Sharing bike trainer between 11spd and 8spd bikes I have an 11spd 11-30 cassette both in my bike and in my direct bike trainer.
My girlfriend recently purchased a road bike with an 8spd 11-34 cassette. What are our options for sharing the trainer? Our bike sizes are fairly different (54cm vs 44cm).


(I guess this is "easy" but not sure if practical): Swap the trainer cassettes every time: I put on the 11spd and use it, she takes it off and puts the 8spd cassette before using the trainer.
Keep the 8spd on the trainer. Will the 11spd bike work with the 8spd trainer? Maybe in ERG mode with no shifting?


What other options would you recommend?
The 11 speed shifter, derailleur and chain will not shift properly with the 8 speed cassette.

As the number of sprockets in cassettes increased, the spacing was made progressively narrower, and chains were made progressively also. Your 11 speed shifter/derailleur combination will not move the chain far enough to shift properly on an 8 speed cassette.

If you leave the shifter in a single gear it will run, but you'll have to mess with the indexing to get it correct on the sprocket. You'll then have to re-adjust the indexing when you go back to the 11 speed cassette.