Renthal 150U-520-51GBBU Ultralight Blue 51 Tooth Rear Sprocket Renthal chainwheels are chosen by more factory motocross and supercross teams than any other brand available. Renthal Ultralight rear chainwheels are manufactured to extremely tight tolerances from a special material developed to withstand the rigors of supercross, motocross and enduro racing. The 7075 T6 aluminum alloy rear chainwheels are 66% lighter than steel, yet incredibly durable. Their advanced tooth profile, concentricity and accuracy of fit provide maximum power transfer and long-life. Renthal's combination of a very high strength base material and an exceptionally hardwearing surface result in significantly increased chainwheel life over other brands.