Great solution for Shimano cassette users who require that one more, extra sprocket on steep climbs. Very simple to install, it fits behind 11-36 cassettes, displacing one sprocket from the main cluster.
There are two ways to fit 40T sprocket to your drivetrain. You can remove the stock 17T sprocket and one spacer from your cassette, then put 40T on the freehub and mount everything else back (except 17T). OR, by removing 11T sprocket. Second option is a lot better in our opinion, but requires our special lockring to hold 13T sprocket (lockring is available in options).
If you don't know which option would be better for you, then just go for a ride and observe which of these two sprockets (11 &; 17) you will miss less. Study shows, around 90% of mtb riders do not use 11T sprocket in terrain.
For running a 1X system, please use medium cage rear derailleur. When in use with 2x10, most recommended is long cage. Please note: weight limit is 100kg (220lbs).