7-speed freewheel to Wahoo Kickr Core - what to know?

Question

I have a 7 speed hybrid Infinity Boss three 7 speed with a Shimano TZ21 14-28T Freewheel and am trying to figure out what I need to get this bike on a refurbished kickr core which I'm planning on buying. I've been trying to do research, but am still overwhelmed with figuring out what it's going to take to get this dang costco bike on the kickr core. I think that I basically have 2 options:

1. Purchase a 7 speed cassette with a 4.5mm spacer to get it to fit a 8-10 cassette and then another 1.8mm spacer to get it to fit the 11. It sounds like to do this, I'll have to do some weird thing to file notches in the 4.5mm spacer?
2. Purchase an 8 speed cassette, but only be able to use 7 of the cogs. I think that's how it works at least? It feels like the main appeal to going this route is that 8 speed cassettes seem more "standard", and I don't have to do the hacky filing thing.

Is there anything I'm missing here? And further, is there else I should consider when buying a new cassette, and is there any reason to get an 8-speed over a 7-speed or vic versa? For a 8-speed I'd be tentatively planning on purchasing something like a Shimano HG51 8-Speed 11-28T or a Shimano Alivio CS-HG200 7-Speed 12-28T (basically the first 2 shimano cassettes I found on Amazon).

Answer 1

Forget about anything involving 8-speed, it's not part of this conversation in any way.

Filing anything is also not part of this conversation.

Say you had a 7-speed cassette bike you wanted to put on a direct drive trainer. In most cases (as in for most hubs that cassette might be mounted on, using a proper 4.5mm 7-speed conversion spacer if applicable), you get your no-adjustment wheel interchange by putting on the 1.85mm spacer (the 11 speed to 8/9/10 adapter spacer) plus the 4.5mm spacer (4.5mm is the spacer dimension that creates interchangeability between Shimano 7-speed cassette hubs and Shimano 8/9/10 hubs).

However, you don't have a cassette hub. You have a freewheel hub and so chaos reigns, because freewheel hubs and freewheels are not as closely standardized. In other words, the point of using the specific size spacers above is that by doing so, you create a known distance between the center plane of the first cog and the dropout surface, which works because the hub dimensions and the cassette mounting-surface-to-largest-cog-center-plane dimensions all stick pretty close to Shimano-derived standards. To my awareness, this level of dimensional cloning isn't really present among 7 speed freewheels and their hubs.

This is the minimum hassle way of making this work:

1. Make the bike with its present freewheel and hub have a perfectly adjusted rear derailleur. In other words, make the limit screws and cable tension ideal.
2. Have your 1.85mm and 4.5mm cassette spacers at hand, and also have a bunch of various thickness ones such that you can get to any total spacer stack needed.
3. Mount a 7-speed cassette with the 1.85 and the 4.5mm spacers. This is a baseline to get the cassette in more or less the right place. Take the chain off the bike. Put the bike on the trainer. Shift the bike on to the smallest cog RD position.
4. Measure or estimate with a caliper how far in or out the small cog needs to move to align with the guide pulley of the RD, to the same standard as you would be shooting for with any RD adjustment. You could also do the same thing by measuring the center plane of the cog to the dropout inner surface on both the bike and the trainer.
5. Modify your spacer stack as needed to shift the cassette around to create drop-in interchangeability.

In my opinion, from painful experience, the wrong way to do this is to put a spacer of any kind under the freewheel on the wheel. Sometimes they get squished and the hub blows up or the freewheel gets cross-threaded.