Converting 105 11s mechanical to di2?

Looking at the upgrade on my bike this year, it needs new front and rear derailleurs anyway so good time. And it’s a di2 compatible frame too, so no issues there. But a few questions:

  • does anyone have experience using the 11s chainrings with the 12s chain? I have new chainrings and would prefer reuse if possible

  • it seems like all the kits are sold with brake calipers, but I already have a disc 105 group set. Would I be ok piecing the kit together without the calipers to save some $ or is there a reason to get those.

And, are there any good guides on this upgrade out there on the internet right now? My searching is coming up a bit empty right now.


My guess is that the 11 teeth are wider, so mostly likely in my opinion is:

  • They might not fit
  • They might fit, but too tight, leaving to issues.

What makes you think you need 2 new derailleurs? They’re servicable parts.

Because he wants to swap from the current 105 11-spd mechanical to 105 12-speed Di2, so they are not at all compatible.


I think his question is why he needs new derailleurs in the first place - making the switch to 12 speed more sensible. The OP notes “I need new front/rear derailleurs anyway, so good tim[ing]”

Not the exact experience but Ive ran an 11sp chain on a 10sp crank for 1000s of miles no problem. I think my bike that did 41,000 miles had an 11sp chain on the its 10 spd crank, as does my current commuter (circa 9,000miles) and my new bike which is 11sp crank failed with the problem that sh1tmano says does exist and I had no problems running a 10spd crank on it for a bit with the 11sp chai. Im not sure about SRAM it flat too system but chains are identical on the inside and only the side plates are thinner to allow them to fit into a tighter spaced cassette. If I recall some bike guru recommended running an 11spd chain on a 10 sp system to improve shifting. I expect it’ll be the same for a 12sp chain on a 11sp system.

Front one, you are probably correct that it’s fine. Rear, probably serviceable but been through a couple crashes, has probably 20-25k miles on it, and the spring is just not crisp anymore. But really I love fiddling and this is a good excuse lol. Not particularly logical given the cost of a rear 105 11s derailleur :rofl:

It’s odd to ‘need new front and rear derailleurs’. Unless bent, they will last years and years, with occasional pulley replacement in the rear. But if you want to upgrade, yes, you can piece the kit together without buying new calipers. Levers alone are a little harder to find, but I just did this myself with an upgrade to GRX levers from ~2013 era di2 levers.

The best guide I know of for di2 is the bettershifting site.

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So sounds like I could “probably” run the chainrings, but shifting might suffer and I might run some risks down the road. Given that many of the packs include chainrings, probably best to just replace again then just to be safe.

Not IME of running an 11spd chains on a 10sp systems as I say above if anything it’ll improve shifting running a lighter narrower chain :wink:

If I ever get round to upgrading to Di2/EPS I am intending to leave my current Dura Ace/ Chorus Cranks on the bikes.

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Are there any non-Shimano 12-speed chainrings out there that fit on 11-speed Shimano cranksets and will cooperate with 12-speed di2?

I don’t know if that is really going to save you a lot of money, if any. When you buy a groupset as a package, you get a discount compared to buying components piecemeal. The crank isn’t that expensive compared to the rest.

Also, Shimano makes a cheap 12-speed, non-Hollowtech II crank in addition to its 105-branded R7100-branded 12-speed crank. The cheaper crank costs $110 on amazon, including chainrings. If you are willing to shell out $33 more, you can get the more expensive R7100-branded cranks.

So overall, I don’t think this is worth it.

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