I have a bike which I use as normal as well as cycling indoors with my tacx flux smart trainer (direct drive, take the back wheel off). Without the trainer, my bike runs smoothly with no issues.

When it's connected to the trainer, it makes a clunking noise then it feels like the chain slips (the tension loosens, like i'm pedaling but the back wheel isn't moving) and it goes 'clunk, clunk, click' and after the click it rides fine for a bit then I have the same issue again. Sometimes this happens under high load but not always, sometimes it just happens when cycling at the same level. Whilst it feels like the chain is skipping a gear, it isn't. I've filmed it and it doesn't move.

  • I've bought a new cassette for the trainer, it has the same number of gears (9), however, it has 29 teeth instead of 27 on the largest sprocket. I was told this wouldn't be an issue.

  • I have tightened my cassette as hard as I can and it isn't loose when I shake it around on the trainer.

  • I have checked the derailleur and it is aligned perfectly with the cassette and the gears shift without issues.

  • I have taken the free hub out and put some lithium grease on it but with no luck.

  • I have measured the chain at a bike shop, they confirmed it was not stretched.

What can I do next? What could it be?

Videos of this happening,

  • Worn chain on newer cassette is pretty likely. You could switch the cassettes to test. May 9, 2020 at 3:51
  • Or replace the chain. To measure chain-wear there are cheap specific tools that are better than rulers, BTW.
    – Carel
    May 9, 2020 at 7:28
  • ok, I will check the chain stretch - but why wouldn't I experience the same thing when i'm cycling normally without the trainer if it is the chain ?
    – rusty009
    May 9, 2020 at 13:20
  • 1
    Sure, added the videos of the issue.
    – rusty009
    May 9, 2020 at 15:11
  • 1
    @rusty009 that's the thing, the pawls need to spring back freely to engage, cleaning/greasing pawls should normally help though (unless grease excessively sticky). N.B. you linked to a Campagnolo compatible freehub, if that's the one you ordered you'll need to send it back to exchange for a Shimano compatible one, to match your cassette. If you ordered a Shimano one, ignore this letter
    – Swifty
    May 9, 2020 at 17:19

1 Answer 1


I had the same thing. It's the screw on spacer on the axle (cassette side coming loose. Screw it back in again and solved. For me anyway.

Use the spacer on the non-drive side to get the internal distance between your rear stays just right before clamping down the quick release. Mine works loose from time to time and the bike becomes loose. Check it before you ride - it's never worked loose in a single training session for me.

It's not your cassette if the sprockets don't wiggle when you try to move them. It's not the chain unless it's skipping over the sprockets - almost (not entirely) impossible.

I'm disappointed with my Tacx. Blood noise, squeaky, creaky and comes loose.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.