How do disc brakes and ABS work?

What does a disc brake actually do?

A moving car carries energy, and the brakes have to get rid of it. They do it with friction: pads rub on a disc that turns with the wheel, and the car’s movement becomes heat in the disc and the pads. One hard stop from motorway speed can warm a front disc by tens of degrees in a few seconds, and repeated stops on a long descent can take it to several hundred, which is why front discs are vented, with cooling vanes between their two faces.

The front brakes do most of the work. As the car slows, its weight shifts forwards onto the front tyres, so they can take more braking force than the rear ones. That is why the front discs are usually larger, and why the rear brakes on many small cars are still drums.

How much force is inside a brake caliper?

More than you might expect. The clamp is simple hydraulics: pressure multiplied by the area of the piston. The app’s caliper has a 57 mm piston with an area of 2,552 mm², so each bar of line pressure pushes it with 255.2 N. At 60 bar, a firm stop, that is 15.3 kN.

The torque that slows the wheel follows from the clamp. Multiply by two, because the disc has two friction faces, then by the pads’ friction coefficient of 0.40 and by the effective radius of 124.6 mm. At 60 bar the model makes 1,526 N·m at that wheel, which is 4.8 kN of braking force where the tyre meets the road. These are the same numbers the app’s brake test reads out.

Why do most cars use a floating caliper?

A floating caliper needs only one piston. It sits on the inboard side of the disc and pushes the inner pad. The housing is mounted on two greased guide pins, so the reaction to that push slides the whole housing inboard, and its fingers pull the outer pad onto the other face with the same force.

It is compact, cheap and works well, as long as the guide pins can slide. When a pin seizes or its rubber boot splits and lets water in, the caliper stops floating, one pad does most of the work, and the brake can drag. Performance cars often use fixed calipers instead, with pistons on both sides of the disc.

How do pads and discs wear?

Both are meant to wear. The model’s pads start with 11 mm of friction material and are replaced at 2 mm. A small spring-steel tab on the inner pad touches the disc at about 2.5 mm and squeals as a warning. As the pads wear, the piston creeps out of its bore, by up to 18 mm here, and has to be pushed back before new pads fit.

The disc wears too. It starts at 26 mm thick and is replaced below 24 mm. If it wears unevenly or runs out of true by more than 0.04 mm, the pads meet a surface that is thicker in some places than others, and you feel it as judder through the pedal and the steering wheel.

How does ABS know a wheel is about to lock?

Each wheel has a toothed tone ring on its hub and a sensor next to it. On the app’s model the ring has 48 teeth and the sensor sits 0.8 mm away. Every tooth that passes makes one pulse, so the frequency of the signal is the wheel’s speed.

The ABS unit compares the wheels with each other and with how fast the car can possibly be slowing. A wheel that slows much faster than that is about to lock: the tyre is starting to slide instead of roll.

What does ABS do when a wheel starts to lock?

A tyre grips best with a little slip, turning slightly slower than the car is moving. Once it locks, the grip falls and the wheel can no longer steer the car. ABS keeps each wheel near that best slip. Its hydraulic unit has valves for every wheel: it can hold the pressure, release some of it back to a pump, and let it build again.

That cycle repeats many times a second while you hold the pedal down, which is the pulsing and buzzing you feel in an emergency stop. Keep pressing hard and steer. In the illustration above, with typical values for a dry road, the car with ABS stops from 100 km/h in about 43 m and keeps steering; with the wheels locked it slides about 53 m.