40T-Rex ratio expander sprocket (14)
Hope isn’t the first to offer an add-on range expanding sprocket, but this 40T item works well, and is more affordable than a specific groupset.
Increasing the lowest gear from 36T up to 40T means that running a single chainring and losing the attendant complication and weight is a viable prospect.
While SRAM’s 42T 11-speed option requires a new drivetrain and freehub, this simply adds to the back of your existing cassette, removing one of the smaller sprockets lower down the range to compensate for the additional width. You’ll need a different sprocket depending on which S supplied your original cassette, with SRAM X5 to X9 level and Shimano XT and 10-speed XTR clusters supported.
The 40T range isn’t as big as XX1 and doesn’t match OneUp’s 42T similar replacement sprocket, but the lower range means shifting is less compromised when used with a normal mech. Adjust clearance by setting the B-tension screw further in, which on our SRAM X7 and Shimano SLX derailleur test setups made a marginal difference to the speed of shifts and it required a high standard of setup.
Shifting on and off the T-Rex cog was fine and despite being alloy, premature wear hasn’t been an issue. The removal of the lower (usually 17T) cassette cog gave a noticeable ‘gap’ in ratios as you move up and down, especially when cranking hard, but it’s a small price to pay.