A brake disc is usually an iron (or carbon composite) disc that is attached to the suspension of a car, motorcycle or even bicycle and is gripped by brake pads held in a brake caliper.
The friction between the brake disc and the pads is what brings the car to a halt.
The brake disc has numerous advantages over the brake drum, because it dissipates heat quickly, giving strong and consistent braking performance.
Disc brakes also give the driver more ‘feel’ at the pedal, allowing him or her to more easily modulate the pressure for a smooth stop.
In higher-performance applications, a brake disc will actually comprise two discs attached side by side by various rotors.
This, in effect, makes the disc ‘ventilated’ further helping cooling and performance.
Brake discs are also commonplace on motorcycles, where they are drilled to improve wet-weather braking performance.
Instead of the brake pad having to compress any water on the surface of the disc, which hampers stopping ability, the water is pushed into the holes on the disc, thereby allowing the brake pads to grip the disc as intended.
As a way of avoiding the stresses associated with excess heat build-up during braking, some discs are sometime mounted almost loosely on to the hub via coarse splines.
This loose mounting allows allows the disc to expand evenly with the heat, and stops it transferring too much heat to the hub on which it is mounted, which could cause a failure.
Disc brake technology has advanced to the point where it is not only used on cars and motorcycles – pedal cycles now commonly have disc brakes as standard because these provide stronger and more consistent performance in adverse weather.
Such braking systems are also light enough that they don’t impact on the overall performance of the bicycle.