Put the future of shifting under your control with Shimano's Dura-Ace Di2 electronic rear derailleur. Feather-light cold-forged aluminum and carbon fiber construction offer exceptional weight savings and extreme durability. The Di2 9070 rear derailleur is optimized for Shimano's 11-speed drivetrains, and uses their proprietary E-Tube wiring technology for waterproof connections and simple installation. Sealed pulley bearings ensure smooth operation, and crash protection mode moves the derailleur inward and out of harms way if your bike takes a tumble.
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: 44/29 chainrings and a 11-28 cassette: (44 - 29 = 15) + (28 - 11 = 17) = 32-tooth chain-wrap capacity.