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I recently converted my bike with 9 speed 105 from a triple to a double. I installed my new 34/50 crankset and noticed that chain would slip across the teeth of the 34t chainring whenever under serious load. It's completely fine on the 50t ring. I've been looking around for answers and the most common explanation is the the chainring and/or chain is worn out and needs to be replaced. The thing is, the chainring and chain are new and showing no signs of wear. I though it might be slack in the chain, but I remeasured it and its the correct length. Is it possible that my 9s chain just isn't compatible with this particular 34t ring?

Any help would be great, I'm pretty stumped.

BTW this is the new crankset I swapped. https://www.aliexpress.com/item/3256802561706591.html?spm=a2g0o.order_list.0.0.21ef1802Xiq4n2

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  • Is the chain badly worn?
    – Daniel R Hicks
    Jul 19 at 16:17
  • Did keep the previous derailleur? Possible that the different cage design causes interferences.
    – Renaud
    Jul 19 at 16:38
  • If both chain rings came with the crank set and they're not worn out, then if the chain works well on one chain ring, it should also work well on the other chain ring. You can check the condition of the chain with a chain tool, and what nightrider says, the chain may be too long for your smaller chain ring.
    – Christine
    Jul 19 at 18:20
  • The chain has no stretch and less than 100 miles on it.
    – pazzmaul
    Jul 20 at 17:11
  • I'm also using the original 9s 105 derailleur, but I don't understand how that could effect the front chainring.
    – pazzmaul
    Jul 20 at 17:13

2 Answers 2

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Smaller front chainring leaves more free chain so check if the derailleur tension spring is still active and not completely out of range. This would manifest the most obviously with the smallest sprocket.

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So, I was running out of ideas and though maybe the sprocket just wasn't compatible with my 9 speed chain. I swapped it for an old 11 speed at the same length and it fixed the problem! Really weird.

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