How do car brakes and ABS work?

The principle

Brakes. A car’s brakes don’t make the car “stop” by magic: they turn its motion into heat, and they need a huge squeeze to do it. Most of the trick is multiplying the small push from your foot.

Step 1: The pedal is a lever. The pedal swings on a pivot at the top. Your foot presses about 4 times farther from the pivot than the rod that goes to the brakes, so the rod gets about 4 times the force: about 300 N (the weight of about 30 kg) becomes about 1,200 N. It’s like using a long spanner: a long handle turns a small push into a big one, but your foot moves farther.

Step 2: Brake fluid multiplies it again. A brake booster behind the pedal adds about 3.5 times more, so about 4,200 N reaches the master cylinder. Brake fluid can’t be squeezed, so whatever the small piston there (about 22 mm across) pushes, the fluid passes on through the pipes. At the wheel it pushes on a bigger piston (about 54 mm across) with about 6 times the area, so the force grows about 6 times: roughly 25 kN, about 2.5 tonnes. It’s like pressing a small syringe that is connected to a big one: the big plunger pushes harder, but moves much less.

Step 3: The pads turn motion into heat. That piston presses a brake pad onto the spinning disc, and the caliper pulls a second pad onto the other side. The rubbing slows the disc, just like rubbing your hands together makes them warm. One stop from 100 km/h warms a typical disc by a few tens of degrees; many hard stops in a row can heat it past 500 °C, hot enough to glow.

Step 4: Lock-up and skid (without ABS). The tyre grips best while it is still rolling. If you brake so hard that the wheel stops turning, the tyre slides over the road like a shoe on ice. A sliding tyre grips less and, worst of all, a sliding front wheel can’t steer the car.

Step 5: ABS lets go and grabs (with ABS). A sensor at each wheel counts how fast it turns. When a wheel slows down much faster than the car, as if it is about to lock, ABS briefly lowers the brake pressure on that wheel, lets it speed up again, then reapplies. It does this up to about 15 times a second, which is the buzzing you feel in the pedal. The wheel keeps rolling, so it keeps its grip and you can still steer around things.

Step 6: Inside ABS: two valves and a pump. For each wheel, the ABS unit has an inlet valve (normally open, so the brake works as usual), an outlet valve (normally shut), a small tank and a pump. One ABS pulse has three parts:

1. Build: the inlet valve is open, fluid flows in, the pressure rises and the piston clamps the pads.
2. Hold: the inlet valve shuts, so the pressure stays the same, like pinching a garden hose.
3. Release: the outlet valve opens and some fluid runs into the small tank. The pressure drops, the piston backs off a little, and the wheel speeds up and rolls again. The pump then pushes that fluid back up toward the master cylinder, on the pedal side of the inlet valve. That push is the pulsing you feel in the pedal: it is not a fault, it means ABS is working.

Without ABS or with ABS?

• Without ABS: Upside: simpler, cheaper and nothing extra to go wrong. Downside: in a panic stop the wheels can lock, the car slides and you can’t steer; the driver has to ease off by feel.
• With ABS: Upside: the wheels keep rolling, so you can brake hard and steer at the same time, and on most roads the car stops shorter. Downside: more parts and sensors, and on loose gravel or deep snow a stop can be a bit longer than with locked wheels, which dig in.

Try it yourself: on a bike, squeeze the rear brake gently and the wheel slows while rolling; squeeze it hard on gravel and it skids, and the back of the bike wants to slide sideways. ABS is like a very fast finger easing that lever off and on again.