Shimano may have taken its sweet time when jumping into the world of 12-speed, but patience is a virtue, and it seems like our wait was certainly rewarded, as its new XTR 12-speed groupset soars right back to the front of the cutting edge, while offering even better shifting performance than before. But just because Shimano soared into the future doesn't mean its designers didn't still keep an eye out for those who are still all-for the two-by persuasion, and the XTR RD-M9120 Rear Derailleur drives the point home. This two-by optimized rear derailleur is specifically designed to pair Shimano's 10-45-tooth 12-speed cassette with a double-ring crankset, offering more gears than most of us know what to do with, while keeping things quiet and smooth. The M9120 Rear Derailleur sees improved chain stability and reduced friction thanks to larger 13t pulley wheels, making it better able to withstand full-force pedaling while shifting gears as smoothly as ever. Shimano was also able to decrease the rear derailleur tension in the lower gear positions, preventing the high chain tension that causes excess friction and noise, and thus a loss of efficiency. The Shadow RD+ clutch system provides excellent chain retention in rough terrain, and keeps annoying chain slap to a minimum. Lastly, the addition of a protective bumper to the inside of the derailleur cage quiets operation even further by cushioning the sound of any chain rub. Because the RD-M9120 is optimized for a two-by system, it only comes in Shimano's SGS model. This means the derailleur has plenty of range to take on the new 10-45t Ryhtym-Step 12-speed cassette, and allows for some play in the front when you're swapping from the little ring to big ring.