 20plug 2 and pin 2, pin 5 (plug 1). The ECU actuatesthe control valves according to the preselectedprogram map (sport/comfort) so that the body vi-brations are damped and evened out.

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18.9 Wheels and tyres

18.9.1 Wheels

Requirements on the wheels● Low weight

● Large diameter for large brake discs

● High dimensional stability and elasticity

● Good heat dissipation properties (frictional heat)

● Easy replacement of tyres and wheels in theevent of damage

Structure of the wheelThe wheel consists of the rim and the wheel discwith a centre hole and bolt holes. Instead of awheel disc, there may be a wheel spider, or the rimmay be connected to the hub by steel spokes. Thewheel is secured to the flange of the wheel hub(Fig. 1), which pivots about the kingpin, with wheelnuts or wheel bolts. The brake drum or brake discis also bolted to the wheel-hub flange. If the bear-ing is open, a hub cover protects the bearing andis also the mounting location for the grease reser-voir.

RimsThere are rims which are fixed to the wheel discand those which can be removed. We also make adistinction between single-piece rims (drop-centrerims) and multi-piece rims, which are used oncommercial vehicles (see Chapter Commercial ve-hicle technology).

Drop-centre rims. Single-piece drop-centre rimsare used almost exclusively on passenger vehi-cles. They are a single-piece cast or forged out oflight alloy and are riveted, welded or bolted to the

wheel disc or the wheel spider (Fig. 2). The crosssection of the rim may be symmetrical or asym-metrical.

Hump rim. If tubeless radial-ply tyres are used,drop-centre rims which have a continual raisedsection = hump (H) on the bead seat near to therim well (Fig. 3).If this raised section is not rounded, it is known asa flat hump (FH). Both types are designed to pre-vent the tyre bead from being pushed from thebead seat into the rim well by the large lateralforces which occur when the vehicle is corneringat speed. The air escapes suddenly on tubelesstyres, which could result in a serious accident.

Dimensions and designations on rimsThis data is standardised. The rim designation isstamped on each wheel by the manufacturer. It ba-sically consists of two dimensions: the rim width ain inches and the rim diameter D in inches. The twodimensions are separated by an “x” on drop-cen-tre rims. Code letters after the rim width indicatethe shape of the rim flange, code letters after therim diameter indicate the type of rim.

Example:

61/2 Rim width in inchesJ Code letter for the dimensions of the rim

flangex Single-piece rim (drop-centre rim)15 Rim diameter in inchesH A hump on the outer bead seatRO 35 Offset 35 mm

6 1/2 J x 15 H RO 35

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McPhersonsuspension strut

Wheelbolt

Wheelhub

Hubcap

Disc wheel

Steering knuckle

Controlarm

Drive shaft

Lower sup-porting joint

Kingpin

Brakedisc

Fig. 1: Passenger-car wheel with bolted wheel hub

D = Rim diameter

a = Rim width

Valvehole

Rim well

Bead seat Rim flange

a

D

Fig. 2: Single-piece symmetrical drop-centre rim

Hump Flat hump

Rim well

Fig. 3: Asymmetrical hump rim

Other rim designations:

H2 Hump on both sides

FH Flat hump on the outer bead seat

FH2 Flat hump on both sides

CH Combination hump:Flat hump on the outer bead seat and humpon the inner bead seat

EH Extended hump

SDC Semi-drop-centre rim

TD Special rim with reduced a reduced flangeheight to improve the ride comfort of thetyre. A groove in the bead seat accommo-dates the tyre bead so that the bead cannotjump out if the tyre is depressurised. The rimwidth and diameter are given in mm.

Offset

Selecting a wheel with a different rim offset maychange the track width.

Note about changing the rim: If the track widthchanges, other geometrical dimensions, such asthe kingpin offset and camber, will also change.

Positive offset. The inner contact face is moved tothe outer section of the wheel in relation to thecentre of the rim.

Negative offset.The inner contact face is moved tothe inner section of the wheel. Using rims with anegative offset increases the track width of a mo-tor vehicle.

Types of wheelsDisc wheels are pressed out of steel sheet or castor forged out of light alloy, e.g. GK-AlSi 10 Mg.Benefits of wheels made from light alloy:

● Low weight (small unsprung mass)● More effective brake ventilation and heat dissipa-

tion

Lightweight wheels made from newly developedsteels, e.g. DP 600 or HR 60, can have thinner wallsand have become up to 40 % lighter compared toprevious steel wheels made from RSt 37.

18.9.2 Tyres

Requirements on the tyres● To support the weight of the vehicle

● To absorb and damping jolts from the road

● To transfer drive, braking and lateral forces

● Low rolling resistance (low friction and heat de-velopment)

● Adequate service life

● Quiet and low-vibration rolling

StructureThe tyres include the inner tube and valve, the tyreitself and the rim band. The latter is now only usedon mopeds and motorcycles with wire-spokedwheels to protect the inner tube from being dam-aged by the nipple heads of the wire spokes. Theinner tube must correspond to the tyre size. In thistype of tyre, the inner tube must always be re-placed at the same time as the tyre.

The tyre (Fig. 2) consists of:

● Carcass● Protector with tread

● Bracing layer (on radial-ply tyres)

● Side wall● Beads with inserted wire-spoked cores

● Airtight rubber layer

Carcass. This is constructed of rubberised cord fi-bres made from nylon, rayon, steel, polyester oraramid. The fibres are laid on top of each other inlayers, either radially – at right angles to the direc-tion of travel – or diagonally – in a point towardsthe running direction. The fibres are wound aroundtwo steel rings (bead cores) and are fixed in placeby vulcanisation.

Protector. This consists of several layers of fabricand rubber cushions. It damps impacts and pro-tects the carcass.

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This is the measurement from the centre of therim to the inner contact face (wheel-mountingplane) of the disc wheel (Fig. 1).

Positive offset Negative offset

Innercontactface

RO

Innercontactface

RO

Fig. 1: Rim offset

Bead seat

Rim flange

Valve

Bead

Tread Protector

Side wallBead core

Airtightrubber layer

Bracing layer

Hump

Carcass

Fig. 2: Structure of a tyre

Bracing layer.This consists of several layers of steelwires, textile fibres or aramid fibres embedded inrubber. The bracing layer lies over the carcass andis made in such a way that the wires or fibres cross.In high-speed tyres, the bracing layer may be fold-ed (Fig. 1 ), thereby increasing the stability.

Tread. This has grooves. The longitudinallygrooved tread provides the tyre with cornering sta-bility and the cross-groove tread transfers motiveforces. The arrangement of the tread has a consid-erable impact on aquaplaning, rolling resistanceand the noise characteristics of the tyres.

If the road is wet, a wedge of water can form be-tween the tyres and the road surface at highspeeds. This eliminates the road-surface adhesionand renders the vehicle unsteerable. The groovesin the tread must be of a certain shape and depthto dissipate the water quickly.

The minimum tread depth of 1.6 mm prescribed bylaw is not sufficient to prevent aquaplaning inmany cases.

Side wall. Lower side walls increase the rigidity ofthe tyre, which improves steering precision but re-duces ride comfort.

Bead.This has the task of keeping the tyre firmly inplace on the rim so that braking, motive and lateralforces can be transferred to the road. It is thereforemade to be particularly rigid using cables madefrom steel wire (bead core). On tubeless tyres, ithas the additional task of sealing the tyre onto therim.

Dimensions and designations on the tyreTyre size. This is given as 2 measurements: tyrewidth in inches or mm and rim diameter in inchesor mm.

However, these numerical values do not corre-spond to the actual dimensions of the tyre. Exactvalues must therefore be taken from the standardtable. All measurements apply to tyres that are in-flated to the standard pressure and unladen(Fig. 2).

Aspect ratio. In order to distinguish between dif-ferent tyre types, e.g. balloon tyres and low-cross-section tyres, the ratio of the tyre height H to tyrewidth W is established. This is given as a percent-age in tyre designations.

On modern tyres, the width is greater than theheight. If the height of the tyre is 80 % of the width,for example, the ratio height to width = 0.8 : 1. Asthe percentage is used in the tyre designation,these would be known as 80s tyres.

Effective radius. A vertical tyre under load has asmaller radius (distance from the centre of thewheel to the road surface) than an unladen tyre.This is known as static radius rstat (Fig. 3).

When the vehicle is in motion, the compression ofthe tyres is eliminated by the centrifugal force andthe radius increases again. This is known as dy-namic radius rdyn.

Dynamic rolling circumference Udyn. This describesthe distance that the tyre covers with one revolu-tion at a speed of 60 km/h when it is bearing theload specified in the standard and inflated to thespecified air pressure. The accuracy of the speed-ometer reading depends on the rolling circumfer-ence. The static radius and the dynamic rolling cir-cumference are given in tyre tables.

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Bead wrapping

Bead coreBead apex

Carcass,2-ply rayon

Folded aramid bracing layer

Airtightrubber layer

Fig. 1: Arrangement of the carcass and bracing layer ina tyre

CompressionW

HD

Load

r r sta

t

Fig. 2:Tyre measurements Fig. 3:Tyre under load

Tyre speed category. This classifies tyres for pas-senger vehicles and motorcycles according to theirmaximum permissible speed. Each maximum per-missible speed is given a code letter, a selection ofwhich is shown inTable 1.

Tyre load-bearing capacity (Table 2). This is shownby the load index (LI). This is a code number andindicates the maximum load-bearing capacity ofthe tyre at the standard pressure.

For some tyres for commercial vehicles, the PRdesignation (ply rating) is also given. 8 PR meansthat, due to the rigidity of its carcass, a tyre canbear the same load as a tyre with 8 layers of cottoncord even though it has fewer layers.

Tyres with the designation Reinforced or ExtraLoad have a reinforced carcass. This means thatthey can bear greater loads at a greater air pres-sure. The load index is higher on these tyres.

Examples of tyre designations

R = radial-ply tyre195 = nominal tyre width 195 mm; 60 = aspect ratio 60 %; 15 = rim diameter 15”88 = load capacity 560 kgH = maximum speed 210 km/h.

195 / 60 R 15 88 H

This tyre has a dual designation for the speed. Thepart in brackets means that the tyre has a maxi-mum speed of 270 km/h (W) at a load index of 102.If the vehicle is approved for greater speeds, thevehicle manufacturer must issue an approvalwhich sets out the permissible load-bearing capac-ity and speed.

Tyre designations (Fig. 1). In accordance with ECEregulation no. 20 (ECE = Economic Commissionfor Europe), the information listed in Fig. 1 must beused for a tyre designation. The full tyre designa-tion must be taken from the motor vehicle technol-ogy book of tables.

Tyre typeWe distinguish between balloon tyres, super low-pressure tyres, low-cross-section tyres, super low-cross-section tyres, 70, 60, 50, 40, 35 tyres, etc.,according to the aspect ratio of the tyres (Fig. 2). The ratio of tyre height to tyre width variesbetween the individual shapes, which results indifferent handling characteristics. They vary froman almost round profile (balloon) to an ever flatterand wider cross-section. Wider treads and lowerside walls result in better driving safety, which isvery important as the speed increases.

335 / 30 ZR 18 (102 W)

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Maximum Speed Maximum Speed tyre speed symbol tyre speed symbol

in km/h in km/h

160 Q 240 V

180 S 270 W

190 T 300 Y

210 H over 240 ZR

Table 1: Speed categories

The tyre load-bearing capacity depends on thetyre type, maximum speed, tyre pressure andcamber. These must be determined from the ve-hicle.

ReinforcedTyre size (Extra Load)

LI kg bar LI kg bar

135/80 R 13 70 335 2.4 74 375 2.8

185/70 R 14 88 560 2.5 92 630 2.9

195/65 R 15 91 615 2.5 95 690 2.9

205/50 R 16 87 545 2.5 91 615 2.9

Table 2:Tyre load-bearing capacity LI (selection)

Note: the tyre load-bearing capacity of thesetyres is reduced by 5 % every 10 km/h above240 km/h.

Code letter for permissible top speed(210 km/h)Code number for tyre load-bearingcapacity (615 kg)Rim diameter in inches

Code letter for radial-ply tyre

Height-to-width ratio in %

Tyre width in mm195

65

R15

91H

Fig. 1: ECE tyre designation

Super low-cross-section

tyre

Tyre width

Tyre

heig

ht

70s tyre 50s tyre

Fig. 2:Tyre cross-sections (selection)

Balloon tyres (height to width = 0.98 :1) e.g. 4.50-16,have good suspension characteristics but poor cor-nering stability due to the large tyre height.

Super low-pressure tyres (height to width = 0.95 : 1)e.g. 5.60-15, are distinguished from balloon tyresby their wider shape and smaller inner diameter(up to 15").

Low-cross-section tyres (height to width = 0.88 : 1)e.g. 6.00-14, have a width in 1/2" gradations. Theymay also be marked with the letter L (low).

Super low-cross-section tyres (height to width~ 0.82) e.g. 165 R 13, were manufactured as cross-ply tyres and, from 1964, as radial-ply tyres (80styres).

70s tyre (height to width = 0.70 : 1) have a heightthat is 70 % of the width. This is what gives thetyres their designation. They have the advantageof increased road grip and vehicle stability. Thehigh lateral forces allow greater cornering speeds.

50s tyres (height to width = 0.5 : 1) e.g. 225/50 R 16,have a height that is only 50 % of the width. Therim diameter is increased since the rolling circum-ference of the tyre remains constant, compared to195/65 R 15 tyres.

Advantages:● Larger and higher performance brake discs with

better ventilation can be fitted.

● Not sensitive to lateral deformation as the cross-section is low and flat.

● High lateral stability when steering into bends;occurrence of large lateral forces even at smallwheel-slip angles, allowing high corneringspeeds.

● Increased resistance to lateral twisting.

● More precise response to steering movements.

Disadvantages:● Poorer aquaplaning characteristics

● Lower internal suspension, loss of comfort

● Greater steering effort required

Tyre contact patch (tyre contact area,positive tread)As the tyre width increases, so does the contactpatch of the tyre on the ground (Fig. 1). A largercontact area increases the friction force so that thetyre grip is increased under fast cornering and un-der braking. Coulomb's law, which states that thefriction force depends only on the normal force(vertical load) and the friction coefficient, only ap-plies to tyres to a limited extent. When rubber-elas-tic materials rub against coarse surfaces (roads),the size of the surfaces rubbing against each otherdue to the indentations is significant.

Negative tread. This is made up of lateral, longitu-dinal and diagonal grooves between the individualtread bars. If the tyre contact patch is large, theproportion of negative volume must be increasedin relation to the tyre contact patch to prevent thetyre aquaplaning due to increased water absorp-tion. The effectiveness of the tyre for winter drivingis also increased by the higher ground pressure.

Air-pumping effect. Deformation of the tyre con-tact patch while the vehicle is in motion can createenclosed cavities, depending on the layout of thenegative tread, which can abruptly fill with air andthen empty again. This results in considerable dri-ving noise.

Tyre constructionA distinction is made between crossply tyres andradial-ply tyres according to the carcass structure.

Crossply tyres. The fabric plies are laid diagonallyon top of each other so that the cord fibres form apoint (cord angle) of 26° to 40° along the directionof travel (Fig. 2). The smaller the cord angle, theharder the tyres, the better the lateral stability andthe greater the possible cornering speeds. Cross-ply tyres are primarily used on motorcycles (seeChapter Motorcycle technology).

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Super low-pressure tyre 70s tyre

Fig. 1:Tyre contact patches

35°…38°

Normal tyre "S" tyre

30°…34°

Fig. 2: Cord angle on crossply tyres

Radial-ply tyres (Figs. 1 and 2). All the cord threadson the carcass are situated next to each other andare arranged radially, i.e. at 90° to the direction oftravel. A belt made of several layers of fabric orsteel cord or aramid at an angle of approximately20° to the direction of travel is fitted between thecarcass and the tread of the tyre, so that the treadbecomes only very slightly deformed when the ve-hicle moves away. Fig. 1 shows 2 criss-cross steelcord and 2 circumferential nylon belts at 0°. Thenylon bracing layers at 0° enable the tyre to with-stand higher speeds.

The side walls of radial-ply tyres compress, the de-formation is mainly limited to the flexing zone.

At lower speeds, radial-ply tyres run firmer thandiagonal tyres, thanks to the reinforcement belt. Atgreater and higher speeds, the springiness of thesoft carcass comes into play, meaning that the ra-dial-ply tyre operates more quietly than the diago-nal tyre. In addition, the bracing layer producesgood lateral stability and thus high lateral forces.

Tubeless tyres (Fig. 2). An airtight rubber layermade of butyl prevents the air from escaping. Nev-ertheless, pressure is lost through diffusion of airmolecules over time. The tyre pressure must there-fore be checked regularly.

If the tyre is filled with nitrogen instead of air, itwill last longer, because nitrogen molecules arelarger than air molecules. The rubber valve fitted inthe rim must also be perfectly sealed. Tubelesstyres bear the inscription “Tubeless” or “sI”.

The benefits of tubeless tyres:

● Less heat build-up because there is no frictionbetween tyre and tube.

● Lighter weight and easier to assemble.

Wheel-slip angleIf a moving vehicle is affected by disruptive forces(wind force, centrifugal force), a wheel-slip angle ap-pears and the lateral forces at work in the tyre con-tact areas counterbalance these disruptive forces.

The lateral force in the tyre builds up through thedeformation of the tyre contact area, when corner-ing, for example. As soon as a wheel-slip angleforms, the tyre tread moves further and furtheraway from the line along which it normally lieswhen in contact with the road surface, in the cen-tre of the tyre (Fig. 3).

This creates a deformation in the tyre. The furtheraway the tread moves from the centre line, thegreater the deformation. The sum of these applicationforces is the lateral force, which takes effect at thecentre of gravity of the deformed tyre contact area.

If the wheel-slip angle continues to increase, a slid-ing friction occurs in the rear section of the tyreand the application force is alleviated. However,the lateral force continues to increase because theadhesion area is still larger than the sliding area. Ifthe wheel-slip angle still continues to increase, thesliding area becomes larger than the adhesionarea and the lateral force is alleviated.

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0° nyloncover (2-ply)(0° bracinglayer)

20° steelbracinglayer

Carcass,2-ply rayon

Fig. 1: Structure of a radial-ply tyre

Negativetread

Bead core

Positivetread

Bracinglayers

Innerrubberlayer

Bead

Flexing zone

Tread

Sidewall

Bead zone

CarcassSidewallzone

Fig. 2:Tubeless radial-ply tyre

The angle between the actual direction of mo-tion and the rim plane (following the line of therim) is called the wheel-slip angle α (Fig. 3). Atyre can only transmit lateral force if it runs atan angle to the direction of travel.

Rim plane

Direction of motion

FS

Wheel-slipangle

a

Adhesionarea

Sliding area

a

0

1

2

34

FS

Tyrecontactarea

Fig. 3:Wheel-slip angle α

When cornering, the wheel load on the outer cor-nering wheels of an axle is increased, whereas thewheel load on the inner cornering wheels is de-creased. The higher the wheel load becomes, thegreater the build up of lateral force in the tyre.Strong lateral forces must also be built up in widetyres in the event of high wheel loads and lateralacceleration, thus increasing safety when corner-ing, whereas with super low-cross-section tyres,such as 165/80 R 13 tyres, the lateral force is actual-ly decreased (Fig. 1).

Winter tyres (M+S tyres). In contrast to the coarsestudded tyre treads that were previously used, to-day's tyre treads have small tyre tread grooveswith many fine fins. The fine fins give the tyre abetter grip on snowy or slippery road surfaces inwinter. To keep the rubber on the tread surfaceelastic at lower temperatures starting from ≤ 7 °C,silicic acid (silica) or natural rubber is added. Thishas the following benefits:

● Better adhesion between tyre and lining

● Lower rolling resistance

● Good service life of tyre tread (less internal heatformation)

Winter tyres with a tread depth of less than 4 mmare no longer sufficiently winter-proof.

Tread wear indicators (Fig. 2).These are elevations inthe tread-groove base. If the tread wears down to thelegally prescribed minimum tread depth of 1.6mm,the height of the tread wear indicators will be levelwith the tread. The position of these indicators in thetyre tread is marked with the letters TWI (tread wearindicator) or with a triangle, on the tyre wall.

Due to the high risk of aquaplaning, at high speedsin particular, and due to the increased braking dis-tance on wet roads if tyres have low tread depths(Table 1), it is advisable to change the tyre if thetread wear indicators are in contact with the roadsurface.

Wheel balancingThe mass of a wheel when turning is never evenlydistributed. In the areas where the mass is greater,an imbalance appears, in other words, centrifugalforces develop which increase more, the greaterthe mass and the higher the engine speed (Fig. 3).

Static imbalance. If, for example, rubber is wornon a section of the tread as a result of lockingbrakes, this produces a centrifugal force in the sec-tion opposite, which can cause the wheel tobounce off the road surface at higher enginespeeds. This fault can be viewed by spinning thewheel.

To ensure that the wheel remains stationary ineach position when spinning, the sum of all themoments of inertia around the wheel rotational ax-is must equal 0.

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7,000

6,000

5,000

4,000

3,000

2,000

1,000

9,0007,0005,0003,0001,000 N

N

195/60 R14

185/70 R13

165/80 R13

Wheel-slip anglea = 5°

Wheel force F

Late

ral

forc

e F

S

Fig. 1: Lateral force build-up for radial-ply tyres

Tread wear indicator – TWI Tyre tread

Tread-groovebase

1.6 m

m

Fig. 2:Tread wear indicator

Tread depth Braking distance in m (wet road surface)(mm) 20 40 60 80

7

5

3

2

1.6

Table 1: Braking distance when braking from 100 km/h to 60 km/h

0

100

200

300

400

500

600

700100g

50g

25g

50 100 150 200km/h

N

Driving speed v

Cen

trif

ug

al

forc

e F

C

Fig. 3: Centrifugal forces on a tyre with designation195/65 R 15

M1 = M2 G1 · r1 = G2 · r2

A balancing mass m2 with weight force G2 must befixed on the rim opposite the heaviest section ofthe wheel. This mass must be large enough tomake the existing torque M2 correspond with thetorque M1. The wheel is then statically balanced(Fig. 1).

Dynamic imbalance. A wheel's imbalance weightm1 is seldom at the same level as its balanceweight m2 affixed to the rim. The wheel is staticallybalanced, but at higher engine speeds, the cen-trifugal forces m1 and m2 produce a torque in linewith the axle and cause the wheel to wobble. Inthis case, the wheel has a dynamic imbalance. Ifthe imbalance weight m1 is level with the wheel-mounting plane, then only the torque MC2 (Fig. 2)will take effect.

Attaching a second balance weight m3 to the in-side of the rim can cause the existing torque MC3

to balance the torque MC2, the wheel is then dy-namically balanced. (Fig. 3). The size and positionof the balance weights m2 and m3 are determinedon balancers.

If a wheel is out-of-round despite being balanced,a radial tyre runout could be the problem. If the ra-dial tyre runout protrudes by more than 1 mmfrom the tread surface, attempts must be made toreduce the radial tyre runout by turning the tyre onits rim (matching).

18.9.3 Run-flat systems

Run-flat systems are wheel/tyre systems withlimp-home characteristics.

A distinction can be made between 2 possible ap-plications:

● Systems which can be used with conventionalrims.

● Systems composed of special rims and corre-sponding tyres.

The use of compressed-air monitoring systems isa requirement for both systems. The driver mustknow about the pressure loss in the tyre in order toadjust the speed and continue driving.

Systems with conventional rimsConti Support Ring (CSR). A light metal ring withflexible mounting is fitted on the rim (Fig. 4). Whenair is lost, the tyre is supported against the ringwithout touching the inside of the tyre walls andcausing irreparable damage to the tyre throughfriction-induced heat. It is possible to drive on forapproximately 200 km at reduced speed. The addi-tional weight per wheel is about 5 kg. Tyres with aheight/width ratio of > 60 are suitable, otherwisethey cannot be fitted.

Self-Supporting Run-Flat Tyres (SSR, DSST*). Theside walls of these tyres are reinforced with rubber(Fig. 5). At zero pressure, the tyre can still be support-ed on its bead, so that the bead does not slide downinto the rim well. It is possible to drive on for approx-imately 200km at a speed of 80km/h. This side wallreinforcement can also reduce comfort due to the in-creased transmission of bumps in the road surface.

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Wheel axis

M2

M1m1

m2

G1

G2r1

r2

M1 = M2

G1 • r1 = G2 • r2

Fig. 1: Balancing (static)

Wheel axis

MC2

m1

m2

FC1

MC2 = FC2 • rC2

Wheel-mounting plane

rC1 = 0

FC2

rC2

MC1 = 0

Fig. 2: Dynamic imbalance

MC2

m1

m2

FC1

FC2 • rC2 = FC3 • rC3

Wheel-mountingplane

FC2

rC2

Inner sideof rim

MC3

MC2 = MC3

FC3

rC3

m3

Wheel axis

Fig. 3: Balancing (dynamic) * DSST Dunlop Self Supporting Technology

Run-flat systems can either aggravate or pre-vent critical driving situations caused by a sud-den air loss in the tyre, particularly at higherspeeds. It is normally possible to reach the clos-est workshop without having to change the tyre.

Metal ring

Normaltyre

DSSTtyre

Fig. 4: CSR system Fig. 5: DSST system

Systems with special rims and tyresPAX system. The PAX system is composed of aspecial rim with a flexible insert and the corre-sponding tyre with vertical anchorage on the rim(Fig. 1).

Rim.The rim is very flat and has one small mount-ing groove in place of the rim well. Rim flanges arenot supplied, both humps are on the outside of therim. Rim diameters for large brake discs can beachieved due to the flat shape of the rim.

Tyre. The tyre has shorter side walls, which in-crease its rigidity. Lateral forces can cause less de-formation to the tyre contact patch, thus improv-ing the road adhesion and reducing the rolling re-sistance.The tyre bead is in a groove outside on the humps.All the forces working on the tyres produce a ten-sile strength in the carcass, meaning that the beadis always pressed into the groove (Fig. 2).This verti-cal anchorage ensures that the bead cannot slip outof the rim, even when the tyre is at zero pressure.

Flexible insert. The insert is an elastomer ringwhich is pushed onto the rim. Thanks to its highloadbearing capacity it supports the tyre whenpressure is lost, meaning that at a speed of80 km/h, approximately 200 km more can still becovered.

Size designations for PAX systems

205 Tyre width in mm650 Outside diameter of the tyre in mmR Radial structure440 Average rim seat diameter in mmA Asymmetrical seat

205/650 R 440 A

18.9.4 Compressed-air monitoring systems

The following types of compressed-air monitoringsystems are used in motor vehicles:

● Indirect measuring systems

● Direct measuring systems

Indirect measuring systemsWhen pressure is lost, the tyre's rolling circumfer-ence, which increases the engine speed in relationto the other tyres, is reduced. The engine speeds aredetermined via the ABS or ESP sensors. However,the driver is not warned until there is a difference inair pressure of more than 30% between the tyres.

Direct measuring systemsThe pressure is measured directly by sensors inthe tyre. The following functions are fulfilled:

● Continued monitoring of tyre pressure whilstdriving and when the vehicle is stationary.

● The driver is given early warning in the event ofa pressure loss, reduced pressure and flat tyre.

● Automatic individual wheel recognition andwheel positioning.

● Diagnostic procedure for systems and compo-nents in the workshop.

The system is composed of:

● 1 tyre-pressure sensor per wheel

● Antennae for tyre pressure monitoring

● Instrument panel with display

● ECU for tyre-pressure monitoring

● Function-selector switches

Tyre-pressure sensor. This sensor is bolted to themetal valve (Fig. 3) and can be reused when chang-ing the tyres or wheel rims. In addition, a tempera-ture sensor, transmitting antenna, measuring andcontrol electronics as well as a battery with a servicelife of approximately 7 years are integrated. Sincethe filling pressures are altered by temperature influ-

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Flexible insertRim edgeprotector

RimMounting grooveHump

Fig. 1: PAX system

Inner stop

Bead core

Outerhump

Tensile strengthin carcass

Fig. 2:Vertical anchorage of the tyre bead

Compressed-air monitoring systems are de-signed both to recognise the air loss in the tyreand also to warn the driver.

Metal valve

Antenna

Tyre-pressure sensor

Fig. 3:Tyre-pressure sensor and antenna

ences, the pressures and temperatures recorded inthe ECU are set to a standard temperature of 20°C.

ECU. The ECU obtains the following informationfrom the transmitting antenna:

● Individual identification number (ID code), usedfor individual wheel recognition.

● Current inflation pressure and current tempera-ture.

● Condition of the lithium battery.

The ECU evaluates the signals transmitted by the an-tenna for the tyre pressure monitoring and impartsthe information for the driver on the display screen,according to the importance of this information. Ifwheels are changed on the vehicle, for example,from the front axle to the rear axle and vice versa,the ECU must be recoded with the new pressures.

Individual wheel recognition. The sensors belong-ing to the vehicle are recognised by the ECU andstored. The sensors are recognised when the vehi-cle is being driven, to avoid interference from sen-sors on cars parked nearby.

System messages, top priority (Fig. 1). These mes-sages are intended for when driving safety is nolonger guaranteed. They are displayed to the dri-ver if, for example:

● … signal threshold 2 is undershot (0.4 bar belowthe stored setpoint tyre pressure of 2.3 bar).

● … signal threshold 3 is undershot (minimumpressure limit value, 1.7 bar in the diagram).

● … a pressure loss is greater than 0.2 bar/minute.

System messages, second priority (Fig. 1). Theyare displayed to the driver if, for example:● … signal threshold 1 is undershot (0.2 bar below

the stored setpoint tyre pressure of 2.3 bar).● … the difference in pressure on the wheels of

one axle is 0.4 bar.● … the system is switched off or has a fault.

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To avoid damaging the sensor irreparably whenchanging a tyre, the tyre must be pressed downon the side opposite the valve.

ACTUALtyre pressure

2

bar

5 6 8 10 12 14s

SETPOINT tyre pressure

Signal threshold 2Signal threshold 3

Case 2 Rapid pressure loss > 0.2 bar/minin example 0.4 bar/min

Case 1 Rapid (sudden) pressure loss

2.3 bar2.1 bar1.9 bar1.7 bar

1

Signal threshold 1

Time

Co

m-

pre

ssio

n

Fig. 1: Diagram of system messages

WORKSHOP NOTES

● Secure the motor vehicle against rolling awaybefore removing the wheels.

● Only use wheel rims which are relevant for thetyre you are using (vehicle documents).

● Check the rims for cracks and remove traces ofrust.

● Tighten the wheel nuts to the specified torquein a diagonal pattern.

● Note the specified air pressure to avoid loss ofoperating life.

● Using a mixture of diagonal- and radial-plytyres is not permitted for passenger cars.

● Only use tyres of the same type and with thesame tread on one and the same axle.

● Label the wheels after removing them andstore them in a room that is cool, dry and dark.

● Do not stand wheels up and do not pile morethan 4 tyres on top of each other.

REVIEW QUESTIONS

1 From which components is a wheel constructed?

2 Which types of rim are there?

3 Why are rims with humps used?

4 What are the benefits of using wheels made oflight-metal alloys?

5 From which components are tyres made?

6 What does “dynamic rolling circumference of atyre” mean?

7 What are the advantages and disadvantages of a50 series tyre?

8 How are radial-ply tyres designed?

9 What is a “tyre-contact area”?

10 Explain the tyre designation195/65 R 15 86 T M + S.

11 What is a tread wear indicator and how is itsposition on the tyre indicated?

12 What is the “wheel-slip angle”?

13 Why do wheels need to be balanced?

14 What does “dynamic imbalance” mean?

15 How can a radial-tyre runout be removed?

16 What are run-flat systems?

17 How is the PAX system designed?

18 What are the tasks of compressed-air monitoringsystems and where should they be fitted?

19 What are the benefits of direct-measuringcompressed-air monitoring systems in relationto indirect-measuring systems?

18.10 Brakes

Brake systemsService brake system. This system enables thespeed to be reduced, if necessary, until the vehicleis stationary. The vehicle must stay firmly in laneduring this process. The service brake is operatedcontinuously with the foot (foot brake) and im-pacts on all the wheels.

Auxiliary brake system.This system must fulfil thefunctions of the service brake system when it ismalfunctioning, possibly to less effect. It does nothave to be an independent third brake, for the in-tact circuit of a dual-circuit service brake system ora graduated parking brake system is sufficient.

Parking brake system. Its function is to secure astopping or parked vehicle from rolling away, in-cluding on a sloping road surface. Its componentsmust be able to work mechanically for reasons ofsafety. In passenger cars, it is normally operated instages by a coupling lever (handbrake) or a pedalvia linkage and control cables. It works on thewheels of one axle only.

Continuous brake system. Its function is to keep thespeed of the vehicle to a prescribed value whendriving downhill (third brake). It is a requirement formotor busses whose weight is mgvwr > 5.5t and othervehicles with a weight of mgvwr > 9t.

Antilock-braking system (ABS), also anti-skip sys-tem. The ABS measures the wheel slip automaticallyduring braking, regulates the braking pressure andthus prevents the brakes from locking. The ABS is alegal requirement on vehicles weighing mgvwr > 3.5 t.

Structure of a brake system (Fig. 1)A brake system consists of:

● Energy supply equipment

● Control equipment

● Transmission equipment

● Possible supplementary equipment for trailervehicles, for example, trailer control equipment

● Parking brake

● Service brake

● Possibly also brake pressure control, such as anABS, for example

● Wheel brake on the front axle and rear axle

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Brakes are used in a vehicle for deceleration, forbringing the motor vehicle to a halt and for se-curing it against rolling away. When a vehicle isbraked, the kinetic energy is converted into heat.

Structure

Energysupply

Controlequip-ment

Trans-missionequip-ment

Servicebrake

Braking-forcecontrol,e.g. ABS

Wheelbrake

Front brakes

Rear brakes

Muscular energy

Hydraulic energy

Pneumatic energy

Brake pedal - master cylinderBrake pedal - brake booster,master cylinderBrake pedal - service-brake valveBrake pedal - electr. potentiometerLever for parking brakeEngine-brake actuation

Transmission media (mechanical,hydraulic, pneumatic,electropneumatic)

Brake lines

Brake hoses

Valves (relay valve, overload-protection valve, solenoid valve)

Supplementary equip-ment for trailer vehicles

Parking brake

Fig. 1: Structure of a brake system

Legal requirements (extracts)The legal requirements for brakes on motor vehi-cles are set out in the German National Road TrafficLicensing Regulations (StVZO), in EC directivesand the ECE regulations.

Specified brake systems (§ 41 StVZO)

Motor vehicles from classes M and N must havetwo separate brake systems (service brake sys-tems, parking brake systems) or a brake systemwith two separate control units. Each control unitmust be able to function if the other fails.

One of the brake systems must function mechanical-ly and be able to secure the vehicle against rollingaway (parking brake system). If more than twowheels can be braked, the same brake areas andmechanical transmission equipment can be used.

Motor vehicles from classes M2/3 and N2/3 and with amaximum speed of more than 60km/h determinedby the model must be fitted with an ABS system.

Continuous braking action (Directive on Approxima-

tion of European Community Laws RREG71/320EC)

Motor vehicles from class M3 with a permissibletotal weight from 5.5 t (except for city buses) andvehicles from class N2.3 with a permissible totalweight of more than 9 t must have a continuousbraking action (continuous brake) for long down-hill gradients. The braking action must be designedsuch that it limits a fully-laden vehicle that is beingdriven on a gradient of 7 % for a distance of 6 kmto a speed of 30 km/h.

Stop lamps (§ 53 StVZO)

The service-brake operation must be made visibleby two red stop lamps to the rear on class L (vmax >50 km/h), M, N and O motor vehicles. Since 18. 3. 93class M1 vehicles have been allowed to have a thirdstop lamp in the centre at the rear. This third stoplamp is a legal requirement on all vehicles whosefirst registration was after 1. 1. 2000.

Inspection of motor vehicles and trailers (§ 29 StVZO)

Owners of vehicles and trailers must establishwithin specified intervals and at their own costwhether the motor vehicles comply with the regu-lations. A distinction can be made here between:

General inspections GI: to check the vehicle's road-worthiness in accordance with § 29 StVZO (appen-dix VIII).

Safety inspections SI: the chassis and suspensioncomponents are subjected to a comprehensive vi-sual, operation and function check (e.g. brakes,steering, tyres).

Minimum braking § 29 StVZO (GI guide line)(Table 3). The minimum braking rate can be calcu-lated from the measured values determined on thebrake dynamometers. Formula:

Types of brake system according to energy supply

Muscular energy braking. The braking force is ap-plied by the driver and enhanced by the mechani-cal and hydraulic transmission ratio.

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L Motorcycles and three-wheelers

M M1 Passenger car with up to 9 seats includingdriver's seat

M2 Motor bus with > 9 seats and up to 5 tgross weight

M2 Motor bus with > 9 seats and up to 5 tgross weight

N N1 Heavy goods vehicle up to 3.5t gross weight

N2 Heavy goods vehicle > 3.5 t and up to 12 tgross weight

N3 Heavy goods vehicle > 12 t gross weight

O Trailer and semi-trailer

Table 1: Motor-vehicle classifications (extracts)

Class of vehicle Time interval MonthGI SI

L 24 –

M1 24 (36) –M1 Passenger transportation 12 –

(e.g. taxi, hire car)

M2, M3 in the 1st year 12 –in the 2nd and 3rd year 12 6from the 4th year 12 3

N1 24 –N2, N3 12 6

O to 750 kg 24 (36) –O > 750 kg to 3.5 t 24 –O > 3.5 t to 10 t 12 –O > 10 t 12 6

Values in ( ) are those for the initial inspection after the vehiclewas first registered.

Table 2:Type and time interval of inspections(extracts)

z = � 100 %Sum of the wheel brake forces����

Vehicle weight force

Class of vehicle Service Parking brake system brake system

M1 Passenger car 50 16

M2, M3 Motor bus 50 16

N1 Heavy goods vehicle, with gross weight up to 3.5t 50 16

N2, N3 Heavy goods vehicle > gross weight 3.5t 45 16

Table 3: Minimum braking z in %

Assisted braking (power-assisted brake). In additionto muscular-energy braking, the braking force is alsoenhanced by other energy sources (vacuum pressure,hydraulic-accumulator pressure, compressed air)

Externally-powered brake (compressed-air brake).The driver controls the braking force. The braking en-ergy (compressed air) is not generated by the driver.

Overrun brake. When the tractor vehicle is braked,the trailer comes closer due to its inactive state(overrun). The braking energy is generated on thetrailer wheel brake via the towbars.

Types of energy transmissionMechanical transmission through pedal, lever,towbar and control cables, for example, when ap-plying the parking brake in a car or the overrunbrake when towing a trailer.

Hydraulic transmission caused by fluid pressure inthe brake line, e.g. in a passenger-car service brake.

Pneumatic transmission caused by compressedair in the brake line, e.g. in commercial vehicles.

Electrical transmission caused by electrical leads, forexample, or the magnetic field in an electric retarder,for commercial-vehicle continuous brake systems.

18.10.1 Braking

Braking duration

Reaction time tR.This is the time needed by the dri-ver between recognising a hazard and operatingthe brake pedal (reaction). The reaction time de-pends very much on the physical and mental con-dition of the driver. It can be lengthened signifi-

cantly as a result of alcohol consumption, drugconsumption and also tiredness.

Braking time t.The sum of the response, thresholdand delay times is known as the braking time t.

Response time tAn. This is generated by the clear-ance in the brake system, e.g. pedal idle travel,clearances.

Threshold time tSw. The pressure in the brake sys-tem is increased during the threshold time and thedesired braking deceleration is achieved.

Delay period tV. The braking deceleration re-mains constant until the vehicle comes to a halt.

Braking distance

Further factors which influence the braking dis-tance are:

● Road conditions, e.g. dry, wet, icy

● Tyre condition, e.g. tread depth, tyre pressure

● The condition of the brakes, e.g. worn, stiff,damaged, corroded

● The condition of the brake pads, e.g. wet, glazedover, oily

● The brake type, e.g. drum or disc brake, com-pressed-air brake, SBC

● The weight of the vehicle, weight distribution,e.g. when towing a trailer

● The condition of the shock absorbers

18.10.2 Hydraulic brake

StructureThe hydraulic-brake system (Fig. 2) consists of thebrake pedal, tandem master cylinder with brakebooster, wiring system (possibly with brake-pres-sure reducer), brake cylinder with wheel brakes.

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During braking, the braking action does nottake effect until an obstacle has been recog-nised. The total duration (stopping time tA) of abraking procedure is determined by the reac-tion time tR and the braking time t (Fig. 1).

I : Detection of dangerII : Start of braking by driverIII : Start of braking action

IV : Full braking actionV : Vehicle stopped

Time t

Bra

kin

g d

ece

lera

tio

n a

01234567

10

0

10

20

30

40

50

m

70

1 2 3 4s

ttA

tAn tSw tvtR

sR

ss

H

I

II

III

ms2

Dis

tan

ce d

riven

s

IV

V

Fig. 1: Braking

The braking distance is dependent on the dri-ving speed. Under normal conditions, doublingthe speed will increase the braking distance byfour times.

Brake disc

Brake line Wheel-brakecylinder

Brake shoeBrakebooster

Brake pedal

Drum brake(Simplex)

Brake circuit 2

Brake circuit 1 Brake-pressurereducer

Brake drum

Tandemmaster cylinder

Brake calliper Return spring

FA Disc brake RA

Brakecylinder

Fig. 2: Hydraulic-brake system

Wheel brakes. Normally, all wheels have disc brakes,older and smaller vehicles have drum brakes on therear wheels. For reasons of safety, a dual-circuitbrake system with tandem master cylinder is a re-quirement. If a brake circuit fails, it is still possible tobrake the vehicle using the other brake circuit.

Operating principleThe operating principle of the hydraulic brake isbased on Pascal's law:

The force with which the brake pedal presses onthe plunger in the master cylinder generates thefluid pressure. The fluid pressure takes effectthrough the brake lines and generates the applica-tion forces (contact pressures).

The hydraulic power transmission normally in-volves a transmission of force (Fig. 1).

The forces interact like the plunger surfaces, in otherwords, the strongest force is created on the largestsurface. The plunger travels, on the other hand, be-have in the opposite way to the forces. So, an actu-ating force of 1,000N with a plunger travel of 8mmon the master cylinder on the four wheel-brakecylinders, for example, produces a total force of4,000N and a corresponding plunger travel of 2mm.

The work performed (W = F · s) is therefore thesame on the master cylinder and the wheel-brakecylinders.

The hydraulic brake can work at high pressures of upto about 180bar. This explains the small dimensionsof the hydraulic construction components. The hy-draulic brake remains maintenance-free for a longerperiod of time. Since brake fluid is almost impossibleto compress and the clearances are small, only smallquantities of brake fluid are moved. The pressure in-crease is very fast and the brakes respond quickly.

18.10.3 Brake-circuit configuration

Hydraulic service brake systems are split into 2 cir-cuits. This means that a sufficient braking action isstill produced if one circuit fails. There are 5 de-signs (Table 1).

Vehicles with ABS control systems normally usethe II (black-white) and X (criss-cross) brake-circuitconfigurations.

18.10.4. Master cylinder

Its tasks are:

● To achieve a rapid pressure build-up in eachbrake circuit.

● To achieve a rapid pressure reduction so that thebrakes are rapidly released.

● To balance the volume of the brake fluid duringa temperature change and when the clearance isincreased because the brake pad is worn.

StructureThe tandem master cylinder (Fig. 1, Page 455) con-tains two plungers arranged one behind the other -the push-rod plunger and the intermediate plunger,which is stored in fluid. The plungers form two sepa-rate pressure chambers in one housing. Bothplungers are designed as double plungers, meaningthat there is a ring-shaped castor chamber betweenthe front and rear sealing section of each plunger.This chamber is always filled with brake fluid via thesnifter bore. The primary cup seal is located at thefront of each plunger and seals the pressure chamber.

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The pressure on fluid which is enclosed on allsides acts evenly on all sides.

Master cylinder

F1 = 1,000 N

Wheel-brake cylinder F2 = 4,000 N

1,000 N 1,000 N 1,000 N 1,000 N

s

4·s

Fig. 1: Diagram of a hydraulic brake

1 2

1 2

1 2

1 2

1 2

Abbreviation RemarkDesign Use

II Front/rear axle configura-(TT) tion. Each circuit drives one

axle. For rear-wheel drivewith ABS. (black-white)

X Diagonal configuration.Each circuit drives onefront wheel and the rearwheel diagonally oppo-site. All-wheel drive andfront drive with ABS andnegative kingpin offset.

HI One brake circuit drives(HT) the front and rear axle, the

other drives the front axleonly. Seldom used. (4-2)

LL Each brake circuit drivesthe front axle and onerear wheel (triangle). Seldom used.

HH Each brake circuit drivesthe front and rear axle.Seldom used.

Table 1: Brake-circuit configurations

Only tandem master cylinders are used, be-cause the law requires that two separate brakecircuits are used. This cylinder is operated bythe brake pedal via the brake booster.

The push-rod plunger is sealed at the rear by thesecondary cup seal. The separating cup seals theintermediate plunger against the push rod circuit.The intermediate plunger has a slot into which acentral bore hole runs at the front. The centralvalve rests in this bore hole. A stop pin which leadsgoes through the slot on the intermediate plungerkeeps the plunger in the cylinder and forms thefront and rear stop.

Central valve. This is used on vehicles with ABSsystems and assumes the function of the balanc-ing port. There are also tandem master cylinderswhich have a central valve on both plungers.

Operating principleRest position. The plunger springs press theplungers against their stop. The primary cup sealon the push-rod plunger releases the balancingport and the intermediate plunger is placed at thefront of the stop pin. This means that the centralvalve (Fig. 2) is opened by the valve pin which fitsinto it and assumes the function of the balancingport. Both pressure chambers are now linked tothe expansion tank. The volume of the brake fluidcan be balanced during a temperature change, forexample.

If the balancing port is closed because the push-rod plunger is in the wrong rest position or due tocontamination, it will not be possible to balancethe brake fluid. The fluid expands due to heat,which then increases or automatically triggers thebraking action.

Brake actuation. When the vehicle is braked, the pri-mary cup seal (Fig. 3) on the push-rod plunger trav-els over the balancing port and seals the pressurechamber. The filler shim thus prevents the balancingport from pressing in to the filler bores and from be-coming damaged. The intermediate plunger is nowsomewhat displaced by the brake fluid. The stop pinreleases the valve pin and the central valve isclosed. Pressure builds up in both brake circuits.

Advantages of the central valve● The primary cup seal has a longer service life

because the sealing lip cannot be damaged bythe balancing port.

● In ABS systems, the primary cup seal would bepressed into the balancing port by pressure-peak reverse movements in an ABS control sys-tem and would thus be damaged.

Releasing the brake. The plunger is pushed backup by the fluid pressure and the plunger springs.The primary cup seal on the push-rod plungerfolds down, the filler shim lifts up and the brakefluid flows from the castor chamber through thefiller bores into the expanding pressure chamber(Fig. 4). The intermediate plunger returns to itsoriginal position. The pressure chambers arelinked to the expansion tank by the central valveand the balancing port. The pressure falls and thebrakes are released.

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Expansiontank

Contact plate

Float

Cylinderhousing

Plungerspring

Centralvalve

Valve pin Longitudinal slot in piston

Separatingcup seal

Push-rodplunger

Intermediateplunger

Plasticbushing

Secondarycup seal

Balancingport

Primarycup seal

Plungerspring

Stop pin

Primary cup seal

Snifterbore

Circ. 1Circ. 2

Fig. 1:Tandem master cylinder

Valve spring

Valve seal

Plungerspring

Centralvalve

Plunger spring

Stop pinPrimary cup seal

Primary cup seal

Fig. 2: Rest position

Snifter bore

Support ring Filler bore

Primarycup seal

Balancing port Snifter bore

Filler shim

Fig. 3: Brake position

Push-rod plunger

Folded filler shim

Fig. 4: Release position

Failure of circuit 1 (Fig. 1)The push-rod plunger is pushed up to the stop onthe intermediate plunger. The actuating force nowacts directly on the plunger for intact circuit 2,where it generates braking pressure.

Failure of circuit 2 (Fig. 1)The intermediate plunger is pushed forwards bythe fluid pressure in circuit 1 until it meets thestop. It seals the intact circuit 1 to the non-tight cir-cuit 2. The pressure now builds up in circuit 1.

Tiered tandem master cylinder (Fig. 2)This master cylinder was developed for II systems(TT, black-white) with front axle/rear axle brake-cir-cuit split. The cylinder diameters are graduated,meaning that the diameter of the intermediateplunger, which works on the rear axle brake circuit,is smaller than the diameter of the push-rodplunger. In intact brake circuits, the same pressureis created in both circuits when the vehicle isbraked. The larger push-rod plunger diameter inthe front axle brake circuit pushes back a greatervolume of fluid when the vehicle is braked, caus-ing the brakes to respond faster. If the front axlebrake circuit fails, the push-rod plunger is pushedonto the intermediate plunger when the vehicle isbraked and the plunger's plunging force now actsdirectly on the intermediate plunger. The pedaltravel is lengthened and a higher pressure is creat-ed in the rear axle brake circuit due to the smallerdiameter of the intermediate plunger without thepedal force being increased. If the front axle circuitfails, a sufficient braking action is still achievedwith the rear axle brakes.

Tandem master cylinder with riveted plunger springThe screw-riveted compression spring keeps theintermediate plunger and the push-rod plunger thesame distance apart when they are in the rest posi-tion (Fig. 3).This causes the pressure to build up evenly in bothbrake circuits when the brake is actuated. If thebraking pressure is increased, the intermediateplunger is no longer moved by the plunger springsbut by the brake-fluid pressure.

18.10.5 Drum brake

Nowadays, drum brakes (Fig. 4) are predominantlyused as brakes for rear wheels of passenger carsor in commercial vehicles.

Structure and operating principleThe brake drum fits snugly on the wheel hub. Thebrake shoes and the components which generate theapplication force are found on the brake anchor plate.The brake anchor plate is fixed to the wheel suspen-sion. When braking occurs, the brake shoes and theirpads are pressed against the brake drum by theclamping fixture, thus generating the friction required.The application force can be created hydraulically bythe wheel-brake cylinder (service brake) or mechani-cally by the control cable and the tension lever, ex-pander lever or brake shoe expander (parking brake).

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Brake position,circuit 1 leaking

Brake position,circuit 2 leaking

Circ. 2 Circ. 1 Circ. 2 Circ. 1

Fig. 1: Failure of a brake circuit

Intermediateplunger

Stop pin Push-rod plungerCentralvalve

Longitudinalslot in plunger

Plasticbushing

Valvepin

Balancing port

Primarycup seal

Fig. 2:Tiered tandem master cylinder with central valve

Push-rod plungerRivetedplunger spring

Snifter boreBalancingport

Connecting screw

Stop sleeve

Intermediateplunger

Intermediate-plunger spring

Fig. 3:Tandem-brake master cylinder with rivetedplunger spring

Brake drum

Brake shoe

Brake anchor plate

Wheel-brakecylinder

Expander lever

Returnsprings

Holding spring

Fig. 4: Parts of the drum brake

Features:● Self-reinforcement

● Dirt-proof design

● Parking brake easier to use

● Long idle time of brake pads

● Pad replacement and maintenance is costly andtime-consuming

● Poor heat dissipation

● Tendency towards fading

DesignsAccording to the actuation methods and brake-shoesupports, it is possible to distinguish between:

● Simplex brakes

● Duo-servo brakes

Simplex brake (Fig. 2). This brake has one overrun andone trailing brake shoe.To tension the brake shoes, adouble-acting wheel-brake cylinder, brake shoe ex-pander, S cam, expanding wedge or expander levercan be used. Each brake shoe has a fixed pivot or ful-crum point, such as a support bearing.

Simplex brakes have the same effect when drivingforwards as they do when reversing but have onlyreduced self-reinforcement (Fig. 1). The pad wearon the overrun brake shoe is greater. A parkingbrake is easy to use.

Duo-servo brake (Fig. 3). The self-reinforcement ofthe overrun brake shoe is used to press down thesecond overrun brake shoe. The support bearing isfloating.The support is provided by the double-act-ing wheel-brake cylinder. The braking action is thesame when driving forwards or reversing. It is of-ten used as a parking brake in cup washers (Fig. 6).A control-cable-actuated brake-shoe expander isthen used in place of a wheel-brake cylinder.

Clamping fixturesThese are intended to tension or expand the brakeshoes and press them onto the brake drum.Wheel-brake cylinders are normally used with hy-draulic brakes (Fig. 1, Page 458). With mechanicallyoperated parking brakes, a tensioning lever (Fig. 5)or a brake-shoe expander (Fig. 6) is used.

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Bra

ke c

oeff

icie

nt

C

0.2 0.4 0.60

1

2

3

4

5Duo-servobrake

Simplexbrake

Disc brake

Friction coefficient μ

Fig. 1: Brake coefficient C

Self-reinforcement (Fig. 4). The friction createstorque which pulls the overrun brake shoe intothe drum and strengthens the braking effect.This reinforcement is expressed by the brakecoefficient C (Fig. 1). The pressing force on thetrailing brake shoe is then reduced.

Fading. This is an abatement of the braking ef-fect caused by overheating, e.g. during longbraking. The friction coefficient in the pad de-creases at high temperatures or high slidingspeeds. The brake drum can also become de-formed to a conical shape, because the heatsupply to the wheel hub is more efficiently car-ried off. The brake area then becomes smaller.

Fixedsupportbearing

Double-actingwheel-brake cylinder

Fig. 2: Simplex brake

Floating support bearing

Fig. 3: Duo-servo brake

Trail-ingbrakeshoe

Directionof brake-drumrotation

Overrunbrakeshoe

M

Fig. 4: Self-reinforcementof the drum brake

Tensioning lever

Returnspring

Controlcable

Readjustmentcaps

Fig. 5: Clamping fixturefor parking brake

Expander

Brake cable

Backplate

Brake lining

Backplate Expander

Fig. 6: Parking brake integrated into the cup washer

Wheel-brake cylinder In the double-acting wheel-brake cylinder (Fig. 1),the pressure generated in the master cylinder actson the plungers and generates an applicationforce. The plungers are sealed by rubber sleeves.Dust caps prevent dirt from entering. On the backof the wheel-brake cylinder are threaded boreholes which fasten it to the brake anchor plate andthe brake line connection. A bleeder valve isscrewed in at the highest point.

Brake drum (Fig. 4, Page 456)Features:● High wear resistance

● Inherent stability

● Good heat conductivity

Substances:● Cast iron with flake graphite

● Malleable cast iron

● Cast iron with nodular graphite

● Cast steel

● Combined casting of light alloy and cast iron

The brake drum must run centrally and free fromrunout. The brake area is finely spun or ground.

Brake shoes (Fig. 4, Page 456)Brake shoes maintain their rigidity due to a T-sec-tion and are cast from a light metal alloy or weldedfrom pressed steel. At one end they have a bearingsurface for the mostly slotted pressure pins on thewheel-brake cylinder. A bolt is fitted at the otherend, or the end of the shoe is flush with the fixedsupport bearing. The shoes can therefore be cen-tred in the drum. They fit better and the pad wear ismore even.

Adjusting componentsThe clearance between the brake pad and thebrake drum is increased by the brake pad wear.This also increases the pedal idle travel. The brakesmust therefore be adjusted on a regular basis, ei-ther by hand or using an automatic adjusting com-ponent.

18.10.6 Disc brake

Disc brakes are designed as a fixed-calliper or float-ing-calliper brake (Fig. 1). The brake plungers arelocated in the brake calliper. They press the padsagainst the brake disc when the vehicle is braked.

Features:

● No self-reinforcement due to the even brake ar-eas. This requires greater downforces and there-fore brake cylinders whose diameters (40 mm to50 mm) are larger than the diameters of thewheel-brake cylinders in the drum brake and ad-ditional brake boosters are required.

● Good metering of the braking force, because theabsence of self-reinforcement and the minorchanges in friction coefficient ensure that hardlyany fluctuations occur in the braking.

● Efficient cooling.

● Low tendency towards fading.

● Higher brake-pad wear due to the high down-forces.

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Pressureelement

Dust capPlunger

Grooved-ring sleeve

Fig. 1: Double-acting wheel-brake cylinder

REVIEW QUESTIONS

1 What are brakes for?

2 Which types of brake systems can be distin-guished according to their method of use?

3 Explain the structure of a hydraulic brake system.

4 Which brake systems are specified in vehicleclasses M and N?

5 How can brake systems be distinguished ac-cording to their mode of operation?

6 What are the functions of the master cylinder?

7 How does the primary cup seal work?

8 What are the functions of the central valve?

9 How does the tandem master cylinder workwhen one of the brake circuits fails?

10 What is the advantage of tiered master cylin-ders?

11 Which brake-circuit configurations are there?

12 What are the features of drum brakes?

13 List the distinguishing features of differenttypes of drum brakes.

14 What does “fading” mean?

Floating-calliper brakeFixed-calliper brake

Brake anchorplate

Floating calliper

Plunger

BracketBrake disc

Brake pads

Plunger

Brake fluid

Fig. 2: Disc brakes

● Easy maintenance and pad replacement.

● Automatic adjustment of clearance.

● More heat generated by the brake fluid, becausethe pads fit tightly on the brake plungers. Dan-ger of vapour bubbles.

● Good automatic cleaning due to centrifugal force.

● Tendency of vapour-bubble formation becausethe brake plungers fit tightly against the brakepad.

● The parking brake requires great effort.

Designs Fixed-calliper disc brake. Two- and four-cylinderfixed-calliper disc brakes are normally used (Fig. 1).

The fixed brake-cylinder backplate (fixed calliper) isbolted onto the wheel suspension. This backplategrips the brake disc like pliers. It consists of onetwo-piece housing. Each housing section containsbrake cylinders which are situated opposite eachother in pairs. They contain the brake plungers withsealing ring, protective cap and clamping ring. Thebrake cylinders are linked by channels. The bleedervalve sits on top of the housing.

When the vehicle is braked, the brake-cylinderplungers press against the brake pads. The brakepads are then pushed against the brake disc onboth sides.

Plunger reset (Fig. 2)A rectangular rubber sealing ring used to seal theplunger is located in a groove in the brake cylin-der. The inner diameter of the sealing ring is some-what smaller than the plunger diameter. It there-fore encompasses the plunger with its pretension.

The braking movement of the plunger deforms thesealing ring elastically due to its static friction andthe plunger stroke. When the pressure drops in thebrake fluid, the sealing ring returns to its startingshape or position. This also removes the plungerfrom the clearance of about 0.15 mm and releasesthe brake disc. This is only possible with completepressure reduction in the wire system and ease ofmovement of the plunger and pads.

Expander spring. It fits the brake pads onto theplungers and thus prevents the pads from knock-ing and chattering.

Floating-calliper disc brake (Fig. 1, Page 460) This consists of two main components, the bracketand the housing or floating calliper and has thefollowing features:

● Low weight

● Small size

● Good heat dissipation

● Large pad surfaces

● Takes up less space.

● Reduced tendency towards vapour-bubble for-mation, as only one or two of the brake cylin-ders are on the bracket side.

● Maintenance-free housing versions, thereforenot sensitive to dirt and corrosion.

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Locking pinFixed calliper Brake lining

Expanderspring(cruciformspring)

Protective cap

Sealingring

Plunger

Backplate

2-cylinder

Fixedcalliper

Plunger

Ex-panderspring

Internal ventilation

Brake discwith holes4-cylinder

Fig. 1: Fixed-calliper disc brake

Release positionBrake position

Plunger

Sealing ring

Fig. 2: Plunger reset

Bracket.The bracket is fixed to the wheel suspension.The housing is fitted within the bracket. Floating-cal-liper disc brakes with various guides are used, such as:

● Guide teeth

● Guide pins

● Guide pins and guide teeth combined

● Guide pins with retractable floating calliper

Floating-calliper disc brake with guide teeth(Fig. 1)Bracket.The bracket has two teeth on each side.

Housing. The housing is kept in the bracket by theguide teeth which fit into its semicircular grooves,thus enabling it to slide back and forth.

Guide spring. The guide spring presses the hous-ing onto the bracket teeth to prevent clatteringnoises from occurring.

Floating-calliper disc brake with guide pins(Fig. 2)This brake has two guide pins bolted onto the hous-ing on the cylinder side of the bracket. The bracket hastwo bore holes which contain sliding inserts made ofTeflon, for example. The housing is kept in these boreholes by the guide pins and can slide back and forth.

Braking.The plunger in the housing presses the in-ner brake pad against the brake disc once theclearance has been overcome. The reaction force

pushes the housing in the opposite direction. Theplunger in the housing now also presses the outerbrake pad against the brake disc once the addition-al clearance has been overcome. Both brake padsare pushed against the brake disc with the sameamount of force.

The guide teeth support the inner pad directly, theouter pad is supported against the housing by theperipheral force.

If guide pins are used, both brake pads are sup-ported on the housing. When the brake is released,the return forces of the sealing ring restore theclearance, with the support of the expander spring.

Brake disc (Fig. 1, Page 459)The brake disc is normally disc-shaped and is madeof cast iron, malleable cast iron or cast steel. In racingcars, this disc can also be made of composite materi-als reinforced with carbon fibres or ceramic carbon.

Internally ventilated brake discs. These discs areused when the brakes are subject to a very greatload. They contain radially mounted air ductswhich are designed such that a fan effect is pro-duced during revolutions. This produces a more ef-ficient cooling effect. Sometimes, the brake areaeven also contains bore holes and possibly alsooval-shaped grooves. This ensures that water isdrained away more rapidly if the brake is appliedwhen the discs are wet. The brakes respond evenlyand the risk of fading is low. At the same time, thebore holes also bring about a reduction in weight.

18.10.7 Brake pads

The friction material uses the braking force to gen-erate considerable friction with the brake disc orbrake drum. This then converts the kinetic energygenerated by the vehicle into heat. With drumbrakes, the brake pad is riveted or adhered to thebrake shoe. On disc brakes, the pad is adhered tothe steel brake-pad backplate.

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Bracket withguide teeth Housing

Guide spring Locking pin

Expander spring

Brake pads

Brake pads

Housing

Locking pin

Expander spring

Bracket

Fig. 1: Floating-calliper disc brake with guide teeth

Brake lining

Housing(calliper)

BracketGaiter seal

Guide pin

Fastening screw

Cover plate

Fig. 2: Floating-calliper disc brake with guide pins

Visual check. Monitoring of the brake-fluid level inthe expansion tank; looking for damp, dark patch-es on the brake cylinders and connection pointsand for corrosion on the brake lines/state of thebrake hoses (chafe marks, bubbles, animal bites).

Functional test. This includes checking the pedaltravel by activating the service brake system. If thepedal travel slowly increases, this may be a resultof a leaking primary cup seal or a leaking centralvalve. If the pedal travel is too great or it is onlypossible to build up the pressure by pumping, thecause could be air bubbles or the clearance beingtoo great.

Leak tests (Fig.1). A pressure-tester tool and a ped-al holder are required. Before the tests, the brakesystem and the pressure tester tool filled withbrake fluid must be bled.

Low-pressure test. The low/high-pressure mano-meter combination of the pressure-tester tool isconnected to the bleeder valve on a wheel brakeand the pedal holder used to apply a pressure ofbetween 2 bar and 5 bar. This pressure should bemaintained for 5 minutes. The entire systemshould be left untouched for this time. If the pres-sure falls, there is a leak.

Electric contacts can be incorporated into discbrake pads for a wear indicator.

Requirements of the friction lining:

● Very stable at high temperatures, considerablemechanical strength and long operating life.

● Constantly high coefficient of friction even athigh temperatures and sliding speeds.

● Not sensitive to water and dirt.

● No glazing at high thermal load, good heat con-duction.

High-pressure test. Using the pedal holder, thebraking pressure is set to a value of between50 bar and 100 bar. Within 10 minutes, this set pres-sure may drop by a maximum of 10 %. If there is alarger pressure drop, this means there is a leak.

Filling and bleeding the brake system (Fig. 2).Thiswork can be carried out by one person with fillerand bleeder apparatus. A bleeder pipe and a trans-parent bleeder hose with collector are the tools re-quired. For vehicles with ABS, observe the brakebleeding instructions.

Connect the filler and bleeder apparatus to thebleeder pipe on the expansion tank and attach thebleeder hose with collector to a bleeder valve.Now open the shutoff cock on the filler hose of theequipment and then open the bleeder valve, untilnew, clear brake fluid flows out without bubbles.Then close the bleeder valve. Repeat the processfor all bleeder valves. Finally, close the shutoffcock. Before removing the bleeder pipe, open ableeder valve briefly and release the pressure.

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Tandemmaster cylinder

High-pressuremanometer

Combinationpressure-tester tool

Front brakecircuit

Rear brakecircuit

Low-pressuremanometer

Vacuum-pressuremanometer

Fig. 1: Leak test

Tandem master cylinder

Shutoffcock

Filling pressure

Safety valve

Operating pressure

Pressureregulator

Filler hose

Filler andbleederapparatus

Bleederbottle

Brakecylinder

Wheel-brakecylinder

Fig. 2: Bleeding with apparatus

Brake lining materials. Brake linings contain, forexample:

● Metals such as steel wool and copper powder.

● Filler materials such as iron oxide, barite, micapowder and aluminium oxide.

● Anti-friction agents such as coke dust, antimonysulphide and graphite.

● Organic substances such as resin filler material,aramid fibres and binding resin.

Brake pads have a friction coefficient of approxi-mately μ = 0.4. They are heat-resistant to approxi-mately 800 °C.

18.10.8 Diagnosis and maintenance of the hydraulic brake system

Work on the wheel brakesBrake drums and brake discs. During a brake check,you must check these for ridges, out-of-roundnessand knock. Brake discs with lateral runout that istoo great must be replaced. The disc-brake pads,sliding calliper and floating calliper must movesmoothly. Brake drums and brake discs that areout-of-round or have ridges must be skimmed orturned down. Observe the maximum skimmingmeasurement or the minimum disc thickness, thebrake discs and/or brake drums may need replac-ing. Brake drums or brake discs with cracks and/ordamaged brake callipers must be replaced.

Brake pads. The thickness and oiling must bechecked and the pads must be replaced if neces-sary.

Brake testThe brake tests are mainly carried out on brake dy-namometers.

The following are measured for each wheel:● Braking force● Rolling resistance ● Fluctuation of the braking force, e.g. in the case

of an out-of-round drum● Occurrence of incipient lock

Dynamic brake analyser (Fig. 1).This has two iden-tical sets of rollers so that the brakes for bothwheels on an axle can be tested at the same time.These each drive one braked wheel during the test.The drive rollers on one side are driven together.The third roller is a sensor roller. It automatically

activates the dynamometer and the locking protec-tion. The braking force (peripheral force) of everywheel is measured.

The braking factor z is mainly determined as a per-centage (see Page 452).The brake-force differential(for the service brake system) of an axle must notbe greater than 25 %. Special test instructionsmust be observed for motor vehicles with perma-nent all-wheel drive and variable engine torquedistribution.

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WORKSHOP NOTES

● Check the fluid level in the expansion tankduring each check. On disc brakes, the sunkenfluid level can be a sign of considerable liningwear.

● The thickness of the drum brake linings can bechecked using inspection holes.

● To check the brake drum, remove it; clean thebrake of wear debris. Vacuum off the wear de-bris, do not blow it out.

● The brake pads must be replaced on thebrakes of one axle at the same time.

● Renew the brake fluid in accordance withcompany regulations, e.g. yearly.

● Do not use drained brake fluid; store it inmarked containers and have it disposed of bya disposal or reprocessing company.

● Keep greases and oils away from the brakecomponents.

● Only use the specified brake fluid to refill.

● Only use brake cleaner, possibly alcohol(methylated spirits) for cleaning.

REVIEW QUESTIONS

1 What are the features of disc brakes?

2 How is a fixed-calliper disc brake constructed?

3 How is the clearance set for disc brakes?

4 What are the different types of floating-calliper discbrakes, as distinguished by the calliper guide?

5 How is the braking factor determined?

6 Describe the braking procedure for the floating-calliper disc brake.

7 What requirements are made of brake pads?

8 What inspections are carried out on the hydraulicbrake?

9 How is the hydraulic brake filled and bled?

10 What measurements can be taken on the dynam-ic brake analyser?

Fig. 1: Dynamic brake analyser

18.10.9 Power-assisted brake

Vacuum brake boosterFor motor vehicles with a spark-ignition engine, thevacuum pressure can generally be taken from theinduction pipe. The small pressure difference be-tween the air pressure and the intake manifold pres-sure of approximately 0.8 bar requires the workingplunger to have large surfaces, so that the plungerrod force can be increased fourfold, for example.

For diesel engines, the pressure difference is gen-erated by a vacuum pump driven by the engine.

Structure (Fig. 1). The master cylinder is usuallyflange-mounted to the reinforcement housing. Theworking plunger divides the housing into a vacu-um chamber and working chamber. The workingchamber is connected alternately via a vacuumpressure and outside air valve with the outside airor with the vacuum chamber. The double valve isactuated by the brake pedal via the plunger rod.This plunger rod presses on the master cylinderpush rod via valve plungers and the rubber reac-tion shim. The working plunger and its boostingforce also presses on the push rod.

Operating principleRelease position (Fig. 1). The outside air valve isclosed, the working chamber is connected via theopen vacuum valve to the vacuum chamber. Bothsides of the working plunger have the same pres-sure of approximately pabs = 0.2 bar.

Partially braked position (Fig. 2). During braking,the push rod is moved forwards and the vacuum

valve is closed. The reaction shim is squeezed bythe valve plunger and the outside air valve isopened. The pressure difference which arises inthe working chamber compared to the vacuumchamber has the effect of a boosting force on theworking plunger. This is pushed forwards with thetiming case and push rod until the reaction forcefrom the master cylinder is equal. When the pushrod is still, the reaction shim expands again andpresses on the valve plunger. This closes the out-side air valve. The booster force on the workingplunger and push rod remains constant.

Full braking (Fig. 3). At full pedal force, the reactionshim is constantly being squeezed by the plungerrod and the counterforce from the push rod,whereby the outside air valve is constantly open.The pressure difference (Δp = 0.8 bar) between thetwo chambers is the largest possible and thelargest booster force is therefore exerted on theworking plunger and push rod.

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To create the assistance (power-assistance), a vacu-um pressure or hydraulic brake booster is connect-ed to the master cylinder of the hydraulic brake.

To non-returnvalve

Vacuum pressure

Vacuumconnection

Reactionshim

Push rod

Plunger return spring Roller diaphragm

Poppet valve

Plungerrod

Rest position

Vacuumchamber

Working-pressure chamber

Brakepedal

Filter

Vacuum valve (open)

Valveplunger

Outside air valve (closed)

Air pressure

Compression spring

Working plunger

Fig. 1:Vacuum brake booster

Vacuum pressure Air pressureReduced air pressure

Fig. 2: Partially braked position

Vacuum pressure Air pressure

Fig. 3: Fully braked position

Hydraulic brake booster (Fig. 2)The system (Fig. 1) consists of the high-pressureoil pump for the power steering, the hydraulic ac-cumulator, the pressure-regulated oil-flow con-troller and the hydraulic brake booster with tan-dem master cylinder and the oil supply reservoir.

Operating principleThe high-pressure oil pump delivers oil to the hy-draulic accumulator. The oil compresses the nitro-gen inside it using a diaphragm and charges theaccumulator with a pressure of up to 150 bar. Thebrake booster and the hydraulic-accumulator pres-sure-oil chamber are connected via an electric line.

Brake position. By applying the brake, the controlplunger (Fig. 2) of the brake booster is moved. Itcloses the return passage and opens the inlet pas-sage. The working chamber is supplied with pres-sure oil and assists the working plunger. The mov-ing working plunger closes the inlet passage thusenabling a variable boost depending on the pedalforce.

Release position. When the pedal force is released,the control plunger closes the inlet passage andopens the return passage. The hydraulic fluid canflow back to the supply reservoir. The resettingspring pushes the working plunger into its originalposition. If the engine fails, there is still pressureoil for approximately 10 brake applications.

Pneumatic brake boosterPneumatic brake boosters (Fig. 3) can be fitted tovehicles with a combined compressed-air/hy-draulic brake system. With an operating pressureof approximately 7 bar, great booster forces can beachieved on small vehicles.

Function. When the brakes are applied, the valvetappet is moved by the plunger rod. The valve tap-pet comes into contact with the valve plate andtherefore closes the outlet valve. Simultaneously,the valve plate is raised and the inlet valve thereforeopens. The supply pressure surges into the workingchamber and has the effect of a booster force on theworking plunger. The moving working plunger clos-es the inlet valve again. This results in a variablebooster force which is directly related to the pedalforce. When the pedal force is released, the valvetappet closes the inlet valve and opens the outletvalve. The pressure in the working chamber is dis-charged and the working plunger is moved back toits original position by the resetting spring.

18.10.10 Braking-force distribution

The axle load displacement that occurs duringbraking depends on the level of braking decelera-tion, the load, the load distribution on the vehicleand the height of its centre of gravity. If the brakesare applied when the vehicle is being driven in astraight line, the front wheels are under load andthe rear wheels are relieved. If the brakes are ap-plied when the vehicle is cornering, the wheels onthe outside of the bend are subjected to an addi-tional load. The brakes are usually designed in sucha way to provide optimum effect at medium decel-eration and medium load. When braking sharply,however, the rear wheels may lock and the vehiclecould skid. Brake-pressure reducers reduce thisdanger and are used on vehicles without ABS.

Brake-pressure reducer (Fig. 1, Page 465). This con-trols the braking pressure of the rear wheels in thebrake line. They are braked with only slightly in-creased pressure as of a certain changeover pressure.

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Hydraulicbooster

Hydraulicaccumulator

Power-steering gearHigh-pressureoil pump

Supply reservoir

Tandem master cylinder

Oil-flowcontroller

Fig. 1: Hydraulic brake-booster system

Releaseposition

To master cylinder

To supplyreservoir

Fromhydraulicaccumulator

Inletpassage

Workingchamber

Push rod

Working plunger Control plunger

Returnpassage

Fig. 2: Hydraulic brake booster

Workingplunger

Partially brakedposition

Workingchambere.g. 3 bar

Supply e.g. 7 bar Inlet valve

Valve tappetAir pressureVentilation

Filter

Outletvalve

Fig. 3: Pneumatic brake booster

Pressure characteristic in the brake system with-out braking-pressure control. The blue line showsthe routing of the braking pressures during actualbraking. The same braking pressure is exerted onthe front and rear axles until the changeover point(e.g. 40 bar). After the changeover point, further in-crease of braking pressure on the rear axle is re-duced. The rear axle is prevented from locking.

Optimum braking is when the braking pressure onthe rear axle increases further at the start of brak-ing than it does on the front axle. This is shown inFig. 1 for a laden and an unladen vehicle. When thevehicle is laden, the wheel contact forces aregreater and therefore enable stronger brakingforces, generated by the higher braking pressuresin the wheel brake cylinders.

Load-sensitive brake-pressure reducer (Fig. 2).This has the same effect as the normal brake-pres-sure reducer, but after the changeover point, thebraking pressure during braking is controlled ac-cording to the load and axle-load shift.

The braking pressure within the control range is al-ways adjusted to the ideal pressure with the load-sensitive shift of the changeover pressure.

18.10.11 Mechanically operated brake

Mechanically operated brakes are often only stillused as the parking brake in vehicles with a hy-draulic service-brake system and as the servicebrake in light motorcycles and single-axle trailers.

The efficiency of the mechanical load transmissionis low (according to the maintenance status, only ~ 50 %). In winter, load transmission componentsmay freeze together in wet weather or frost.

Brake cables.These are steel cables which are rout-ed over rollers in pipes or flexible metal hoses(Bowden cables). To reduce friction and protectagainst icing and corrosion, these are coated withplastic. Tensioning bolts are attached to adjust thebrake cables.

Brake compensator (Fig. 3).This is required so thatthe same forces are exerted on the wheels of oneaxle.

Overrun brake (Fig. 4). This is used for trailers.When the tractor vehicle is braked, the trailer runsonto the tractor vehicle. The shear force of the trail-er causes the pull rod to be pushed against a com-pression spring. The movement caused is createdby a reversing lever and a control cable on an ex-pander which creates an application force on thebrake.

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Blow valve

Stepped plunger

Plunger spring

RA To rear axle

MC From master cylinder

Unreducedpressure

Reducedpressure

Changeoverpoint

loaded

Ideally unloaded

Ideally

Braking pressure FABra

kin

g p

ress

ure

RA

Fig. 1: Brake-pressure reducer

Brake cable,front

Brake cable, rear

Rubber grommet Bracket

Adjusting screw

Clamp

Tensionspring

Spring hook

Compensating lever

Fig. 3: Brake compensator

Wheel

Brake cables

ExpanderCompensatingelement

Drum brake

Tear-offcable

Towingattachment

Handbrakelever

Reversing lever

Pullrod withgaiter seal

Fig. 4: Overrun brake

Rest position

Valve"open"

Housing

Ring surface

To rear axle

Steppedplunger

MC RA

Braking pressure FA

Changeover pointloaded

Changeover pointunloaded

Controlrange

Idea

llyloaded

Ideally unloaded

Bra

kin

g p

ress

ure

RA

Fig. 2: Load-sensitive brake-pressure reducer

REVIEW QUESTIONS

1 What types of brake booster are used in hydraulicbrakes?

2 What pressure difference is used in the vacuumbrake booster?

3 How does the vacuum brake booster work inemergency braking?

4 What are the components that make up a hy-draulic brake-booster system?

5 What is understood by dynamic axle-load dis-placement when braking?

6 What are the basic types of brake-pressure reduc-er used?

7 How does the brake-pressure reducer work?

18.10.12 Basics of the electronic chassiscontrol systems

The following control systems are used:

● ABS (Antilock-Braking System), prevents wheellocking during braking.

● BAS (Brake Assistant), detects emergency situa-tions and brings about shorter braking distances.

● SBC (Sensotronic Brake control), reduces brak-ing distances and increases the directional sta-bility when braking in bends.

● TCS or ASC (Acceleration Skid Control), ELSD (Elec-tronic Limited-Slip Differential), prevents wheelspinning when pulling away and accelerating.

● VDC (Vehicle Dynamics Controller such as ESPor DSC), prevents the vehicle from skidding.

Every vehicle movement or change in movementcan only be achieved by forces on the wheels.These are:

● Peripheral force as motive or braking force. Thisacts on the longitudinal direction of the tyre.

● Lateral force, e.g. caused by steering or externalinterferences such as crosswind.

● Normal force caused by vehicle weight. This actsat right angles to the road surface.

The strength of these forces depends on the roadsurface, tyre condition/type and weather influ-ences.

The possible load transmission between the tyresand road surface is determined by the frictionforce. Optimum transmission of the loads can onlyoccur as a result of static friction between the tyresand road surface. The electronic control systemsutilise the static friction optimally.

The peripheral force is transferred via static fric-tion as a motive (FA) or braking force (FB) to theroad surface. Its size is equal to the normal force FN multiplied bythe coefficient of friction μH (μIce = 0.1 to μDry = 0.9).

FA, B Motive force, braking force

FN Normal forceμH Coefficient of friction

The coefficient of friction μH (grip value) is deter-mined by:

● Material pairing, tyres and road surface

● Occurring weather influences

Kamm's friction circle (Fig. 1). The largest forcetransferable onto the road (Fmax = FN · μH) is shownas a circle. For a stable driving condition, the re-sulting FRes of peripheral force FU and lateral forceFS must lie within the circle and therefore be small-er than Fmax.

If the peripheral force FU reaches its maximum as aresult of spinning or locked wheels, no lateralforce FS can be transferred. The vehicle can then nolonger be steered.

If when cornering at maximum cornering speedthe lateral force FS is at its maximum, the vehiclecannot be braked or accelerated, as it would other-wise break away at the rear.

Slip (Fig. 2). While a tyre rolls, elastic deformationsand sliding occur. If, for example, a braked wheelwith a rolling circumference of 2 m covers a dis-tance of only 1.8 m during a turn, the travel differ-ence between the tyre circumference and brakingdistance is 0.2 m. This corresponds to a slip of 10 %.

If a wheel locks or spins when it is being driven,there is a slip of 100 %.

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Electronic control systems should guaranteesafe control of a motor vehicle during braking,accelerating and steering.

FA, B = μH · FN

Kamm'sfriction circle

Greatesttransmissibleperipheral force FU

Greatesttransmissible

lateral force FS

Greatest transmissibleforce on tyre FRes

Normal force FN(tyre load)

FResmax = μ · FN

Fig. 1: Forces on the wheel, Kamm's friction circle

A slip-free transmission of force between thetyres and the road surface is not possible, be-cause the tyre is not interlocked with the roadsurface and always slides a little when drivingor braking.

Vehicle speed Brakedwheel

Tyrecircum-ferenceBraking forceRoad surface

Travel in 1 rotation = 1.8 m

Slip 0.2 m = 10 %

Rolling circumference = 2 m

Fig. 2: Slip on the braked wheel

Relationship of forces on the wheel and slipThe relationship between motive force, brakingforce, lateral force and slip when driving straightahead is shown in simplified form in Fig. 1. Even atlow-slip values, the braking force increases steeplyto its highest level. It then falls again a little when theslip values increase further. The routing and highestvalue of the motive/braking force curve depend onthe friction coefficient of the tyre on the road surface.The highest value lies between 8% and 35% slip. Thefirst area of the curve is called the stable area be-cause the wheel remains stable for driving and steer-able. This is where the wheel has the best force trans-mission. Electronic control systems therefore workin this control range. For large slip values, the lateralforce decreases significantly, the vehicle can nolonger be steered and the driving characteristics be-come unstable. The control systems in the vehiclemake sure that the stable area is kept to.

18.10.13 Antilock-braking system (ABS)

Antilock-braking systems (ABS), also known as an-ti-skip system (ABV), are used in hydraulic brakesystems and air-brake systems for brake-pressurecontrol.

StructureAn ABS consists of the following components:

● Wheel sensors with pulse rings

● ECU

● Hydraulic modulator with solenoid valves

The solenoid valves are selected by the ECU inthree control phases; pressure build-up, pressureholding and pressure reduction. They prevent thewheels from locking.

ABS systems have the following features:

● Lateral forces and directional stability remainthe same, whereas the risk of skidding is re-duced.

● Vehicle is still steerable and obstacles can thusbe avoided.

● An optimum braking distance can be achievedon normal road conditions (no gravel, snow).

● “Flat spots” on tyres are prevented as thewheels do not lock.

Antilock-braking systems.They can be differentiat-ed between according to the number of controlchannels or sensors, and according to the type ofcontrol in

4-channel system with 4 sensors and X (diagonal)or II (black-white, TT) brake-circuit configuration.Each wheel is controlled individually.

3-channel system with 3 or 4 sensors and X (diag-onal) brake-circuit configuration. The front wheelsare controlled individually and the rear wheels al-ways together.

Individual Control (IC). The greatest possible brak-ing pressure for each wheel is adjusted here. Thismeans that the braking force is at its maximum.Because the wheels of an axle can be braked withvarying forces, e.g. due to a road surface that is icyon one side, there is vehicle torque on the verticalaxis (yaw moment).

Select-Low Control (SLC). With SLC, the wheel deter-mines the common braking pressure of an axle withthe low road-surface adhesion. The yaw momentwhen braking on road surfaces with varying road-surface adhesion is lower, because the braking forceson the rear wheels are approximately the same.

Operating principleThe brakes are mainly applied when there is lowslip. The ABS does not therefore take effect. TheABS closed-loop control circuit (Table 1, Page 468)is only activated and wheel locking prevented dur-ing sharp braking and when there is significantslip. The ABS control range lies between 8 % ... 35 %slip. Below approximately 6 km/h, ABS is generallydeactivated so that the vehicle comes to a stop.

There is a toothed pulse ring around each wheelwhich creates alternating voltage by induction in aspeed sensor. The frequency of the alternatingvoltage is a measurement for the wheel speed. TheECU can therefore determine the acceleration ordeceleration for each wheel.

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Mo

tiv

e o

r b

raki

ng

fo

rce

& l

ate

ral

forc

e

Slip

ABS operating range

Lateral force

Stable Unstable

0 50 100%

Motive or braking force

358

Fig. 1: Forces on the wheel subject to slip

During braking, ABS systems control the brak-ing pressure of a wheel according to its grip onthe road surface in order to prevent wheel lock-ing.

Only moving wheels can be steered and cantransfer lateral forces.

The front wheels are generally controlled indi-vidually and the rear wheels are often con-trolled according to the Select-Low principle.

Brakecylinder

Pulse ring

Control CheckMonitor

Compute Warn

Closed-loopcontrolparameter

Mastercylinder

Solenoid valve

Control parameter

Tyre

Sensor

Controller

Road surface(interference factor)

Referenceparameter

ECU

Closed-loopcontrol path

Pressure build-up.The pressure created in the mastercylinder is transferred to the wheel-brake cylinder.

Pressure holding. If a wheel tends to lock duringbraking and exceeds a predefined slip, this is de-tected by the ECU. It switches the solenoid valve ofthe wheel to pressure holding. The connectionmaster cylinder – wheel-brake cylinder is interrupt-ed. The braking pressure remains the same.

Pressure reduction. If the slip and therefore the incip-ient lock continue to increase, the switch to pressurereduction is made. A connection from the wheel-brake cylinder via the return pump to the mastercylinder is therefore made. The slip is reduced. If theslip falls below a particular threshold, then the ECUswitches the solenoid valve back to the pressurebuild-up. The control cycle is repeated (4…10 timesper second) as long as the brake is applied.

ABS with return in a closed circuit.During pressure reduction, brake fluid is taken inby a pressure accumulator. At the same time, thereturn pump pumps it back to the respective mas-ter-cylinder brake circuit.

Structure (Fig. 1).This ABS has the following com-ponents in addition to the usual brake system:

● Wheel sensors ● ECU

● Hydraulic modulator ● Warning lamp

Wheel sensors (Fig. 2). These are on every wheel.For each sensor there is one pulse ring around thewheel. Inductive speed sensors or Hall-effect sen-sor are used.

ECU.This processes the incoming signals from thesensors, determines the necessary settings for thesolenoid valves and adjusts these accordingly. Thefunction of the ABS system is constantly moni-tored by self-diagnosis.

Warning lamp. Upon starting, this signals the op-erational readiness of the ABS. It lights up shouldthe ABS control system fail. The vehicle can still bebraked fully.

Hydraulic modulator with return pump. This con-tains solenoid valves for control, an accumulatorfor brake fluid for each brake circuit and an electri-cally-driven return pump. The pump is activatedvia a relay and always runs during the ABS controlsystem.

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Closed-loop Friction pairing of tyres and road sur-control path face, wheel contact force (wheel load)

Interference Road conditions, state of brakes,factor weight distribution of vehicle,

tyre condition (air pressure, tyre tread)

Controller In ABS ECU (comparison of setpoint/actual values)

Closed-loop Rotational speed or change in speed control of wheelparameter

Reference Braking pressure specified by pedal parameter force

Control Braking pressure in brake cylinderparameter

Table 1: ABS closed-loop control circuitBrakecalliper

ECU

Warning lamp

Brake pedal

Tandem master cylinder

Hydraulicmodulator

Sensor

Pulse ring

Brake systemABS hydraulicsABS electronics

Brake disc

Final drive

Fig 1: ABS with return in a closed circuit (illustration)

Sensor

Coil Pulse ring

Fig. 2: Inductive wheel sensor (speed sensor)

ECU

Sensor

Winding

Pulse ring

Solenoid valve Pressureaccumulator

Return pump

Electric motor

Pressure build-up

M

Fig. 3: 3/3 solenoid valve - operating principle

Operating principle with 3/3 solenoid valves (Fig. 3, Page 468).For braking-pressure modulation in the ABS con-trol system, the ECU triggers a 3/3 solenoid valvein the hydraulic modulator for each channel. In ac-cordance with the three control phases, the mastercylinder is connected as follows:

● To the wheel brake cylinder for pressure build-up

● No connection for pressure holding

● To the return pump for pressure reduction

Operating principle with 2/2 solenoid valves(Fig. 1)In this system, the hydraulic modulator isequipped with smaller, lighter and faster switching2/2 solenoid valves. Each control channel now re-quires an inlet valve and an outlet valve.

The ECU switches the solenoid valves in the con-trol phases as follows:

● Pressure build-up. Inlet valve (IV) open, outletvalve (OV) closed.

● Pressure holding. Both valves closed.

● Pressure reduction. Inlet valve closed and outletvalve open. The running return pump pumps theexcess brake fluid from the accumulator back in-to the relevant brake circuit.

ABS with return in an open circuit and 2/2 solenoid valves (Fig. 2)During a control action, the excess brake fluidflows back into the expansion tank at zero pres-sure. The hydraulic pump is selected by the ECUusing the position of the pedal-travel sensor. Itpumps the missing volume of brake fluid out ofthe expansion tank at high pressure back into therespective brake circuit and therefore brings thebrake pedal to its basic position. The pump is thendeactivated.

StructureThe system is composed of:

● ECU● Wheel sensors● Actuating unit ● Hydraulic unit● Warning lamp

ECU.This processes the sensor signals and passesthem on as control signals to the solenoid valves.The signals from the travel sensor control the hy-draulic pump in the ABS control system. Faultsand malfunctions are detected by the ECU, ABS isswitched off and the ABS warning lamp isswitched on.

Wheel sensors. These are on every wheel andtransmit the wheel speed.

Actuating unit. This consists of a vacuum brakebooster, which has an integrated pedal-travel sen-sor, and the ABS tandem master cylinder with ex-

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Brakepedal

Tandemmaster cylinder

Brakecircuit 1

Brake circuit 2

Expansiontank

Vacuumbrakebooster

Damper

Returnpump

Accumulator

Inlet valve (IV)

Non-return valveOutlet valve (OV)

FL RL

FR RR

M

OV

IV IV

OV

IV

OV

Fig. 1: ABS with closed circuit and 2/2 solenoid valves(hydraulic circuit)

Brakepedal

ABS tandemmaster cylinder

Non-return valve

Inlet valve (IV)

Outlet valve (OV)

Supplyreservoir

Hydraulic pump Electr. motor

Pedal-travel sensor

Vacuumbrakebooster

FR RR

FL RL

MIV

OV

IV

OV

IV

OV

Fig. 2: ABS with open circuit (hydraulic circuit)

pansion tank. The pedal-travel sensor reports theposition of the brake pedal to the ECU.

Hydraulic unit. As the engine-pump unit, it in-cludes a dual circuit electrically-driven hydraulicpump and the valve block. This has two 2/2 sole-noid valves for each closed-loop control circuit. Aninlet valve (IV) and an outlet valve (OV) with a par-allel selected non-return valve.

Operating principle of the ECUIf the ECU detects an incipient lock, e.g. on the frontleft wheel, then the inlet valve closes and the outletvalve opens. The brake fluid now flows at zero pres-sure back into the expansion tank. When switchingto pressure build-up, the outlet valve closes andthe inlet valve opens. The brake fluid missing fromthe brake cylinder will be added by the mastercylinder plunger. The master cylinder plunger andthe brake pedal move slightly as a result. The travelsensor informs the ECU. This switches the hydraulicpump on. It pumps fluid back until the original ped-al position has been reached again.

Electrical circuit of an ABSThe schematic diagram (Fig. 1) shows a 4-channelABS with return in a closed circuit with eight 2/2 solenoid valves and 4 sensors.

When the ignition switch is switched on, the con-trol winding in the electronic protection relay issupplied with the voltage from terminal 15, theECU switches and connects to terminal 30 (posi-tive) via pin 1 (plug-in connection on the ECU). Atthe same time, the warning lamp lights up be-cause it is connected to terminal 15 to the positiveand via terminal L1 to the valve relay and via thediode to earth. The ECU now checks the ABS forfaults. If everything is OK, it connects across pin 27and returns the control winding in the valve relayto earth. The valve relay switches. Pin 32 on theECU receives positive from terminal 30, as does

the cathode of the diode. The warning lamp goesout. The solenoid valves are now at the positive.

Should the ECU detect a risk of locking in the valverelay, pin 28 is returned to earth. The motor relayswitches on the return pump. The valve relay cannow be switched to the control phases by connect-ing to earth at pin 35 or 37 in the control phases.

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3015

Plug-in connectionECU

Warning lamp

Brake-light switch

1

311 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

30 15 87

14 28 25

Electronicprotectionrelay

Motor relay ValverelayM

27 29 32 10 5 4 11 21 7 3 24 25 15

Electr. motor (return pump)

30 85 86 87 87 86 85 L1

Solenoid valves

Hydraulic modulator Plug-inconnections to ECU

Speedsensor

X2

Protected

n

FL

X3

FR

X4

RL

X5

RR

G1

D+/61n n n

2 18 35 19

1 5 3 7

FL RL FR RR

36

2

38

6

39

8

37

4

Fig. 1: Schematic diagram for a 4-channel ABS

Before disconnecting the ECU, the ignitionmust be switched off.

WORKSHOP NOTES

Check of the electrical systemThis can be carried out using a voltage or resis-tance measuring device, a test diode or specialtest equipment.

1. Inspection. Power supply ECU:Ignition “on”; between pin 1 and earth, U > 10 V.

2. Valve relay function:Pin 27 at earth, ignition “on”; senses theswitching of the relay, or between pin 32 andearth, U > 10 V. Electric circuit control winding: ignition “off”,resistance measuring device between pin 1and 27, R ≈ 80 Ω.

3. Speed sensor VR resistance:Ignition “off” between pin 11 and 21, R = 750 Ω … 1.6 kΩ.Function: turn wheel, between pin 11 and 21e.g. at 1 rotation of wheel/second U > 30 mValternating voltage.

4. Motor relay function:Ignition “on”, pin 28 at earth, senses the trip-ping function, or between pin 14 and earth, U > 10 V, return pump runs (noise).

18.10.14 Brake assistant (BAS)

Many drivers brake quickly in critical situations butdo not depress the brake pedal enough. The brak-ing distance is therefore longer which can lead tocollisions.

StructureThe brake assistant (Fig. 1) consists of the follow-ing components:

● BAS ECU ● Solenoid

● Travel/pedal sensor ● Release switch

Operating principleThe movement of the pedal causes a change in re-sistance in the pedal sensor. This is reported to theBAS ECU. If the ECU detects that the pedal is sud-denly applied, for example during panic braking,then the solenoid is activated. This vents the work-ing chamber of the brake booster to create the fullforce of the booster. The result is emergency brak-ing. The ABS prevents the wheels from locking. Thesolenoid is only switched off via the release switchonce the brake is released and the brake pedal hasreturned to its initial position.

For data exchange, the BAS ECU is connected viathe CAN bus to the ECUs for other electronic chas-sis control systems, e.g. ABS, TCS, ESP.

If the ECU detects faults, the brake assistant isswitched off. The failure is displayed with a yellowwarning lamp.

18.10.15 Traction-control system (TCS)

This stabilises the vehicle in the longitudinal direc-tion, the cornering stability is maintained and thevehicle is prevented from breaking away at thepowered axle.

The TCS is an enhancement of ABS. Both systemsuse common sensors and actuators and often havea common ECU where the data exchange is usuallycarried out via a CAN bus. When the vehicle is be-ing driven with snow chains, the TCS can be deacti-vated. A distinction can be made between:

● TCS systems with engine intervention.

● TCS systems with brake intervention, otherwiseknown as ELSD Electronic Limited-Slip Differential.

● TCS systems with engine and brake intervention.

Advantages● Improvement of traction when pulling away or

accelerating.

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The brake assistant immediately makes sure inthe case of panic braking that there is maxi-mum brake boosting effect, which means thatthe braking distance is considerably reduced.

Travel sensor

Tobrake mastercylinder

Diaphragm disc

Solenoid coil

Release switch

Workingchamber

Vacuum chamberECU

Solenoid

Fig. 1: Brake assistant

REVIEW QUESTIONS

1 Which electronic chassis control systems are used?

2 Which forces take effect on the vehicle wheel?

3 What is understood by slip?

4 What is the slip range in which the vehicleremains steerable and stable?

5 What are the tasks of an ABS?

6 Name the components of an ABS.

7 Name and explain the terms of the ABS closed-loop control circuit.

8 Name the control phases for ABS.

9 What are the essential differences of thehydraulic-ABS concepts?

10 How does the brake assistant work?

The TCS system prevents the drive wheels spin-ning when pulling away and accelerating.

Brake master cylinder

ABS low-pressure accumulator

Suctionvalves

Hydraulicpump P

Deliveryvalve

Inlet valveIV

Speed sensor,drive wheel, right

Hydraulicchangeovervalve

Checkvalve CV

Controlline

ABSoutletvalve

Wheelbrake

Pressure-limiting valve

ABS/ELSDECU

ELSD pressure build-upELSD pressure holding

InductionELSD pressure reduction

Fig. 2:TCS/ELSD brake circuit of a wheel

● Increase of driving safety at high motive forces.

● Automatic adjustment of engine torque to thegrip ratios.

● Driver information about reaching dynamic lim-its.

TCS with brake intervention/electronic limited-slip differential ELSD

Structure (Fig. 2, Page 471)Hydraulic system. This is composed of a hydraulicpump with suction and delivery valves, inlet andoutlet valves, a hydraulic changeover valve and acheck valve with pressure limiter.

Electrical system. This is composed of ABS/TCS(ELSD) ECU and wheel-speed sensors.

Operating principlePressure build-up. If a driven wheel spins, this isdetected by the ECU with a speed sensor. It acti-vates the hydraulic pump and the check valve. Thecheck valve (CV) closes and the pressure generat-ed by hydraulic pump P brakes the spinningwheel.

Pressure holding.The inlet valve (IV) is closed.

Pressure reduction. If the wheel has stopped spin-ning, then the inlet and check valves are openedand the pressure is relieved to the expansion tankvia the master cylinder.

TCS with engine and brake intervention

The system works with engine or brake interven-tion, according to the driving situation. The block di-agram Fig. 1 shows the collaboration of engine andbrake intervention for preventing unreliable wheelslip when pulling away (TCS operation/ELSD opera-tion) or in overrun mode (EDTC operation).

Structure (Fig. 2).● ABS/TCS EDTC ECU

● ABS/TCS hydraulic unit

● Electronic accelerator pedal with ECU

● Setpoint generator, servo-motor and throttlevalve

Operating principle (Fig. 1, Page 473)All wheel speeds are entered and processed in theABS/TCS ECU. If one or both wheels tend to spin,then TCS control is activated.

Control when pulling awayIf a wheel is threatening to spin, then the brakingtorque control overrides because it is important tohave as much traction as possible. If, for example,the rear right wheel (RR) starts to spin, then pumpP1 is activated via the ECU. The intake solenoidvalve Y15 is opened, the changeover valve Y5 andsolenoid valve Y10 on the rear left wheel (RL) areclosed. The pump-interior pressure thereforebrakes the wheel (RR). With the solenoid valvesY12 and Y13 in the hydraulic unit, the brakingtorque can be controlled through pressure build-up, pressure reduction and pressure holding.

Control when drivingIf, for example, both wheels are threatening to spin,then the drive-torque control overrides to gain opti-mum traction. The throttle-valve position is returnedwith a servo-motor and the moment of ignition de-layed, whereby the drive torque is reduced.

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controls

reduces

Drive torqueClosed-control loop

Intervention in enginemanagement v > 40 km/h

Electronic acceleratorpedal/EDTC

Brake interventionv < 40 km/h

Speed differential, wheel slip

Engine interventionBrake intervention

Optimaldirectional stability

Maximumpossible traction

Braking torqueClosed-control loop

controls

TCS

monitors

ABS

Fig. 1:TCS block diagram

ABSTCS

ABS TCSEDTC

Accelerator pedal

Brake control circuit

RA

Brake unit withbrake-light switch

TCS ON/OFF(snow-chain switch)

FASpeedsensor

Electronics

Hydraulics

Pressure accumulator

Pump

ETC

Engine control circuit

Emergency drive unitThrottlevalve

Servo-motor

Electronics

Setpoint generatorM

Fig. 2:TCS-system overview

An electro-hydraulic system is used as a start-ing-off aid. The lock effect is created as a resultof brake intervention on the spinning wheel inorder to achieve better traction.

If the wheels spin despite this, the braking torquecontrol is activated by feeding braking pressurefrom pump P1 via solenoid valves Y10 and Y12 tothe rear wheels until the wheels stop spinning. Thedirectional stability is increased.

OverrunIf slip occurs during sudden deceleration causedby the braking effect of the engine on the drivewheels, the ECU detects this and activates the en-gine-drag torque control (EDTC). By activating theservo-motor, the throttle valve is moved to such anextent and the engine speed therefore increasedthat there is no longer slip on the drive wheels.

TCS warning lamp. This informs the driver in thecase of TCS closed-loop control and if the systemfails.

18.10.16 Electronic Stability Program ESP, VDC

In the Electronic Stability Program (ESP) (Fig. 2),the following systems work together:

● Antilock-braking system (ABS)● Automatic braking-force distribution (ABV)● Traction-control system (TCS) with engine-drag

torque control (EDTC)● Automatic regulation of yaw moment (GMR).

With a networked data bus, the systems controlthe brake intervention depending on the wheelspeed, braking pressure, yaw rate, steering angle,lateral acceleration and defined program maps.

Operating principleThe signals from the sensors, e.g. wheel speed,steering movement and lateral acceleration arerecorded by the ECU as actual values and com-pared with stored setpoint values. If the actual val-ues deviate from the desired and actual course(setpoint value), then one wheel is braked specifi-cally so that the vehicle remains stable.

The ESP system decides …

● … which wheel is braked and how sharply.● … whether the engine torque is downrated.

Understeer. If the vehicle tends to understeerwhen cornering or during a swerve to avoid an ob-stacle (Fig. 3), then it would be pushed straightahead by the front axle. The ESP system controlsuses a presupply pump (Fig. 1, Page 474) to controlthe braking pressure of the rear wheel in the insideof the bend. The yaw moment created as a resulttwists the vehicle on the vertical axis and counter-acts the understeering.

Oversteer. If the vehicle tends to oversteer (Fig. 3),then the front wheel on the outside of the bend, forexample, is braked by the system, therefore stabil-ising the vehicle.

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M

RL RR

Y11 Y13

Y10 Y12

Solenoid valve

Pump

P1

Y15 Damper

Y5

Intakesolenoid valve

Changeovervalve

Pressure accumulator

RA circuit

To FAcircuit

Pressurebuild-up RRInduction

PressureholdingPressurereduction

Fig. 1: Hydraulic circuit diagram of a brake circuit

Through the specific braking of individualwheels, the vehicle can be stabilised laterallyand longitudinally. This therefore prevents thevehicle turning on a vertical axis.

Steering-wheel-angle sensor

Hydraulic control unitwith integrated controller

Enginemanagement

2 pressure sensors ontandem master cylinder

Yaw-rate sensor Wheel-speedsensor

Lateral-accelerationsensor

ABS: Antilock Braking System+ ABV: Automatic regulation of braking-force distribution+ TCS: Traction Control System+ GMR: Automatic regulation of yaw moment= ESP: Electronic Stability Program

ABV

GMR

ABSESP

TCSESP

Fig. 2: Components of the ESP system

+ +

Vehicleundersteers

Vehicleoversteers

Fig. 3: Understeering and oversteering vehicle

Hydraulic circuit diagram (Fig. 1)The brake circuit of a wheel is shown here.

Pressure build-upIf ESP intervenes in the control, P1 draws in thebrake fluid from the supply reservoir and suppliesit to pump P2. This guarantees that the systemquickly builds up braking pressure in the brake cir-cuit even at low temperatures. The return pump P2works in the same way, increasing the brakingpressure further until the wheel is braked. Thehigh-pressure switching valve Y1 and the inletvalve Y2 are therefore opened. The outlet valve Y3is closed and the switching valve Y4 is blocked.

Pressure holdingIn this control phase, the high-pressure switchingvalve Y1 and the inlet valve Y2 are closed. The brak-ing pressure remains constant.

Pressure reductionIn this phase, the outlet valve Y3 and the switchingvalve Y4 are opened. The brake fluid is returnedthrough the return pump back to the master cylin-der.

18.10.17 Sensotronic Brake Control (SBC)

StructureThe SBC system (Fig. 2) essentially consists of thehydraulic unit with pressure accumulator, actuat-ing unit, ECU and speed and yaw-angle sensors.

Unlike a conventional brake system, where allwheels are first subjected to a high braking pres-sure as quickly as possible and then a pressurecontrol is carried out, with SBC the braking pres-sure of the individual wheels is controlled individ-ually. Sensors determine the current driving situa-tion and the ECU calculates the optimum brakingpressure from this for each wheel. In this way, it ispossible to brake the left-hand wheels moresharply which are subjected to heavier loads dur-ing a right-hand bend, for example. When brakingin bends, this results in an optimum braking factorand stable driving characteristics.

In addition to the functions of a conventional hy-draulic brake system, SBC can, for example, adoptthe following functions:

● Holding the vehicle on an incline (hill-starting).

● Applying the footbrake and handbrake until thediscs and drums are dry in wet conditions.

.

● Softstop to prevent diving under braking.

● Filling the lines when rapid deceleration is de-tected, therefore faster pressure build-up duringan emergency braking manoeuvre.

● Automatic adaptive speed and distance control(ACC).

The system does not need a brake booster. In thecase of an electronics failure, restricted braking cantake place via an emergency hydraulic connection.

FunctionFig. 1, Page 475, shows the structure of the SBChydraulics. The driver operates the brake pedal,therefore generating a braking pressure in bothbrake circuits in the master cylinder. The pressureis recorded by pressure sensor b1.

SBC normal brakingThe ECU closes the hydraulic connection to thefront axle by supplying both isolating valves y1,y2.

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Return pump

Inlet valve

Mastercylinder

Presupply pump High-pressure switching valve

Exhaustvalve

Switchingvalve

Brake pliers

P1Y1

Y3

Y2

Pressurebuild-upPressurereductionPressureholding

Y4

P2

Fig. 1: ESP-system hydraulic circuit diagram

The Sensotronic Brake Control SBC (or EHB) isa “Brake by Wire” system. This means that thedriver's wish to brake is transmitted via an elec-trical wire. The system incorporates the func-tions of ABS, TCS, BAS and ESP.

Hydraulic controlunit with SBC ECU

Actuating unit Speed sensor

ECU box withESP ECU

Yaw-angle sensor

Fig. 2: Components of the SBC system

The brake system pressure supply is now providedby the pressure accumulator 3. The storage pres-sure is generated by the electrically driven hy-draulic pump m1 and measured by the pressuresensor b2. This can be up to 150 bar. Should thestorage pressure sink below a particular value,then the hydraulic pump is reactivated.

The ECU calculates the optimum braking pressurefor each wheel and adjusts it accordingly using in-let valves y6, y8, y10, y12 and outlet valves y7, y9,y11, y13. Pressure sensors b3, b4, b5, b6 report theactual values of the individual wheel brake cylin-ders to the ECU.

Balance valves y3, y4 balance the pressure for thewheels of one axle during a brake application.They are activated and closed during brakingwhen cornering and in the Electronic Stability Pro-gram. It is now possible to regulate the brake pres-sure individually for each wheel.

The two media isolators 7, 8, prevent nitrogenfrom entering master cylinder 1 from a leakingpressure accumulator 3.

Emergency-braking manoeuvre if SBC failsThe two isolating valves y1, y2 are not energisedand therefore remain open (Fig. 2).

The braking pressure generated by the driver inthe master cylinder is directed to the brake cylin-der at the front axle. The rear axle is unbraked. Be-cause there is no brake booster, the braking effectis low. The vehicle speed is therefore restricted bythe engine ECU to a maximum of 90 km/h.

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Returnline

S9/1

B37/11

Suction line

2

MC2 MC1

b1

y1 y2

y7 y9y6 y8

y3

b3

7

5b

b4

8

5a

FL FR

y11 y13y10 y12

y4

b5

6b

b6

6a

RL RR

b2

m1

3

Media isolator

Balancevalve

Pressure accumulator

Braking pressure SBCAdmission pressure

Balancevalve

Isolating valves

M Pressure sensor

Pressure sensor

Pressure sensor

Hydraulicpump

Fig. 1: SBC hydraulic circuit diagram, normal braking

Returnline S9/1

B37/11

Suction line

2MC2 MC1

b1y1 y2

y7 y9y6 y8

y3b3

7

5b

b48

5aFL FRMediaisolator

Balancevalve

Brakingpressure,emergencybraking =admissionpressure

Isolating valves

Fig 2: SBC hydraulic circuit diagram, emergency brakingmanoeuvre

REVIEW QUESTIONS

1 What is understood by a traction-control system?

2 What are the advantages of traction-controlsystems?

3 What are the components in the TCS systemrequired for the braking-torque control loop?

4 Describe the function of the TCS system withengine and brake intervention.

5 What are the advantages of an Electronic Stability Program?

6 How does the ESP/VDC system work if thevehicle is oversteered?

7 What are the extra functions of SBC in additionto a hydraulic brake system?

8 Explain the function of SBC.