Weird gear slipping issue on steep climbs with Neo 2T

Not sure if I should be positing this here, on the Tacx forum or on a general cycling forum but as it’s indoor-specific this seems a good place to start.

I have this annoying issue with gear changing / gears slipping on very steep climbs in Zwift (e.g. Alpe du Zwift). It happens when I’m in the small ring and changing to larger sprockets on the left side of the cassette, i.e. between about 34-21 and 34-28.

I’m using mechanical 11 speed Campagnolo Record (HO rear derailleur) with a Tacx Neo 2T trainer that’s fitted with a Shimano 11sp 11-28 cassette.

What will happen is that I’ll be grinding the gears and changing to a larger sprocket - the chain will flip up to the bigger cog but immediately slip back down again, as if the indexing is out of adjustment. But it’s not - shifting is perfect in other circumstances, and if I try to compensate with the indexing it just screws up the shifting entirely. It tends to start happening half way up a long, steep climb and get slightly worse as the summit approaches. It’s really odd!

I have a few hypotheses. Perhaps it’s just because I’m shifting under high load, although I do try to ease off, and I’m pretty sure I don’t have the same problem outdoors in similar situations. Maybe it’s due to incompatibility between the Campagnolo chain / rear mech and the Shimano cassette - but shifting is perfect in most circumstances, and I use a SRAM cassette without issues outdoors with the same bike. Or perhaps it’s due to flex in the Neo 2 trainer, which does have a bit of “rock” built in. But as far as I can tell the cassette and hub assembly rock with the bike, indepedently of the rest of the trainer.

I’m completely flummoxed. The chain is more than long enough, and the derailleur is appropriately adjusted.

First off, are you certain that your bike is ‘square’ on the trainer and that the quick release is adequately tightened to prevent the frame from moving on the QR? If all of that is good, have you checked to be sure your derailleur hanger isn’t bent/cracked? And does this happen among ANY of those gears, or only when shifting to the 28? And does the same thing happen when shifting to these gears when not under a hard load?

Yup, bike is square in the trainer and QR really tight. Also pretty sure the hanger is fine.

It happens between the largest 3 or 4 sprockets, when shifting from a smaller to a larger sprocket. It doesn’t happen when shifting under a gentle load, or at least not when not half way up the Alpe… Although I’d rarely be in that gear combination unless I was on a steep slope, TBH - almost necessarily it’s never going to be under a hard load when in these gears unless there is sugnificant resistance (I can’t spin that fast/powerfully!). But it doesn’t happen when spinning and changing the gears to test them when not under load.

You know what it reminds me of? When a gear cable is fraying and about to break, every time you change to a larger sprocket it stretches a bit more, so often then jumps back down again. It’s not quite like the indexing being out, because usually when that’s the case it teh chain just won’t move to the larger sprocket, while in this situation it moves, but moves down again just after I take my finger off the lever… First time it happened I thought that was the issue, but the cable is fine. Then I thought maybe the cable was slipping in the RD bolt, but that’s not the issue either.

It’s as if the cable tension is being momentarily reduced due to some weird flex somewhere, or else that the chain just doesn’t want to stay on those sprockets when under load.

Well, those top sprockets are the ones where the chain will be most out of alignment, so adjusting the positioning of the derailleur does seem like the best option. You say you’ve tried that, but then it puts everything else out of whack, though, which seems odd. Any chance you’ve somehow put a 10-speed spacer in there somewhere? I’m not really sure what else might be happening here without actually seeing it in person.

Yes, it’s weird. I think if it was a spacer issue I would notice an obvious difference in shifting between the bike on the trainer and having a proper wheel on (I’d need to re-index slightly). But eveything shifts fine without adjustment until I’m cranking it out half way up the Alpe… Also, the chain alignment is worse when in teh big ring / big sprockets, but I’ve never noticed the same problem there (although that could easily just be because I’m hardly ever in those gears, especially on long, continuous climbs).

If I try reindexing on the fly while riding (tightening the cable), it just throws the shifts in the other direction out (as you’d expect), without solving the problem.

Out of curiosity, is the cassette much newer than the chain? Just wondering if an older Campy (?) chain paired with a newer Shimano cassette isn’t pushing your system over into sub-optimal performance under strain.
Any chance of putting the bike’s Campy cassette onto the trainer and testing to see if performance improves significantly? (I’m guessing that you’d need to change the freehub on the Neo 2T to do that, so probably not.)

I’m out of my depth here, never having mixed Shimano with anything else, but it seems not to be a black-and-white issue. Lots to take in here, in case you haven’t already seen it: Mixing road groupsets: what works together and what doesn’t - CyclingTips

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Not a massive difference in chain/cassette age. Yes, using a Shimano cassette because I’d need to buy a new freehub for the Tacx to use a Campy one (not supplied with the trainer…).

Still, I’m thinking that might be the issue. Shimano cog ramps just a little bit too eager for the Campy chain when under load…?

Or could it be frame flex on the trainer affecting the effective cable length, especially when out of the saddle…?

On the same bike, outdoors, do you notice any issues that could be attributed to excessive frame flex?

Had similar symptom when I forgot to torque up cassette after a change. The flex with torque made the larger sprockets move out of alignment more than the smaller ones and start catching the next gear - that was only a mtb cassette though.

I hadn’t thought of checking that the cassette was torqued up properly and had high hopes for that being the problem… But no… (I mean, it wasn’t super-tight but it wasn’t loose).

Haven’t noticed any issues with frame flex outdoors.

Totally mystified!

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A quick google and it looks like shimano and campagnolo cassettes are not interchangeable (article does state they are close enough for short term/emergency use).

In addition, Campagnolo’s chains differ from Shimano/SRAM in overall width, and the company uses variable spacing for its sprockets (Shimano/SRAM cassettes have uniform spacing).

From Mixing road groupsets: what works together and what doesn’t - CyclingTips

Yeah, officially they aren’t but it works, at least with mechanical shifting. The non-uniform spacing means it can be a little less than 100% perfect shifting between a couple of the smaller cogs, but nothing you would notice if it’s well adjusted. I use SRAM Red 11sp cassettes on the road without any issues.

But I am wondering now if there is a problem with the Shimano cassettes specifically and Campagnolo chains when in those big cogs and the chain is under heavy load.

Got any friends that would have a 12-sp chain from their mtb/road bike that you could try? Being narrower width could help and may be worth a shot.

I’m going to just do it properly and get a Neo 2T Campy freehub and Campy Centaur 11sp cassette. May or may not sort it out, but it’ll narrow things down a lot if if doesn’t!

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So I changed both the freehub and cassette (to Campy versions). Noticed when doing this that while the old Shimano cassette was properly torqued onto the freehub, the freehub itself was loosely torqued onto the Neo 2T… Hadn’t realised until I changed it that it was torqued on as opposed to just pressed on (but I’ve never removed it so it must have been supplied loosely fitted).

I’m assuming now that this was significantly contributing to the problem - in combination with changing gears at high loads when climbing (at 100% resistance).

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Well, never thought that could even be a thing on modern trainers … thanks for looping back Neil.

When I say the freehub I mean the freehub endcap - it screws on as opposed to being pressed on:

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Thanks for the update, @Neil_Bell. Interesting, regarding that freehub endcap. Perhaps it was a cumulation of very small outlying issues, exacerbated by that endcap not being fully tightened, that produced the frustrating performance. It’s probably worth taking the cassette off in a few months’ time to check to see if the freehub endcap is staying torqued in place.

After the parts changes, is the shifting now all as good as when you’re out on the road?

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So far it seems good - I’ll know next time I do a full-on effort on the Alpe…

It’s worth noting: the NEO2T supports thru-axles natively. If configured for those, a 5mm hex wrench won’t work for tightening end caps: you need a 17mm cone wrench to engage flats on the ends of the thru-axle end-caps. Support: Thru-Axle Bike Installation on a Tacx® NEO 2T - YouTube

I’ve personally experienced drivetrain issues with the end-caps being too loose - this was before I knew they needed to be more than “hand tight”.