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I have recently replaced a 9 speed chain with a 10 speed one on a 9 speed cassette. I read this is compatible. Shifting is fine and quieter but it is now jumping on the smaller cogs 6-9. Is this a common occurrence? Never had any skipping on 9 speed chain though it was a lot noisier.

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    Jumping meaning it skips over the teeth under load? Or it tries to climb to the next gear? If you look at it from the back, is the derailleur+chain perfectly aligned with the cogs? How worn is the cassette?
    – Michael
    Jan 30 at 9:02
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    How worn was the old chain - might be you need new cassette as well.
    – mattnz
    Jan 30 at 9:34
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    A ten speed chain is too narrow for a 9 speed cassette.
    – Carel
    Jan 30 at 10:16
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    @Carel: For some people on the internet it seems to work fine (some even claim better shifting performance). Though it’s true, 9 speed sprockets are 1.78mm wide while 10 speed are only 1.6mm with a correspondingly narrower chain but apparently still wide enough for 9 speed sprockets.
    – Michael
    Jan 30 at 10:25
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This feels like a new chain on a worn cassette problem. The fix is to replace the cassette, and soon before your chain suffers accelerated wear.

Assuming you have 9 speed shifter and rear mech still, the slightly narrower chain won't really make any difference while pedalling, and shifts might be slightly sloppier.

I run a 10 speed chain on a 9 speed cassette without issue, because its what I had at the time.

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    @elpocketo I bet if you install new 9 speed chain on the same old 9 speed cassette, it will still skip. The chain and cassette wear into each other, so replacing one can require replacing the other.
    – Criggie
    Jan 31 at 10:28
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    On closer inspection whilst riding it seems that the chain is skipping up and down gears rather than over the cassette. Perhaps the ten speed chain is too thin.
    – elpocketo
    Jan 31 at 18:58
  • @elpocketo Assuming you have a 9 speed cassette still, it shouldn't make a difference. The rear mech sounds like it needs reindexing, ie, realigning to the cogs in the cassette. You can do this with the barrel adjuster built into the rear mech.
    – Criggie
    Jan 31 at 21:59
  • @elpocketo that is wear - the worn 9 speed chain fits the worn 9 speed cassette. A new unworn chain would skip too. You're past the point of saving the cassette, so there's nothing wrong with wearing both out together. Just be aware that there's a slow degradation in reliability and shifting over time, and you will have to replace cassette and chain together in the future.
    – Criggie
    Feb 1 at 18:45
  • I have a feeling this issue has been caused by a recent change of gear levers to an indexed thumb shifter. I had to wind the b screw most of the way out to get it shifting well and I have a feeling this has taken tension off the chain and made it abit slacker in the smaller cogs. I could be wrong but this is my feeling. I might try to remove a link and see if this improves matters.
    – elpocketo
    Feb 1 at 22:06
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After trying many things I think I've found what the underlying issue is.Its not the chain because was even still doing it on a different 9 speed chain.The issue i think lies with a gear shifter by sunrace,an indexed manual thumb shifter.I dont know the reason why but it slips into other gears when peddling hard.its not the cassette as nothing is skipping over the teeth.

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  • Strange. Usually a chain is only really too long if your rear derailleur is completely "folded" and can’t take up any more slack. Usually you’d see it visually hanging down and/or it would be very easy to press it towards the chain stays (and you’d get a lot of chain slap on bumpy roads as a result).
    – Michael
    Feb 3 at 13:31
  • By the way: I don’t think you need an equation for this. Make sure the derailleur is not fully extended in the large-large combination (there should roughly be enough remaining capacity to press the chain down to the chain stay) and that there is still a nice bit of tension on the chain in the small-small combination.
    – Michael
    Feb 3 at 18:57

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