Chain jumping on rear cassette

spatch1969
spatch1969 Posts: 5
edited March 2009 in MTB workshop & tech
Hi,

I am hoping that you can help with an odd problem I am having. All of a sudden, after the bike has been laid up for a while the chain is jumping intermittently off 6th, but only when on the middle or small chain ring. I will pedal for anything like 4+ revolutions and at some point the chain will jump. The rear mech is adjusted properly, all other gears are fine and it shifts smoothly as well. I have checked the chain for tight spots but is fine.

I had a propbem with my old Deore thumb shifters where it would rattle in one gear only, and it turned out to be a worn index position on the shifter. The problem I currently have is different as it is quiet in 6th, until it jumps and it is then fine for another few revs.

Originally (when I was on rollers) it seemed to jump every 8 revs (I could count them) which led me to think it might be a combination problem bertween chain and cassette (i.e when a certain point on the chain corresponded with a certain point on the cassette, then it would jump), however, on the road it doesn't seem so repeatable.

The bike is a home built Zaskar Race with LX chainset and front mech, XT rear mech and an LX (I think) 7 speed cassette running a HG chain. Bike was built up about 5 years ago, and has been fine. I have not done a massive amount of cycling on it and there is no evidence of wear on the cassette (you can still clearly see the manufacturing marks on the teeth).

I do not want to wholesale start changing items as I cannot afford this, plus I do not see why this should have suddenly started happening.

Any ideas would be greatly appreciated.

Thanks
Mark

Comments

  • supersonic
    supersonic Posts: 82,708
    Is the cassette fully tightened? Also check carefully for any bent teeth.
    GT ZASKAR CARBON TEAM

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  • cullen_bay
    cullen_bay Posts: 256
    bent/ missing teeth on the casset of cranks can do that, maybe the tension on the mech isnt right
  • spatch1969
    spatch1969 Posts: 5
    Thanks for that. The cassette is tight and there is no play with any of the gears, and the teeth on 6th do not seem to be bent - there is some very slight variance but this is just the same as on the other gears and I assume this is to do with gear change assistance.

    Any other ideas???

    Cheers
  • cullen_bay
    cullen_bay Posts: 256
    is it a new chain/cassette? also, the teeth might be worn down too much, those are the most likely thing, if you really want to stop the clicking, get a new chain and cassette, never just get one, because it doesnt work
  • Mister Mental
    Mister Mental Posts: 63
    new chain and cassette
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    Resident Grumpy Smurf
  • spatch1969
    spatch1969 Posts: 5
    The chain and cassette are not new, but have done little mileage. The manufacturing marks on the teeth are still very evident across the whole profile and have not worn away. Certainly, swapping out the chain and cassette would probably solve the problem but neither are worn, and I do not want to change out perfectly serviceable items - hence looking for any advice on the forum.

    All I can summize is that either the chain and/or sproket have got damaged somehow. I have a spare chain so will swap this out to see if it improves the situation. Then perhaps try to get a new 6th gear if it does not.

    Thanks for all the replies.
  • spatch1969
    spatch1969 Posts: 5
    Thought I would just follow this up. Turned out to be the chain.

    Mark.

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