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I have been fixing up a Northrock XC6 bicycle.

Symptoms:

Chain clunk when trying to start moving. It feels like the chain is slipping. It only happens under load, I cannot get the same symptom by running the wheel by itself. This makes it impossible to get started. However, the same problem does not occur when at higher speeds.

Background:

I just installed a new derailleur, new chain, and new pedals. I needed a new derailluer because a similar problm happened and bent the old one and broke the chain.

What is wrong with my bike? Thanks!

  • What's the mileage on that cassette? They're wear items too, so it pays to record the date/distance when you change chain/cassette/tyres/pads/cables. – Criggie Aug 15 '18 at 10:14
  • Try starting in a different (higher/lower) gear than you normally start in. I bet the problem reduces or vanishes, which implicates the gear chosen. – Criggie Aug 15 '18 at 10:15
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Cassette sprockets are highly worn and chain is slipping off of sprocket teeth, or (and more likely I think) the freehub freewheel mechanism is slipping, either due to pawls not extending properly or damage to pawls or body.

Sometimes freewheel mechanisms get gummed up with dried out grease which makes the pawls stick. Liberally applying solvent can sometimes unstick them, but I'd look into disassembly and regreasing.

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When a chain wears down it gets longer. The greater distance between the links doesn't match the spacing between the teeth. This means the chain links rub more on the teeth and wear them down faster. If you have just replaced the chain without the cassette, it could mean the teeth of the cassette are worn and don't align correctly with the new chain.

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  • Good answer - I'd also add that trying to use a new chain on an old worn block/cassette will drastically accelerate chain wear. – Criggie Aug 15 '18 at 10:16

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