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Shimano Ultegra Di2 Rear Derailleur

Shimano Ultegra Di2 Rear Derailleur
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Ride into the future with Shimano's incredible Ultegra Di2 Electronic Rear Derailleur. Instead of a cable and spring, it's driven by an intelligent 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 1,250 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. And, because there are no cables and housing to stretch, wear or fail, you'll also love Di2's amazingly durable, reliable and silent operation.

An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 53/42 chainrings and a 12-23 cassette: (53 - 42 = 11) + (23 - 12 = 11) = 22-tooth chain-wrap capacity.

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