Car Parts and Accessories

Rear Auto disc brakes

WHEN DISC brakes are used on the rear wheels of a car, the parking brake is often incorporated into the disc braking system. Several methods are used to achieve this. Early systems used a cable-actuated lever which mechanically pushed the pads against the discs. The lever is situated adjacent to the caliper’s piston.

Two systems currently incorporating the parking brake into a rear-wheel disc use a helical screw or a separate drum brake. The separate drum brake method has a disc with an offset centre which contains the brake shoes and operating cam normally found inside a conventional drum brake. Operating the parking brake brings the brake shoes into contact with the drum and does not affect the disc brake operating system.

The helical-screw method has a lever attached to a screw which bears on the rear of the sliding caliper assembly. When the parking brake is operated, the lever turns the screw so that the pads clamp the disc.

To reduce weight at the wheel, some cars with independent rear suspension have the disc brake rotors and calipers mounted inboard of the hubs. Usually they are adjacent to the differential.

Anti-lock Auto brakes

When a car’s brakes are applied too vigorously the tyres will start to skid. A car with its wheels locked and tyres skidding will invariably take a greater distance to stop than one with the brakes held at the point of lock-up, but with the wheels still turning.

Several car makers now offer an anti- lock or anti-skid braking system. Sensors on the wheels monitor wheel rotation speed and compute any difference which may indicate that one of the vehicle’s wheels is starting to skid. At this point the computer intervenes and releases the car’s brakes for a fraction of a second, then permits them to be applied again. Pulsation continues until the vehicle is brought to a halt, or the brakes are released.

Under severe braking the driver is made aware of the operation of the anti-lock braking system by the pulsation of the brake pedal. While the pedal is pulsating it is usually possible to retain steering control over the car’s direction. On dry sealed (bitumen) surfaces, braking distances may be significantly shorter, while on wet or slippery surfaces the difference could depend very much on driver skill, but steering control will be maintained on a car with anti-lock braking.

Auto Servo-assisted systems

A Servo mechanism fitted to the braking system reduces the physical effort the driver has to use on the brake pedal.

The servo cylinder contains a piston or a diaphragm. When air is exhausted from one end of the cylinder and atmospheric pressure is admitted to the other end, the difference in the pressures on the two sides of the piston (or diaphragm) can be used to help the driver to apply the brakes. The amount of servo assistance increases with the physical force used on the pedal.

All servo units are powered by the engine. The more common type of servo uses the partial vacuum in the inlet manifold; others have a separate vacuum pump.

In one simple arrangement, the engine exhausts air from both sides of a diaphragm (or main piston) which is held in a state of balance until the brake pedal is used. This admits atmospheric pressure on one side, while a partial vacuum remains at the other side. As a result, the diaphragm moves, applying force to a slave piston, which then adds to the pressure applied by the driver.

If the servo fails for any reason, the driver can still apply the brakes, but the pedal effort required is greatly increased.

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