Hi there any thoughts or advice on this would be great.
My other half wants to get in to training on her bike Indoors, to get fitter to ride better outside.
We would both ideally like to upgrade the dumb trainer we have currently been using, to a smart direct drive trainer, because she doesn’t have a power meter.
The sticking point is that she has a 9 speed bike and I have 11 speed.
It’s a bit of a headache I know.
It’s possible.
Would you consider rollers instead?
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If you’re both just going to be doing erg workouts then you might be able to get away with it, as you don’t need to change gear. A 9 speed chain may struggle running on an 11 speed cassette (the chain is wider than an 11 speed chain), but an 11 speed chain will run (on a single cog with no gear changing) on a 9 speed cassette.
Assume you mean you have direct drive dumb trainer? Because if it’s wheel-on, there will obviously be no problem at all.
At home we share a trainer between 3 of us, one is one 10 speed, 2 11 speed. just go up and down a couple gears until the gear meshes well. we use erg or resistance mode anyway so gears are not needed!
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Is it worth just swapping out the casettes? That doesn’t seem like too much hassle to me…
Yeah, a direct drive trainer won’t work easily in that situation because 9-speed chains are quite a bit thicker/wider than 11-speed chains. It won’t line up with the cassette properly.
I think the easiest solution would be a wheel-on smart trainer like a Kickr Snap or Tacx Vortex or Flow. I would also suggest getting a couple of trainer wheels and tires, which also means an extra cassette for each bike (if you don’t already have this stuff), but ultimately it’ll still be a wash in terms of cost compared to a direct drive smart trainer anyway.
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Both 11 speed and 9 speed chains have an internal width of 3.2mm. Only the outer width is narrower on 11 speed. That means an 11 speed chain would work just fine on a 9 speed cassette on at least one of the gears. You might have to adjust a barrel adjuster a tiny bit to get a perfect match, but at least I have to do that anyways between my 2 wheelsets and the kickr core.
I wouldn’t hesitate to go with a 9 speed cassette on the trainer if I was planning on doing all my training in erg mode and hence not needing to switch gears. I wouldn’t want to switch cassettes before a trainer ride, it would be just one more possible excuse to skip a training session.
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Ah, that’s a good point. I was thinking about this the other way around—an 11-speed cassette wouldn’t work as well (or at all?) with a 9-speed chain.
Good call.
I’ve actually got a 9 speed and 11 speed bike here… And old chains for both of them. I just did some tests:
9 speed chain on 11 speed cassette: won’t fit for example on 12 tooth cog due to 11 and 13 being too close on the sides. You might get away with for example on 28 cog since the 25 next to it is so small it doesn’t get in the way 
. Very sketchy though.
11 speed chain on 9 speed cassette: 
seems to work just fine.
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From my understanding if you put a 9 speed cassette on a new direct drive trainer you would need a spacer to take up the extra space, as most DD trainers will come with 11 speed hub bodies. But the spline pattern should be the same, therefore you should be able to do it.
Easier solution: Buy her a new 11spd bike!
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My wife and I have been sharing our standard trainer (Elite Super Chrono) for years. She with 10-speed Shimano, me on 11-speed Campagnolo. Neither of us has a powermeter. We’re progressing just fine. Maybe start like that and see where it gets you?
My wife and I share our elite drivo, I just bought an old Boardman for £100 as a turbo mule. No changing out bikes.
You should just get her a power meter.
The direct drive trainer is optional at this point - but if you get it, keep the dumb trainer and both of you can work out at the same time.
The other ridiculous thought that I have - is to get one of the fully bike trainers that’s easily adjustable. Obviously, not the cheap solution there. 