Ride into the future with Shimano's amazing electric-shifting Dura-Ace Di2 Rear Derailleur. Instead of a cable and spring, it's driven by battery power and its servo motor so shifts are identical, fast and precise every time. The derailleur also boasts a light, tough cold-forged aluminum and carbon construction. Plus it has 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. The derailleur is even designed to move inward under impact to reduce the potential for damage in a crash.
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.
Largest cog: 28 tooth