The Shimano RD-9070-SS is an electronic Di2 rear derailleur from the Dura-Ace 9070 series, designed for 11-speed road bikes. It mounts to a standard derailleur hanger and pairs with Shimano Di2 shifters and batteries for electronic shifting. The short cage suits racers and riders prioritizing low weight and fast operation over wide gear range, handling cassettes up to 28T max cog, 16T max front difference, and 33T total capacity.

Road cyclists building or upgrading electronic drivetrains choose this for its light 215g weight from carbon link plate and titanium hardware, plus features like multi-shift programming via E-Tube software. It offers quick shifts under load compared to mechanical derailleurs, ideal for sprints and climbs, though lacks a clutch so chains may slap on rough roads. Install with care per Shimano specs to avoid B-tension issues; compatible with Dura-Ace 9000/9070 components.