How Do Car Brakes Work


If you really take the time to think about it, it’s pretty amazing that we can stop the forward motion of a four thousand pound car by just pressing on a pedal with one foot. Of all the things we ask of our vehicles, please stop is the most important request of all. So how is it possible that so little effort can generate so many results? The answer is hydraulics.

Here’s How Hydraulics Works

Hydraulics uses pressurized liquid to create power. This works because liquids cannot be compressed. So if you try to compress a liquid that is inside a sealed container, it will push out on all sides of the container, looking for a place to go.

Think of squeezing a plastic water bottle. If the cap is removed the water will shoot out of the top. If the cap is tightly secured , you will only be able to squeeze the bottle so far. Since water doesn’t compress, hydraulics stops you from collapsing the bottle. If you leave the cap loose, and squeeze the bottle real hard, hydraulic pressure will blow the cap right off the bottle.

So when you step on your brake pedal, you are pressurizing the entire brake system, applying the brakes.

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Applying Hydraulics to Brakes

The end game of all this hydraulic pressure is kinetic energy. Kinetic energy is created by friction, and friction is what actually stops the vehicle. All four wheels of your car are equipped with either a brake rotor or a brake drum. The rotor or drum are attached to, and spin with the wheels.

So when the rotor or drum slows down, so does the wheel. So when you apply your brakes, high temperature resistant brake shoes or brake pads press against the rotor or drum creating friction, slowing down and eventually stopping your car.

On drum brakes the shoes are actuated by hydraulic wheel cylinders. Disc brakes use hydraulic calipers. Wheel cylinders and calipers are the sealed containers that we talked about earlier.

They are filled with brake fluid. When the fluid is pressurized, it pushes out on all sides of the container, forcing out a piston that engages the pads or shoes. Much like squeezing the water bottle caused the loose cap to pop off. The pads or shoes press against the drum or rotor, stopping the car.

It all Starts with Your Foot

The force that pressurizes the wheel cylinders and calipers comes from the master cylinder. The force that pressurizes the master cylinder comes from the brake pedal. Which makes your foot the initial force that puts this whole chain of events into motion.

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The master cylinder is another sealed container filled with brake fluid. When you push the brake pedal you are pushing a piston up against the fluid, pressurizing it. This pressure is forced out of the master cylinder through the brake lines, much like when we squeezed the bottle with cap removed.

Some sections of the brake lines are steel, other sections are rubber. The lines attach to the wheel cylinder and calipers. So by pressurizing the master cylinder, the whole brake system is pressurized.

When the brake pedal is released the piston pulls back in the master cylinder, relieving pressure. This allows the pistons in the wheel cylinders and calipers to pull back, releasing the brakes.

Drum Brakes and Disc brakes

Here’s the difference between drum and disc brakes. A brake drum is shaped kind of like a bowl and spins with the wheel. The brake shoes are located inside the drum and are pushed out against the drum by the wheel cylinders.

Disc brakes use rotors which are heavy metal discs, usually around an inch thick. They too spin with the wheels. The brake pads are attached to the caliper, one on each side of the rotor. So when the caliper is pressurized, the pads clamp down on the rotors from the outside. Either way, it’s friction causing kinetic energy, stopping the car.

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We tend not to think much about our brakes till they stop working. It’s easy to take for granted that the car will stop every time we push on the brake pedal. If you’ve ever had brakes fail, you’d never feel that way again. Having the brake pedal go straight to the floor without even slowing the car down is an experience that you will never forget.

Unusual brake noises or a low brake pedal are your brakes giving you a heads up that something bad could happen.  So heed the warning signs.

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