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Technical FAQ: How does Di2 know?

Shimano's Wayne Stetina talks 11-speed Di2 conversions and Zinn answers a question about crank length's impact on bike fit

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Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
A few weeks back, there was a photo taken at the Amgen Tour of California of a Dura-Ace Di2 bike with a 10-speed cassette (I can’t find the link; bike was the Shimano NA head).

It brings up the interesting question. Could an 11-speed Di2 shifter be placed in a 10-speed mode? It seems that if the derailleur is a servo, the controller could do any number of spacing. Is this what we saw in the photo? I guess it would be up to Shimano if they wanted to support this, but it would make the upgrade much easier for those who have a pile of wheels.

Dear Jeff,
It was Wayne Stetina’s bike you saw in the photos, and he rode most of every stage of the Amgen Tour of California ahead of the race. He’s Shimano’s R&D director, and those of you who’ve been around know that he is not only very fast now despite approaching 60, but he also has impressive racing palmarès extending back to the early 1970s. He also likes to tinker and experiment with his bike, and the bike he had at the Amgen Tour of California is a good example. The things he did to his drivetrain are by no means recommended by Shimano or in any way endorsed or suggested or advertised by the company.

First of all, there is no 11-speed Shimano for sale yet, and there is no 11-speed 9070 Di2 available for any field testing yet. Only Mechanical 9000 is out in the field.  Stetina was riding 9000 mechanical at the Amgen Tour of California.

During the Amgen Tour of California, Stetina was riding WH-7900 wheels. The spline is 1.85mm — too short to engage 11th cog. So he was riding a CS-9000 11-28 cassette as a 10-speed 12-28 with a 1mm 10-speed spacer behind the 28T to prevent the cassette lockring bottoming before the cassette is tight (due to the narrower spacing 11-speed spacing).

When Stetina tried the CS-7800/7900 cassette on a 9000 bike just to see how many gears would work, he found that it shifted as well as a CS-9000 cassette. He found it to have a wider adjustable range than the 7900 drivetrain in spite of the wrong spacing, because the narrower chain doesn’t touch larger sprockets in the 13/14 when adjusted too tight in order to be tight enough to avoid autoshift out of the larger cogs. During the Mt. Baldy stage, Stetina rode a 10-speed XT 11-32 modified to 12-32 for the steep final pitches above Baldy Village. He reported flawless shifting with the B-Tension screw maxed on the shortest rear derailleur hanger available from Specialized. 

To answer the question Jeff asked about Di2, the system needs to be told if it’s a 10-speed or 11-speed rear derailleur. The shift lever will shift either. Another Shimano employee tried and, according to Stetina, he could not re-program his RD-6770 (Ultegra Di2) to shift 11-speed cassette spacing (with a CN-9000 chain). I have yet to receive or test or wrench on an 11-speed Shimano group, mechanical or Di2 (the latter is months away), so it’s unknown at this time if the RD-9070 Di2 rear derailleur can be reprogrammed to shift 10-speed spacing. But using the limit screws and what Stetina’s bike at ATOC demonstrates about 9000 mechanical, I am confident that I can adjust 11-speed Di2 to shift a 10-speed cassette made from an 11-speed one (thus usable on any wheel with a Shimano-compatible freehub) as Stetina has done, including 12-32 for ultra low range climbing gears, just as well as his 9000 mechanical shifted with such a cassette. 
― Lennard 

Dear Lennard,
I recently had an expensive bike fit (Retül) done with my primary bike of the past several years which has a 170mm crankset. Due to a set of mechanical issues with this bike that were going to cost a lot to fix, I decided to purchase a new bike and sell my old one. However, my new bike came stock with 172.5mm cranks. I’m wondering if it’s possible to calculate what my new fit should be given that crank length is the only dimension of my fit which I can’t duplicate from my previous bike.

The guy who did the fitting for me insisted that my overall fit would “completely change” with my longer cranks, but I’m wondering if I couldn’t simply adjust my seat height and setback to compensate for the slightly longer cranks. I purchased my new bike online and would rather not pay for an additional bike fit at a local shop or purchase a new 170mm crankset. In case you’re wondering, I’m 5’9″. I purchased my previous bike used, and it came with 170mm cranks. There’s no reason from a bike fit perspective why I “need” 170mm cranks. In fact, 172.5mm cranks probably make more sense. Does 2.5mm in additional crank arm length really “change everything” in my fit as my bike fitter insisted?

Dear John,
Well, I do a lot of testing of different crank lengths AND I’m very particular about my riding position. And I find that, over a considerably greater length difference than a mere 2.5mm, I can push my saddle forward with a longer crank half the length difference and then lower my saddle to my old seat height less the length difference, and it works well for me. Obviously, I do try to keep the reach from saddle to handlebar about the same, so sometimes a longer stem is required. I generally don’t lower the stem along with the saddle, since the longer crank makes my knees come up higher, hence tightening the hip angle and the hamstrings accordingly. I find it feels generally just about right without changing handlebar height.

I would at least try that, if I were you. If you find it to not be to your liking, you can always invest in another professional fit down the road.