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. It runs off the Di2 rechargeable Li-ion battery pack (sold separately), which attaches to the bottle cage bolts, holds a charge that lasts for approximately 1,500 miles, and is completely sealed from the elements. Plus, the derailleur is designed to move inward under impact to reduce the potential for damage in a crash. The derailleur also boasts a light, tough carbon pulley cage and the chain-wrap capacity for standard and compact cranksets. And, there aren't any cables or housing to stretch or fail, so you get peace of mind.
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