Hi all, I just bought a new zwift hub. I have an older Trek mountain bike that has a 7 speed (what turns out to be) freewheel. I thought I would just be able to pull the cassette off the bike and put it on the trainer but it turns out the it is threaded not splined like the Zwift hub 
Any ideas on how to make this bike work? It has a Shimano 14-34t with threaded freewheel, any ideas on an adapter or cassette that would work with this? Not sure if the teeth numbers are important here (newb). Any help is appreciated.
You can buy a 7 speed Hyperglide cassette for it. You will need additional spacers under the cassette to make up the difference between 7 and 8 speed. Other than that it should work fine. I think you’ll need spacers in the neighborhood of 3.5mm in addition to the extra spacer that comes with the Hub. A bike shop can help if you run into difficulty with it.
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Thanks for the answer. Just to make sure I’m understanding, sounds like the number of teeth (I’m seeing mostly 11-28 for the hyperglide) doesn’t effect whether it works with the existing trainer and bike setup.
7 speed cassettes with different gearing will still be compatible. An 11-28 will have a higher high gear (so you can pedal harder and go faster) and also a higher low gear (so you’ll run out of easy gears sooner on hills compared to your existing gearing). Whether that matters depends on how strong you are and whether you ride the steepest hills. If the new cassette shrinks the small cog from 14 teeth to 11 teeth, you may find the chain is slack in the smallest cogs and might rub on the derailleur. If that happens the chain can be shortened. If the existing cogs and chain are worn, the chain may skip on the new cassette. If that happens you would need a new chain.
7 speed cassettes from other companies like SRAM and SunRace will also work. The description will usually say they are Shimano compatible.
Thank you, that was exactly the information I needed. I wasn’t sure if I would have to do modifications to it if I changed the sizing that much. A change in the chain seems pretty easy and it’s probably due for a new one anyway. Someone messaged me and brought to my attention the fact that there are 12-32s widely available still. The explanation of the gearing was very informative, I didn’t know that stuff. The bike shop wanted me to bring the whole bike and trainer in which was a little more than I wanted to do so this information is really helpful and helps so that they don’t look at me like I’m from outer space when I walk in. Thanks for taking the time to educate a new person on this stuff! I included a photo of the decommissioned bike which is patiently awaiting its new cassette it’s overkill for a trainer but this is what I have!
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Looking at these pictures I’m sure you have a great spread of front chain rings and I doubt you are going to run out of ‘easy’ or ‘hard’ gears no matter which 7 speed cassette you end up going for.
Of course this is dependant on how much you can cross from large to small and from small to large but if you add in trainer difficulty I don’t think you are going to have any problems. I’m sure @Paul_Southworth can add to this when he sees the photos ( what are the tooth sizes of your front rings?)
Changing chains can be easy if you have the correct tools but it can go wrong quite easily on modern chains and trying to rectify can then weaken the strength of the chain.
Although the bike shop might give you some ‘alien’ looks (they shouldn’t) it might be cost effective to take everything in and let them sort it out.
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Yeah I would guess gearing will be fine with any cassette. If anything the smaller cassette might be better since the jumps between gears will be smaller.
Derailleur adjustment will probably be required after installing it on the trainer, which might be a reason to take it all to a bike shop. These videos might help.
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Thanks for the video, this might be the next thing I need to do. I did take the whole thing to a bike shop and I’m glad I did as they got the right number of spacers and were able to put the cassette on right then (they replaced with a 12-28). Somehow though it feels a bit off. There’s a slight grinding sensation in the pedals. They didn’t put a new chain on so starting there and will take a look at the derailleur next. The shifting also seems worse although still functional. All that being said, I rode about 10 min with it and WHAT A DIFFERENCE from my old trainer! I had a Bkool Smart GO (the worst) and I’m positive now that it was making all my workouts 4x harder than necessary, I could never clear 120W of power! 10min on this one and I’m at 200W (new at it but good for me!).