I am trying to make a cheaper (500ish) bike work with a Tacx Flux (2018 version). According to their website, the Tacx Flux only supports 8-11 speed cassettes, but my bike has a 7 speed freewheel (14-28T) on it. So I looked up and found out that I am able to use a 4.5 spacer to fit the 7 speed cassette on the Flux's cassette hub. However, all the 7 speed cassettes that I found online have 11 or 12 teeth top cogs, while the derailleur on my bike requires a 14 teeth top cog. I was thinking of just replacing the top cog with a 14t one, but I couldn't find a way to just replace one cog in a cassette. I am new to this stuff and wasn't sure if my understanding of the situation was correct. The Shimano website states that the top sprocket max and min for my derailleur is 14T (https://bike.shimano.com/en-EU/product/component/tourney/RD-TY200-GS.html) Does this mean that I have to use a 14-28T cassette for my bike to work or am I understanding this incorrectly?

Thank you in advance!

  • Haven’t watched the vid, but the trick may be to use spacers. Someone else free to watch and add this to an answer. youtu.be/Jrd5XAQfgaA
    – Weiwen Ng
    Nov 28, 2020 at 20:25

2 Answers 2


You're trying to put a bike with a 7 speed freewheel into a trainer that expects an 8-11 speed cassette which is fixed to the trainer.

Your option of fitting a 7 speed cassette and a couple of spacers is a good workable solution.

The freewheel design is limited to a 14 tooth smallest cog (ok some 13 tooth ones existed but they were rare) So your derailleur has a good spacing for a 14 tooth small cog. When the 7 speed cassette is in the same relative place, the smallest tooth cog has 11 or 12 teeth, so will be a little smaller than expected. This should still work okay, but there will be slightly less chain engagement.

I would focus on getting the trainer's cassette's left/right position exactly the same as whats in your wheel. That way moving between road and trainer does not require adjusting your shifter/derailleur. If you have friction shifting this is less of an issue but you still want them close.

Also, do try and match your 7 speed cassette's big cog with the 28 tooth one on the rear wheel. Again it helps with chain length and not jumping over-the-top.

  • 1
    There’s actually such an abomination with a 11t small cog: em3ev.com/shop/dnp-11t-freewheel
    – MaplePanda
    Nov 28, 2020 at 18:03
  • I've put an 80s Peugeot friction RD 2x6 racer of the FLUX, juggling with spacers to move the cassette as far out as possible. For finer adjustment, the thin spacers from a worn 10/11 speed cassette are very practical. You may have to re-adjust the LO position on the RD precisely to keep the chain from climbing over the biggest cog.
    – Carel
    Nov 28, 2020 at 20:02

I don’t think anything bad will happen if you use a cassette with 12t smallest cog. At the worst shifting to it won’t work reliably but you probably don’t need the cog on the trainer in the first place (and using it often would wear it down relatively quickly due to the low tooth count).

  • 1
    I usually keep the chain on the (two) middle-most cogs. Shifting is of no great use on a direct drive trainer.
    – Carel
    Nov 28, 2020 at 20:09

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