Chain being thrown off

Discussion in 'Bicycle Mechanics and Repairs' started by chemswot, 6 May 2010.

  1. chemswot

    chemswot New Member

    This happened today off to the inside toward the bottom bracket. I'm wondering what causes it. Now I bought this bike second hand - the LBS straightened the middle chainring and adjusted the limit screw so that it only slowly changed to the smallest ring, but it still comes off occasionally (less so now, but still is annoying on a hill and gets my hands dirty...).

    Can it be solved with a new chainset, as they suggested? Any other ideas as to what it could be? Does it just happen occasionally to anyone? Might just carry some disposable gloves...

    Can't use one of those chain catcher things like the Jump Stop - the mounting on the derailleur is in the way...
     
  2. Gerry Attrick

    Gerry Attrick Lincolnshire Mountain Rescue Consultant

    I'd still suspect your lower limit screw is incorrectly adjusted. Try turning it clockwise half a turn at a time, riding between adjustments to check if it is ok.
     
  3. fossyant

    fossyant Ride It Like You Stole It!

    Location:
    South Manchester
    Cables need to be clean..had new cables ?

    Also mech adjusters will need moving in a bit - you shouldn't need a chain catcher on a well set up system, but if you have a carbon bike, it's worth having one..just stops the clear coat mess it makes......

    If the chain rings aren't straight running, you need new ones.
     
  4. OP
    OP
    chemswot

    chemswot New Member

    The chain already rubs against the cage when it's on the lowest gear. But I'll try when I'm out tomorrow, thanks. Hopefully it won't affect the shifting too much.

    Could it be anything else?
     
  5. OP
    OP
    chemswot

    chemswot New Member

    Just saw your post Fossyant - cable is newish. Replaced that a month or so ago. What do you mean by the mech adjuster? The barrel adjuster?
     
  6. fossyant

    fossyant Ride It Like You Stole It!

    Location:
    South Manchester
    Mech adjuster is the pair of screws on the front mech near where you pedal.. have a look but the one that stops the mech moving in too far to the frame needs screwing in...a little bit....so it stops it.... have a good look at what is going on...you will see..simple mechanics...........
     
  7. g00se

    g00se Über Member

    Location:
    Norwich
    Could the bare cable be sticking somewhere? Could the front derailieur be missaligned laterally? That is, it might be touching towards the rear of the front mech but the rear of the front mech is sticking out a bit?
     
  8. RedBike

    RedBike New Member

    Location:
    Beside the road
    Assuming the limit screws are set correctly and the mech is straight then either your chainring is bent or theres some unwanted flex in the bottom bracket.

    As you pedal look down and watch the chainrings. If they appear to be moving in and out relative to the frame then that's your problem.

    You will probably need a new bottom bracket and or chainrings. Your unlikely to need a new chainset
     
  9. sayek1

    sayek1 Well-Known Member

    Location:
    Glasgow
    My problem turned out to be "chain suck". Look on the internet as there is a vry good explanation somewhere.

    This might not be your problem (caused by a couple of things including worn/bent front cogs, chain wear & others) but worth checking.

    I have now learnt how to remove freewheels & casesttes since reading this - videos on the intranet - great stuff.
     
  10. Cubist

    Cubist Still wavin' Moderator

    Location:
    Ovver 'thill
    If it only happens occasionally, is there a common factor, for example it comes off if you're in the smallest sprocket? In which case it's simply a bit of crossover. Try changing to bigger sprockets at the rear before changing onto the granny....it will smooth things out ayway.
     
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