How do disc brakes stop a vehicle?

1. A car is rolling down the street when the driver sees a family of ducks waiting to cross. She presses the brake pedal. What happens inside the wheels?
2. The pedal pushes a piston into a tube filled with brake fluid. Liquid cannot be squashed, so the push travels along the tube to every wheel at once.
3. A metal disc spins with the wheel. A clamp called the caliper sits over it. The fluid pushes pistons in the caliper, and they press a brake pad against each side of the disc.
4. The pads rub hard on the spinning disc. This rubbing is friction. It slows the disc, and the wheel with it, and it turns the car's motion into heat. The disc can get hotter than an oven.
5. Slowing the wheels is only half the job. The tires must grip the road. On a wet road they grip less, so the car needs more room to stop. On ice it needs much, much more.
6. The car stops in good time, and the ducks waddle across. Pedal, fluid, pads, disc, tires: every part did its job, and the motion ended up as a little heat.

Tap each part. Use the round window to see inside the caliper.

• Brake pedal. The brake pedal is a lever. It makes the push from the driver's foot stronger and passes it on to the brake fluid.
• Brake fluid and line. Brake fluid fills thin tubes that run to every wheel. Because a liquid cannot be squashed, a push at one end comes out at the other end right away.
• Caliper. The caliper is a strong metal clamp that sits over the edge of the disc. It holds the pads and the pistons that push them.
• Piston. A piston is a round plunger inside the caliper. Brake fluid pushes on its back, and it pushes the pad against the disc.
• Brake pads. Brake pads are blocks of tough, grippy material on steel plates. One pad presses on each side of the disc. They slowly wear away and are replaced.
• Disc. The disc, also called a rotor, is a flat metal ring bolted to the wheel. When the pads slow the disc, they slow the wheel.
• Wheel hub. The hub is the middle part that turns on the axle. The disc and the wheel are both bolted to it, so they always spin together.

Spin the disc up, then push the pedal down. Compare a gentle squeeze with a hard one, and a dry disc with a wet one. Watch the speed and the heat.

Experiment: What happens to the disc when the pads squeeze it to a stop?

1. Press Spin it up, then push the pedal about halfway.
2. Watch the speed dial and the heat dial until the disc stops.
3. Spin it up again and squeeze harder. Then try a wet disc.

What we found: The disc gets hot every time. The pads rub the spinning disc, and that rubbing turns the motion into heat. A harder squeeze stops it sooner. A wet disc is slippery at first, so it takes longer.

Turn on JavaScript to try it yourself.

Drag the Brake here sign to the spot where the driver should start braking, then press Go. Stop the car in the box before the line: first on a dry road, then on a wet one.

Anything that is moving has energy of motion. A heavy car going fast has a great deal of it. To stop, the car has to get rid of that energy, and the brakes do this by rubbing.

When the driver presses the pedal, brake fluid pushes pistons inside a clamp called a caliper. The pistons squeeze two pads against a metal disc that spins with the wheel. The rubbing between pad and disc is called friction. Friction slows the disc and makes heat, just as your hands get warm when you rub them together.

So the motion does not vanish. It turns into heat in the discs, and the air flowing past cools them down again. After a long, steep hill the brakes can be very hot. Never touch a car's wheels or brakes after a drive.

The brakes only slow the wheels. It is the tires gripping the road that slow the car. On a dry road the grip is strong. On a wet road it is weaker, and on ice it is very weak, so the car slides much farther. That is why drivers slow down and leave more room in rain and snow.

Key words

Friction

The rubbing force between two things that touch. It slows them and makes heat.

Caliper

The clamp over the disc that holds the pistons and the brake pads.

Brake fluid

The liquid that carries the push from the pedal to the pistons at each wheel.

Kinetic energy

The energy something has because it is moving.

Grip

How well the tires hold on to the road.

Stopping distance

How far a vehicle travels from the moment the brakes go on until it stands still.

Check your thinking

1. What do the brake pads do when the driver presses the pedal?

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   They squeeze the spinning disc from both sides. The pads press on both faces of the disc, and the rubbing slows the wheel.

2. A car brakes to a stop. Where has its energy of motion gone?

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   It has turned into heat in the brakes. Friction turns the motion into heat. Energy never just disappears.

3. Why does a car need more room to stop on a wet road?

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   The tires grip the wet road less, so they cannot slow the car as quickly. The brakes slow the wheels, but only tire grip slows the car. Less grip means a longer slide.

4. Why can a push on the pedal reach all four wheels at once?

   Show the answer

   Brake fluid cannot be squashed, so it passes the push along the tubes. A liquid keeps its volume, so pushing one end of a full tube pushes the other end right away.

5. A car doubles its speed from 50 to 100 kilometers per hour. About how does its braking distance change?

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   It becomes four times as long. Kinetic energy and braking distance both go with the square of speed: 2 squared is 4.

6. What does the disc spin together with?

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   The wheel. The disc and the wheel are bolted to the same hub, so slowing the disc slows the wheel.