Brake Systems

Brake Systems

The brake system is designed to slow and halt the motion of a vehicle. To do that, various components within a hydraulic brake system must convert the momentum of the vehicle into heat. They do so by using friction.


Friction is the resistance to movement exerted by two objects in contact with each other. Two forms of friction play a part in controlling a vehicle: kinetic, or moving; and static, or stationary. The amount of friction, or resistance to movement, depends on the type of materials in contact, the smoothness of their rubbing surfaces, and the pressure holding them together (often gravity or weight).

For example, slide your hand lightly across a polished table top. Very little effort is required because of the smooth texture of the rubbing surfaces and the small amount of pressure exerted. Now, rub you hand over a piece of sandpaper. Feel the difference in friction? The rough texture of the sandpaper resists the movement of your hand.

Rub any surface long enough or hard enough and you begin to feel heat. Friction always converts moving, or kinetic, energy into heat. The greater the friction between two moving surfaces, the greater the amount of heat produced. As the brakes on a moving automobile are actuated, rough-textured pads or shoes are pressed against rotating parts of the vehicle, either rotors (discs) or drums. The kinetic energy, or momentum, of the vehicle is then converted into heat energy by the kinetic friction of rubbing surfaces and the car or truck slows down.

When the vehicle comes to a stop, it is held in place by static friction. The friction between the surfaces of the brakes, as well as the friction between the tires and the road, resist any movement. To overcome the static friction that holds the car motionless, the brakes are released. The hear energy of combustion in the engine crankcase is converted in to kinetic energy by the transmission and drivetrain, and the vehicle moves.

Static friction also plays an important part in controlling a moving vehicle. The rotating tires grip the road, and the static friction between these two surfaces enable the driver to control the speed and direction of the car. When the brakes are applied, the kinetic friction of the rubbing brake components slows the rotation of the tires. This increases the static friction between the tires and the road, decreasing the motion of the car. If the kinetic or sliding friction of the brake components overcomes the static friction between the tires and road, the wheels lock up and the car begins to skid. Static friction then exists between the components in the brakes and kinetic friction between the skidding tires and the road. The car is out of control. Obviously, the most effective braking effort is achieved just below the brake component kinetic friction levels that result in wheel lockup. This is the role antilock braking systems play in modern vehicles. By electronically pumping the brakes on and off may times each second, antilock systems keep kinetic friction below the static friction between the tires and road.

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