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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?

  • Have you checked the frame for cracks? I had a similar issue it turned out to be a crack on the bottom side of the chain stay. In my case it only skipped under high load while on the smallest cog. – mikes Jul 23 '18 at 21:55
  • I could not find any visible cracks – duggulous Jul 23 '18 at 22:19
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    I think you need to replace the LBS. – Daniel R Hicks Jul 24 '18 at 1:39
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    Does it also skip on larger cogs? – Robert Lee Jul 24 '18 at 9:24
  • Can you post a picture of the rear derailleur B screw? If that's not in very far, your derailleur may not be putting enough tension on the chain. – Andrew Henle Jul 24 '18 at 11:15
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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)

  • Or just crank in the B screw... – Andrew Henle Jul 24 '18 at 11:15
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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.

  • Seems more likely to occur in the middle and large front rings. I don't think the chain length is wrong - in big/big it looks like it won't tolerate losing more than one or two links. I'll try to find a more exact way of measuring... – duggulous Jul 23 '18 at 22:22
  • @duggulous in big/big That is the best way to measure minimum chain length. It's by far the easiest, and it's the most accurate in reality because it's the only measurement that matters. – Andrew Henle Jul 24 '18 at 11:12
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I see that 2 chain rollers come from beneath improperly engaged, and those are the ones that cause the skipping.

enter image description here

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.

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    It's hard to tell, but it looks to me like the derailer is not properly centered under the cog. And it could be a artifact of the video, but it kind of looks like the cluster is "dancing around" a bit, like maybe it's not tightly held on the hub. – Daniel R Hicks Jul 24 '18 at 1:44

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