Shimano Ultegra RD-R8150 Di2 Rear Derailleur - 12-Speed, Medium Cage, Black

The RD-R8150 12-speed rear derailleur is the central element of the Shimano Ultegra Di2 road bike groupset when it comes to semi-wireless as well as fully-wired shifting. It has even more to offer than a conventional rear derailleur, since it communicates wirelessly with the shift/brake levers via the built-in wireless chip in the disc brake version. In the rim brake version with a cable connection to the STIs, it is responsible for coupling the components. This is done via a function button, which is also used to adjust the rear derailleur. You can also use the button to check the battery level, change the shift mode from manual to Synchro and Semi-Synchro Shift, and establish a Bluetooth connection to a bike computer or smartphone. An LED display helps you to do this. The rear derailleur is connected to the battery via a cable, from which a cable also runs to the front derailleur and possibly to the STIs. It has a charging port that allows you to easily recharge your circuit using the matching charging cable.