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A closeup of a Porsche sports car front wheel showing the brake disk behind

Brakes

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by Chris Woodford. Last updated: July 11, 2017.

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 this Porsche sports car is the small, metal wheel just behind the silver spokes of the outer, alloy wheel. When you put the brakes on, a brake pad (red) clamps onto this metal wheel to slow you down.

The science of stopping

A parachute coming in to land, seen from beneath

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.

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 airplane, 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, most vehicles simply use brakes.

Different brakes for different machines

From cars and trucks to planes and trains, brakes work in a similar way on most different vehicles. There are even brakes in wind turbines! Here's a quick comparison of some common brake systems.

Bicycle

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.

A closeup of bicycle brake blocks

Steam locomotive

The brakes on a steam locomotive work the same way as a car's and are even more obvious. You can see the brake just behind the wheel in this photo. It clamps against the locomotive's driving wheels to slow them down. Since there are no tires on the wheels, the friction that stops the train comes from the immense weight of the locomotive pressing the metal wheels down onto the track.

A closeup of a steam engine wheel with the brake shoe

Motorcycle

Motorcycles typically have disc brakes comprising a rotor and a brake block. The rotor is a disc with holes (or slots) in it mounted on the side of the wheel. A brake pad, operated by a cable, jams against the rotor to slow it down by friction. The holes in the rotor help to dissipate the heat generated.

Motorcycle brake rotor, brake block, and cable

Airplane

Airplanes have brakes inside their wheels to help bring them to a stop on the runway, but they can also use air brakes to increase drag (air resistance) and slow themselves down—a bit like parachutes. Jet fighters often have a speed brake, which is a large metal plate just behind the cockpit that can be hydraulically raised to increase drag and braking.

Photo by Vincent Parker, US Air Force.

An F-15E Strike Eagle jet fighter airplane raises its aerodynamic speed brake to slow down as it comes into land.

Wind turbine

Wind turbines have brakes to stop their rotors (propellers) turning too quickly. The brake is mounted inside the nacelle (the square-shaped casing behind the propeller that contains the gearbox and generator). Most turbines have an anemometer on them to measure the wind-speed. If it rises above a safe level, the brakes come on automatically and bring the rotors to a standstill. It's a shame, because higher wind speeds mean more energy could be produced. But safety always comes first!

Photo by Matthew Bates courtesy of US Air Force.

Wind turbine brake

A closer look at car brakes

Primitive friction brake for a car from 1910 patent by John Stawartz, US Patent 960,426.

Artwork: Early car brakes were amazingly primitive by today's standards. Here's a simple friction braking system from 1910 invented by John Stawartz of Homestead, Pennsylvania. When you pull on the brake lever (yellow), a giant brake "shoe" (blue) drops down under the back wheel (brown). As the car drives onto the shoe, the shoe's teeth (red) bite into the road and the car comes juddering to a halt. Artwork from US Patent 960,426: Automobile Brake by John Stawartz, courtesy of US Patent and Trademark Office.

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 alloys, ceramics, or composites.

How car brakes work

Artwork explaining how hydraulics works

Artwork: 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 force you supply.

In theory...

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.

In practice...

Animation showing the key parts of a car's braking system and what happens when you press the brake pedal

  1. Your foot pushes on the brake pedal.
  2. As the pedal moves down, it pushes a class 2 lever (a kind of simple machine), increasing your pushing force.
  3. The lever pushes a piston (blue) into a narrow cylinder filled with hydraulic brake fluid (red). As the piston moves into the cylinder, it squeezes hydraulic fluid out of the end (like a bicycle pump squeezes out air).
  4. The brake fluid squirts down a long, thin pipe until it reaches another cylinder at the wheel, which is much wider.
  5. When the fluid enters the cylinder, it pushes the piston in the wider cylinder (blue) with greatly increased force.
  6. The piston pushes the brake pad (green) toward the brake disc (gray).
  7. When the brake pad touches the brake disc, friction between the two generates heat (red cloud).
  8. The friction slows down the outer wheel and tire, stopping the car.

This shows the basic principle of a hydraulic braking system; in practice, there's a little bit more to it. The brake pedal actually operates four separate hydraulic lines running to all four wheels (we're just showing one wheel here for simplicity). Instead of a single cylinder, there's usually one main cylinder (sometimes called the master cylinder), operated by your foot and the brake pedal, and then one secondary cylinder (or slave cylinder) on each wheel. By making the main cylinder smaller than the secondary cylinders, we can amplify the braking force that the driver applies. Finally, for added safety, hydraulic brakes typically have two separate hydraulic circuits in case one of them fails.

Who invented hydraulic brakes?

Malcolm Loughead of Detroit, Michigan invented "fluid-operated" (hydraulic) brakes in 1919—and here's one of his improved designs from the mid-1920s. It uses the momentum (moving power) of the car to provide the force that pushes the hydraulic piston into the cylinder, giving a kind of power-assisted braking. Loughead and his brother Allan were airplane pioneers and the founders of the Lockheed Corporation (originally known as the Loughead Aircraft Manufacturing Company).

Early fluid-operated hydraulic brake from 1929 patent by Malcolm Loughead, US Patent 1,732,309.

Artwork: When your foot presses the brake pedal (red), it pushes on a lever (light blue) that applies a friction belt (dark blue) to a drum (orange) surrounding the vehicle's drive shaft (yellow). As the belt locks on, the force from the moving drive shaft tugs it to the right, pushing a lever (green) also to the right. This forces a piston into a hydraulic cylinder (purple), pushing hydraulic fluid down a pipe (orange) that applies the brake (dark blue) to the back wheel (gray). Artwork from US Patent 1,732,309: Fluid-operated Brake by Malcolm Loughead, courtesy of US Patent and Trademark Office.

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Text copyright © Chris Woodford 2008, 2017. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008/2017) Brakes. Retrieved from http://www.explainthatstuff.com/brakes.html. [Accessed (Insert date here)]

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