Shimano's unbelievable Di2 technology has made the leap to the trails with their XTR Di2 drivetrain. This rear derailleur features all of the race-day performance you've come to expect from XTR, and the unrivaled stability, speed, and performance of an electronic drivetrain. Since there aren't any cables to stretch or housing to create friction, shift performance stays amazingly perfect, delivering flawless shifts any time, under any condition. System communication with the front derailleur eliminates chain rub and, if you prefer, the rear derailleur can be optimized to use Shimano's Synchronized Shift technology.
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.