Optimized for impact survivability the 12-speed XTR RD-M9250 medium cage rear derailleur is the choice for both Shimano's top enduro athletes and weight-conscious XC racers alike especially when matched with the 9-45t compact cassette (cassette sold separately). Featuring all-new wireless Di2 shifting automatic impact recovery a redesigned wedge shape and 23 millimeters of additional ground clearance for a bash-free ride this rear derailleur stands out for the speed of its shift and its ability to keep riders competitive on the trail.Wireless Di2 paired with HYPERGLIDE+ technology result in smooth precise and instantaneous shiftsWide link structure skid-plate design durable carbon plate and solid pulley add robustness and longevityAutomatic Impact Recovery function instantly returns the derailleur motor to its original position after impactShadow ES design provides a slimmer stronger evolution of Shimano's pioneering chain-stabilizing Shadow RD+ conceptWedge shape of Shimano's most survivable rear derailleur to-date avoids more direct impacts and smoothly glances off rocks if there is a hitTucked safely inside an enclosed space the battery won't budge even in rough terrain while remaining completely removable for charging