# How Hydraulic Brakes Work

Q:
Describe briefly how a hydraulic car brakes functionsP
- Janeth Joseph (age 16)
Dar-es-salaam, tabata, Tanzania
A:

Hello Janeth,

A hydraulic braking system is a brake pedal attached to a piston full of a nearly incompressible brake fluid connected to another piston near the wheels attached to brake pads which push on the rotors on a wheel. When you push down on the brake pedal of a car, you make some fluid pressure P=Force/Area where the force is the force of your foot on the pedal (equalt to the force of the pedal back on your foot) and the area is the cross sectional area of the piston. That pressure is a lot higher than the pressure of your foot on the pedal, because the foot-pedal contact is much more spread out than the little piston area. Fluids will transmit pressure throughout the whole fluid, so the piston at the other end will push harder on the brake pads, pressing them against the rotors. The friction between them causes the rotational energy of the wheels to turn to heat and stop the vehicle.

Since the brake fluid is nearly incompressible, you don't have to press the pedal far to make the pressure go way up. Squeezing liquid into just a bit smaller volume makes a big difference in pressure. (If air gets in your brake system, then you have to push the pedal much farther because air is very compressible.)

I hope this helps,

Erik  (mods by Mike W.)

(published on 04/28/2013)

## Follow-Up #1: bubbles in hydraulic brakes

Q:
in the hydraulic brake system of a car, if there are bubbles in it, how could we compress them ? The pressure inside the bubbles are atmospheric pressure. Could the pressure we use to brake exceed the atmospheric pressure( in order to compress the bubbles)?
- Levy (age 16)
Malaysia
A:

The pressure in the bubbles will go up when you step on the brake pedal, just as the pressure in the brake fluid goes up. The problem is that the bubbles will compress a lot as the pressure goes up, unlike the fluid, which compresses very little. That means that you have to press the pedal down much farther than you would if there were no bubbles. That can make braking much slower. If there are too many bubbles, then even if you push the pedal down as far as it can go you won't compress them enough to raise the pressure much. Then the brakes won't work.

Mike W.

(published on 06/01/2013)

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