I have a 'new to me' 2021 Norco Bigfoot 2. When I got it I gave it a quick once over and everything seemed in perfect order, very minimal wear, and didn't even need to tweak the indexing... worked perfect. We finally got some snow and I took it for 2 leasurely 10km rides this last weekend, again all worked perfect. Getting back to the parking lot at the end of the second ride the chain dropped off the chainring. No reason for it to, it was flat and smooth. I put the chain back on (yes I'm aware of needing to line up the narrow/wide) and it dropped off in half a revolution of the crank. Tried again, I had to give the chain a good press to fully seat all around the chainring, but it immediately rode right off the chainring. Tried a third time and same thing.

Thankfully we were back at the parking lot so I walked the bike back to the truck. It was driving me nuts trying to figure out why it would be doing that. When I got the bike home and on the work stand it was doing the same thing. It was like the chainring is designed for the typical 0.500" pitch but the chain was 0.490". To try to observe better what was going on I used my fingers to guide the chain to keep it riding on the chain ring. It was riding on top of the teeth, not really meshing as before. After about 3 revolutions of the crank the chain just fell onto the chain ring and worked as it should. Now I can't make it work anyway but properly. Good that it works, Bad that I now have trust issues.

WTF happened? Anyone seen this before?

Ice affecting the chain rollers?

Nope. It was a pristine day. About -2°C, sunny, no snow or ice anywhere on the bike.

[edit] FWIW the previous ride was -5°C and there were no issues.[/edit]

Check chain "stretch."
how clean/dirty is the chain and associated parts?

Not clear. Is the chain falling off when riding, or only.when you're off the bike?

If the latter, consider that a certain amount of upper loop tension keeps the chain tugged down on sprockets. Absent that a chain, possibly stiffened by cold lube can start riding high leading, to it falling off.

you may have a bent tooth on the chainring

/markp

is the rear axle pivoting around. Check that the wheel stays consistent within the stays, both side to side & front to back.

I'd start by measuring the chain for wear and then making sure the upper and lower stops are set on the frnt DR. If that all checks out take a really close look at the chainring. Does it jump both or just the large? There's a reason it's not always working properly. Your mission, if you decide to tackle it, is to find out why. Good luck, Nice bike BTW

No front mech, it’s a 1X

if it was still playing up I was also thinking chain stretch or maybe worn chainring and new chain where the chain was rising up the teeth (or misplaced N-W but he covered that) but the fact it fixed itself is just very odd.

edit: and the other thought I had was maybe an untightened quick link but this was after a ride where it was fine so can’t be

New chain on an older chainring? This could cause your 0.490 chain on 0.500" teeth effect only now it's 0.500 chain on 0.510 teeth. Or a bent tooth as mentioned above. Or narrow-wide issues? Perhaps that narrow-wide chainring and that brand/model of chain? (I"ve never done them so that's a pure guess.)

Based on how manually pressing it around the ring solved the problem, I wonder if it might have been partly frozen or at least stiffened by the cold.

Small stone wedged in the chain?

My bad. I shoulda clicked the link.

Suspend the bike (workstand, skyhook, whatever) and watch the chain mesh with the chainring as you turn the crank slowly. The problem should become apparent to the naked eye.

Yeah this is what I was doing when then it miraculously fixed itself. I can't find a single thing wrong. Pulled the crank, removed and inspected chainring, so far nothing. Chain is in mineral spirits to fully strip and relube, nothing unusual in the griity bits at the bottom of the jar. Just for an extra step, I also tore down the RD.... more gunk than I like to see on the jockey wheels, but nothing that isn't normal.

I really hate that I can't find anything wrong. It's going to take some time for me to stop worrying about that every time I go out..