Shimano craftsmanship reaches new heights with the XTR 12-speed groupset. These Hollowtech II crank arms feature a lighter and stiffer design, and simple-to-change direct-mount chainrings with a single key release, allowing you to feel things out with your new 12-speed configuration and find the perfect gear ratio for you. If you're a little confused by Shimano's offerings of both the FC-M9120 and FC-M9100, you're not alone. With two offerings in this top-tier package, the differences may be difficult to spot at first glance. Shimano builds the M9120 with a beefier, more robust design than the XC-oriented 9100, ideal for riders charging hard and having up-close-and-personal encounters with rowdy sections of trail that give your crankarm the occasional bash. On top of the beefier construction, the M9120 crankarms boast a wider Q-factor that lands in at 168mm, in comparison to the 158mm of the 9100. This wider Q-factor increases stability when you're descending at high speeds and offers a bit more chainstay clearance for beefier 148-millimeter Boost spacing in the back. As its name suggests, this particular XTR M9120 Crankset is intended solely for bikes with Boost 148-millimeter rear axles, with its 52-millimeter chainline reflecting the outboard move to accommodate Boost axle spacing. The crank arms come alone, without chainrings, but can be paired up with either one-by or two-by options in a huge range of sizes to best suit your riding needs. Additionally, Shimano is offering the XTR 9120 in a range of sizes from 165mm to 175mm, so shorter riders, or those with gravity-oriented appetites who want a little more clearance, can be decked out with the perfect crank length, too.