Manual Adjustment

Manual Adjustment

Adjustment through a hole in the back plate is often used. This involves moving a type Brake adjustment hole

Square type adjuster

of nut on a threaded bar, which pushes the shoes out as it is screwed along the thread. This method is similar to the automatic adjusters. An adjustment screw on the back plate is now quite an old method. A screw or square head protruding from the back plate moves the shoes by a snail cam. As a guide, tighten the adjuster until the wheels lock, and then move it back until the wheel is just released. You must ensure that the brakes are not rubbing as this would 835

Self-Servo Action ฀฀ The precise way in which the shoes move into contact with the drum affects the power of the brakes. If the shoes are both hinged at the same point then the system is said to have one leading and one trailing shoe. As the shoes are pushed into contact with the drum, the leading shoe is dragged by the drum rotation harder into contact, whereas the rotation tends to push the trailing shoe away. This 'self-servo' action on the leading shoe can be used to increase the power of drum brakes. This is required on the front wheels of all-round drum brake vehicles.

Twin Leading Shoe Brakes The shoes are arranged so that they both experience the

self-servo action. The shoes are pivoted at opposite points on the backplate and two wheel cylinders are used. The arrangement is known as twin leading shoe brakes. It is not suitable for use on the rear brakes

Twin leading shoe system

because if the car is travelling in reverse then it would become a twin trailing shoe arrangement, which means the efficiency of the brakes would be seriously reduced. The leading and trailing layout is therefore used on rear brakes, as one shoe will always

be leading no matter in what direction the vehicle is moving.

Leading and Trailing Shoe Brakes The standard layout of drum brake systems is normally:

Twin leading shoe brakes on the front wheels Leading and trailing Leading and trailing shoe brakes on the rear system

wheels. Disc brakes are now used on the front wheels

of all light vehicles but many retain leading and trailing shoe brakes on the rear. In most cases, it is easier to attach a handbrake linkage to the system with shoes on the rear. This method will also provide the braking performance required when the vehicle is reversing.

Hand Brake Linkages ฀฀ Inside a brake drum, the hand brake linkage is usually a lever mechanism as shown here. This lever pushes the shoes against the drum and locks the wheel. The hand brake lever pulls on one or more cables and has a ratchet to allow it to be locked in the on position. There are a number of ways in which the hand brake linkage can be laid out to provide equal force, or compensation, for both wheels:

• Two cables, one to each wheel • Equaliser on a single cable pulling a ‘U’ section to balance

effort through the rear cable (as shown here) • Single cable to a small linkage on the rear axle.

Disc Type Handbrake Some sliding caliper disc brakes incorporate a handbrake mechanism. The footbrake operates as normal. Handbrake operation is by a moving lever. The lever acts through a shaft and cam, which works on the Sliding caliper parking brake

adjusting screw of the piston. The piston presses one pad against the disc and because of the sliding action, the other pad also moves.

Handbrake Drum in Disc ฀฀ Some manufacturers use a set of small brake shoes inside a small drum, which is built in to the brake disc. The caliper is operated as normal by the footbrake. The small shoes are moved by a cable and lever.

Summary ฀฀ In summary, remember that the purpose of the braking system is to slow down or stop a vehicle. This is achieved by converting the vehicle’s movement energy into heat. Friction is used to do this. Braking system developments have improved efficiency, reliability and ease of servicing.

 Describe the leading/trailing layout of the shoes on drum brake systems when used front and rear.

 State why disc brakes are self-adjusting

 Look back over the previous section and write out a list of the key bullet points here:

HYDRAULIC COMPONENTS

Principle of Hydraulic Braking ฀฀ Shown here, is the principle of hydraulic brakes. The movement of the piston, labelled 2, causes an equal force in all parts of the system. The pistons, labelled 1, move a shorter distance. If larger area pistons are used, the force at the brakes can be increased. This is called a liquid lever and acts in addition to the leverage of the brake pedal.

Braking System ฀฀ A complete braking system includes a master cylinder, which operates several wheel cylinders. The system is designed to give the power amplification needed for braking

Master cylinder

the particular vehicle. On any vehicle when braking, a lot of the weight is transferred to 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 braking. This reduces the chance of the rear wheels locking and skidding.

Wheel Cylinders Brake shoes can be moved by double or single-acting wheel cylinders. A common layout is to use one double acting cylinder and brake shoes on each rear wheel

Slave cylinder components

of the vehicle, and disc brakes on the front wheels. A double acting cylinder simply means that as fluid pressure acts through a centre inlet, pistons are forced out of both ends.

Fixed caliper

Floating caliper

Disc Caliper Piston ฀฀ Disc brake calipers are known as fixed, floating or sliding types. The pistons are moved by hydraulic pressure created in the master cylinder. A number of different calipers are used.

Some high performance

Sliding caliper

calipers include up to four pistons. However, the operating principle remains the same.

Brake Fluid Always use new and approved brake fluid when topping up or refilling the system. Manufacturers’ recommendations must always be

A common

followed. Brake fluid is hygroscopic, which means type of

brake fluid

that over time, it absorbs water. This increases the risk of the fluid boiling due to the heat from the brakes. Pockets of steam in the system would not allow full braking pressure to be applied. Many manufacturers recommend that the fluid be changed at regular intervals. Make sure the correct grade of fluid is used. The current recommended types are known as DOT4 and DOT5.

Brake System Shown here are the main arts of a typical modern braking system. A separate mechanical system is a good safety feature. Most

Braking and

vehicles have the mechanical parking brake working other

components

on the rear wheels but a few have it working on the front - take care. Note the importance of flexible connections to allow for suspension and steering movement. These flexible pipes are made of high quality rubber and are covered in layers of strong mesh to prevent expansion when under pressure.

Tandem Master Cylinder ฀฀ Safety is built into braking systems by using a double acting master cylinder. This is often

described as tandem and can be thought of as two master cylinders inside one housing. The pressure from the pedal acts on both cylinders but fluid cannot pass from one to the other. Each cylinder is then connected to a separate circuit. These split lines can be connected in a number of ways. Under normal operating conditions, the pressure developed in the first part of the master cylinder is transmitted to the second. This is because the fluid in the first chamber acts directly on the second piston.

Master cylinder operation Circuit Failure ฀฀ If one line fails, the first piston meets no

restriction and closes up to the second piston. Further movement will now provide pressure for the second circuit. The driver will notice that pedal travel increases, but some braking performance will remain. If the fluid leak is from the second circuit, then the second piston will meet no restriction and close up the gap. Braking will now be just from the first circuit. Diagonal split brakes are the most common and are used on vehicles with a negative scrub radius. Steering control is maintained under brake failure conditions.

Multi-Circuit Systems There are three common ‘splits’ used on modern braking systems. The first two types listed are the most common:

• Diagonal split type, where if a fault occurs, the driver loses half of the front and half of the rear brakes • Separate front and rear, where if a fault occurs, the

driver loses all of the front or all of the rear brakes • Duplicated front, where if a fault occurs, the driver loses

the rear and part of the front or part of the front brakes only. Special front calipers are required when using this method.