Condensed Answer: An Ultegra R8000 rear derailleur can be combined with 6800 shifters because all of the components are designed for 11-speed road bikes.
Consequently, the rear derailleur has the needed rear shift ratio to be integrated successfully into an 11-speed road drivetrain. Adjustments, however, could be necessary to ensure smooth performance.
Rear Shift Ratio and Cable Pull
To understand the properties that affect the compatibility between a rear derailleur and a shifter, it’s necessary to become acquainted with the terms rear shift ratio and cable pull.
Modern shifters are indexed. Their travel is pre-determined and segregated into clicks. Each click equals a gear shift.
The shifter controls the rear derailleur by pulling or releasing the gear cable connecting the two units. Each click releases or pulls a pre-determined amount of gear cable to initiate a shift. That value is known as cable pull and depends on the number of speeds, the bike type (road or MTB), and the shifter’s manufacturer.
The movement of the rear derailleur is also pre-determined via the rear shift ratio.
The rear shift ratio indicates how much the rear derailleur moves per 1mm pulled or released by the shifter.
Just like the cable pull, the rear shift ratio depends on the number of speeds, the bike type, and the producer of the component. The combination of pre-determined cable pull and rear shift ratio ensures that the derailleur moves precisely up and down the cassette.
In the case of the Shimano 11-speed road derailleurs, the rear shift ratio is 1.4. This means that the derailleur moves 1.4mm per 1mm of cable pull.
The cable pullo of Shimano Road 11-speed derailleurs is 2.7mm.
And since those two values are the same across the 11-speed road groupsets, it’s possible to combine Ultegra R8000 with 6800 shifters.
Front Derailleur Compatability
The above principles apply to the front derailleur and front shifter. In other words, a front 6800 shifter works fine with a front R8000 derailleur from the perspective of cable pull and shift ratio.
However, there is a crankset difference between the R8000 and 6800 series that creates front-shifting issues.
In the case of R8000, the smaller front chainring is 0.4mm closer to the frame. Consequently, the profile of R8000 front derailleurs is slightly different so that the unit can move closer to the seat tube.
In the case of some frames, a 6800 front derailleur may fail to move sufficiently to prevent chain rubbing when combining the small chainring with the larger cogs of the cassette.
For that reason, it’s not recommended to combine a 6800 front derailleur with R8000 cranks.
6800 and R8000 cassettes are interchangeable because both series fit within the 11-speed Shimano road segment.
An incompatibility can occur if you try to mix a wide-range cassette (e.g., 11-34) with a short cage derailleur that has a maximum cog capacity of 30 or 32 teeth.
Cranks and Bottom Brackets
The bottom brackets of R8000 and 6800 are Hollowtech 2. Thus, they’re interchangeable too.
As explained above, however, the spacing of R8000 cranks is slightly different primarily to ensure a straighter chain line.
In some cases, a 6800 derailleur will be unable to move inboard enough to ensure proper shifting without chain rubbing. Thus, it would be inaccurate to say that 6800 and R8000 cranks are interchangeable.
Since both series are designed for 11-speeds, so are the chains and are therefore interchangeable. (Note: It will be necessary to re-adjust the chain length when switching chainrings and/or cassettes.)
The mechanical brake levers and calipers of R8000 and 6800 are compatible with each other.
Shimano’s compatibility chart indicates that the hydraulic ST-RS685 levers (R6800) are compatible with the BR-RS805 (R8000 calipers) too.