All disc brakes use a rotor and pads. A rotor is a flat, round piece of steel, roughly shaped like a pancake and located directly behind the wheel. When you depress the brake pedal, the pads squeeze the rotor, so the friction stops the wheel from turning. Engineers understand that pads wear out, so they design the system so pads are easily replaced. However, engineers further understand that rotors wear out, too. Because of the wear factor, rotors are manufactured with extra steel on the contact surfaces, so they can be repaired.
Anticipating the Future
Engineers and designers anticipate the future. They understand pads and rotors wear out. Theoretically, rotors should last a long time. In fact, many do. If you change your brake pads as soon as a squeaking is heard, the pad won’t wear completely through. If a pad wears through, the metal backing contacts the rotor, leading to scoring and deep gouging of the rotor surface.