Another noobie question

[Halfmanhalfbike]

Another dumb question which you guys will be able to answer in a second

Now that my rear jockey wheel prob is sorted the only thing wrong now is that when I freewheel the chain comes off the big sprocket. No probs when under tension.

Is it just that the chain is too slack ?

Advice appreciated. If I can fix this one I will have stripped down a bike to it's basic frame + bits and re-assembled it all to work again. A bit of a minor miracle for a mechanical moron like me!

 

[Tynan]

useless too

my wild guesses will be chain tension as you say and rear mech not being in line

 

[spandex]

Is the chain / jockey wheels mucky?

 

[spandex]

Trying the adjuster barrel half a turn clockwise or anticlockwise and see if that helps.

What will that do?

 

[spandex]

Yes I do

It will mess up the gears and not fix the problem

 

[spandex]

Ok

 

[jpembroke]

You say sprocket so I assume you are talking about the cassette and not the large chainring. Anyway, in lowest gear set the limiter screws on the rear mech using a screwdriver (for large sprocket turn limiter screw for low gears - it may have an 'L' by it). This will set the limit of the rear mech arm i.e. the point beyond which it can't move. Then do fine tuning with barrel adjuster (best to do this with a middle gear selected e.g. 4th or 5th). Seems odd that it comes off when freewheeling but not when under tendion; it's usually the other way round. The chain should be static when freewheeling so unlikely to unship. Anyway, set limiter screw and then adjust.

 

[spandex]

You say sprocket so I assume you are talking about the cassette and not the large chainring. Anyway, in lowest gear set the limiter screws on the rear mech using a screwdriver (for large sprocket turn limiter screw for low gears - it may have an 'L' by it). This will set the limit of the rear mech arm i.e. the point beyond which it can't move. Then do fine tuning with barrel adjuster (best to do this with a middle gear selected e.g. 4th or 5th). Seems odd that it comes off when freewheeling but not when under tendion; it's usually the other way round. The chain should be static when freewheeling so unlikely to unship. Anyway, set limiter screw and then adjust.

That is why it is not the limit screws or the cable tension as they just come into play when the chain is moving.

 

[jpembroke]

It may be a knackered freehub i.e. the freehub does not actually freewheel but continues to turn slow as the wheel rotates. That would certainly cause the chain to unship.

 

[jpembroke]

That is why it is not the limit screws or the cable tension as they just come into play when the chain is moving.

I agree but it's always worth starting with the simplest (and cheapest!) options and working from there. If the jockey wheel is set too far towards the wheel then it's possible - unlikely I admit - that a jolt could unship the chain from the sprocket.

Another possibility is that the mech is damaged and is not taking up the slack as it should. The chain may be the correct length but the chain tension would be affected. Mind you, I'd expect this too be more of an issue in higher gears (i.e smaller sprockets) where the mech has to take up more slack.