Brake disc (rotor)

The brake disc, also called the rotor, is a steel disc bolted to the wheel hub. When you brake, the caliper presses the pads onto both faces of the disc, and friction turns movement into heat. Many bikes have two front discs and one at the rear.

Discs are often floating: the braking surface is attached to an inner carrier with buttons that let it expand with heat without warping. Holes and wave-shaped edges help shed heat, water and dust and reduce weight.

A disc must be replaced when it is worn below the minimum thickness stamped on it, or when it is warped, which shows as a pulsing lever. Never let pads wear down to the backing plate, as metal-to-metal contact quickly ruins the disc.