It's been a little while since we've seen an update to Shimano's coveted XT group. In fact, the last time we saw big changes was when things shifted to 11-speed, which should make it no shock at all that Shimano's new XT RD-M8100 Rear Derailleur comes along with the launch of 12-speed XT. The new derailleur packs in a new, larger 13-tooth pulley, and carries on with the proven Shadow RD low-profile tech that keeps things tucked out of harm's way when the trail gets loose, wily, and filled with menacing boulders hungry to take a bite out of bike components. The new derailleur comes in two versions, the M8100 and M8120. This particular model is built for one-by drivetrains, to be paired up with Shimano's new 12-speed cassette. It's capable of running both the 10-45t and 10-51t cassette, but won't fair well if you're hoping to run a two-by crank setup. The derailleur brings a new level of silence to the drivetrain thanks to decreased tension in the lower gear positions, plus an added bumper on the pulley cage. The shifting is a bit faster than on the 8000-series derailleur thanks in part to the larger pulley. One thing that we're thankful has stayed the same through the generations is the stabilizer switch that we saw trickling down from XTR in the previous model. When engaged, this switch helps to eliminate chain slap and dropped chains by increasing spring tension. This is a valuable option for suspension systems that require chainstays and pivots located close to the rear derailleur. The RD M8100 keeps a good thing going with some of the previous model's improvements, including the direct cable path. Your shift housing can enter the stop on the derailleur without the traditional loop of excess housing that often was the victim of contamination in yester-years. Shimano sticks with the old-reliable aluminum components in the XT derailleur. All of the linkage plates, knuckles, and cage sides are made from it, so while it may lack the carbon-fiber-laden aesthetic...