The hope 40 t rex expander sprocket is a shimano or sram specific 40 tooth sprocket that has been designed allowing the benefit that wouldnt be possible with a larger sprocket size or with a universal type design all you have to do is replace your 17 tooth sprocket with the t rex hope have precisely matched the tooth profiles and shifting points to work with each specific cassette with perfect mating of support tabs reducing flex and aiding shift performance lower b tension adjustment is nee