Brakes

Last updated: May 7, 2008.

You're driving along quite happily when, all of a sudden, a dog runs out into the road just in front of you. You have a split second to react to what's happened. When you stamp on the brakes, you confidently expect they'll bring you to a halt in time. How can you be so sure? Because brakes use the power of science and thankfully, for the most part, science doesn't let us down!

Photo: The brake disc on a car is the small, metal wheel just inside the silver spokes of the outer, alloy wheel. When you put the brakes on, a brake pad clamps onto this metal wheel to slow you down.

The science of stopping

If you're moving, you have energy—kinetic energy to be precise. Kinetic energy is simply the energy an object possesses because it has both mass and velocity (speed in a certain direction). The more mass you have (effectively, the heavier you are) and the faster you're going, the more kinetic energy you have.

That's all well and good, but what if you suddenly need to stop? To change from moving quickly to not moving at all, you have to get rid of your kinetic energy.

If you're jumping from an aeroplane, the best way to lose energy is with a parachute. This giant sack of fabric drags behind you, slowing you down, reducing your velocity, and therefore helping to get rid of your kinetic energy. That means you can land safely. Drag-racing cars and land speed record cars also use parachutes to stop but, in practice, bicycles and cars simply use brakes.

Photo: Coming in to land: a parachute "brake" reduces your velocity and kinetic energy so you can land more safely. Photo by Senior Airman Micky Bazaldua courtesy of US Air Force.

Bicycle brakes

If you ride a bicycle, you know all about brakes. If you want to stop suddenly, you squeeze the brake levers on the handlebars. Thin metal cables running to the back and front wheels pull on small calipers, forcing thick rubber blocks to press against the wheels. As they do so, friction between the blocks and the metal wheel rims generates heat, reducing your kinetic energy, and bringing you safely to a stop.

Photo: The rubber shoes of this bicycle's brakes clamp the metal rim of the wheel to slow you down.

Car brakes

Most cars have two or three different types of braking systems. Peer through the hubcap of a car's front wheels and you can usually see a shiny metal disc just inside. This is called a disc brake. When the driver steps on the brake pedal, a pad of hard-wearing material clamps onto the brake disc and rubs it to make it slow down—in a similar way to bicycle brakes.

Some cars have disc brakes on all four wheels, but many have drum brakes on the back wheels, which work in a slightly different way. Instead of the disc and brake block, they have shoes inside the hollow wheel hub that press outwards. As the shoes push into the wheel, friction slows you down.

A car's handbrake applies the two rear brakes (disc or drum) in a slower, less forceful way when you pull on a lever located between the front seats.

A speeding car has loads of energy and, when you stop, virtually all of it is converted into heat in the brake pads. The brakes can heat to temperatures of 500°C (950°F) or more! That's why brakes have to be made of materials that won't melt, such as metals, ceramics, or composites.

Braking hard

Imagine how much force you need to stop a fast-moving car. Simply pressing with your foot would not generate enough force to apply all four brakes hard enough to bring you quickly to a stop. That's why brakes use hydraulics: a system of fluid-filled pipes that can multiply force and transmit it easily from one place to another.

When you press on the brake pedal, your foot moves a lever that forces a piston into a long, narrow cylinder filled with hydraulic fluid. As the piston plunges into the cylinder, it squirts hydraulic fluid out through a long and narrow pipe at the end (much like squirting a syringe). The narrow pipe feeds into much wider cylinders positioned next to the car's four brakes. Because the cylinders near the brakes are much wider than the one near the brake pedal, the force you originally applied is multiplied greatly, clamping the brakes hard to the wheels.

Caption: When your foot presses the brake lever, brake fluid squeezes out of a narrow cylinder, through a tube, into a much wider cylinder. This system, known as hydraulics, greatly increases the pushing force.