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I've a folding Tern Link D8 that I've started customize by replacing some parts...what is driving me nuts is a strange chain skipping that happens on some gears and only when I apply some force (starting from moderate hill)

On the drivetrain I've replaced:

  • crankset ( FSA Carbon Pro Team Issue ) - USED
  • bottom bracket ( fsa platinum Isis drive) - NEW
  • cassette (Shimano XTR CS-M950 8 speed cassette 12-30T ) - USED
  • rear derailleur ( Shimano XTR M950 8 speed ) - USED
  • Sram gripshift with the right index ratio for Shimano - NEW
  • Shimano chain for 8 speed - SOMEWHAT USED

What I've tried without any noticeable difference until now:

  • Chain tension: I've removed or added a couple of links
  • Adjusted the rear derailleur as per manual (hanger alignment and L/H-screws)
  • checked for stiff links
  • switched with the original rear derailleur ( Neos )
  • checked the cassette teeth...there is some wear but plenty of life here

The skipping is happening without changing gear, and seems a sort of "chain that jump on the chainring" feel. On the smallest chainring usually it wont happen...I've made a video to better understand what kind of skipping is, but even in slowmotion I can really grasp what happens.

slow motion

original

Can you give me some suggestions?

SOLVED: the culprit were the worn teeth on the cassette: look at the pic below, I've highlighted the increased gap that cause chain to skip

green=still good
red= too worn

enter image description here

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    My guess as it is, is slightly too much tension on the rear derailleur cable. – alex Jun 27 '16 at 9:22
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    @alex Added the original video, about the tensions I'm not sure...as I've wrote I've already removed and added a link with any difference – Decagrog Jun 28 '16 at 4:34
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    @Criggie Added a pic of the cassette, there is just a bit of wear on the teeth but imho are not so bad to cause a skippin...I'll check for the chain or at least replace (again :( ) with a new one – Decagrog Jun 28 '16 at 4:34
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    @Carel Yep I've worked on barrel adjuster correctly, even with someone on the seat to compensate a possible frame flex – Decagrog Jun 28 '16 at 4:35
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    That video of the wobble looks like a bent axel. – alex Jun 28 '16 at 4:36
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With reference to the new/used bits of your question, note that using worn consumables (chain, cassette etc.) on a bike is never a good idea, largely because you're in the dark as regards how much life is left in them. So my best guess is that the chain or maybe both chain and the cassette needs replacing for new parts.

Assuming that these parts are actually ok, then the problem becomes one of poor mechanics. Two things spring to mind:

(1) the chain you used as a replacement is the wrong length, and/or (2) you did not adjust the derailleurs correctly after your chain/cassette changes.

And are you sure you have fitted things to this bike which are compatible with those you removed?

  • I can give a try replacing the chain but is somewhat new (i think about 200-300km), I've played a lot with the rear-derailleur settings, since also with the original Neos there's the skipping I'm not sure is the culprit. Instead I could give a try replacing the cassette...this XTR M950 is not very worn (teeth are ok), but I've noticed that some chainring in the middle are wobbling a bit...could this be cause of some issue? – Decagrog Jun 27 '16 at 13:58
  • Cassettes are meant to be tight, so one that wobbles doesn't sound too healthy. You should aim to try and get that sorted. – PeteH Jun 27 '16 at 15:18
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    Added a short video about the wobbling, is not a single chainring but the whole cassette when is in freewheel spin...probably due to a bad hub bearing? Could be the cause? – Decagrog Jun 28 '16 at 4:27
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    Sorry for the late reply but I managed to change the cassette just yesterday...in the end the problem was caused by the worn cogs @PeteH was right, if you look closely you will notice the teeth of 4° and 5° gear are very thin, also the near ones are worn, instead the 1° , 7° , 8° are still good. I've attached a pic with highlight on the increased spacing between the worn teeth, this caused the skipping issue! – Decagrog Jul 13 '16 at 18:24
  • I'm glad you got yourself sorted, well done! – PeteH Jul 13 '16 at 18:31

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