Did you know most 10-speed and 11-speed Di2 hardware is cross-series compatible? You can actually mix different series and even different generations of Di2 hardware, as long as you make sure you follow the Di2 compatibility chart!
Hidden somewhere on the Shimano E-Tube Project website is a Di2/e-Tube compatibility chart. This useful PDF helps you select your components, making sure they're compatible before spending $$$ and installing them on your bike.
Want to use the mountain bike Di2 display on your road bike? You can!
Feel like upgrading your derailleurs but not your shifters? Sure! You can!
12-speed Di2 compatibility
Before I get started though, a quick note on 12-speed Di2 compatibility. Shimano released their 12-speed DURA-ACE and Ultegra Di2 groupsets to the world on September 1st, 2021.
While Shimano has been pretty good with backward compatibility in the past, these new groupsets actually feature very limited compatibility with the older Di2 series. There is a page on 12-speed compatibility, so read that if you're looking for 12-speed Di2 information.
The remainder of this page applies to 11-speed and 10-speed Di2 compatibility, not the newer 12-speed Di2.
Compatibility chart? Where? How does it work?
The latest version of the compatibility chart can be found on the Shimano e-Tube project website. At the time of writing the latest version of the chart is 5.0.1.
In the image above I have selected the SM-EW90 Junction box, R9150-series Dura-Ace shifters and two 11-speed derailleurs; the RD-6870 Ultegra rear derailleur and the Ultegra FD-R8050 front derailleur. Note that I could also have selected the Dura-Ace FD-R9150 front derailleur if I wanted to.
Shimano recommends you use components all from the same series, because that’s when they perform best. However, this isn’t feasible for everyone and fortunately, they allow you to use different generation components on the same bike..
The only thing you can never do is mix different speed derailleurs or mtb / gravel / road derailleurs. All derailleurs have to support either 10-speed or 11-speed and from the same ‘category’.
Mixing MTB and Road/GRX components
So.. can you use mountain bike di2 hardware with road hardware? Sort of… it’s complicated. You can definitely use road shifters on a mountain bike – but would you really want to? However, let’s say you wanted to build a 1×11 Di2 setup? You’d probably want to take advantage of the Clutch System Shimano equipped the mountain bike rear derailleurs with, but use your road shifters. You should be able to do this no problem! Alternatively you can use a GRX rear derailleur with road shifters, or the Ultegra RX805 clutch rear derailleur.
The only real limitation is that derailleurs you use have to be either road/gravel or mountain bike derailleurs – you cannot mix these derailleurs. Other than that you’re free to do whatever you want.
E-Bikes, Di2 hubs, motor units
While the hubs are Di2-specific, they are not actually connected to the Di2 system. They're just designed to work with the Di2 motor units and can handle the forces the motor units produce.
When it comes to compatibility though, looking up information on these units can be a little confusing. If you look up the hub, you'll find only a couple of entries.
So what gives? Are the hubs only compatible with Bosch E-Bikes? Are the hubs not compatible with current parts at all?
Don't worry - everything is fine. You see, for compatibility purposes, that hub does not matter at all. Di2 compatibility for these systems is determined by the drive unit. Ignore the hub, just look at the MU-UR500/510 and the MU-S705 entries instead.
Like you’ve read above, I've linked the Shimano Di2 compatibility charts. Click the image below to view the full compatibility chart PDF. Note that the pages relevant to your setup depend on which series hardware and, perhaps more importantly, which battery or battery mount you’re using.