Hope T-Rex Cog Sram 40T Precise matching of the tooth profiles and shifting points specific to each cassette Hard anodized 7075 T6 Aluminum Allows you to upgrade your existing 11-36 10spd cassette to give an 11-40T range Perfect mating of supporting tabs to reduce flex and aiding shift performance T-Rex installs behind your existing cassette and replaces one of the smaller sprockets, usually the 17T