Renthal 1500-520-49GPSI Twinring Silver 49 Tooth Rear Sprocket Unique 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