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I would like to upgrade my basic wind trainer to a Zwift Hub smart trainer. I'm using a 1980s 18-speed Mongoose Hilltopper with a thru axle (148mm). The Hub offers Shimano 8, 9, 10, 11, and 12 speed cassettes for mountain or road bikes. Since my rear wheel has 6 sprockets, how do I figure out which cassette to get, or if I can use this (or any other) smart trainer with my old bike? And if not, any general guidelines for a cheap-ish replacement? Zwift chat has been utterly useless in helping me with this.

Thanks for any advice.

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    If you have a 1980s mountain bike with a six-speed cluster, I am reasonably certain you don't have through axles. You might have solid axles.
    – Adam Rice
    Mar 8, 2023 at 16:35
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    Best option is buy a newer bike with a suitable cassette. You are likely to find one at your local recycle center or on ebay/craigslist for next to nothing.
    – mattnz
    Mar 8, 2023 at 18:34

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I mostly know just know 6-speed freewheels. Those obviously cannot be used on a cassette-hub trainer.

There are some old 6-speed Uniglide cassettes and they used a different kind of hub and will most likely not be compatible with the trainer hub. Sheldon Brown mentions that a Uniglide hub will not accept a modern cassette, but does not mention the opposite direction.

Now I found a 6-speed HG (HyperGlide) cassette CS-HG30. That might work but it will be very hard to find. It would require a spacer. I see some on e-bay and they are quite expensive.

A wheel-on trainer would have been simpler, but I also am not sure if you will be able to physically fix the bike on the trainer (or any other trainer).

You might be able to just use some random 8-speed cassette instead and just don't shift at all at the rear. That limits what you can do with the trainer. But check that you will be even able to fix the bike properly on the trainer.

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There's not an easy way of doing this that maintains easy interchangeability with your existing wheel and freewheel.

Bicycles with thru-axles didn't exist in the 80s. You have a quick release or nutted axle. Your first challenge will be to determine what the frame spacing on your bike is, and then put the correct endcaps on the trainer to match it.

Old 6-speed freewheels didn't have standardized cog to cog spacings. New ones do, but there are no off-the-shelf cassettes that match it. So in other words, there is nothing you can put on that will "just work" with your existing setup.

You could put a 7-speed cassette on the trainer, plus a 4.5mm cassette spacer, plus the 1.85mm spacer that your trainer probably came with. The chain will need to be a 7/8-speed. The cassette's total width (distance between the plane of the big and small cog) will be be close to that of your 6-speed freewheel. Thus, with this setup you would likely be able to adjust the rear derailleur to work correctly with the cassette. You will have to choose one whose large cog doesn't exceed what the derailleur can handle. Get one with a 28t if you want to be safe and avoid having to figure that out.

You could get a 7-speed cassette where the first six cogs would be satisfactory for the trainer, grind out the rivets that hold it together, discard its big cog and existing spacers, and then add spacers that make it duplicate the exact cog-to-cog spacing of whatever freewheel you have. The spacers would have to be exact, so this would likely take a long time and be expensive. But, it is the only route to get something that matches what you have in terms of drop-in interchangeability.

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You might have some luck fitting a 7 or 8 speed cassette to your trainer.

This is because 6-7-8 speed chain is all very similar in width, and every time an additional gear is added, the cassette gets one-chain-width thicker. By contrast, in 8-9-10 speed the chain gets thinner, and the cassette stays the same thickness as gears increase.

6 speed cassettes seem to be unavailable, and 7 speed are uncommon. Most bike shops won't stock such things any more, but I found one at https://www.aliexpress.com/item/1005002362793253.html

You'll need to use spacers on the freehub first, potentially quite a thickness somewhere around 6-12 mm worth. It might be possible to tweak the trainer so the bike's indexed gear positions just line up.

If you have friction shifting, this is even easier.

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Definitely a cassette - look in the middle and see the splines, not a thread.

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