We made the rotor hat (the aluminum part) 0.005 inches thinner than the flange on the hub. That way, when the wheel is clamped on to the hub, the hat still has 0.005 inches of play. The brake hat can now heat up and expand away from the hub without warping the rotor. Another really cool benefit is there is a tremendous break in the thermal paths from the rotor to the hub--so you don't overheat your hub bearings under hard braking.
