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Disc brakes use a clamping action to produce friction between the
wheel and the suspension members which hold the wheel. Firmly mounted
to the spindle, the caliper works like a c-clamp to pinch the rotor
which is attached to the spinning wheel. "Floating" calipers allow
themselves to move slightly when the brakes are applied, because only
one pad moves (in relation to the caliper). If the caliper is
solid-mounted, there are pistons on each side of the rotor. These are
called "dual cylinder" or "dual piston" calipers, and are standard
equipment on many performance cars. Inside the caliper, the
piston(s) press against the pads due to the pressure generated in the
master cylinder. The pads rub against the rotor, slowing the vehicle.
Because disc brakes can fling off water more easily than drum brakes,
they work much better in wet conditions. They allow better airflow
cooling, which also increases their effectiveness. Some high
performance disc brakes have drilled or slotted holes through the
face of the rotor, which helps to prevent the pads from "glazing"
(becoming hardened due to heat). Disc brakes were introduced as
standard equipment on most American cars in the early seventies.
The disc (rotor) is a heavy plate that attaches to the spindle to
provide a two-sided braking surface. Fluid from the brake line flows
into a cylinder mounted on the side of a clamp-like caliper. Part of
the fluid pressure is exerted against a piston, which forces the
brake shoe and pad against the inside surface of the disc. The fluid
also exerts pressure in the other direction against the back of the
cylinder. This back pressure causes the whole arm of the caliper to
move sideways, bringing an outboard shoe and pad tight against the
outside of the disc to provide additional stopping power.
Ventilation slots around the outside rim of the disc allow friction
heat that is generated to be transferred to the air quickly.
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