Shimano's electric-shifting 11-speed Ultegra Di2 Rear Derailleur is driven by battery power and a servo motor, so shifts are fast, precise, and perfect every time. It's made of light, tough cold-forged aluminum while mid-cage and short-cage models give you your choice of cassette. Since there aren't any cables or housing to stretch or fail, maintenance is dramatically reduced and shifting accuracy is unprecedented. This high-tech derailleur is even designed to move inward under impact to reduce the potential for damage in a crash.