The Shimano Deore XT RD-M8100/M8120 rear derailleur is designed to provide smooth and stable shifting for 12-speed MTB drivetrains. The Shimano XT RD-M8100 Rear Derailleur has been upgraded from 11-speed to 12-speed and has the capacity to go up to a 51 tooth cog on the rear. The RD-M8100 is designed for 1x12 drivetrains and comes with the SGS long cage.