Purpose of braking system

o Stop the vehicle by converting the kinetic energy of the vehicle to heat

energy.

o Heat energy is created in the brakes by friction.

o Friction is created between a moving and a non-moving surface at each

wheel to generate the heat.

o Disc and drum brakes are the most common type of braking systems

used.

o Factors Affecting Braking

o Number of wheels braking.Weight of vehicle.Type of friction

material.Surface area of friction material. Size or discs or drums Tire

traction.Road surface. Load transfer.Incline or decline of road. (gravity)

Engine braking. Pressure applied

o Service brakes. It’s the primary braking system using a pedal connected

to a hydraulic system causing it to operate.

o Parking brakes. It’s mechanically applied by a lever or pedal.

• The purpose of the braking system is of course, to stop the car.

• Brakes are used on all wheels and are hydraulically operated.

• Two common types of brake assemblies are used.

• Disc Brakes

• Drum Brakes

Disc Brakes

• Uses a rotor that spins with the wheel and a stationary caliper to press

friction material against the spinning rotor.

• Used on most all front brakes and some rear brakes.

Drum Brake

• Uses a drum which spins with the wheel. Stationary brake shoes are

pressed out from the inside to cause friction.

• Used on rear brakes of many cars.

Hydraulic Braking system

Working

o The master cylinder displaces brake fluid under pressure to brake system.

o When brake pedal is depressed, push rod moves the primary piston

forward in the cylinder.

o Hydraulic pressure created and the force of the primary piston spring

moves the secondary piston forward.

o Forward movement of the pistons causes primary cups to cover bypass

holes, hydraulic pressure builds up and is transmitted to the wheel

cylinders.

o pedal retracts, the pistons allow fluid from the reservoir to fill the

chamber

o Special sensors within the master cylinder used to monitor level of fluid

in reservoir, and alerts driver if pressure imbalance develops.

o standard dual master cylinder gives front and rear brakes separate

hydraulic systems.

o Brake lines are steel tubing with copper and lead coatings to prevent rust

and corrosion. As the brake pedal is depressed, it moves pistons within

the master cylinder and forcing hydraulic brake fluid throughout the

brake system and into the wheel (or brake) cylinders.

o The pressure placed upon this fluid causes the cylinder pistons to

move, forcing the brake shoes or friction pads and brake drums or rotors

to slow the vehicle.

Anti – Lock Braking System

• Helps driver stop under control

• Keeps brakes from locking up

• Pulses brakes

• Enables car to be turned

Car Body Types

Sedan

Sedans are cars that typically have four doors, though; they can have two

doors, and conventional trunks.

Small sedans are compact in size, and generally have less rear passenger

room than other types of sedans.

The next size sedan is the family or mid-sized sedan. They typically have

more room than small sedans for supporting passengers and cargo.

Large sedans are both larger in size and in price. They offer greater leg

room and can comfortably seat five.

Coupe

This is a type of car that has a fixed roof and two full-sized passenger

doors that can seat two to four people.

For four passenger coupes, the rear seats are often smaller than the two

front seats.

Autotropolis indicates that the coupe body type is a compact and sportier

version of the sedan type.

Van

A van is a box shaped vehicle that usually comes with sliding rear side

doors for passengers.

It sits higher off the ground than sedans or coups and typically has extra

cargo space for the transporting of goods for family or business.

Vans come in various sizes from full sized to mini, which are most often

associated with

Families

Hatchback

This body type is less than 178 inches with a height of less than 64.4

inches,

They have five seats and a lift rear door. The cargo area is accessible to

the passenger area.

A Hatchback car has a sloping back with a hinged rear door that opens

upward.

The rear deck lid and window lift open as a unit.

The rear door leads into the car.

Sports Utility Vehicle

SUVs, are generally higher than most cars and have greater carrying

capacity in terms of cargo than other types of cars

In certain SUVs there are up to three rows of seats to sit seven to eight

people.

Because of their weight they also consume more fuel than minivans or

wagons, according to Consumer Reports. Many SUVs are also four-

wheel drive.

Pickup trucks

Pickups, or trucks, either have a cab for passengers or no cab, and an

elongated, uncovered cargo bed in the back.

This body type is ideal if you need to haul large items, but it is also

commonly used as a passenger vehicle as well.

Crossovers

Crossovers are a combination type of car with characteristics of both the

SUV and regular vehicles.

They are built on a car-based platform, but have the appearance of an

SUV or wagon, most often without the four wheel drive capabilities.

Convertible

A convertible is a car body style with a folding or retracting roof.

The collapsible roof section is typically made from flexible canvas or

vinyl, although plastic, aluminium and steel have occasionally been used

in elaborate folding designs.

Convertibles are usually 2 door models, only a few 4 door models exist.

Vehicle classification

Sports Those cars with significant high performance features

Luxury Higher-end cars that are not classified as sports

Large Length more than 495.3 cm (195 in) and wheelbase

more than 279.4 cm (110 in)

Midsize Length 457.3–495.3 cm (180–195 in) and wheelbase

266.8–279.4 cm (105–110 in)

Small Length less than 457.2 cm (180 in) and wheelbase less

than 266.7 cm (105 in)

Classification Code Curb weight

Passenger cars: mini PC/Mi 1,500–1,999 lb (680–

907 kg)

Passenger cars: light PC/L 2,000–2,499 lb (907–

1,134 kg)

Passenger cars: compact PC/C 2,500–2,999 lb (1,134–

1,360 kg)

Passenger cars: medium PC/Me 3,000–3,499 lb (1,361–

1,587 kg)

Passenger cars: heavy PC/H 3,500 lb (1,588 kg) and

over

Sport utility vehicles SUV -

Pickup trucks PU -

Car class Total passenger and cargo (litres)

Two-seaters Any (designed to seat only two adults)

Minicompact Less than 2,407

Subcompact 2,407–2,803

Compact 2,832–3,087

Mid-size 3,115–3,370

Large 3,398

Small station wagons Less than 3,681

Mid-size station wagons 3,681–4,502

Large station wagons 4,531 or more

Bus Building

Preparation of KITS

o Kits are readymade pre-welded angles.

o Kits are meant for uniform model buses.

o ADVANTAGES:

o The main advantage in the kit model is that the company need not wait

for the chassis from the customer.

o Working days are less in doing kit model buses.

Preparation of Floor kit, Roof kit, LEFT & RIGHT KITS and Assembling of

kits

WELDED ANGLES furnished with SAL WOOD

Use fibres for front COWL and Das Board

o To avoid water leakage fibre reinforced plastics are used.

o This fibre materials reduces weight and helps in mileage

o These fibres are made out of readymade moulds of different shapes.

o A chemical cote called PVA is applied. This cote will be converted as

thin polythene sheets within few minutes.

o Then a mixture of Gel Cote, Catalyst, Resin and COBOLT are applied.

o This fibre is a non heat conductor.

o Can repair the damages easily,need not be replaced entirely

o FIBRE MADE FRONT COWL

o FIBRE used for FRONT COWL & DASH BOARD

Flooring o DENSIFIED PLYWOOD are fixed for flooring .

o Each PLYWOOD are attached through BEEADING.

o Diamond shaped, plain,round designs plywoods are available.

ROOFING

PLYWOOD FIXED BEFORE ELECTRICAL WORKS

COMPLETION OF ROOFING WORK after ELETRICAL WORKS

ELECTRICAL WORKS

This works would be done after space allocated for

TV sets,cabinet for players & speakers Tube Lights & Lamps

in roof Decorative lights placed in roof Lamp placed in Steps

FITTINGS

Seats, Lights, IGR & PostLaft, Crathy, Curtains,

INNER FITTINGS

Driver partition, Seats, Post,IGR,Crathy, Handle, Lightings, Sound

system, Video provision

Battery box, Lafts on either sides, Ventilator

DRIVER PARTITION

Route board & connection board, Almira, Engine bannet,

Mirror, first aid box, bottle holder,

Player cabinet, UPS,Swith board, Seat facing driver, OUTER

FITTINGS

Front & rear glass, Doom Light, Indicator s, Wiper, Name board, Rear

view mirror

Windows , Entrance shutter, Ladders, luggage scrollers, Rear

light , WATER

LEAK TEST

This is used to test water leakage after sheet metal is fixed

Water will be sprayed out from EVERY NOOK & CORNER

This work will be held before painting

PAINTING

This process is done after fitting stage

Financial risk is included in this work

Moderate climate is friendly to painting process

RISK FACTORS :-

Air pollution

Rainfall

Workers in-alertness, Unusual sunlight

Active Safety

In the automotive sector the term Active Safety (or Primary Safety)

refers to safety systems that are active prior to an accident

safety systems that help avoid accidents

Used to describe systems that use an understanding of the state of the

vehicle to both avoid and minimize the effects of a crash. brake assist, traction control systems Electronic Stability Control systems, that interpret signals from various

sensors to help the driver control the vehicle.

sensor-based systems such as Advanced Driver Assistance

Systems including adaptive cruise control and collision warning/avoidance

• good visibility from driver's seat,

• low noise level in interior,

• legibility of instrumentation and warning symbols,

• early warning of severe braking ahead,

• head up displays,

• good chassis balance and handling,

• good grip,

• anti-lock braking system,

• Electronic Stability Control,

• Chassis assist,

• intelligent speed adaptation,

• brake assist, traction control,

• Driver assistance • which help the driver to detect obstacles and to control the vehicle.

• Driver assistance systems include:

• Automatic Braking systems to prevent or reduce the severity of collision.

• Infrared night vision systems to increase seeing distance beyond

headlamp range

• Adaptive headlamps control the direction and range of the headlight

beams to light the driver's way through curves and maximize seeing

distance without partially blinding other drivers

• Reverse backup sensors, which alert drivers to difficult-to-see objects in

their path when reversing

• Backup camera

• Adaptive cruise control which maintains a safe distance from the vehicle

in front

• Lane departure warning systems to alert the driver of an unintended

departure from the intended lane of travel

• Tire pressure monitoring systems or Deflation Detection Systems

• Traction control systems which restore traction if driven wheels begin to

spin

• Electronic Stability Control, which intervenes to avert an impending loss

of control

• Anti-lock braking systems

• Electronic brake force distribution systems

• Emergency brake assist systems

• Cornering Brake Control systems

• Pre-crash system

• Automated parking system

•

• Electronic Stability Control Improves Safety - The Motor Vehicle

Safety Equipment Exists Now to Help Reduce Crashes and Rollovers •

• Passive Safety (or Secondary Safety), which are active during an

accident. To this category belong seat belts, deformation zones and air-

bags, etc.

• passenger safety cell

• deformation zones

• seat belts

• air-bags

• laminated glass

• correctly positioned fuel tanks fuel pump kill switches

• ECU (Electronic Control Unit)

• Is a generic term for any embedded system that controls one or more of

the electrical system or subsystems in a motor vehicle.

Types of electronic control units

• ESP (ESC) stands for Electronic Stability Program

Electronic stability control (ESC), also referred to as electronic stability

program (ESP) or dynamic stability control (DSC), is a computerized

technology that improves the safety of a vehicle's stability by detecting and

reducing loss of traction (skidding). When ESC detects loss of steering control, it

automatically applies the brakes to help "steer" the vehicle where the driver

intends to go. Braking is automatically applied to wheels individually, such as

the outer front wheel to counter oversteer or the inner rear wheel to

counter understeer. Some ESC systems also reduce engine power until control is

regained. ESC does not improve a vehicle's cornering performance; instead, it

helps to minimize the loss of control. According to Insurance Institute for

Highway Safety and the U.S. National Highway Traffic Safety Administration,

one-third of fatal accidents could be prevented by the use of the technology.

• Electronic stability control (ESC), also referred to as electronic

stability program (ESP) or dynamic stability control (DSC), is a

computerized technology

• It improves the safety of a vehicle's stability by detecting and reducing

loss of traction (skidding)

• When ESC detects loss of steering control, it automatically applies the

brakes to help "steer" the vehicle where the driver intends to go. Braking

is automatically applied to wheels individually.

• Some ESC systems also reduce engine power until control is regained.

• ESC does not improve a vehicle's cornering performance; instead, it

helps to minimize the loss of control.

It is used on vehicles to prevent ―spinning‖ during heavy braking and

steering

ESP utilises w/speed sensors to monitor the road wheels, as well as a yaw

speed sensor to detect its level of movement through the z axis (spinning) and a

steering wheel angle sensor. It also uses traction control and anti-lock braking

systems; it can’t work on its own.

• Understeer occurs when you go around a corner much too fast and the

front wheels don’t have enough traction. As a result you end up going

forward instead of turning.

• Oversteer is the opposite, the car turns more than the driver intended to

causing the rear wheels to slide and the car to spin.

• Understeer is common on front wheel drive cars

• Oversteer is common on rear wheel drive cars

Sensors Used

o Wheel speed sensor monitors the rotational speed of the road wheels,

and detects when the vehicle is de-accelerating (slipping)

o Yaw Speed Sensor, detects rotation around the z axis (spinning)

o Steering angle sensor measures the angle of the steering wheel, to

monitor what direction the driver wishes to steer

o Traction Control is used in order to drop acceleration from the wheel

that is deemed to be slipping.

o ESP also uses ABS to activate the brakes on individual wheels at the

required level to prevent the driver from losing control

o

Control Units in a Modern Automobile

Airbag control unit (ACU)

door locks, electric windows, courtesy lights, etc.

Convenience control unit (CCU)

Door control unit (DCU)

Engine control unit not to be confused with electronic control unit, the

generic term for all these devices

Electric Power Steering Control Unit (PSCU)— Generally this will be

integrated into the EPS powerpack.

Human-machine interface (HMI)

Powertrain control module (PCM):

Seat Control Unit

Speed control unit (SCU)

Transmission control unit (TCU)

Brake Control Module (BCM; ABS )

Battery management system

Key elements of an ECU Core

Microcontroller

Memory

Inputs

Supply Voltage

Digital inputs

Analog inputs

Outputs

Relay drivers

H bridge drivers

Injector drivers

Logic outputs

Communication links

CAN

Serial

USB

Flexray

Connector

Housing

ECU and Sensors

• Modern automotive control systems consist of a network of electronic

sensors, actuators, and computer modules designed to regulate the power

train and vehicle support systems.

• A computer processes the input voltage signals it receives by computing

what they represent, and then delivering the data in computed or

processed form.

SENSORS are essential components of automotive electronic control

systems.

Sensors are defined as

“Devices that transform (or transduce) physical quantities such as pressure

or acceleration (called measurands) into output signals (usually electrical) that

serve as inputs for control systems.”

The engine control system includes:

sensors for the detection of the engine operating modes

electronic control unit (ECU) which elaborates the signal values

supplied by the sensor, according to defined control strategies and

algorithms, and defines the actions to be delivered to the actuators

actuators which have the task to actuate the defined commands

DRIVING FACTORS LEADING TO INCREASED USE OF SENSORS

Power Train

• Low emission

• Fuel economy

• Best in class driveability

Chassis

• Vehicle safety and comfort features

Body

• Safety

• Security

• Comfort

• Convenience

THE FOUR BASIC COMPUTER FUNCTIONS

All computer systems perform four basic functions:

input,

processing,

storage, and

output.

• Input

– A signal from a device that can be as simple as a button or a switch

on an instrument panel, or a sensor on an automotive engine.

• Processing

The term used to describe how input voltage signals received by a computer

are handled through a series of electronic logic circuits maintained in its

programmed instructions

• Storage

– The place where the program instructions for a computer are

stored in electronic memory.

Output

After the computer has processed the input signals, it sends voltage signals

or commands to other devices in the system

DIGITAL COMPUTERS

• The digital computer can process thousands of digital signals per second

because its circuits are able to switch voltage signals on and off in

billionths of a second.

• The voltage signal or processing function is a simple high-low, yes-no,

on-off signal.

• Central Processing Unit (CPU)

– The CPU can be considered the heart of a computer because it

performs the essential mathematical operations and logic

decisions that make up its processing function.

• Computer Memory

Other IC devices store the computer operating program, system sensor input

data, and system actuator output data information that is necessary for CPU

operation.

Major areas of systems application for automotive sensors

MAP (manifold absolute pressure) sensor

• This sensor detects engine load.

• The computer uses this information for fuel delivery and

for onboard diagnosis of other sensors and systems such as

the exhaust gas recirculation(EGR) system.

• MAF (mass airflow) sensor.

• This sensor measures the mass (weight and density) of the

air entering the engine.

• The computer uses this information to determine the

amount of fuel needed by the engine.

• A mass air flow sensor (MAF) is used to find out the mass

flow rate of air entering a fuel-injected internal

combustion engine.

• The air mass information is necessary for the engine

control unit (ECU) to balance and deliver the correct fuel

mass to the engine.

• Air changes its density as it expands and contracts with

temperature and pressure. In automotive applications,

air density varies with the

ambient temperature altitude and the use of forced

induction, which means that mass flow sensors are more

appropriate than volumetric flow sensors for determining

the quantity of intake air in each piston stroke.

ECT (engine coolant temperature) sensor.

• This sensor measures the temperature of the engine

coolant needed by the computer to determine the amount

of fuel and spark advance.

• This is a major sensor, especially when the engine is cold

and when the engine is first started.

• The coolant temperature sensor is used to measure the

temperature of the engine coolant of an internal

combustion engine.

• The readings from this sensor are then fed back to

the Engine control unit (ECU).

• This data from the sensor is then used to adjust the fuel

injection and ignition timing.

• On some vehicles the sensor may be used to switch on the

electronic cooling fan. The data may also be used to

provide readings for a coolant temperature gauge on the

dash. The coolant temperature sensor works using

resistance. As temperature subjected to the sensor

increases the internal resistance changes. Depending on the

type of sensor the resistance will either increase or

decrease.

O2S (oxygen sensor)

• This sensor measures the oxygen in the exhaust stream.

• These sensors are used for fuel control and to check other

sensors and systems.

Exhaust Gas Treatment- Catalytic Converters

• It’s a Emission control device, it helps to reduce - carbon

monoxide, un-burnt hydrocarbons and nitrogen oxide

molecules. The converter uses two different types of

catalysts, a reduction catalyst and an oxidization catalyst.

Both types consist of a ceramic structure coated with a

metal catalyst, usually platinum, rhodium and/or

palladium. The idea is to create a structure that exposes

the maximum surface area of the catalyst to the exhaust

stream

• The reduction catalyst is the first stage of the catalytic

converter. It uses platinum and rhodium to help reduce the

nitrogen oxide emissions. When such molecules come in

contact with the catalyst, the catalyst rips the nitrogen

atom out of the molecule and holds on to it, freeing the

oxygen in the form of O2

• The oxidation catalyst is the second stage of the catalytic

converter. It reduces the unburned hydrocarbons and

carbon monoxide by burning (oxidizing) them over a

platinum and palladium catalyst. This catalyst aids the

reaction of the CO and hydrocarbons with the remaining

oxygen in the exhaust gas.

EGR

• Exhaust Gas Recirculation (EGR) is of critical importance

in diesel engine emission control.

• The recirculation of inert exhaust gas displaces some of

the oxygen from the intake air charge and lowers peak

flame temperatures by absorbing heat.

• As combustion temperatures decrease, NOx production is

also reduced. When less NOx is produced in the engine

cylinders, there is a reduced burden on the vehicle’s after

treatment system.

• While the newest diesel engines now rely on some form of

after treatment to deal with NOx, they all still utilize EGR

as well.

TP (throttle position) sensor

This sensor measures the throttle opening and is used by the

computer to control fuel delivery as well as spark advance and

the shift points of the automotive transmission/transaxle

VS (vehicle speed) sensor

• This sensor measures the vehicle speed using a sensor

located at the output of the transmission/transaxle or by

monitoring sensors at the wheel speed sensors.

Height Sensor

Crank Position Sensor

• A crank position sensor is an electronic device used in

an internal combustion engine to monitor the position or

rotational speed of the crankshaft.

• This information is used by engine management systems to

control ignition system timing and other engine

parameters.

• The crank sensor can be used in combination with a

similar camshaft position sensor to monitor the

relationship between the pistons and valves in the engine,

which is particularly important in engines with variable

valve timing.

• This method is also used to "synchronise" a four

stroke engine upon starting, allowing the management

system to know when to inject the fuel. It is also commonly

used as the primary source for the measurement of engine

speed in revolutions per minute.

• Common mounting locations include the main

crank pulley, the flywheel, the camshaft or on the

crankshaft itself. This sensor is the most important sensor

in modern day engines. When it fails, there is a chance the

engine will not start or cut out while running.

Automotive Air-Conditioning

• The air conditioning system shares air ducts, controls and

a blower with the heater, but has its own refrigerant

system. The air conditioning system is comprised of the

following components:

• 1. Compressor

• This is a pump that pressurizes and circulates the

refrigerant through the system.

• 2. Condenser

• As the refrigerant flows through the condenser, it gives off

heat and converts from gas to liquid.

• 3. Receiver/Dryer or Accumulator

• This is where clean, dry refrigerant is stored as it

circulates through the system.

• 4. Evaporator Core

• Refrigerant enters the evaporator core as a liquid. Heat

from the air converts the liquid refrigerant back into a gas,

which cools the air before it enters your car.

Air-Bag System Supplementary Restraint System for driver and/or passenger safety in

case of a crash.

Basic Mechanism: A thin nylon bag in the steering wheel / above glove

compartment inflates in the event of an impact and prevents the

driver/passenger from hitting the steering wheel/dashboard.

3 Main Components: 1) Airbag module

2) Diagnostic Unit

3) Crash sensors

Contains both inflator unit and light-weight fabric airbag and is located

either inside:

1) Steering wheel hub

2) seat or door.

Airbag:

Thin nylon fabric bag folded neatly into steering wheel.

It inflates to the size of a large beach ball on impact.

The sensor

is the device that tells the bag to inflate.

Inflation happens when there is a collision force equal to running into a

brick wall at 10 to 15 miles per hour (16 to 24 km per hour).

A mechanical switch is flipped when there is a mass shift that closes an

electrical contact, telling the sensors that a crash has occurred.

The sensors receive information from an accelerometer built into a

microchip.

Inflator unit:

Contains a number of sodium azide pellets which are electrically ignited

to produce N2 that then fills the airbag.

Both airbag and inflator unit are for single deployment only – i.e. have to

be replaced after a crash

whole process happens in only one-twenty-fifth of a second

The air bag system ignites a solid propellant, which burns extremely

rapidly to create a large volume of gas to inflate the bag.

The bag then literally bursts from its storage site at up to 200 mph (322

kph) - faster than the blink of an eye!

A second later, the gas quickly dissipates through tiny holes in the bag,

thus deflating the bag so you can move.

Immobilizer Is the latest security device introduced by the Indian Automobile

Industry for cars.

This device is situated at the ignition point where you insert the key.

The Key (called Transponder Key) has a chip inside its head containing

a fixed code.

When the key is inserted the devices decodes the code and starts the

engine.

In case if you insert a key (normal key without a code) will only open the

doors of the car but will not start the engine.

TRACTION CONTROL An enhancement of an existing ABS system that prevents wheel spin

while accelerating on wet or slick surfaces.

It uses the same wheel speed sensors to monitor wheel speed during

acceleration, but requires some additional control solenoids and a pump

to apply braking pressure to control wheel spin.

The traction control system brakes the drive wheel that's starting to spin

to shift torque to the opposite drive wheel that still has traction.

Most traction control systems only operate at speeds up to about 30 mph.

Additional control strategies that some traction control systems use to

limit wheel spin include reducing the throttle opening, upshifting the

transmission, retarding spark timing and deactivating fuel injectors.

Steering mechanism

Definition and Purpose The function of Steering system is thought of simply as that of providing

means whereby the driver can place a vehicle as accurately as practicable where is desired to be on the road, so as to avoid other road users and obstructions. It must also keep the vehicle stable on course regardless of irregularities in the surface over which the vehicle is travelling.

When a vehicle is turning, front wheels are not pointing in the same

direction as shown in the figure below.

Illustration of different angles when the wheels turn

For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

Rack and Pinion Rack-and-pinion steering is quickly becoming the most common type of

steering on many vehicles. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.

The pinion gear is attached to the steering shaft. When you turn the steering

wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. This illustrated on the figure in next page.

5

1

The rack-and-pinion gearset does two things:

· It converts the rotational motion of the steering wheel into the linear

motion needed to turn the wheels. · It provides a gear reduction, making it easier to turn the wheels.

Rack and Pinion steering type

On most cars, it takes three to four complete revolutions of the steering

wheel to make the wheels turn from lock to lock (from far left to far right).

The steering ratio is the ratio of how far you turn the steering wheel to how

far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio.

Generally, lighter, sportier cars have lower steering ratios than larger cars and

trucks. The lower ratio gives the steering a quicker response - you don't have to turn the steering wheel as much to get the wheels to turn a given distance - which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.

Some cars have variable-ratio steering, which uses a rack-and-pinion gearset

that has a different tooth pitch (number of teeth per cm) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel's turning limits.

5

2

Recirculating-ball steering Recirculating-ball steering is used on many trucks and SUVs today. The

linkage that turns the wheels is slightly different than on a rack-and-pinion system.

Recirculating-ball Steering System

The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Recirculating-ball gearbox

Instead of the bolt directly engaging the threads in the block, all of the

threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel - without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.

Power Assisted Steering There are a couple of key components in power steering in addition to the

rack-and-pinion or recirculating-ball mechanism which are the pump and the rotary valve. When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.

Rack and pinion power assisted steering

A power-steering system should assist the driver only when he is exerting

force on the steering wheel (such as when starting a turn). When the driver is not

exerting force (such as when driving in a straight line), the system shouldn't

provide any assist. The device that senses the force on the steering wheel is

called the rotary valve.

The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of

metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

The input from the steering shaft forms the inner part of a spool-valve

assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has.

As the bar twists, it rotates the inside of the spool valve relative to the

outside. Since the inner part of the spool valve is also connected to the steering

shaft (and therefore to the steering wheel), the amount of rotation between the

inner and outer parts of the spool valve depends on how much torque the driver

applies to the steering wheel.

When the steering wheel is not being turned, both hydraulic lines provide the

same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

The hydraulic power for the steering is provided by a rotary-vane pump (see

diagram below). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.

Hydraulic pump

As the vanes spin, they pull hydraulic fluid from the return line at low

pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds.

The pump contains a pressure-relief valve to make sure that the pressure

does not get too high, especially at high engine speeds when so much fluid is being pumped.

Power steering in a recirculating-ball system works similarly to a rack-and-

pinion system. Assist is provided by supplying higher-pressure fluid to one side

of the block.