The 11 speed chains are narrower than the 10 speed chains so I don't think there should be any issue with using the R3 with the old cassette. and as long as your RD is indexing fine, then I don't think there is any risk of harming it either. I'm not mechanic though, so some others that chime in may have more information that might indicate otherwise

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the world's still turning? >>>>>>> the world's still turning

Thx for the chain insight, dang I was going to grab one of my old waxed chains, thanks for the heads up on that.

Just to be clear; you want to use a bike with 11 speed shifters on the trainer which is equipped with a 10 speed cassette?

If that's the case the shifting will not be perfect (and may be poor) as the 11 speed shifter pulls the derailleur a different amount than a 10 speed.

That being said I've run my trainer bike (10 speed) on my kickr (11 speed cassette) for almost three years. The shifting is not great but I ride almost exclusively in ERG mode.

As stated above 11spd chain on 10spd cassette is ok (IME).

That's the question, I'm mostly ERG as well so I think I'm ok. Thx

You’ll be just fine. My H1 is set up as 11 speed and my road bike is 10 speed. I just choose the gear that gives as straight as possible of a chain line and don’t ever shift and only run ERG with it.

If you do that it’ll be fine.