Hi, newbie here. Getting back into cycling after a very long absence but will be starting with indoor training until I have lost weight, built up stamina and endurance when I will then treat myself to a new bike next summer. (assuming no more pandemic’s) I have been loaned a six month old Tacx Flux s with an 8 speed cassette already attached. My current bike (hybrid) has a 7 speed cassette. Can I use my bike on the trainer or will I have to get a different cassette although I’m told only 8.9.10.11 is compatible. Apologies if this a really daft question but the world of wheel off trainers is new to me. Thanks in advance.
So I don’t have a ton of experience with 7 or 8 speed systems but if you are using ERG mode then you should be able to find a gear that won’t have the chain rubbing on the neighboring cassette sprocket and you should be find.
Also, based on 5 min of research 7 and 8 speed systems are so close in terms of chain size and sprocket spacing that you may find that you will have zero problems. and a 7 speed cassette is smaller than an 8 speed. You can fit a 7s on an 8s freewheel by using a 4.5mm spacer. So if you really feel the need to you should be able to just add a spacer and a 7s cassette. Though there my be a certain reason why Tacx only lists the trainer as 8 speed and not 7 speed compatible.
Thank you mwglow15 for the quick response. I will hook it up and see what happens…should be a laugh. 
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I’m sort of extrapolating my use of a 10 speed on an 11 speed cassette when I visit my parents and use my mom’s Kickr. I just shifted around till I found a cog that was quiet and used that with little issue. It’s possible that you will need to give the barrel adjuster a small turn to get it to line up well but should be relatively painless. Good Luck! Hope it works easily!
Well I did it, no major dramas… It seems to be ok for now. Given that I am right at the start of a long recovery I will not be thrashing it anyway. Just for the record I really am clueless when it comes to all of this so I have no idea what a barrel adjuster is or where to find it but thank you for thinking i did… 
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Just as @mcneese.chad posted both GCN and Park Tools have tons of videos on youtube to answer any of your bike tech or mechanic questions!
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