Chain slipping/skipping on sprints while on trainer

I have no problem sprinting on the road (races, etc.), but any time I put more than 600 watts on my trainer, the chain skips/slips. Not entirely off, just a link or two, but enough to really disrupt my sprint.
Because it happens under heavy load, I can’t actually see or tell where it’s happening. I can’t even tell if it’s the front chainring or back cassette.!
My chain is not worn (less than .5) and I don’t see anything obvious on the teeth of the chainring or cassette. Seems more likely it’s on the cassette as there are less teeth gripping the chain, but also a tighter bend radius…so maybe not? Could it be that my indexing is slightly off and under load causes issues? It doesn’t appear to be changing gears though during the skip, just temporarily losing “traction” so to speak.
Just change the cassette as it’s the cheapest thing?
Put a video camera on it and play back in slow motion to see where it skips?
Other suggestions?

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Your rear derailleur needs adjustment. Just use the barrel adjuster to move it up/down the cassette as needed.

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even if the slip isn’t from one cog to the next? It stays on the same cog the entire time.

What trainer do you have? On rear-wheel-off trainers like the Kickr, there is a belt drive that skips if you ramp up the torque too fast. You can adjust the belt tension to minimize this, but there are tradeoffs.

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For example:

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I have the 2017 KICKR.
I doesn’t feel like it’s a belt slip (I can hear the metal-mash skip of the chain), but I suppose it’s possible.
I’m thinking I need to set up the gopro on hi-speed and just video it.

The 2017 Kickr is still the old clogged belt design. If you can make that skip, you are built like a juiced up power lifter below the belt. Its practically impossible for that design to slip without breaking something.

Well, I’m not on the gear or built like a power lifter, but I have a 2017 version, which skips when I try to do heavy stomp sprints in ERG or slope mode. I have just sort of given up trying to do high torque sprints and now slowly wind up 20-30 second sprints if I’m sprinting on the trainer.

Sprints are more fun outside anyway, and I enjoy the erg on the kickr for disciplined intervals.

My point was that it’s very unlikely that the skipping is the belt of the trainer. Unless the tension has slipped way out of adjust, it just not practical to expect.

However, the skipping is most likely at the chain and cassette. Source can vary but a poorly adjusted reward derailleur, worn chain / cassette or other mismatch is the most likely cause of drivetrain skipping.

Well, replaced the front chainrings, rear cassette, and chain. Seems better and I was almost ready to declare victory, but then…BAM, skipped again.
I’ve tried a few clicks here and there on the barrel adjuster with no luck.
Will try the belt tension tomorrow, but I really don’t think it’s the belt slipping, I can all but see the chain moving and the sound is pretty clearly with the chain.
Pretty frustrated as anything approaching a sprint is impossible.

Just wondering if the spacing of the cassette on the trainer is just slightly different to on your road wheels? And therefore the RD is not correctly aligned on the trainer also?

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4 guesses.

  1. Your chain is too long
  2. Your derailleur is old and weakened. When you apply force, it’s moving the cage back and popping in and out of a partial shift. This, I believe, even happens with crappy derailleurs like Shimano Tourney.
  3. something is bent, check the hanger
  4. something is bent, check the derailleur cage. Look at it from the rear towards the crankset, the jockey wheels should not be askew, even slightly.

have someone record a video in slow motion, 240fps. You had mentioned that, and that would be good to see, but I’m betting it’s one of my 4.

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OK, have the video. First, thanks to everyone for helping me troubleshoot. Can’t easily get this setup to a bike shop to troubleshoot during distancing, so thank you for helping me self-wrench.

Few thoughts:

  1. This wasn’t normally what happens. Normally it skips, this time it came clean off. Not sure if it’s getting worse or…
  2. Normally I sprint at the end of a race when I’m pretty gassed. I came in here pretty much to make the video, so it’s possible I was just putting down more force. Maybe I was nervous to make sure it happened on camera…who knows. But while before it wasn’t clear where it was happening, seems pretty clear now it’s front chainring.
  3. The chainring is BRAND NEW. It’s the same model that was supplied with my cranks/powermeter when I bought it from power2max many years ago.
  4. No visible bent parts on front chainring. Also, was happening on old one and new one, so seems unlikely?
  5. From the video it looks like the bottom is unloading and becoming slack. I don’t really know what causes that? I rotate 3 chains, it happens in all, and they are all under .5 (not stretched/worn).
  6. The gopro can only do 240 fps in HEVC, so you have to have that codec to play it. Had I known what was happening, I would have put the gopro at an angle from the front of the bike facing back to avoid the shoes–sorry, next time.

Files are here:
Original Front chainring view:
Youtube of front chainring view (Youtube lowers quality and only 60fps but more compatible):

Rear view original:
youtube of rearview (1080p60fps):

Some still photos from the video feed.

The one that is most interesting to me is the crop where it looks like the chain is riding up on the teeth of the chainring–which is exactly what I suspected before (when it was just slipping and not coming entirely off). You can hear the “bang” as the chain slides and catches into the next set of grooves.

I’m running into the same problem but I use a taxc flux s.
I read somewhere that it might also be caused by the frame flexing, I’m using an older aluminium cyclocross frame (so a bit longer chainstays, which might flex a bit more). And I think it might be caused by the flex of my frame. I also tryed a new cassette, chain and I have a fairly new derailleur and brand new charin rings but its still slipping.

Might have to try and record the chain slip to see if we are having the same problem.

The chain is coming off (or riding up) the chain ring at the same point in your pedal stroke every time.

The rear view video shows the frame flex as you’re putting down the power. This probably only happens indoor on the trainer because you’re flexing the frame more/harder than when you’re sprinting outdoors (where you can rock the bike).

I have a p2m/rotor3d+ combination like you, and I’ve noticed that my chain ring is wavy (look down at the rings as you pedal lightly) through the pedal stroke. I don’t recall where, but I’ve read reports of other people with p2m spiders that were warped, which would cause the chainrings to bend, resulting in the waviness. Luckily, for me, it hasn’t caused issues (I noticed this outside the warranty period). Just something else to look into.

The exact same thing was happening to me on my mtn bike a few years ago, it was due to the chain ring being mounted backwards.

Update: as a last resort I bought a new chain and fingers crossed it solved the issue. I can’t explain it as the chain had essentially zero stretch/wear, no bad links, etc.

But with a new chain I can hit full sprints at 1300 watt spikes with no slippage.
I rotate several chains, so I’m curious to see how it happens with other used-but-not-worn chains.

I thought about the frame flex but 1)it works fine with new chain (and same frame, obviously) and 2)I’ve used this same frame on this trainer for years. Maybe it’s a contributing factor?

I’ve had that same P2M spider for years. Maybe this new chainring is more susceptible? Not sure. I have a sneaking suspicion that this chain didn’t really “solve” the issue, maybe just covered it up.