Car Parts and Accessories

A disc break consists of a cast-iron disc which rotates with the car wheel. Part of the disc is surrounded by a caliper which contains a cylinder and a piston, pipe connections to the hydraulic system, and friction pads which press against the disc to halt or slow down the car.

Dust and moisture are prevented from entering the cylinders, or piston housings, by rubber sealing rings.

As only part of the disc is covered by the caliper, the disc is more easily cooled by air than is possible with a drum brake, and water is quickly spun off. A back-plate —sometimes called a splash shield may protect the inner face of the disc not covered by the caliper.

When the brake pedal is applied, hydraulic pressure forces the piston out from the cylinder to press the inner pad against the inner face of the disc. At the same time, the cylinder which is part of the caliper moves in the opposite direction pressing the outer pad against the outer face of the disc.

The disc expands slightly with the heat, so instead of moving away from the pads— as a drum moves away from the linings— the disc moves nearer to them.

Shim plates—thin sheets of metal which have sprung ends—help to keep the pads firmly in position, which reduces squeal.

The pads can be seen through an opening in the caliper and are easily changed when worn. Each pad is held in place by two retaining-pins, which pass through holes in the caliper, the metal backing plates on the pads and the shim plates. The pins are secured by spring clips.

The friction pads on disc brakes arc made from an extremely tough compound of materials bonded to a steel backing plate. The hacking plates take the torque reaction on the caliper during braking. The pads are usually segmental in shape, but in some cases can be square, rectangular or oval.

The four-piston Auto caliper

One form of disc brake uses two single- piston cylinders. The hydraulic fluid which operates them is fed directly to one cylinder and linked to the other by a bridge pipe. A variation of this has fluid fed to both cylinders through a passage in the caliper casting.

The pistons in a disc-brake caliper are made of steel and are plated on their outer cylindrical surface with a hard coating to resist wear and corrosion.

To limit the amount of heat transmitted from the disc through to the fluid the poisons are machined in the form of a cup —the open end making contact with the backing plate of the friction pads.

The arrangement of the pistons in the caliper varies according to the type of braking performance required or the design of the car in which they are used.

The four-piston caliper is a very efficient arrangement because with two small pistons one each side of the disc– opposite each other and exerting the same pressure—larger pads can be used giving a greater effective friction surface area.

Vented disc keeps Auto brakes cool

Many tires of disc brakes are fitted to cars marketed in South Africa. They use the same principle of gripping the edge of a disc, but have minor differences of design.

Disc brakes are less susceptible to the effects of heat than drum brakes, because on most cars the discs are adequately ventilated by air flowing over them.

With the heavy-duty requirements of a modern braking system, often operating in high ambient temperatures, a ventilated disc rotor keeps the braking system cool.

To increase the surface area of a disc and simplify cooling, it is cast to appear like two thin metal plates with metal ribbing joining them together.

The ribs are arranged to encourage air flow through the disc and rapidly cool the disc rotor like a fan.

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