Zwift Hub classic switch to 12 speed


I’ve been looking through the forum but couldn’t find a solution to my problem, so here it goes:

I purchased the Zwift hub few years ago with a 11 speed cassette (SunRace)
Just recently I got a new bike which is 105 Di2 12 speed.
So now I tried replacing the SunRace 11 Speed with the 105 CS 7100 12 Speed cassette which came with the bike and I get a lot of noise and shifting issues.
The same setup works just fine with my rear wheel installed.

Now I’m wondering whether there is some spacer I’d need to remove/replace for this to work.
Or do I have to adjust my rear derailleur - which would be annoying as I’d have to change it back when riding outside.
Or am I missing something else here?

Many thanks in advance!

You’re not missing anything - no spacer is called for with that installation. However if you just need to shift it by 1mm or so it should be safe to add a thin spacer like that to either the trainer or your rear wheel, depending on which way it’s off. There is some variation between different hubs on the market, and trainers, in terms of exactly where it puts the cassette. A second wheelset for your bike with a different brand of hub might have the same kind of issue.

Thank you for the clarification!
Do you think it would work to use one of the spacers from the old SunRace cassette (the black ones between the cogs) and put it on the freehub first before sliding on the cassette?

That may be too thick. You don’t want to add a lot because you need the lockring threaded far enough into the freehub to keep it secure. Not enough threads and you can strip it, which may damage either the freehub or the lockring. Any bike shop should have smaller spacers, and probably some free advice.

Right, will try that and report! Thanks again!

Quick update:
Zwift support offered me to try the usually included SunRace 12 speed cassette for free. Currently waiting for that one to arrive.
In the meantime I ran into some other issue with the Zwift Cog.
It makes strange noises under load.
I already ruled out other factors by switching back an forth between Cassette/Freehub and the Cog.
Noises will only be there when using the cog and at around 130W and above.
I made a Video of it as well:

https:// youtube .com /shorts/aqUw_xsNAnY?si=Wh0VkP7BY1cdo7L1

Sorry - links not allowed - therefore split up

Any Idea whats causing this?

I already tried applying grease before installing the Cog and on the Axle with no success.

A little hard to hear–what noise are you thinking of?

IIRC, the cog is a little thicker than a standard cassette cog (I don’t own one), some have reported more noise just from that.

Hi, @Martin_Kropf, Welcome to the Zwift forums!
Fernanda here, part of the support specialist team at Zwift.

I appreciate very much the video you shared regarding the noise you are experiencing with the Zwift Cog, I can understand how concerning this could be, after checking the video, can I kindly suggest that to verify if the tighten of the drive side adapter is not higher to 6Nm, f you don’t have a torque wrench for verifying this, as an idea you can check if after reinstalling the drive side adapter by finger tighten it, then you should tighten only half a turn with the wrench provided with the Zwift Hub.

Give it a try and let us know how it goes for you!