With the move to 12-Speed, and many other changes across the component range, with the updated Di2 system, there is no more crucial component to the overall use and adjustment of the shifting system than the rear derailleur, and with the Ultegra R8100-series this workhorse component delivers. Shimano dedicated years of research, development, and testing to refine this Di2 road system and the RD-R8150 Rear Derailleur features prominently in terms of performance, capability, and actually replaces the Junction A box system of the previous versions. At 262 grams (listed), it is also reasonably light weight while still housing the new charging port, LED indicator, D-Fly communication system, and Function button. Ultegra components are known for durable materials, consistent flawless performance and functionality. The materials have been selected for the best combination of these characteristics, yet the overall function and design come right down from the Dura-Ace version. This R8150 Di2 shifting system delivers 58% faster rear shifting compared to the R8100 Di2 and while some may point out that Shimano continued with a partially wired system over full wireless, the engineering rationale has been strong and sound based on battery requirements.

In the R8150 Di2 set-up the shifter levers, which have a very low power requirement, employ a secure wireless communication system amply powered via coin cell batteries in each shifter body. The front and rear derailleurs handle the motors and actual shifting action and they are still powered by a new cylindrical internal seatpost battery (BT-DN300) which features three wiring ports for the SD300 e-Tube wires. Shimano rates the battery level at about 2000km of use, meaning you rarely have to charge the battery, which is much more powerful. As such, each derailleur has it's own wire that goes directly to the battery. Why is there a third wire port on the BT-DN300? Well, Shimano allows you wire the shifters in as well if you choose, but with so many cockpit set-ups now requiring a fully internal routing into the fork from the bar-stem, and down into the frame, the wireless option is the simplest and least challenging way to set-up your R8150 Di2 system. By moving the charging and control access to the rear derailleur, Shimano also delivers easier adjustment, a more stable charging connection, and overall simplicity that current Di2 users will deeply appreciate.

Now built in a single cage length model that can handle 12-speed Hyperglide+ road cassettes in the 11-28, 11-30, and 11-34 tooth cog options, the R8150 ensures road cyclists can meet their higher cadence pedaling styles and overall gearing range requirements in conjunction with the proper chaining pairing. The 11T composite Guide and Tension pulleys spin smooth and fast, while offering serious longevity and chain engagement. These ride in a strong and resilient cage, with both plates in aluminum that deflects torsion and allows for a specific shaping that ensure proper alignment. Steel hardware, a steel B-axle, and four flouric coated link bushings keep the alloy and GFRP body construction functioning and moving with precision and fluidity for thousands of miles of reliable performance.

Along with the release of the R8100-series components, Shimano also created their new generation E-Tube app which truly simplifies the set-up, shift button customization, battery level check, and diagnostic process, with a much more effective user interface for iOS and Android devices via the system's Bluetooth Low Energy communication channel. While an upgraded e-Tube PC site is still an option, most users find the app to be the easiest option available.