Shimano has been hard at work refining their highly reliable and successful Ultegra Di2 group. This latest 6870 version is 11-speed compatible as well as featuring lighter with a slimmer profile making it more like its flagship Dura-Ace big brother. It is offered in a SS version that works up to a 28T cog and a new GS version that works up to a 32T cog. The GS version can also be used with Dura-Ace 9070 Di2 to use a 32T cog with a Dura-Ace (there is no 9070 GS rear derailleur).The best thing about Ultegra Di2 is you get Dura-Ace Di2 performance at a fraction of the price.
Most derailleurs use a spring and cable tension to move the chain from cog to cog. The Shimano Ultegra RD-6870 Di2 rear derailleur employs highly innovative digital technology where an electronic signal is sent to a worm drive motor move the chain from gear to gear. Shifts are lightning-quick with no hesitation. Performance is on par with Dura-Ace Di2 except the Ultegra uses a slightly bulkier motor and has an alloy cage instead of carbon.
For those concerned with damaging the derailleur in a crash, Shimano's engineers included a servo-saving mechanism that allows the rear derailleur to handle a big side impact with minimal damage. Shifting is completely customizable. Shifting speed and the amount of cogs shifted with a press and hold can be tuned to each rider's preferences.
The Ultegra Di2 rear derailleur is one part of the Di2 system including shifters, derailleurs, battery, battery charger, and several cables/wires. The Ultegra Di2 rear derailleur is not compatible with Dura-Ace Di2 shifters or first generation cables.
Select SS for up to a 28T cog or the GS for up to a 32T cog.