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
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.