The hope t rex ratio expander allows you to upgrade your existing 11 36 10spd cassette to give an 11 40t range the t rex simply installs behind your existing cassette and replaces one of the smaller sprockets usually 17tshimano and sram specific 40t sprockets have been designed this gives us the following benefits which wouldnt be possible with a larger sprocket size or with a universal type designprecise matching of the tooth profiles and shifting points specific to each cassetteperfect mat