With the release of the XTR M9100 12-speed drivetrain, Shimano jumps right back to the front of the pack by expanding the available gear range and offering better shifting performance than ever before. The new XTR RD-M9100 Rear Derailleur is specifically optimized for Shimano's new wide range cassettes, and features several updates that ultimately lead to smoother and quieter drivetrain operation. It comes in two different cage lengths depending on what gear ratio you choose, letting you select the best option to suit your riding style and preferences. The M9100 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. There are two options available depending on your desired setup. The longer cage SGS model is required for the 10 - 51 tooth XTR 12-speed cassette, but also works with the 10 - 45t Rhythm-Step 12-speed cassette. The GS model features a shorter cage with 28mm more ground clearance for rocky, rooty, and aggressive terrain. It's designed exclusively for the 12-speed 10 - 45t Rhythm-Step cassette.