Shimano Ultegra Di2 RD-6770 Rear Derailleur

Even as the lust remains for their first electronic groupset, Shimano hits us with Ultegra Di2. We don't feel like they've muddied the water, only that there's a value priced option for those of us who couldn't make the jump to Dura-Ace Di2. This Ultegra Di2 RD-6770 Rear Derailleur offers the same level of performance as the higher priced unit, and that is exactly why it's so appealing. The difference is mainly one of aesthetics and a few grams here and there. Where Dura-Ace Di2 uses custom servo motors, made exactly to Shimano's specs, their Ultegra Di2 derailleurs use off-the-shelf motors to drive the gear changes. The upsized internals determine the larger overall package dimension. You won't look at the Ultegra Di2 rear derailleur and scoff at the size of the housing for the servo motor; it's just not that noticeable. However, if you laid the two side by side, you'd see that Dura-Ace is slimmer and sleeker. On the scale the difference would amount to about 50 grams -- the generic motors being larger and a bit heavier.As you could expect, there are a few major changes between this electronic version and the mechanical Ultegra derailleur. The wire goes into the rear of the derailleur at a right angle to how the cable housing normally attaches, but there remain two limit screws to set the overall swing of the parallelogram. The derailleur spring is gone, replaced by a motor-driven worm drive. Shifting is pushed or pulled a specified distance, in a "programmed motion." This means that the derailleur travels a specified distance between cogs for each shift. It also means that the derailleur will derail the chain, drop or lift it onto the next cog, and then it will self-adjust back to the center. A feature of mechanical rear derailleurs is that they move inward when hit from the outside. In keeping with this, Shimano has developed a "servo saving feature" that allows the derailleur to move inward upon impact without bending the derailleur or wrecking the worm drive.