Sprocket R 520 50T Bk Sc TwinTwinring Rear SprocketInner ring is the same high strength Ultralight 7075 T 6 aluminum used for the standard Renthal sprocket Outer ring is electro less nickel coated steel for ultimate durability and is designed with the minimum amount of material to keep weight down The outer ring is expanded at 200C and then fitted over the inner; this interference fit combined with the wave pattern ensures a permanent bond for the life of the chainwheel The flat