99-19 YAMAHA YZ250: Renthal Rear Sprocket (520 / 49T) (Blue) Renthal chainwheels are chosen by more factory motocross and supercross teams than any other brand available. Renthal Ultralight&trade; 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.