Brake rotors are the parts within the wheels of an automobile which brake pads squeeze in order to slow the rotation of the wheel and bring the vehicle to a stop. Depending on the type of vehicle, its brake rotors can be made in a variety of materials and designs. Some are made of heavy cast iron, while others may be made with carbon, Kevlar®, or other more specialized materials.
Some brake rotors are cross-drilled, meaning that they have small holes drilled through them. This was done originally in racing vehicles, because gas would sometimes become trapped between brake rotors and pads, impairing performance. Since modern brake pads do not present this problem as much as older ones, and due to the fact that brake rotors can eventually crack where holes are drilled, cross-drilling is no longer done on racing vehicles.
Cross-drilling is now more common on motorcycles and high-performance mountain bikes, such as those used for downhill racing. The purpose of this is to aid in the dissipation of heat which could otherwise warp the rotor when the brakes are applied quickly. Brake rotors can also slotted, a process in which shallow grooves are carved into the rotor, accomplishing the same task in racing vehicles that cross-drilling used to. Slotted rotors are not necessary or practical for most cars, since they cause brake pads to wear down quickly, and over-worn brake pads can lead to damaged rotors.
While iron and steel are the most common materials for brake rotors, others can be used in certain vehicles. Reinforced carbon is a common choice for brake rotors in racing vehicles because of their superior performance at high temperatures, compared to iron. Carbon is also much lighter than iron, which is an important consideration in racing.
Slightly less common that carbon rotors are ones made from ceramic materials. Again, the principal advantage in this case is the light weight, as well as low maintenance requirements. They are also strong enough to tolerate high heat, but are significantly more costly than iron.
There are several ways in which brake rotors can be damaged. Excessive rust is a hazard in rotors made with iron, for example. A small amount of rust is universal, but it is possible for the rotor to rust to such an extent that it needs to be replaced. Warping is perhaps the most common rotor problem. When a rotor becomes warped, excessive heat causes a disproportionate expansion of the braking surface of the rotor. This can lead to vibration in the steering wheel and elsewhere while braking.
To correct this problem, the rotor is removed from the car and turned through a machining tool to give it a more uniform surface. This can only be done a certain number of times before the rotor must be replaced. Other problems are somewhat less common, such as cracking in cross-drilled rotors or the scoring that takes place when worn out brakes cause scratches and grooves on the rotor surface.