The brake disc or rotor is the main rotating component of the disc brake unit.
It’s usually made of cast iron because it’s hard-wearing and can resist high temperatures.
On motorcycles, it is often made of stainless steel.
Most brake discs are stamped with the manufacturer’s minimum thickness specification. When the pad wears, if the thickness of the disc were below this minimum, the piston may go beyond the sealing edge.
Ventilated discs can be used to improve cooling. These slots are designed to use centrifugal force to cause airflow when the disc is rotating.
Some discs are drilled or slotted on their friction surface to improve cooling and assist with removing water.