Condensed answer: Ultegra 6870 and Ultegra 8050 are high-end 11-speed road group sets by Shimano. The cable pulls and the rear shift ratios as well as the used technology of both group sets are compatible. Therefore, it’s possible to mix Ultegra 6870 and 8050 components.
It’s important to be familiar with the following terms and principles.
- Indexed Shifting
Modern shifters are indexed. This means that the movement of the shifter from its starting to the end position is segregated into separate clicks that indicate a shift.
In the case of standard mechanical shifters, each click pulls or releases a certain amount of gear cable to trigger a shift. The amount of released or pulled cable is known as cable pull. Its value depends on the number of gears, the bike type (road or MTB), and the producer.
Electronic shifters such as the ones part of Ultegra’s Di2 do not pull a cable. Instead, they control electronic motors located in the front and rear derailleurs. Each click sends an “impulse” via the wiring going from the shifter to the respective derailleur. Each impulse triggers a down or an upshift.
Just like the cable pull version, each click triggers a pre-determined movement of the rear derailleur that the user has no effect over. The rider simply presses the lever to get on another gear.
- Rear Shift Ratio
Derailleurs have a rear shift ratio that indicates how much the derailleur moves per 1mm of cable that’s pulled or released by the shifter. The rear shift ratio depends on the number of gears, the bike type, and the component manufacturer.
Electronic derailleurs work differently, as they are controlled by electric motors. However, they are also programmed to move a pre-determined amount per 1 click of the shifter. If this wasn’t the case, shifting would be inaccurate and poor.
Since the cogs of 11-speed cassettes are spaced the same whether one uses mechanical or electronic shifters, each click of the Di2 system triggers a movement of the rear derailleur matching that of a hypothetical mechanical derailleur.
Shimano Compatibility Chart
Shimano’s compatibility chart indicates that Ultegra’s 6870 shifters are compatible with both Ultegra’s 8050 front and rear derailleurs.
FAQ: What about the wires, junctions, and batteries?
In most cases, those parts are compatible too. However, to be sure, it’s worth it to consult the latest e-tube compatibility chart from Shimano. (Currently, this is the latest version.)
The chart could be somewhat tricky to read, but if you follow the guide below, you will figure it out fairly quickly.
The first rule is that all parts of the chain (starting from the battery) must be shown as compatible in the chart or else the whole chain fails. (The manual states this in the beginning.)
The chain is as follows:
Battery – Junction (A) – Shifters – Rear Derailleur + Front Derailleur – Linkage Device – Wireless Unit
For example, if you want to combine the 6870 series with the 8050, you have the following viable paths:
Step 1: Battery
If you have an external battery, you can combine it will all the possible junctions (A) in the table.
If you have an internal battery, you can’t use it with junction SM-EW67A-E.
Step 2: Shifters and Derailleurs
Once you have determined that your battery and junction A are compatible, it’s time to check the shifters.
The chart indicates that the left (ST-6870-L) and the right (ST-6870-R) shifters from the 6870 series are compatible with respectively the FD-R8050 front derailleur and RD-R8050 rear derailleur.
Step 3: Linkage Device and Wireless Unit
The final step is to determine whether your linkage device and wireless unit complete the chain.