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 17T.
Shimano and Sram specific 40T sprockets have been designed, this gives us the following benefits which wouldn’t be possible with a larger sprocket size or with a universal type design…
Precise matching of the tooth profiles and shifting points specific to each cassette
Perfect mating of supporting tabs reducing flex and aiding shift performance
Lower B-tension adjustment is needed ensuring good shifting throughout the cassette range compared to fitting a 42t
Shimano XT and XTR 11-36 10spd cassettes.
Sram PG1030, PG1050 and PG1070 11-36 cassettes