Dura Ace Di2 with SRAM XX mountain crankset

Di2 and a custom Seven -- doesn't get much better

Alternate gearing is really popular especially here out west where we have lots of hills to contend with.  The triple crank used to be standard fare, but the advent of the compact crankset, and the improvements it brought (smoother shifting, better chainline, less weight) started the extinction of the triple.

The one downside of a compact, however, is that you can't get gearing quite as low as a triple so some triple-to-compact converts were left wanting more.

The use of mountain bike cassettes with road drivetrains solved this issue, though and the idea isn't new -- when everything was nine speed it was pretty easy to do with the right choice in components.  The switch to 10-speed temporarily inhibited these setups, but work-arounds were found and soon enough SRAM developed their 10-speed mountain group and made it fully compatible with their road group -- a great idea in my mind.  So I had a number of clients do SRAM Red drivetrains (compact cranks, shifters, etc) and use a SRAM XX 11-36 cassette with a SRAM XX derailleur.  Now with their WiFli setup being ubiquitous, this is even easier to do now with many more component choices.

One drawback to using a mountain cassette and a road compact crank, at least from the standpoint of a road rider, is the large steps between gear shifts on an 11-36 or 11-32 cassette; compared to an 11-23/25/28.  A few years ago, I built a custom Seven (Ti/carbon) for a client who wanted the low gearing without the big steps between gears, and we settled on a hybrid solution -- we went with a SRAM Red 11-28 cassette but instead of the SRAM Red compact crankset, we settled on a SRAM XX mountain cranks with the 28-42 chainring combination.  We still included a mountain component but we did it from the opposite side that you'll normally see.

Climbing shifter added

regular Di2 der with 12-27 cassette

This provided him with the low gearing he wanted (and he had little use for the high end gears of a road crankset anyway) and the small steps between gears.  It worked flawlessly and for 3 years he's been putting many thousands of miles on this setup without any difficulty.

Recently he came in to see me with questions about the functionality of Dura Ace's Di2 electronic setup.  I told him the reviews were fairly glowing -- not only does it sport a high "cool" factor, it works exceptionally well.

The question would be could we get his desired gearing on it?

I did some research and found that a few people had gotten it to function with a 32-tooth cassette in back, but this presented the "big gear step" problem as before.  Then I remembered that K-Edge had modified a setup for mountain bikes, calling it Ki2 -- they adapted the rear derailleur by custom designing a new, longer pulley arm.  After looking at it in depth and a phone call or two it was clear they hadn't modified the front derailleur at all, and it was shifting an XTR 2X crankset (which I believe was 26-39).  I already knew they Di2 front derailleur could shift a 16-tooth difference between chainrings (which is standard compact design 34-50) but with my client's 28-42 SRAM XX crankset, we were only dealing with a 14-tooth drop.

Dura Ace 7970 front chainrings are thicker and stiffer than most -- would the SRAM XXs be stiff enough?

SRAM XX was designed to shift ultra-crisp so the rings are 2mm thicker than most and the special bolt attachment (the BCD) is set wider than regular mountain cranks to further stiffen the drivetrain.

I was pretty confident that it would shift perfectly.  But you never can tell with mismatched components, and if we went ahead on this, I was on  the hook for a few thousand dollars worth of components if it didn't work -- not to mention all the work of repeated tearing down and rebuilding with his SRAM Red components if this experiment failed.

Well, long story sorta shorter, is that it works great.  The Di2 front derailleur shifts perfectly over the SRAM XX crankset.  No modifications or calisthenics necessary to get it working.  Check it out:

Even though the bike is all done and waiting for it's owner to return for it, I can't help but go over to it and take the shifting through it paces just to see it work.  About every 10 minutes I've been compulsively returning to it, just because its fun to see a custom project come together so well.  Nerd, I know.