No one offers more steel pulleys for the Kenne Bell Twin Screw and Eaton Roots superchargers - because pulleys and superchargers are our business and not a sideline. Supercharger/driven pulleys are only 1/3 the diameter of crank/drive pulleys and therefore spin at approximately 3 times the speed of the crank - 750 ft/sec. This equates to over an 1/8 mile per second! Thats a lot of heat and friction.