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Drum brake system

Braking principle

Some systems have all drum brakes. Some have disc brakes on the front wheels and drum brakes on the rear, others have all disc brakes.

A basic braking system has a:

y brake pedal,y master cylinder to provide hydraulic pressure,y brake lines and hoses to connect the master cylinder to the brake assemblies,y fluid to transmit force from the master cylinder to the wheel cylinders of the brake assemblies; and

y brake assemblies drum or disc that stop the wheels.

The driver pushes the brake pedal; it applies mechanical force to the piston in the master cylinder. The piston applies hydraulic pressure to the fluid inthe cylinder, the lines transfer the pressure which is undiminished in all directions within the confines of the brake lines to the wheel cylinders, and

the wheel cylinders at the wheel assemblies apply the brakes.

Force is transmitted through the fluid. For cylinders the same size, the force transmitted from one is the same value as the force applied to the other. Byusing cylinders of different sizes, forces can be increased or reduced.

In an actual braking system, the master cylinder is smaller than the wheel cylinders, so the force at all of the wheel cylinders is increased.

When brakes are applied to a moving vehicle, they absorb the vehicles kinetic energy. Friction between the braking surfaces converts this energy intoheat.

In drum brakes, the wheel cylinders force brake linings against the inside of the brake drum. In disc brakes, pads are forced against a brake disc. Inboth systems, heat spreads into other parts and the atmosphere, so brake linings and drums, pads and discs must withstand high temperatures andhigh pressures.

Backing plate

The backing plate is made from steel and is attached to the steering or suspension componentsby bolts. It then supports the wheel cylinder, brake shoes and levers.

Brake drum &brake shoe

assembly

The brake drum fits over the brake linings and forms the braking surface for these linings.

The brake shoe assembly consists of the steel shoe and the brake lining material.

Wheel cylinder

The wheel cylinder is attached to the backing plate. The pistons push against the brake shoes,which then make contact with the brake drum to slow or stop the vehicle.

Retaining clips &springs

The brakes are held against the backing plate by retaining clips and springs.

The hold down spring is used to retain the the brake shoe in position in relation to the backingplate. During vehicle operation it keeps the brake shoe in position.

Automatic brakeself-adjuster

To manually adjust the brakes it may be necessary to release the adjusting lever away from the

star wheel. To do this, insert a small screwdriver through the adjust slot in the backing plate andpush on the adjusting lever. A brake adjuster can now be used to adjust the brakes in the usualway.

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On modern vehicles this basic system has some refinements, such as a power booster. This helps the driver apply the brakes.

Brake pedal.

The brake pedal uses leverage to transfer the effort from the drivers foot to the master cylinder.

Different lever designs can alter the effort the driver needs to make, by using different levels of mechanical advantage. Brake pedals should bemounted securely, free from any excessive sideways movement, and at a height and angle that will allow the driver to quickly move from pressing the

accelerator, or gas pedal, to applying the brakes.

The pedal is usually suspended from a bracket between the dash panel and the firewall, and works as a force-multiplying lever. If the power assist fails,the pedal's leverage is designed to allow the driver to still generate thousands of pounds of pressure at each wheel cylinder.

Brake pedals must be free to return to their starting position when pressure is removed. This allows the master cylinder pushrod to return to its un-

depressed position.

Changes to how far the pedal can travel or to its resistance if it feels harder or softer than normal can be an indicator of problems such as low fluidlevels or even a leak in the hydraulic system. When ABS brakes are activated during heavy braking, the pulsations of the system can be felt by thedriver through the pedal and this is normal, however, a pulsating feel during normal or light braking can also be an indicator of a potential brakingsystem problem.

Power booster

A power booster or power brake unit uses a vacuum to multiply the drivers pedal effort and apply that to the master cylinder. This increases thepressures available from the master cylinder.

Units on petrol/gasoline engines use the vacuum produced in the intake manifold. Vehicles with diesel engines cannot use manifold vacuum so they arefitted with an engine-driven vacuum pump.

The most common booster operates between the brake and master cylinder. It increases the force that acts on the master cylinder. Whenever thebrake pedal is depressed, a pushrod opens the vacuum-control valve. Vacuum from the engine lowers the pressure in the chamber, forcing thediaphragm forward and increasing the pressure on the master cylinder pistons. The level of assistance this power-boost gives depends on the pressure

applied to the brake pedal.

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Disc brake system

SummaryThe primary components of disc brakes are: the rotor, caliper and brake pads.

Modern passenger vehicles are usually equipped with disc brakes on at least two wheels.

The primary components of the disc brakes are:

y Rotory Caliper

y Brake pads

The rotor is the main rotating part of this brake system. It is hard wearing and resists the high temperatures that occur during braking. Its manufacturerwill specify the minimum thickness for the rotor. Rotors can be of a solid construction or slotted. The slotted rotor is referred to as a "ventilated disc".

Rotor

Disc brakes have rotors to dissipate heat so the brakes work efficiently. In high performance vehicles, therotors are made from composite materials, ceramics, or carbon fiber; otherwise, they're made of iron.

Some of these rotors have directional vanes, which means the disc can only be fitted to one side of thevehicle.

Caliper

The caliper straddles the rotor, and houses the disc brake pads and activating pistons. The caliper is usuallybolted to the steering knuckle or, in the case of a non-steer axle, to a suspension component.

Brakepads

The disc pads are located inside the caliper. The pads clamp onto the rotor to slow or stop the vehicle. Thedisc pad consists of a friction material bonded to a steel backing plate.

OPERATION:

Disc brakes can be used on all four wheels of a vehicle, or combined with disc brakes on the front wheels and drum brakes on the rear.When the brake pedal is depressed, a push rod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder convertsthe force into hydraulic pressure, which is then transmitted via connecting pipes and hoses to one or more pistons at each brake caliper. The pistons

operate on friction pads to provide a clamping force on a rotating flat disc that is attached to the wheel hub. This clamping tries to stop the rotation ofthe disc, and the wheel.

On non-driving wheels, the center of the brake disc or hub contains the wheel bearings. The hub can be part of the brake disc or a separate assemblybetween the wheel and hub with nuts or bolts. On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel. Onfront wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.

The brake caliper assembly is bolted to the vehicle axle housing or suspension. In most cases the brake is positioned as close as possible to the wheel,but there are exceptions. Some high-performance cars use inboard disc brakes on its rear wheels. The makers claim improved vehicle handling for this

design because it reduces unsprung weight.

Applying brakes can absorb a lot of vehicle energy so friction between braking surfaces generates great heat. Brake parts withstand very hightemperatures. Most of the friction area of a disc is exposed to air so cooling is far more rapid than for a drum brake. Unlike with drum brakes, brake fadeis rare. Because of their shape, discs tend to throw off water. So after being driven through water, they operate almost immediately.

Disc brakes need much higher pressures to operate than drum brakes, so almost all disc brake systems need a power brake booster to help reduce thepedal forces that are needed from the driver. Because of the high forces needed to apply a disc brake, using it as a handbrake is less common. Somevehicles build a drum brake into the center of the rear disc to provide for park brake operation.