Chain being thrown off small chainring

Meds1962
Meds1962 Posts: 391
edited May 2010 in Workshop
A problem has developed whereby my chain comes off the small chainring when I'm climbing a steep gradient, usually when I'm out of the saddle but sometimes when I'm seated as well. I don't think it's a general problem of setting the front mech as in all other situations it does not happen.

I'm running a 50/34 with 12 - 25 at the back and the whole setup has been fine until recently. It happened in the 34 / 25 combination this morning but it could be happening with the next sprocket down as well. At least I'm getting plenty of practice unclipping in a split second!

Any ideas please?
O na bawn i fel LA

Comments

  • amaferanga
    amaferanga Posts: 6,789
    Sounds like you need to adjust the inner limit screw as the mech shouldn't allow the chain to derail like that.
    More problems but still living....
  • keef66
    keef66 Posts: 13,123
    Wot he said.

    Plus, if your seat tube will take one, fit a Deda dog fang.
  • John.T
    John.T Posts: 3,698
    Is the chainring worn. This could cause the chain to jump. Not likely to be limit screws if it changes OK but comes off when not changing gear.
  • Meds1962
    Meds1962 Posts: 391
    The front mech set up is fine, about 2-3mm clearance between the cage and chain when it's in 34/25. It throws the chain off the chain ring even if it's two sprockets in from the 25 at the back. The chainset isn't worn and I'm staring to think worn chain and cassette as they are both older than the chainset.

    I'll compare the cassette teeth with the one on my other bike to see if there's much difference.
    O na bawn i fel LA
  • keef66
    keef66 Posts: 13,123
    You're unlikely to be able to see an obvious difference between a worn cassette and a good one. Put on a new chain and see what happens when you stand on the pedals; if the chain jumps on the cassette, it needs replacing.

    I'm not sure a worn chain / cassette would explain your problem though.

    I think the front mech should be 1mm from the chain when on the small ring.
  • Tom Butcher
    Tom Butcher Posts: 3,830
    I'd adjust the limit screw anyway - it may not be the answer but it's such an easy thing to try why not do that first.

    it's a hard life if you don't weaken.
  • ColinJ
    ColinJ Posts: 2,218
    Proper adjustment of the front mech prevents chain-dropping problems most of the time but when you come round a bend and are faced with a 25% wall, a panic shift to the little ring can often drop the chain. Deda Dog Fangs on all 3 of my bikes prevent that problem.
  • John.T
    John.T Posts: 3,698
    This problem is not when changing so is unlikely to be a limit screw one. It can only really be a worn ring or chain that do not get on together.
  • Meds1962
    Meds1962 Posts: 391
    It's definately not mech tolerance, I'll try a new chain and see how it goes.
    O na bawn i fel LA
  • John.T
    John.T Posts: 3,698
    John.T wrote:
    This problem is not when changing so is unlikely to be a limit screw one. It can only really be a worn ring or chain that do not get on together.
    I should add damaged chain or ring to this. Also if you are really unlucky a cracked frame leading to a load of flex.