The disc brake hardware at the wheel consists of a rotor, two brake pads, and a caliper.
Rotor. The disc, also called a rotor, connects to the wheel hub. Friction is produced when the brakes are applied. This friction creates a lot of heat, so rotors are often vented to help with cooling. When inspecting rotors check both sides for thickness, cracks, unevenness, warping, and deep scoring. If a rotor measures thick enough, it may be possible to resurface it smooth. It is recommended to replace or resurface rotors in pairs, so their thickness remains balanced and surface finish is consistent.
Brake Pads. A set of brake pads hug the rotor. As force is applied to the brake pedal, the brake pads hug the rotor tighter causing more friction.
Wear Indicators. Some brake pads are engineered with mechanical or electrical wear indicators as a replacement reminder. A mechanical indicator uses a thin metal strip against the rotor to produce a high-pitched squeal once the pad wears down, without causing rotor damage. The electrical indicator uses an electrical contact in the pad. When the pad has worn to that electrical ground contact it completes a circuit and a dash indicator lamp (also called a MIL – malfunction indicator light) lights up. If you don’t replace worn brake pads you will soon hear a metal on metal grinding noise that will damage the rotors and create a braking hazard.
Caliper. The caliper, a type of hydraulic C-clamp with a piston, holds a brake pad on either side of the rotor. The caliper converts the fluid pressure in the brake lines to the mechanical motion of the pads against each side of the rotor.