All of the rotors that we sell are cast iron with a 6.25" bolt circle. Generally rotors are heat sinks that absorb the heat generated by the friction with the brake pads. The greater the mass of the rotor the more heat that it can store. In the same regard, the faster that a rotor can cool the more friction it can endure. Knowing this, the rotors that will best tolerate severe braking are those that are heavy and cool quickly. Lighter rotors such as .810's should be used only in less demanding applications such as the rear of asphalt cars or in the front under mild braking conditions. Aside from circulating more air, curved rotor vanes help to prevent cracking by not following potential crack lines. 12.19" diameter rotors are well suited for very heavy brake applications such as ASA.