Skipping on new chain, new cassette, new derailleur

Question

For a long time I have been having issues with chain skipping on my 1999 Trek 6500. I took a video to illustrate exactly what I mean by "skipping":

Over the last few months, in the course of trying to fix it, LBS has persuaded me to replace the chain, rear wheel & cassette, rear derailleur, shifters, and cables. Derailleur hanger angle has been checked, limit screws and cable position adjusted. None of these have completely fixed the issue.

As it is now, I can reliably reproduce the issue by riding over a large bump, or doing a bunnyhop to cause a physical shock to the bike, then riding for several seconds while standing up and accelerating hard. If I remove the bump from the equation and just accelerate over smooth pavement, the issue does not occur. There is generally one skip per bump - i.e. After a skip occurs, I usually must induce another bump before it will skip again.

What other candidates are there to investigate as the cause of this issue, and how would one test them?

Answer 1

You could try removing a link or two from your chain to reduce the amount of slack in it. A chain that is too long may cause skipping.

(Ensure you can still reach your entire gear range with the shortened chain length)

Answer 2

If this is happening on a new chain and cassette, suspicion would turn to insufficient chain tension, specifically caused by weak or broken tension springs in the derailleur - but the derailleur has been replaced as well.

Does this problem occur more in the small or middle front ring?

I would check the chain length, just to rule that out.

Answer 3

I see that 2 chain rollers come from beneath improperly engaged, and those are the ones that cause the skipping.

Maybe the upper jockey wheel rests too close to the cogs, or moves up and down too much (possibly hitting the cogs), and somehow is causing a few rollers to engage earlier than they should, before the ones in front of them had time to engage properly.

But before tinkering with that, I would clean and lubricate the chain really well and see if it stops the skipping. The chain might be too rigid and not wrap back properly around cogs or jockey wheels after it has been airborne because of the bump.

A video from the side would also be helpful, to see what happens with the chain around the jockey wheels.

Update: I have discovered something about this kind of chain skipping recently on my bike when using a slightly worn chain. It skips when on the smallest rear cog, same as OP. The chain wear on the problematic chain is 0.4% (i.e that's how much longer it is than its original length), and I believe it is counterfit as well, because it reached this wear level after only 2000km (using waxing as lubrication). I have another chain, which is 0.2% worn, after 4000km, and it doesn't skip. So in my case the chain seems to be the issue. Usually it's the chain-cassette combination, or the difference in wear level of the chain compared to the cassette (new chain on worn cassette, or worn chain on new cassette).