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  • chain comes off, gets jammed

     Gunner updated 11 years, 5 months ago 5 Members · 9 Posts
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  • allenh

    2 August 2009 at 2:37 pm

    my sons bike chain comes off far too often but the real problem is it gets jammed at the front end.

    bike is a GT Aggressor 4, 24″ wheel, 27 speed

    at the front mech the chain drops down between the small chain ring and frame and gets jammed there. there are 4 rivet or bolt heads that the small ring is fixed in place with and the chain slips over these into the channel between the chain ring and frame, and wont come back over them. in this position the chain is not lined up to come out easily either. very rarely i get it in just the right position to ease it out but usually have to use brute force. the frame gets scratched each time and i’m sure its not doing the chain or ring any good. my son is 9 and doesnt have the strength to do this.

    the dealer (halfords) made some adjustments to try to prevent the chain coming off so easily but it still does about once every 2 weeks on average.

    see pic attached

    (imo children’s bikes are overcomplicated by having so many gears – just another thing to go wrong.)

    please can anyone suggest anything i can do to prevent the chain getting jammed? eg a device to buy or make for example that prevent the chain dropping into the well between the chain ring and frame? or something occupying the well to block it?

    many thanks


  • Dale

    2 August 2009 at 2:40 pm

    It sounds like the front derailleur (the bit that moves the chain) is going too far. Although you took it to Halfrauds the chances are they didn’t fix it correctly. If you follow the cable from the derailleur to the lever on the handlebar there should be a nut you can twist. Just tweak this until the derailleur isn’t going too far past the edge of the sprocket.

  • Steve Kish

    2 August 2009 at 3:42 pm

    Front changer just needs moving outwards a bit at it’s slackest point.

    There should be two screws next to each other somewhere near the top of the front mech. Sometimes marked H-igh and L-ow but if not, could be either one, as they vary between different mechs. You need L-ow, of course.

    The way I test it is to change to the biggest ring and put pressure on the thumb shifter – the harder-to-turn of the two screws will be for the big ring, so the other one will be the one that you need for the small ring.

    Put the cassette into the biggest sprocket and smallest chainwheel (lowest of all the gears) and screw in the adjuster for the smallest ring to adjust the front changer so that the chain just lightly touches the inside of the cage when you turn the pedals. You should see the cage move in slightly as you do it.

    This sometimes happens when bottom brackets are changed, as the cranks screw on just a few microns short of the worn old one, thus moving the alignment outwards.

    Take it to Halfords AT YOUR PERIL!! 🙄 Most of the ‘mechanics’ there are monkeys!

  • allenh

    5 August 2009 at 7:07 am

    thanks v muvh for your advice guys. do you mean that the chain shouldnt ever come off if the front mech is adjusted right? in fact the chain has come off when its been on the middle chainwheel, not just on the smaller one, ie the gearchange has been set to “2”. in this position the cage is correctly aligned with the middle chainwheel, so i dont think the problem lies with incorrect adjustment of the front mech.

    if it were possible, by adjusting the front mech, to stop the chain ever coming off at the front, that would be fine. but as i said the real problem is that when it does come off it gets trapped between the small chainwheel and the frame, and once there my son has no hope of getting it out himself.


  • badblood

    5 August 2009 at 8:49 am

    Could it be that the chain needs shortening? Which gear is he in at the back when this happens? Photos of the bike and drivetrain from the side would be useful.

  • Dale

    5 August 2009 at 8:55 am

    If you have it setup correctly it shouldnt come off no, but as said above if it’s coming off in the second ring on the front I would imagine its a bit slack and may need a link taking out.

  • Gunner

    5 August 2009 at 12:39 pm

    I also can’t understand why it would be soming off in the middle ring at the front and then be getting jammed between the granny ring and the frame. 😕 😕

  • Steve Kish

    5 August 2009 at 6:11 pm

    Dropped mine twice today! Adjustment follows tonight. 😀

  • Gunner

    5 August 2009 at 7:24 pm

    After re reading through this, I agree with all said above.

    The front derailleur needs properly indexing using the high/low screws on the mech itslef to stop this happening.

    Very simple procedure and problem solved.

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