Ride into the 21st century with Shimano's heralded Dura-Ace Di2 Electronic Rear Derailleur. Instead of a cable and spring, it's driven by a servo motor so shifts are identical, fast and precise every time. The derailleur also boasts a light, tough carbon pulley cage and the chain-wrap capacity for standard and compact cranksets. And, since there aren't any cables or housing to stretch or fail, maintenance is reduced dramatically and durability is increased. Furthering the durability, the derailleur is designed to move inward under impact to reduce the potential for damage in a crash.