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Technical FAQ: Di2 XTR opens up gear options for road bikes

Shimano XTR on tandem bikes

Dear Lennard,
Many of us road tandem riders have been waiting for an electronic option.

So do you think the new Electronic XTR derailleurs are compatible with Di2 road shifters? Could I have true electronic 3×11 (or 10) on my road tandem?
— Al

Dear Al,
Yes, XTR Di2 derailleurs are compatible with Di2 road levers, so you can install an XTR Di2 rear derailleur onto your road Di2 tandem. By eliminating all of that cable stretch and friction, this Di2 option will make for quicker and crisper shifting on a wide-gear-range tandem than has ever existed before.

But you can forget about having “a true electronic 3×11 (or 10),” because there is no road Di2 lever with three clicks. So you would have to go to a 2X system. And skip to the next question below for details about making it 2 X 11 or 2 X 10.

This XTR/road Di2 combination can offer gear options like a 34-50T crankset with an 11-40T cassette, which is a 120-inch top gear with a 22-inch low gear. And if that’s not low enough, I’ll bet with perhaps only some extra turns of the rear derailleur b-screw, you can use that system on a SRAM XD freehub driver body with an XX or XX1 10-42T cogset. Then you could use the XTR 28-38T crankset and still have a 100-inch high gear (the equivalent of a 46 X 12) along with an 18-inch low gear (equivalent to a 24 X 34). This is an amazing range for tandem riders.
― Lennard

Compatibility between 10-, 11-speed Di2

Dear Lennard,
After my buddy’s Dura-Ace lever failed on him, he inquired about going electronic at our favorite LBS. They told him something interesting that relates to a question I had asked you — can you use 11-speed levers with 10-speed electronic? They said yes — the derailleur is the controlling item. The implication was that the shifter just tells the derailleur to go up and down, so that it doesn’t matter if the shifter itself is a newer 11-speed or an older 10-speed electronic lever. Do you think this might be true?
— Steve

Dear Steve,
Yes, 11-speed Dura-Ace 9070 Di2 or Ultegra Di2 is 100 percent compatible with 10-speed Ultegra Di2. Your LBS is correct that the rear derailleur determines the 10-speed or 11-speed function, and the lever is simply a switch telling it which direction to move. However, you cannot upgrade original Dura-Ace 7970 10-speed Di2 this way, because it is completely incompatible with the current Shimano plug-and-play wiring platform.
― Lennard

Ultegra Di2 TT shifters

Dear Lennard,
I’m trying to retrofit a 10-speed with the newest Ultegra 6871 TT shifters — technically 11-speed stuff. Have you had any new experience rigging this to shift a 10-speed system other than what you wrote about earlier? Specifically wondering about cassette spacing and chainring compatibility.
— Alex

Dear Alex,
Yes, you can use 11-speed Ultegra 6871 Di2 TT shifters with a 10-speed Ultegra Di2 rear derailleur and 10-speed cogset. You won’t need to change anything with the cog spacing. As I said above, you cannot do this with original Dura-Ace 7970 10-speed Di2, because it won’t plug into the system.

The front shifting will be unchanged going from a Di2 road lever to a Di2 TT shifter. You’re not changing the chain, front derailleur, or crank; you’re merely changing the switch that tells the front derailleur when and in which direction to shift.
― Lennard

Small chainring options for Dura-Ace 9000

Dear Lennard,
I am about to build up a new Cannondale Evo Hi Mod with 9000 series Dura-Ace. Mostly a 53/39 crank will be fine, but I’m wondering whether it will shift OK if I were to use a 36-tooth small ring (for a couple of local races). I know this is outside Shimano’s recommended 16-tooth maximum [difference], but I don’t want to use a 12-28 cassette, and I’ve heard of it being run on some bikes.
— Ben

Dear Ben,
Well, I saw Alexander Vinokourov use that combo seemingly without problems in the 2010 Giro on the Monte Zoncolan stage. The timing of the shift ramps and pins on the chainrings will of course not be ideal, since they will not have been designed for that ring pairing, so you can’t expect the same kind of smoothness and speed of front shifting that you can expect with a “system” drivetrain. I’m sure it will shift, even if not quickly. You can avert potential pitfalls if you were to use an inner stop and avoid small-small cross gears.
― Lennard