The Shimano ULTEGRA RD-R8150 Di2 Rear Derailleur 12s boasts Shimano's fastest performance yet with its new 12-speed design and versatile single cage option, while being compatible with 28-34T low sprockets.
Its Shadow design allows for a 58% increase in speed compared to the RD-R9150 model, and reducing the extension(stick out) profile to 33mm from 45mm, -12mm.

SHIMANO SHADOW RD

The mechanism rear derailleur, Shimano Shadow RD, is intended for more aggressive riding. Its super low profile design carries many benefits. Because of its low profile and single tension construction, the derailleur does not hit the chain stay in rough riding conditions. The result is smooth and silent performance.