Car Parts and Accessories

Operation of Drum and Disc Auto brakes

An auto brake works by causing friction between a non-rotating part of the car and a disc or drum which turns with a road wheel. Friction produces the force needed to slow the car, and converts the energy of the moving vehicle into heat which disperses into the air around the auto brakes.

For many years, auto brakes had either an external band that contracted around the drum, or internal ’shoes’ that expanded against the drum’s inner surface, each with heat-resistant linings. Today, the braking material has been placed on the outside of the auto brake, but now pads are forced under hydraulic pressure onto the sides of a smooth rotating metal disc.

Internal expanding drum auto brakes are still used on the rear wheels of some cars — all of which today have disc auto brakes fitted to the front wheels.

In the most basic systems, the auto brake pedal is connected to all four auto brakes which are operated by a single piston in the master cylinder.

The hand lever provides an alternative means for using the rear auto brakes of the car only.

Manufacturers design drum auto brakes so that rain, snow, ice or grit cannot get inside and decrease braking efficiency, for moisture greatly reduces the friction between the linings and the drum. But the shield which protects the drum cannot cope with immersion in water, so after passing through a flood or a ford, a driver should partially apply his auto brakes so that friction and heat will dry them.

Overheating, however, lessens the effectiveness of drum auto brakes and, when excessive, will permanently damage the linings. Temporary loss of effectiveness during prolonged braking, such as on the long descent of a steep hill, is called auto brake-fade.

Disc auto brakes are more exposed to air and lose heat more quickly than drum auto brakes so they are less liable to overheating and fading. Disc auto brakes are used on all four wheels of most high-performance cars today.

A disc auto brake works like the caliper auto brake on a bicycle. The caliper has a pair of auto brake blocks which straddle the bicycle wheel and grip the rim.

A car’s disc auto brake also has a pair of friction pads, but instead of acting directly on the wheel they straddle a metal disc which turns with the wheel. When the driver applies the auto brake, the pads press against the disc and slow the wheel.

The shoes of a drum auto brake can be designed to provide a self-applying or ’self-wrapping’ effect which reduces the pedal effort needed. Disc auto brakes lack this advantage and consequently require `servo assistance’ (in all but the lightest cars) to supplement the physical effort applied by the driver (see p. 206).

Auto Brake Distances

THE DISTANCE it takes to bring a moving vehicle to a stop can be broken down into:

1 The reaction time of the driver, that is, the time it takes him to respond to a situation and apply the auto brakes.

2 The speed of the vehicle.

3 The efficiency of the auto brakes.

Reaction times vary from driver to driver. An alert driver has an estimated reaction time of about 0,5 to 0,6 seconds. This can easily be converted into distance. Stopping distances increase with the square of the speed, so that a vehicle travelling at 120 km/h would require four times the distance to stop than a vehicle travelling at 60 km/h.

The term auto brake efficiency is a bit of a misnomer because what it really refers to is not the efficiency of the auto brakes themselves, but rather the coefficient of friction between the tires and the road surface, that is, the degree of grip between the tires and the road. Auto brake efficiency is expressed as a percentage, and on a dry tarmac road is usually between 80 and 85 per cent.

Auto Brakes Weight Transference

Ideally, the braking effort should be distributed between the front and rear wheels in proportion to the weight that they carry. This will vary according to the design of the car (whether it is front engined or rear engined, for example), the number of passengers and the amount of luggage. But the effect of applying the auto brakes is to throw some of the weight forward: this ‘transfer’ adds to the load on the front wheels and, similarly, reduces the load on the rear wheels.

When the auto brakes are applied hard there is a greater transfer of weight and the rear wheels tend to lock, which often causes the tail of the car to slide sideways. If the front wheels become locked first, the car will usually slide in a straight line, but steering control will be lost. Locking the wheels by heavy braking is much more likely on slippery roads, so a driver should always use auto brakes cautiously on a treacherous road surface.

The designer proportions the braking effect between front and rear wheels to match the weight distribution under average conditions. In some cars the system makes allowance for weight transfer by setting a limit to the maximum braking effort that can be applied to the rear wheels. Beyond this limit, extra pedal pressure acts on the front auto brakes only.

Auto Brake-fade

Excessive heating of the auto brakes following prolonged or repeated braking can result in fade. Heat causes temporary changes in the friction properties of the material used for auto brake pads and linings, and the auto brakes become less efficient as they get hotter. Normal efficiency generally returns when they cool again.

If one auto brake goes on harder than the others, it may fade much sooner and cause uneven braking, which may lead to a skid.

Auto brake pads and linings also wear away faster at higher temperatures.

Tires and Auto Brake

Tires should grip the road well. Badly worn tires may auto brake satisfactorily in dry conditions, but will not do so in the wet.

Water, ice, oil, loose gravel and wet leaves all reduce the friction between the road and the tires. Severe braking may lock the wheels and cause the car to skid. A locked wheel has less adhesion between the tires and the road than a rolling wheel.

Worn shock absorbers can also reduce the braking effectiveness of your car and can cause `stewing’.

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