Rotor

A brake rotor (also called a brake disc) is a flat, circular metal disc attached to the wheel hub that rotates with the wheel. When the brakes are applied, the brake caliper squeezes the brake pads against both sides of the rotor, converting the vehicle's kinetic energy into heat through friction. This is the fundamental operating principle of a disc brake system.

Rotors are typically made from cast iron for standard vehicles, though performance and luxury vehicles may use composite materials or carbon-ceramic rotors. Cast iron rotors come in two main designs: solid rotors (a single disc of metal, common on rear brakes of lighter vehicles) and vented rotors (two discs with cooling fins between them, used on front brakes and high-performance applications for improved heat dissipation).

Over time, rotors wear from the repeated contact with brake pads. Manufacturers specify a minimum thickness (often stamped on the rotor itself), below which the rotor must be replaced. Rotors can also develop issues like lateral runout (warping that causes brake pulsation), scoring (deep grooves cut by worn brake pads), or hot spots (hard areas caused by overheating that create uneven surfaces).

When brake pads are replaced, rotors should be measured for thickness and runout. Previously, rotors were commonly resurfaced (machined smooth) on a brake lathe. However, many modern rotors are designed thinner to save weight and may not have enough material to safely resurface, making replacement the preferred option.