I've been told by many shops that the Ultegra rear derailleur can only handle an 11-32 without the roadlink being used. I almost picked up the 11-34, but was told it might have issues, as the Ultegra 8000 mid-cage was built for that cassette. I think you'd need to use the XT or XTR rear derailleur for the 11-40 or 11-42 cassette. If you, as I do, have a Di2 front derailleur, you're kind of stuck with 11-32/34 (maybe a 36 with the roadlink), at least until Shimano allows the road shifters/front derailleur to communicate with a mountain rear derailleur. I may be completely off on this, if so, I'd also like additional information for a 2x system working with larger range cassettes.
mr_slow, thanks for responding! I am running the Ultegra 6870 Di2 RD, and have been running the 11-36 SRAM cassette for some time with zero issues, you just have to adjust the B screw appropriately. That said, I found out that the 11-36 road cassette wouldn't work on the 9020 MTB freewheel (now obvious to me), so I opted for the new Ultegra 800 11-34 cassette which does fit nicely on the 9020 freewheel. Shifting is perfect, and I am really stoked about this wheelset. Hopefully I won't be disappointed in longevity. What would be even more exciting is if I could run the XT or XTR 11-40 cassette without the Roadlink since I switch out the 650b wheels for the 700c road wheels. Surely someone on here has tried this! Lemme know!
I've been thinking about going with Ultegra Di2 Brake shifters and XT Di2 front and rear der's. This setup apparently is compatible and will allow to run wide spaced 11 speed mtn. bike cassettes. The problem is the XT Di2 has a maximum chainring of 38T. While this fine for mountain bikes -- it sucks for gravel riding. Unfortunately, you can't mix an XT Di2 rear der. with an Ultegra Di2 front der. BTW, I've been running a Ultegra 6800 mech. mid-cage rear der. with a Wolftooth Roadlink and an 11 speed 11-40 mountain bike cassette without any problem. This is on my back up gravel bike which I've set up as a 1x, using a 40T Wolftooth 1x chainring. I may switch to a 36 tooth chainring to get even lower gearing, and slightly lower gear ratio gaps.
I'm running an 11 speed 11-40 cassette with DI2 Ultegra derailluers and R785 levers with a Wolf Tooth road link. The bike is a Seven Evergreen. Everything seems to shift OK. On the front end there's a 46/30 Sugino crank.
