Are you going to resize the chain at all or leave it as is? If you leave it as is, yeah you're going to have suboptimal shifting.
this post was submitted on 05 Aug 2025
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I'd size it appropriately for the bike. The shifting concern is whether the lesser external width would cause issues as it tries to climb the cassette sprockets.
Probably won't make a difference. I run 10 on 11 and have been using no name quick links for years now. They are wide enough they might even work on 8 speed, though 8 to 9 was a bigger width difference than any of them IIRC. Back 10 years ago 10 and 11 speed were actually different. I rode a Shimano rep's 9000 DA bike once and holy f bomb that was impressive shifting in all conditions.
The main thing that will not likely work together is the shuttle ramps on the cassette and chainrings like a properly matched system. You'll be relying on the pins and leverage force. The chain my be harder to shift under load. As it wears it will become harder to firmly and consistently get the correct cog for the shift index, and that will accelerate wear at the end of life and on the cassette. Any mixed drivetrain will likely have this issue. Most bikes ship with mixed drivetrains when new or at least they did a decade ago. So it shouldn't be a huge deal.
You'll be more likely to have rubbing on the end cogs in your FD cage too.
Thanks for the info. I think I'm gonna stick with the 9-speed chain. It's KMC so it's not garbage and it should engage the ramps better than 11. Ordered a Connex link to make it removable for waxing.
I got a couple packs of "ebike" chain that was labelled as 9/10/11s a while back from a friend who got a belt drive after his bike was totaled. I was skeptical but I put it on an electric mid drive 1x9 and human power 2x11 and it seems fine for both and derailleur adjustments seemed normal and unproblematic. I have yet to break any chain in good condition on a manual bike, but the ebike is doing well even in some bullshit load situations after wrecking several generic chains.