How do disc brakes work?

December 10, 2018

All modern cars have disc brakes on front wheels. Some cars are equipped with drum brakes on the rear axle, but the majority of cars have disc brakes on the rear wheels as well. How do disc brakes work?

Disc brakes
Disc brake components

First, let's look at the components. The brake disc (rotor) is mounted on the hub. The brake pads are installed within the brake caliper, on both sides of the brake disc. The way the pads are installed, they can slide on the caliper bracket laterally, towards and away from the disc. The brake caliper has at least one piston. There are cars with two or four or even six pistons per caliper, but here we consider a simple system with a one-piston caliper.

When the driver presses the brake pedal, the brake master cylinder connected to the pedal creates hydraulic pressure. The brake fluid transfers the pressure through the braking system and the brake hose to the caliper. The piston inside the caliper extends and pushes on the backing plate of the inner brake pad. The opposite force pushes the caliper towards the centre of the car. This causes the outer caliper housing to push the outer pad towards the disc. As a result both pads squeeze the disc between them.

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Brake discs (rotors)

brake disc
BMW performance brake disc

The disc is mounted on the hub. The contact surface of the disc and the hub must be clean. A piece of debris stuck between the hub and the disc can cause the disc to wobble side-to-side. The working surface of the disc (see the diagram above) is also very important. If a portion of the working disc surface is rusted or damaged, the brake force is reduced.

Another problem with discs is uneven wear or warping. Discs wear unevenly due to rust or warp after overheating. As a result, the brake disc thickness varies around the disc. This causes the brake pedal pulsation and the steering wheel shake during braking. To restore the working surface, brake discs can be machined. Mechanics use a special brake lathe machine to cut a thin layer of metal from both sides of the discs. With mileage, discs wear out. Manufacturers specify the minimum disc thickness. Mechanics can measure the disc thickness and lateral runout (warping) without removing the disc. If the disc is worn close to or beyond the minimum thickness, it cannot be machined and must be replaced.

Cracks, deep scores and other damage are other reasons for disc replacement. Brake discs must also be replaced if any of the discs is seized on the hub and cannot be removed without damaging it. Both brake discs on one axle are replaced at the same time.

Brake calipers

Read caliper
Rear caliper

Front brake calipers are mounted on the steering knuckles. The rear calipers are bolted to the rear spindles. The caliper slider pins allow some lateral movement of the caliper. This movement is vital for proper brake operation. Seizing of the caliper pins will lead to problems, including overheating of the caliper.

If caught early, the movement of the sticking caliper slider pins can be restored. If a seized caliper pin cannot be freed, the caliper must be replaced. The pads can also seize up inside the caliper bracket. This will cause brake pads to wear unevenly. That's why regular brake inspections are so important. Mechanics clean and lubricate caliper slider pins when they service your brakes or replace the pads and discs (rotors). A brake caliper must be replaced if there is any problem with it.

Brake pads

Brake pads
Brake pads in many cars are installed in the caliper bracket.
Brake pads
Brake pads

A brake pad consists of the friction material molded to a metal backing plate. In many cars, noise-reducing shims are added to the backing plate, see the image. The job of the pads is to stop the disc. Brake pads wear with mileage. To work properly, the pads have to have enough friction material. Car manufacturers specify the minimum thickness of the friction material. If any of the pads is worn out close to that limit, or there is any damage to the pads, brake pads must be replaced. This brake pad is worn out beyond the limit.

One of the common problems with many cars is when one of the pads seizes up inside the caliper bracket. This causes many problems, including the uneven wear of the pads, overheating and noises. If pads are worn unevenly, they also must be replaced. Brake pads are replaced as a set; all 4 pads on the same axle are replaced at the same time.

Brakes are vital to the safe driving. Problems with brakes make a car unsafe. A properly functioning braking system is the difference between stopping in time and hitting someone. We strongly recommend having your brakes inspected at least once or twice a year and to trust your brake job only to qualified professionals.

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