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Rotor or Brake Disc is the rotating part of a wheel's disc brake assembly, against which the brake pads apply. It is typically made of gray iron, a form of cast iron. Some discs are both drilled and slotted. There are three types of disc brake; The fixed caliper disc brake has pistons on both sides of the disc. Some use two pistons, one on each side. Others use four pistons with two on each side. The floating caliper disc brake has only one piston located on the inboard side of the disc. The caliper moves or floats on rubber bushings on one or two steel guide pins. The bushing allows the caliper to move slightly when the brakes are applied. The sliding caliper disc brake type is similar to a floating caliper brake. Both calipers move slightly when the brakes are applied. However, the sliding caliper slides on machines surfaces on the steering knuckle;e adapter or anchor plate