Can I use 11 speed drivetrain and chain with a 9 speed cassette? My smart trainer has a 9 speed cassette attached and my 9 speed bike is in the shop for some time. Thanks!
No.
Never tried it but the indexing is going to be off and will probably drive you crazy. I guess if you just keep it in 1 gear it will be fine, you might get it to work for 2-3 gears but beyond that, I wouldn’t expect much success.
Also, since you’ll likely need to adjust the cable tension to get it to work, you’ll have to readjust when you take that bike off the trainer. If indexing gears isn’t something you know how to do, I’d probably wait for the 9 speed bike to return.
The longer answer to ‘no’ (which is correct!) Is the spacing between each sprocket at the back is larger on 9 speed than 11.
So each shift on an 11 speed shifter will not pull enough cable through and so not shift the mech far enough to align to the next sprocket up or down.
As the previous reply if you’re lucky you may get a couple of gears working with some right faff. But… maybe not even that.
I’m swapping my 2012 P2 10 speed and my 2020 R3 back and forth on my trainer (Tacx) which is using a 10 speed cassette. The 11 speed shifting is a little funky but I have settled on just getting a middle gear to work and then do all my training in erg mode. So far it’s working well and the ride is pretty quiet.
I don’t know if going to a 9 speed is that much of a jump as long as you are happy with doing erg mode instead of shifting.
You can actually do it and not really damage anything, but a large number of gears will not be shifted into properly (lots of clicking when you are pedaling), so you’ll only have a handful that will actually run smoothly.
I did this by accident earlier this year when I got a new TT bike with an 11sp cassette, and ordered what I thought was a cheap 11sp cassette for my Kickr to match from Amazon, but neglected to see there were several versions of the cassette, and I had actually ordered a 9spd.
It actually wasn’t that bad as I recall - I was at first mostly disappointed that I didn’t have ‘perfect shifting’ and that the 2nd fastest gear was totally not usable, but weirdly enough I just avoided the ‘off’ gears and it worked fine. I even swapped that cassette onto a road wheelset not knowing it was a 9spd and did at least 6 hrs of riding with the totally mismatched cassette. Found out shortly after I was using the wrong one when I tried to re-order the cassette for yet another wheelset, and everything worked great one it all matched up.
I wouldn’t recommend it, but it’s not unusable. You’ll have to try it to see if on the erg you can’t stand it or not. If you ride mostly in erg mode, it actually shouldn’t be much of a problem.