How does fluid transmit a braking force?

A brake pedal moves only a little, yet it can make pads press firmly against a turning disc at a wheel. In a hydraulic brake, a liquid-filled connection carries that push from the pedal mechanism toward the wheel. The liquid provides a way to transmit force around bends in a vehicle.

The pedal mechanism pushes a piston into a sealed cylinder. Force spread over the piston's area produces a pressure increase. Pascal's principle says that this increase is transmitted throughout the enclosed liquid. It is the pressure increase that is shared, not an identical force at every piston.

In the ideal example, a 20-newton force acts on an input piston with an area of 1 square centimeter. The pressure increase is 20 newtons per square centimeter. Applied to an output piston of 4 square centimeters, that same increase gives an 80-newton output force. Four times the area produces four times the force.

The larger force comes with less movement. For a nearly incompressible liquid, the volume displaced by the input piston must go somewhere. An output piston with four times the area moves one quarter as far for the same transferred volume. The system trades distance for force rather than multiplying energy; the calculation neglects friction and differences in height.

At a disc brake, the hydraulic push makes the pads press against the rotating disc. Friction opposes its motion and transfers much of the vehicle's lost motion energy into thermal energy. The fluid carries the pressure change; contact between pad and disc supplies the braking interaction. Real stopping performance also depends on the complete brake system, the tires and the road.