Chain keeps coming off

[4F]

Having problems with my boys bike with the chain keep jumping off the smaller of the chain rings. It happens when he is either mounting where he moves the pedals backwards to get his foot into position to move off or when he pedals backwards when moving.

I have tried adjustments on the high / low settings on both the front and rear mech but still having the same problem !

It always seems to come off at the same place on the front ring but the ring and teeth do not appear to be bent or out of shape

It is a new BSO "Universal" which was won on the school raffle so its not possible to take it to the shop where it is purchased.

Please heeeeeeeeeeeeeeeeeeeeeelp

 

[MessenJah]

It is a new BSO "Universal" which was won on the school raffle so its not possible to take it to the shop where it is purchased.

Please heeeeeeeeeeeeeeeeeeeeeelp

Take it to a shop where it wasn't purchased...

 

[4F]

Take it to a shop where it wasn't purchased...

Thanks for the helpful and informative reply. I would rather repair it myself first if possible and was looking for a few pointers.

 

[fossyant]

Stiff chain, poor running rear mech jockey wheels - i.e. causing chain suck or poor chain alignment -

 

[MessenJah]

Thanks for the helpful and informative reply. I would rather repair it myself first if possible and was looking for a few pointers.

Well it's the most logical answer isn't it? If you can't figure out what's wrong with it then it's going to be easier for a mechanic who can actually physically look at the bike, than for someone on the internet who can only see your words.

 

[GilesM]

If it comes off when he back pedals then the chain line must be bad, the rear mech could be badly twisted/bent or the chain may just be a bit slack, does it only happen when it is on the small chainring and small sprockets.

Apologies, but a bit difficult to help with out seeing it.

Giles

 

[4F]

If it comes off when he back pedals then the chain line must be bad, the rear mech could be badly twisted/bent or the chain may just be a bit slack, does it only happen when it is on the small chainring and small sprockets.

Apologies, but a bit difficult to help with out seeing it.

Giles

Hi Giles, only seems to happen on the small chainring and large sprocket. The rear mech does not appear to be twisted however I did wonder this morning whether to double check the length of the chain and possibly take a link out.

 

[briank]

If the chainring and the rear mech are not bent or warped, as you say, then I can't see what else could eb the cuplrit but bad chain line or chain too slack. You wouldn't expect a perfect chain line between either chainring and either of the sprockets at the tip or bottom of the cassette, but a good straight edge should reveal whether the smaller ring is about in line with the one-larger-than-middle sprocket. (A piece of string held taut makes a surprisingly good substitute for a straight edge.) If it is, then the chain must be too loose.
You sound like you know how to check this and change it if need be - bu remember to leave the chain JUST long enough so that when your lad accidentally shifts onto large chainring and largest sprocket he doesn't pull the rear derailleur apart.
If the cassette is too far inboard (? too close to the centre line of the bike) to get a reasonable chain line then I'd go to my LBS and expect to pay. Moving the chainrings in is virtually impossible, and moving the cassette out would require putting spacer(s) behind it - and it might not even be possible to put enough there to make the difference. Perhaps it has the wrong rear wheel? Or the amount of dishing can be changed?
Sorry if this is all obvious...

 

[4F]

If the chainring and the rear mech are not bent or warped, as you say, then I can't see what else could eb the cuplrit but bad chain line or chain too slack. You wouldn't expect a perfect chain line between either chainring and either of the sprockets at the tip or bottom of the cassette, but a good straight edge should reveal whether the smaller ring is about in line with the one-larger-than-middle sprocket. (A piece of string held taut makes a surprisingly good substitute for a straight edge.) If it is, then the chain must be too loose.
You sound like you know how to check this and change it if need be - bu remember to leave the chain JUST long enough so that when your lad accidentally shifts onto large chainring and largest sprocket he doesn't pull the rear derailleur apart.
If the cassette is too far inboard (? too close to the centre line of the bike) to get a reasonable chain line then I'd go to my LBS and expect to pay. Moving the chainrings in is virtually impossible, and moving the cassette out would require putting spacer(s) behind it - and it might not even be possible to put enough there to make the difference. Perhaps it has the wrong rear wheel? Or the amount of dishing can be changed?
Sorry if this is all obvious...

Cheers. I have a day off tomorrow so will have another look and try the string method. Thanks

 

[GilesM]

I was up early this morning working on a bike as my other half was cycling to work, but had rack problems for panniers to carry her work clothes, while I was sorting her rack out, I was thinking about you chain coming off problem, to me it has to be the chain line and/or a slack chain. For the chain to come off the small ring when back pedaling the then the bottom jockey wheel in the rear mech must be too far to the left (left when sitting on the bike in normal position) of the small chainring, remember when back pedaling the chain goes from the rear mech to the bottom of the chain ring before going throug the front mech, so it can't really be a front mech problem, however it doesn't happen on the bigger chain rings when the chain line would be worse for the larger sprockets, however the chain would be tighter on the larger chain rings. Having said all that it does sound odd that a chain on a new bike would be too long, not impossible though, so one other option may be the spring in the rear mech is not keeping the chain correctly tensioned.

Just the thoughts of strange mind when looking at a bike transmission this morning, apologies if I'm stating the bleeding obvious.

Giles

 

[4F]

Well sorted out problem, the chain was too long by 3 links. Now runs like a dream so thanks all

 

[Angelfishsolo]

Congratulation on sorting out the problem. I bet you feel pleased with yourself now for having fixed it and saved some cash.

Well sorted out problem, the chain was too long by 3 links. Now runs like a dream so thanks all