automotive eng

8/3/2019 Automotive Eng

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Automotive Brakes

by

Dr. Amr Ibrahim


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Friction force is used in most of brakes to slow or stop the vehicle

During braking, the kinetic energyis converted into heat


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Automotive brake types:

Service brakes

Parking (or emergency) brakes

Most automotive service brakes are hydraulic brakes

The service brakes on many trucks and buses are air brakes


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Brake fluid

petroleum products such as oils damage rubber seals and hoses in the

braking system Petroleum products are rapidly and selectively absorbed by brake system

rubber parts, resulting in a high degree of softening and general

deterioration of the functional properties of these rubber parts

the main types of brake fluids are glycol(whose base is alcohol) and

silicone basedbrake fluid

the brake fluid is required to have a high boiling point and to remain

viscous to lubricate the pistons in the master cylinder, wheel cylinders, and

calipers


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the disadvantage of glycol is that it has the tendency to attract moisture

from air through rubber hoses, seals, and the vent in the master cylinder

reservoir cap

the moisture reduces the brake fluid boiling temperature and causesmetal parts to corrode

when the brake fluid gets overloaded with moisture, it must be replaced

silicon brake fluid has less tendency to absorb moisture and higher boiling

temperature compared to glycol brake fluid

the main disadvantage of silicone brake fluid is that it aerates easily. The

air remains suspended in the fluid creating foams


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Liquids can transmit both force and motion

Principle of hydraulic brakes

1001*100

1

100

BPF

PA

FP

BB

BA

2002*100

1

100

CPF

PA

FP

CC

CA

505.0*100

1

100

DPF

PA

FP

DD

DA


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The brake pedal increases the force of the drivers foot through

leverage

This force can also increase further by using bigger output pistons

Fi

Foa

b

b= distance from Fo to the pivot

a=distance from Fi to the pivot

iO

iO

Fb

aF

aFbF


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Engineers must consider the principles of force, pressure, and motion

when designing a brake system

if the master cylinder piston area is too small, the developed hydraulicpressure will be very high but the pedal travel will be extremely long

if the master cylinder piston area is too big, it can move a large volume

of liquid but it may not develop enough pressure to exert adequate

braking force at the wheels

Most brake systems with front discs and rear drums have relatively

large diameter master cylinder piston and a power booster to increase the

input force


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Dual braking system

The dual braking system is hydraulically split in two different ways:

Front-rear split

Diagonal split


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Front- rear split

Diagonal split


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Disc brakes

Drum brakes

Service brakes:


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Disc brakes


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Floating-caliper disc brake:


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Fixed-caliper disc brake:


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Disc brake shoe


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Drum brakes


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Wheel cylinder

Brake show

Return spring

Return spring

Brake show

Backing plate

Shoe hold

down spring

and pin

hub


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a

b

1

0

0

baa

FN

NaNbFa

M


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a

b

1

0

0

baa

FN

NaNbFa

M


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Leading-trailing drum brake

Anchor pins

Leading shoe

Trailing shoe

Forward direction


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Double-anchor double-cylinder drum brake


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Duo-servo drum brake

Floating adjusting screw


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Drum brake self adjuster

Adjusting screwAdjuster spring

Adjusting lever

cable


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Parking brake linkage


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Integral parking brakes (rear drum brake)

Parking brake lever

Brake shoe

pivot

Shoe strut or adjuster

A parking brake lever is attached to a brake shoe via a pivot

One end of the shoe strut is attached both the brake shoe and parking lever


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The other end of the shoe strut is attached the other brake shoe


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Parking brake cable

The lower end of the parking brake lever is connected to the hand lever via a

parking brake cable


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Independent parking brakes


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Power brakes

Most vehicles have power-assisted braking

This assist is usually provided by a vacuum brake booster (or servo)

The booster is located between the brake pedal and the master cylinder

Brake booster

Master cylinder


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Basic booster operation


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Braking dynamics

Front engine front drive Front engine rear drive

The location of the car center of gravity depends on the car design

The center of gravity is relatively shifted to the front when the engine is

located in the front and the car is a front wheel drive


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Stationary car:

W= car weight=mg

b=wheel base

Rf= reaction force on the front wheels

when the car is stationary= the static

weight on the front wheels

b

WxR

b

WyR

WxbRWybR

M

rf

rf

0

Rr=reaction force on the rear wheels when the car is stationary= the staticweight on the rear wheels


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Car during deceleration (braking):

ma=inertia force (m=car mass, a=car

deceleration)

Fbf= braking force on the front

wheels

Fbr=braking force on the rear wheels

Ff=the reaction force on the frontwheels during braking=the dynamic

weight on the front wheels

Fr=the reaction force on the rear wheels during braking=the dynamic

weight on the rear wheels

h=the height of the car center of gravity


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b

mahR

b

mah

b

WyFWymahbF

bmahR

bmah

bWxFWxmahbF

M

fff

rrr

0

0

0

ransferc weight tthe dynamib

mah


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During braking:

Dynamic weight at the front wheel=static weight at the front wheel +

the weight transfer

Dynamic weight at the rear wheel=static weight at the rear wheel the

weight transfer


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Braking capacity:

when a driver applies the brakes, the shoes/pads cause the rotating wheel

to slow down relative to the ground

this generates slipping between the road and the tire, and this slip

generates the braking force on the vehicle

as the driver increases the brake force, the slip increases and generates

higher braking forces

if the brake force is increased above a certain limit (called the braking

capacity), the wheel will lock and the tire will completely skid

when the wheel locks and the tire skids, the tire-road is operating at its

dynamic coefficient of friction which is lower than the static coefficient of

friction which exists before the wheel locks up

As a result, the car stops in a longer distance if the wheels lock


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The maximum braking force that the tire-ground contact can support is

determined by the coefficient of friction multiplied by the normal force:

fsbf FF max,

rsbr FF max,

Any further increase in the braking force would cause the tire to lock up

Since Ff>Fr, then the maximum braking force which can be applied on the

front wheels is higher than the maximum braking force which can be applied

on the rear wheels

This means the front wheels will have an increased capacity to provide

braking force


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Proportioning valve:

as less braking force is needed at the rear wheels, equal brake pressure at

the four wheels could cause the rear wheels to lock and the rear tires to skid the proportioning valve has no effect on hydraulic pressure during normal

braking

however, hard braking causes the fluid pressure to go above a certain value

called the split point

the proportioning valve then reduces the amount of pressure increase to the

rear drum brakes


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Metering valve:


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Pressure-differential valve:


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Combination valve:


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Anti-lock Braking System (ABS):

the purpose of the ABS system is to prevent the wheel lockup and skidding the system allows the brakes to apply until the tires are almost starting to

skid

then, the ABS system vary the hydraulic pressure to the brake at each wheel

The ABS consists mainly of:

rpm sensor for each wheel

ABS control module

Hydraulic actuator

the ABS control module receives the electrical signals from the rpmsensors and the stop light switch, and sends signals to the hydraulic unit


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Automotive Suspension

Systems

by

Dr. Amr Ibrahim


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The suspension system is located between the wheel axles and the

vehicle body or frame


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Purpose of the suspension system:

supports the weight of the vehicle

maintains traction between the tire and the road (Watch video)

provides a cushioning action so road shocks (resulting from road bumps

and holes) have a minimal effect on the occupants in the vehicle

allows the vehicle to corner with minimum body roll


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Main components of suspension system:

Springs

The springs support the weight of the vehicle and its load and absorb

road shocks

Dampers (shock absorbers)

spring oscillation occurs after passing bumps or holes

dampers dampen the spring oscillations

dampers allow the basic spring movement but quickly dampen out the

unwanted oscillation that follows


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Types of springs used in automotive suspension systems:

coil spring

leaf spring

torsion bar

air spring


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Coil spring:

it is made of a length of round spring

steel rod wound into a coil coil springs are used widely in

automotive applications due to their

compact size

coil springs are not capable of providing

any location for the axle (control armsmust be used)

Leaf spring:


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Leaf spring:

Single leaf spring is made of a single

plate while multi leaf spring is made

of several flexible steel plates ofgraduated length ( called leaves)

fixed rigidly by the center bolt

the additional leaves make the

spring stiffer allowing it to support

greater loadsas the spring deflects, friction is generated

between the leaves, resulting in some

oscillation damping capability

leaf spring can provide a mounting location

for the axle housing

although leaf springs are simple and cheap,

they tend to be heavy


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The two ends of the leaf spring

are attached to vehicle body

the middle of the spring isattached to the axle housing by

U bolts

The leaf spring can be

mounted below or above the

axle housing


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T i b


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Torsion bar

the torsion bar is a circular rod made of spring steel

one end of the bar is rigidly fastened to the vehicle body

the other end attaches to an upper or lower control arm

as the control arm swings up and down in response to wheel movement,

the torsion bar twists to provide spring action Watch video

Torsion bars


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A torsion bar with no load applied

A torsion bar with a load applied

one end of the torsion bar has a hexagonal head which fits into an


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g

adjustment key

the adjustment key is used to apply the twisting motion needed for vehicle

suspension and ride height adjustment

the amount of torque (twisting motion) is applied to the torsion bar by

turning the adjusting bolt

tightening this bolt will turn the key which will twist the torsion bar

this extra torque will apply more force to the control arm raising the vehicle

to the desired height


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Cross member attached to

the vehicle body

key Torsion bar

Adjusting bolt

t i b b t d l it di ll t l


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torsion bars can be mounted longitudinally or transversely

the main advantages of torsion bars compared to coil springs is the ease of

adjusting the vehicle height and they do not occupy large volume space

Air spring


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Air spring

the air spring is a rubber

cylinder or air bag filled withcompressed air

the air spring is placed between

the vehicle body and the axle

housing or between the body

and the lower control arm

the vehicle height can be adjusted by controlling a solenoid valve at the

top of the air bag which opens to add or release air Watch video

the compressed air is supplied by an air tank connected to a compressor

p g

air suspension systems are used to provide an adjustable suspension


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air suspension systems are used to provide an adjustable suspension

which allows vehicles to sit extremely low during high speed operation

exceeding about 100 km/h for improved aerodynamic performance

the vehicle height can be raised to a level high enough to maneuver overobstacles and inconsistencies in the roadways

the air suspension system can be used with an electronic control unit to

automatically control the vehicle level according to the vehicle weight

failure of an air spring may result in complete immobilization of the

vehicle

the compressor can be damaged due to leaking air springs. The

compressor will burn out trying to maintain the correct air pressure in a

leaking air system

Damper (shock absorber)


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Damper (shock absorber)

shock absorbers are more accurately called dampers because they do not

absorb shocks. The springs absorb shocks

when one of cars wheels hits a bump, the wheel is forced up towards thecar body

as the wheel moves up, it compresses the spring that attaches it to the car

body

after driving over the bump, the spring extends pushing the wheel back

onto the road

with no control, the spring would extend beyond its original length and

then compress again a little less than it did when the wheel first hit the

bump

this process of extension and compression would continue until all the

energy the spring had received from that bump was dissipated

by then the car would be bouncing out of control particularly if the wheels

were hitting new bumps

the shock absorber function is to damp the spring oscillations


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p p g

an oil damper converts the kinetic energy into heat via the friction between

the oil and the damper piston holes

A= chamber above the piston, B= chamber

below the piston, C=reserve cylinder

Compression


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p

The oil in chamber B transfers to

chamber A via holes in piston

The oil in chamber B transfers to

chamber C via deflecting discs in thebase valve

Extension


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The oil transfers from chamber A tochamber B via a valve in the piston

The oil transfers from chamber C

to chamber B via the base valve

h th i t it ifi i t th fl f il d h f i ti


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when the piston moves, its orifices resist the flow of oil, and hence, friction

and heat are created

wheels and suspension systems deflects at many different speeds,

depending on the type and size of bump and vehicle speed

the resistance of the tube (or piston) movement increases with the square

of its speed

for example, if the wheel deflection speed increases 4 times, the tube

resistance is 16 times as great

therefore, if a wheel strikes a large bump at a high speed, the wheel

deflection and rebound can be effectively locked by the damper

therefore the base valve can be made to open in stages according to fluid


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therefore, the base valve can be made to open in stages according to fluid

pressure

during fast upward wheel movement during the compression stroke,

excessive pressure in the lower oil chamber forces the base valve to widelyopen allowing more oil to flow to the reservoir

shock absorber fade


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shock absorber fade

heat is generated inside the shock absorber due to the friction between

the oil and the orifices

the heat is transferred through the outer tube to the outside air (shown

in the photo)

however, excessive heat can decrease the viscosity of the hydraulic oil

the thinner the oil, the lower the resistance to piston movement, and

hence, the lower the damping rate

also increasing the oil temperature increases the chance of oil cavitation


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also, increasing the oil temperature increases the chance of oil cavitation

(forming vapor bubbles) and aeration

mixing gas or air with the oil creates foams which lowers the damping effect

Gas-filled damper


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Gas filled damper

one method of reducing foaming is to fill the space above the oil in the

shock absorber with a pressurized gas such as nitrogen (which can be placed

in a bag)

Air damper


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air dampers have an air bag surrounding the shock absorber which can be

filled with compressed air

the compressed air increases the load carrying capacity of the vehicle whilemaintaining proper rear end height

Suspension types


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Suspe s o types

suspension systems can be classified according to position (front or rear) or

type (dependent solid axle versus independent)

Dependent (solid axle) suspension

a solid axle has wheels mounted to each end of a rigid beam

this system is very robust and usually used when high load carrying

capability is required

the main disadvantage is that the rigid connection results in a transmission

of motion from one wheel the other when the suspension deflects

this system is designed with several arrangements such as:

leaf-spring (or Hotchkiss) suspension

coil-spring (or four link) suspension

Beam axle suspension

Leaf spring (or Hotchkiss) suspension


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Leaf-spring (or Hotchkiss) suspension

Coil-spring suspension


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p g p

two control arms on each side allow up and down movement of axle

housing and prevent forward and backward motion

this arrangement does not prevent the sideward movement of the axle

housing

Four-link suspension system


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Beam axle suspension


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this arrangement is used for the rear suspension of a front-wheel drive car

the beam is called dead beam or dead axle

spring/shock units (struts) are bolted to both ends of the beam and seat upinto the car body

this arrangement is used for the front suspension of a rear-wheel drive car


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this arrangement is used for the front suspension of a rear wheel drive car

Independent suspension


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Independent suspension

in this system, each wheel is independently suspended by one spring. This

means the up and down movement of one wheel does not affect the otherwheel

there are several arrangements which are used with independent suspension

systems such as:

short-arm/long-arm (SALA) or double wishbone system

MacPherson system

SALA (or double-wishbone) system


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this system has 2 control arms look like the letter A.

each control arm has two attachment points in the inner end attached to

the car body which allow the control arms to move up and down with little

resistance

each control arm has a single ball joint in the outer end which allows the

steering movement for the front wheels Watch Video1 Video 2


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Beam type lower control arm


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MacPherson struts


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a strut is a rod or tube that is acted upon compression forces

in automotive suspension, the assembly that combines the shock

absorber with a coil spring is often called a MacPherson strut

the top of the strut mounts to the vehicle body while the bottom

tt h t th f t h l t i k kl th h l h i


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attaches to the front-wheel steering knuckle or the rear wheel housing

only a lower control arm is needed

Representation of a vehicle as a spring-mass system


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the sprung weight is the weight supported by springs

the unsprung weight is the part not supported by springs which includes

the weight of drive axle, axle shafts, wheels, and tires.

the unsprung mass is kept as low as possible

the roughness of ride increases as unsprung weight increases

Vehicle behavior during cornering (body roll)


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When a vehicle turns a corner, a centrifugal force acts on the body and tries

to push it outward


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r

mVFforcelcentrifuga

2

to push it outward

the centrifugal force, which acts at the vehicle center of gravity, tries to

rotate the vehicle body (sprung mass) around a line called the roll axis


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rotate the vehicle body (sprung mass) around a line called the roll axis

the rotating torque=centrifugal force multiplied by the moment arm

the position of the roll axis depends on the type of suspensions at the front

and rear


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and rear

for a car having a solid axle suspension for the front and rear, the roll axis will

be some distance above ground level

for a car having independent suspensions at the front and rear, the roll axis

will be at or near the ground level

for a car having an independent suspension at the front and a solid axle

suspension at the rear, the roll axis will be inclined from approximately the

ground level at the front rising to about axle level at the rear


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When a centrifugal force, F, acts on the body it sets up forces f1 and f2 at

the points of connection of the body and springs.

their resultant is a force F1 equal and opposite to F acting at a point called

the roll center (point O) located on the roll axis

there is a front roll center for the front suspension and a rear roll center

and a rear roll center for the rear suspension. The roll axis passes through

the front and rear roll centers

These two forces F and F1 constitute a couple of magnitude Fh, h being the

perpendicular distance between them

For equilibrium, there must be an equal and opposite couple to balance

th l Fh


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the couple Fh.

This balancing couple is supplied by an increase q in the left hand vertical

reaction and a decrease of the same magnitude in the right hand one.

t

Fhq

qtFh

the body roll can be resisted by either increasing the spring rate (spring

stiffness) or the distance between the two springs (spring base, t)

ideally, the springs should be soft enough to give a good ride to absorb most

of the energy resulting from the road shocks

therefore, an anti roll bar (stabilizer or sway bar) is used to increase the roll

stiffness

a stabilizer bar is a horizontal torsion bar which connects some part of the

left and right sides of the suspension system


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left and right sides of the suspension system

on independent suspension system, the stabilizer bar connects the right

and left lower control arms

most cars have a front anti roll bar

The suspension system geometry of a car is designed to keep the bottom

of the tire parallel with the road for maximum contact patch


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During body roll, the car body is no longer parallel with the road. That

reduces the suspension's ability to keep the tire parallel to the road

Large amounts of body roll cause the wheels to tilt away from the cornerwhich lifts the edges of the tire and reduces the contact patch size.

body roll causes one lower control arm to move upward, twisting the

stabilizer bar


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however, the stabilizer bar resists being twisted. This stiffens the suspension

during turns so less body roll results and helps to maintain as much of the

contact patch as possible


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Vehicle rollover


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when a centrifugal force (F) acts on the vehicle center of gravity, reactionforces (f1 and f2) appear at the wheels

the resultant of these forces (F1) are equal in magnitude and in opposite

direction to F

these two forces exert a torque on the vehicle (FH, H is the height of force F

above the ground)

this torque is balanced by an opposite torque created due the dynamic

weight transfer caused by vehicle cornering

TH


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T

FHp

T= wheel track

when the weight transfer, p, is equal to the static weight, W, the normal force

on the inner wheel becomes zero and the vehicle is at the point of incipient

rollover

in order to reduce the chance of rollover, the vehicle center of gravity needs

to be close to the ground

Active suspension system

the active suspension system is a computer controlled suspension system


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the active suspension system is a computer controlled-suspension system

in this system, hydraulic (or penumatic) actuators are used instead of

springs and shock absorbers

the oil pressure inside the actuator is

regulated by an electronic control unit

(ECU) by controlling the oil flow

through the actuator via servo valves

the ECU receives information

regarding the road conditions and

vehicle dynamics from sensors

conventional suspension systems involve trade offs. Less roll in a corner

requires stiffer springs hence a harsher ride So performance and comfort


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requires stiffer springs, hence, a harsher ride. So, performance and comfort

are at odds to each other

on the other hand, the active suspension system can produce any desiredsuspension movement at any wheel at any time

the active suspension system uses the hydraulic pressure to keep each tire

pushing against the road surface with a constant force

this force changes as the tire moves up and down. A load sensor in the

actuator signals the ECU when the tire force changes

A displacement sensor installed on the actuator to inform the ECU about

the actuator relative position

this enable the ECU to track the extension and compression of each

actuator and to know if the wheel undergoes jounce or rebound

other sensors signal changes in steering position, acceleration,

deceleration, and body movement

the ECU receives these inputs and then signals the proper servo valve to

control the pressure inside the actuator


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control the pressure inside the actuator

for example, during hard braking, the system increases the pressure in the

front actuators and reduces the pressure in the rear actuators in order to

minimize dive and keep vehicle height and control

after braking, valves operate to equalize pressure at the front and rear

actuators

the rate at which the oil is bled from (or fed to) the actuator can be varied

by the ECU at any point during jounce or rebound to produce a variablespring rate effect. This determines the relative softness or harshness of the

ride.

disadvantages include the high cost & hydraulic pump noise and power

consumption

automotive eng

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