Renthal Twinring Heavy Duty Rear Sprocket - Black - 50T 1540-520-50GPBKUnique flat face design keeps weight to a minimum and means the TwinRing is as narrow as a regular chainwheel Inner Ring is high strength Ultralight 7075 T6 Aluminum. Outer Ring is Electroless Nickel Coated steel Outer is expanded at 200C and then fitted to inner, creating a permanent bond for the life of the sprocket Precision CNC machined to extremely tight tolerances Flat socket head screws add additional strength to the chainwheel during side