Shimano Ultegra R8150 Di2 12-Speed Rear Derailleur The R8150 Di2 12-Speed Rear Derailleur is the heart of the latest Shimano Ultegra electronic drivetrain. It now fulfils the job of the junction box and offers Bluetooth and ANT+ connectivity as well as changing the gears. It houses the recharging point for the system’s battery and receives the wireless signals from the shifters for both derailleurs. Connection to Shimano’s app allows some shifting functions to be altered and customised to personal preference, simplifying shifting and allowing the rider to get on with the job of pedalling the bike. Shifting itself is extremely fast and accurate, with the derailleur swapping sprockets at the lightest touch of the shifter’s buttons, helped by four self-lubricating micro-machined fluorine-coated link pivot bushings. The Shadow design is low profile to keep the derailleur out of harm's way whilst a ‘fuse’ function ensures that it remains protected in the event of an accident or impact. There is now only one rear derailleur in the Ultegra groupset, which will work with 12-speed cassettes with up to 34 teeth on the largest sprocket.