Tech FAQ: Mixing Shimano road, MTB Di2 derailleurs

Lennard Zinn addresses questions pertaining to interchanging various Shimano drivetrain parts

Dear Lennard,
I read your post earlier this year that the XTR Di2 rear derailleur would work with the Di2 front shifters. I just got the XTR rear mech [derailleur], and it won’t work. I have the new Ultegra 6870 setup (2 of them on 2 different road bikes).

I called Shimano today and got through to their tech support, and they told me that at present the road shifters are not compatible with the XTR Di2 derailleurs. I asked if they had plans to upgrade the firmware to let it work, given the large number of people who would, like me, want to use a wide rear cassette for some road mountain or tandem bikes. They said, “no current plans.” FYI, when I connect the bike to the Shimano PC interface and set the bike to “MTB,” I can get the rear XTR derailleur to be recognized along with the battery and switch box; but it says, “remove road shifters and derailleurs.” Similarly, if I designate the bike as a “road” version, it says, “remove MTB rear derailleur.” When the XTR rear mech is connected along with the regular Ultegra front derailleur and shifters, not only will the rear derailleur not work, but the front derailleur won’t work either.

Do you know a workaround that caused you to make the comment in Velo about compatibility?
— Mark

Dear Mark,
Thanks for being the guinea pig for us all on this. Your experience came as a surprise to me, since I had understood from Shimano at the XTR Di2 introduction that there would be complete compatibility between road and mountain Di2. I have yet to get any XTR Di2 myself, and I do plan to try various combinations with road components when I do.

Electronic components have doubled the possibilities for incompatibility. They have brought us not only incompatibilities based on intended use similar to those of mechanical systems (yours falls in this category), but also incompatibilities that we constantly run up against with our computers due to software revisions. When it comes to Di2, we can lose compatibility with components that had been happily working together by simply performing a software update.

No, I do not know a workaround other than to run it without a front derailleur. That’s probably not what you want to do, given your desire to greatly widen your gear range.

I got some answers from Shimano about this, and I now know that you cannot mix any road FD (front derailleur) with an XTR Di2 (11-speed) rear derailleur (RD-M9050); it won’t work with any Di2 shifter, road or MTB. This is similar to incompatibility to trying to mix numbers of speeds, as it won’t work to mix any Di2 FD/RD road derailleurs that aren’t both 10- or 11-speed. (Actually, it may work to mix, for instance, an Ultegra FD-6770 front derailleur with an RD-6870 rear derailleur until you update the battery software. After that, it will never work again.)

You can run a full XTR Di2 drivetrain with your Di2 road levers (you could use any Di2 ST or SW Di2 switch, like climbing, sprint, or TT switches with it). That would be analogous to mixing 10-speed ST-6770 (Ultegra Di2 shifters) with 11-speed RD/FD, or 11-speed ST-6870 (Ultegra Di2) or ST-9070 (Dura-Ace Di2) with 10-speed FD/RD-6770 (original Ultegra Di2). Both of these road combinations do work.

You could also run a 1×11 system with that XTR RD. The FD is not needed to make the system work. You could use any ST or SW with it.
― Lennard

Mixing 9-, 10-speed components

Dear Lennard,
I have a Shimano 9-speed triple setup on my hill road bike using an XTR RD and 11/34 XTR cassette. I would like to switch to Shimano 6603 shifters, with a 105 triple FD 10-speed. Will the 6603 shifters function with a 9-speed RD and a XTR 10-speed cassette 11/34? Or will I need a long-cage road RD like RD 6700?
— Dave

Dear Dave,
Yes, you can use a 9-speed XTR rear derailleur with 10-speed Shimano shifters, chain and cassette.
― Lennard