Twinring* Rear Sprocket - 49 Tooth - Black Inner 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 durability and is designed with the minimum amount of material to keep weight down, The outer ring is expanded at 200?C 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 socket head screws add additional strength to the chainwheel during side impacts and eliminates contact with the chain guide, Self cleaning mud grooves between each tooth prevent dirt build upand reduce chainwheel wear and weight, Will outlast a standard all aluminum sprocket by 3 times, yet is much lighter than an all steel rear sprocket