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I have a 2 month old Vodoo Bizango from Halfords in the UK. I have had this problem about 2 weeks into having the bike where if I'm on the highest gear and want to do sprints (standing on the pedals) the chain will choke/make a churning noise and then the pedals will just fly forwards, I have not been able to replicate the same thing when I turn the bike upside and lay its on it's seat. This caused one of my bike chains to snap, supposedly they replaced the derailer and the chain but I'm still having the same problem. It's now started to creep to lower gears now when I'm climbing up hills too.

I'm a complete novice at bikes and the mechanics but I need this sorting, it's my main mode of transports especially to work.

All help is appreciated, thank you.

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  • How many miles on the bike? One first suspects worn cogs, but that would only happen with 2-3 thousand miles wear. Also, a too-long chain is suspect, as is a faulty derailer (lacking spring tension) or simply poor derailer adjustment. Or maybe a bent derailer hanger. – Daniel R Hicks Dec 30 '14 at 13:23
  • Can you elaborate on "fly forwards"? Does it just jump and then catch again, or does it spin freely for a whole rotation or more? It could be worn cogs, but I'd be more inclined to look for a failing freehub. – Deleted User Dec 30 '14 at 17:03
  • The bike has had maybe 200-300 miles at the most and thats really over exaggeration, I am really a bike novice so not sure. it needs to go back for a check but the people in the shop are hopeless. – user3103598 Dec 31 '14 at 12:35
  • @chrisinAk yeah it just seems to free spin for a half rotation and as you get to the bottom (so the free spin starts at the top) it seems to catch again. – user3103598 Dec 31 '14 at 12:36
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Sounds like the problem might be that your freewheel isn't freewheeling.

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  • Or, is freewheeling in both directions, instead of only the one. – Deleted User Dec 30 '14 at 17:51
  • Re-reading the original post I think we're both right. I was thinking his free wheel wasn't letting him coast when he finished his sprint. That could cause the chain to bunch up and break his chain. But that wouldn't be consistent with his pedals flying forwards when climbing up hills. A bad freewheel/freehub could do both (lock up when coasting and slip when applying power). If you have another wheel you could substitute it and see if that fixes it. – Eric Dec 30 '14 at 17:56
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Sounds to me like the chain is jumping on cassette

As the chain and cassette wear they just don't grip each other as well. Under higher force the chain can jump.

It will typically start in the higher gears as you have less teeth engaged.

If you are in 4th 99% of the time it might start in 4th just because it is the most worn.

Compare your cassette to a picture of a new one. It the teeth are sharp and the dip worn long you need a new cassette. And you should replace the chain when you replace the cassette.

worn cassette

It should not wear that much in two months but something to check.

Another cause could be slack in the chain or chain not as tight. In the highest gear you let out the most chain so the derailleur might not be pulling the slack out of the chain as effectively.

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