Chassis systems

8 Chassis systems

8.1 Brakes

Caution and regular servicing is required to ensure the following:

8.1.1 Introduction

● no air must be allowed in the hydraulic circuits (air compresses and would not transfer the

The main braking system of a car works by

force);

hydraulics. This means that when the driver presses ● correct adjustment must be maintained between the brake pedal, liquid pressure forces pistons to

shoe linings to drums and pads to discs (other- apply brakes on each wheel. A handbrake system,

wise the pedal movement would be too large); usually operated by a lever and cables, is used for

● lining materials must be free from contamin- parking. Most hand brakes operate on the rear

ation (such as oil, grease or brake fluid). wheels.

A separate mechanical system is a good safety Disc brakes are used on the front wheels of some

Two types of light vehicle brakes are used.

feature. Most vehicles have the mechanical hand cars and on all wheels of sports and performance

brake working on the rear wheels but a few have cars. Braking pressure forces brake pads against

it working on the front – take care. both sides of a steel disc. Drum brakes are fitted

Note the importance of flexible connections on the rear wheels of some cars and on all wheels

to allow for suspension and steering movement. of older vehicles. Braking pressure forces brake

These flexible pipes are made of high quality rub- shoes to expand outwards into contact with a

ber and are covered in layers of strong mesh to drum. The important part of brake pads and

prevent expansion when under pressure. shoes is a friction lining that grips well and with-

Extra safety is built into braking systems by stands wear.

using a double acting master cylinder (Figure 8.1). This is often described as tandem and can be thought of as two cylinders in one housing. The

8.1.2 Principle of hydraulic

pressure from the pedal acts on both cylinders but

braking

fluid cannot pass from one to the other. Each cylin- der is then connected to a complete circuit. This

A complete system includes a master cylinder can be by a number of methods: operating several wheel cylinders. The system is

designed to give the power amplification needed

● diagonal split;

for braking the particular vehicle. On any vehicle, ● separate front and rear; when braking, a lot of the weight is transferred to

● duplicated front.

the front wheels. Most braking effort is therefore designed to work on the front brakes. Some cars have special hydraulic valves to limit rear wheel

8.1.3 Disc and drum brake

braking. This reduces the chance of the rear

systems

wheels locking and skidding. Figure 8.2 shows a typical disc brake, calliper The main merits of hydraulic brakes are as

pads and disc. The type shown is known as single follows:

acting sliding calliper. This is because only one ● almost immediate reaction to pedal pressure

cylinder is used but pads are still pressed equally (no free play as with mechanical linkages);

on both sides of the disc by the sliding action. ● automatic even pressure distribution (fluid

Disc brakes keep cooler because they are in the pressure effectively remains the same in all

air stream and only part of the disc is heated as the parts of the system);

brakes are applied. They also throw off water bet- ● increase in force (liquid lever).

ter than drum brakes. In most cases servicing is

170 Advanced automotive fault diagnosis

Figure 8.1 Double acting master cylinder

Figure 8.3 Drum brake adjusters – ratchet type

Disc brakes provide for good braking and are less prone to brake fade than drum brakes. This is because they are more exposed and can get rid of heat more easily. Brake fade occurs when the brakes become so hot they cannot transfer energy any more, and stop working! This type of prob- lem can happen say after keeping the car brakes on for a long time when travelling down a long steep hill. This is why a lower gear should be used to employ the engine as a brake. It is clearly impor-

Figure 8.2 Disc brake calliper and brake pads

tant to use good quality pads and linings because inferior materials can fail if overheated.

minimal. Disc brakes are self adjusting and replac- Drum brakes operate by shoes being forced on ing pads is usually a simple task. In the type shown

to the inside of the drum. A common type with just one bolt has to be removed to hinge the calliper

a ratchet for automatic adjustment is shown as upwards.

Figure 8.3. Shoes can be moved by double or

Chassis systems 171

Vacuum chamber with vacuum

Diaphragm

connection

Piston Bell valve

Air filter Piston rod

Push rod

Rear chamber Backing plate

Figure 8.4 Vacuum operated brake servo (booster)

single acting cylinders. The most common lay- The adjustment procedure stated by the manu- out is to use one double acting cylinder and brake

facturer must be followed. As a guide, though, shoes on each rear wheel of the vehicle, and disc

most recommend tightening the adjuster until brakes on the front wheels. A double acting

the wheels lock and then moving it back until the cylinder simply means that as fluid pressure acts

wheel is just released. You must ensure that the through a centre inlet, pistons are forced out of

brakes are not rubbing as this would build up both ends.

heat and wear the friction material very quickly. Drum brakes more affected by wet and heat

As an aid to fault diagnosis, the effects of than disc brakes because both water and heat are

incorrect adjustment are as follows: trapped inside the drum. However, they are eas- ier to fit with a mechanical hand brake linkage.

● reduced braking efficiency; ● unbalanced braking;

● 8.1.4 Brake adjustments excessive pedal travel.

Brakes must be adjusted so that the minimum movement of the pedal starts to apply the brakes.

8.1.5 Servo-assisted braking

The adjustment in question is the gap between the Servo systems are designed to give little assist- pads and disc and/or the shoes and drum. ance for light braking but increase the assistance Disc brakes are self-adjusting because as as pedal pressure is increased. A common servo pressure is released it moves the pads just away system uses low pressure (vacuum) from the from the disc. Drum brakes are different because manifold on one side, and the higher atmospheric the shoes are moved away from the drum to a pressure on the other side of a diaphragm. The set position by a pull off spring. The set position low pressure is taken via a non-return safety valve is adjustable and this can be done in a number from the engine inlet manifold. This pressure dif- of ways. ference causes a force, which is made to act on

● Self-adjusting drum brakes are almost univer- the master cylinder. Figure 8.4 shows a vacuum sal now. On light vehicles a common type uses

servo.

an offset ratchet which clicks to a wider pos- Hydraulic power brakes use the pressure from ition if the shoes move beyond a certain amount

an engine driven pump. This pump will often be when operated. Modern cars frequently have a

the same one used to supply the power assisted self-adjusting hand brake.

steering. Pressure from the pump is made to act on ● Screwdriver adjustment through a hole in the

a plunger in line with the normal master cylinder. wheel and drum is also used. This is often a

As the driver applies force to the pedal, a servo type of nut on a threaded bar which pushes the

valve opens in proportion to the force applied by shoes out as it is screwed along the thread. This

the driver. The hydraulic assisting force is there- method can also have an automatic adjuster

fore also proportional. This maintains the import- fitted.

ant ‘driver feel’.

● An adjustment screw on the back plate is now

A hydraulic accumulator (a reservoir for fluid quite an old method in which a screw or square

under pressure) is incorporated into many sys- head protruding from the back plate moves the

tems. This is because the pressure supplied by shoes by a snail cam.

the pump varies with engine speed. The pressure

172 Advanced automotive fault diagnosis

Figure 8.5 Checking a brake disk with a dial gauge

in the accumulator is kept between set pressures

Figure 8.6 Rolling road brake tester display

in the region of 70 bar.

Warning

4. Check cable operation by using an assistant in If you have to disconnect any components from

the car while you observe. the braking system on a vehicle fitted with an

5. Renew the cable if seized. accumulator, you must follow the manufacturer’s

6. Check hand brake operation and all associated recommendations on releasing the pressure first.

systems.

8.1.6 Brake fluid

8.2.2 Test equipment

Always use new and approved brake fluid when Note: You should always refer to the manufac- topping up or renewing the system. The manu-

turer’s instructions appropriate to the equip- facturer’s recommendations must always be fol-

ment you are using.

lowed. Brake fluid is hygroscopic which means that over a period of time it absorbs water. This

Brake fluid tester

increases the risk of the fluid boiling due to the Because brake fluid can absorb a small amount heat from the brakes. Pockets of steam in the sys-

of water it must be renewed or tested regularly. It tem would not allow full braking pressure to be

becomes dangerous if the water turns into steam applied. Many manufacturers recommend that

inside the cylinders or pipes, causing the brakes the fluid should be changed at regular intervals –

to become ineffective. The tester measures the in some cases once per year or every 30 000 km.

moisture content of the fluid. Make sure the correct grade of fluid is used.

Brake roller test (Figure 8.6)

This is the type of test carried out as part of the