I’m staying with my sister she has her bike on her trainer which is 11 speed. She has a 10 speed bike that might fit me. Would this fit well enough to stay in one gear and just use erg mode? Wouldn’t need to change gear just as long as one fits.
Almost certainly not an issue, just pick a gear combination that is reasonably quiet and stay there.
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#cool thanks, thats kind of what I thought
Oh yes, there is also the possibility that one of the cassette or chain in use is quite worn and the other isn’t, mostly an issue if it is the cassette that is worn as it will cause a newer chain to skip, but that too is usually limited to an individual cog or two.
Only other issue might be cassette size and chain length, but if you pick a cog that’s in the middle of the cassette (not one of the bigger ones), that’s not likely to be an issue either.
Huh… Why not do it properly? What will it need, a 10 speed cassette on the “11 speed” trainer?
Possibly because he’s just visiting and didn’t bring an extra cassette or the necessary tools.
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10 speed chains are also a hair wider on the outside. Same on the inside. If there seems to be unexpected chain noise with proper adjustment, the largest cog will have the least chance of rubbing.
The internal widths of 10sp and 11sp chains are the same, it’s the external width that’s different. But that’s a good point, the chain width may cause rubbing. If the chain fits okay around the big cog, or the smallest cog of course, that would be less rubbing.
The reason I didn’t want to change the cassette was because I’m just visiting, not taking my own tools or kit etc and if it worked then I might use it otherwise I wasn’t too bothered.
But I just put the saddle up really high on my sister’s bike I stead, a little cramped but leg length was fine for a quick spin 
If I follow Paul’s reasoning correctly possibly not the smallest cog of course 
It’s the space between cogs that’s the concern–that space will be smaller on an 11sp cassette, and a 10sp chain will be wider. So the goal would be eliminating rubbing on the neighbor cog. If your biggest cog is sufficiently bigger than the next cog down, there won’t be rubbing at all, as the chain won’t overlap the teeth of the next smallest cog. But if there’s little difference between them (say only 2 teeth difference), you might still be at risk of some rubbing, even with the chain on the biggest cog. At that point, chainline would be where I’d look. If you’re running on the big ring up front, cross chaining to the biggest cog in back won’t be great. But running small-big (small in front, big in back) might not get you a terribly satisfying experience, erg mode or not. Unless you’re the most ‘spin it to win it’ sort of rider 
So big-small might be a better riding experience, and might be a straighter chainline, which would cause the chain to rub less on the nearest cog.
The difference between 10sp and 11sp spacing isn’t so tight that the chain will automatically rub at all times. It’s largely a matter of chainline. Take a 10sp chain straight off an 11sp cog, and there’s every chance you won’t have a problem at all.
Yeah, I meant to suggest this option as well but forgot, indoors it is really only the distance from the bottom bracket (or pedals) to the saddle that is critical. The bars will be in the wrong place but you can just keep your hands off them…
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Yeah, definitely wouldn’t recommend it long term (or rising out of the saddle, my knees hit the handlebars!) But fine for a bit while the niece was being fed/being put down for naps and I had a bit of time to kill.