Dereilleur issue - cage moving forward when freewheeling

[Sheffield_Tiger]

Apt, perhaps given the Google Helpouts thread - virtual assistance?
Just had this on facebook:

Its fine when you peddle but as soon as you stop and the bikes moving the derailleur shoots forward and chain comes off?? Its probably something minor I know but I have no clue about bikes..lol

So...when the bike is moving forward the rear mech jockey wheels pull to the front of the bike and de-tensions the chain?

I've seen this bike as much as you have (i.e. never)

My guesswork is that the B-tension screw has gone awry or even the stop sheared (have a mech in my spares drawer just like that), so the jockey wheel is hitting the sprockets.

Or the freewheel isn't freewheeling and is pulling the chain round but that would loosen the chain at the to, rather than the bottom, surely?

Any other ideas?

 

[Sheffield_Tiger]

Hmm...thinking about it, the second option (seizing freewheel) sounds likely. If the chain loosens at the top then it would tighten at the bottom ("derailleur shoots forward")..

Visualisation of things after the pub sometimes takes a while...

 

[Cubist]

Possibly chainsuck. Would the chain balling up behind the chainwheel drag the rear mech forward in the split second before it jumps off?

 

[slowmotion]

I had that problem with a dodgy freehub. It went away when I changed it, but that isn't necessarily the solution to your problem. Good luck.

Edit: It used to drive me bonkers when I put the pedal at the 60 degree position, waiting to push off from stationary......and find that there was just slack in the chain.

 

[bianchi1]

My guess would be freehub not running free.

 

[marcusjb]

Freehub does sound the most likely culprit for sure. On a work stand, a misbehaving free hub will almost make the bike look like a fixed wheel and the pedals will keep turning when you stop pedalling (you wouldn't notice this when you are on the bike). Holding the pedal and freewheeling can cause the chain to drag forward as described.

What hub is on the bike?

 

[Sheffield_Tiger]

What hub is on the bike?

No idea. Neither has the rider. Probably a screw-on freewheel.

It can only get worse, he sent me the message "don't worry, I'll take it to Halfords".
"Don't Worry" is not normally a suitable prefix for "I'll take it to Halfords"

 

[the_mikey]

It sounds like a failing freewheel.

 

[Gravity Aided]

Might be a sticky pawl in the freewheel, actuated by a spring, which may have gone bad through age or rust. Problem with taking apart the freewheel is the multitudinous assemblage of little bearings and parts. Probably new freewheel is easier and cheaper in terms of time and aggravation. These freewheel episodes can be intermittent, but worsen with time, maybe a little, may be a while. Time Bomb.

 

[actonblue]

+1Freehub. Has it been cold in your part of the world ? Sometimes the grease solidifies in low temperatures.

 

[solidthegreat]

Strip the derailleur and check parts are moving ok. If all ok then check freehub. My money is on that as well but at least you will rule out seized jockey wheels, limiter screws etc.
If you you tube changing freehubs it gives you an idea of how to do it. One of the harder jobs to do but still achievable to a novice with the right tools, just have a go, what have you got to lose if you are thinking of buying new wheels anyway.

 

[Paulus]

My guess is the freehub. Does the chain go really slack if you rotate the pedals backwards when standing still? I would have a go at taking the freehub off and use plenty of penetrating oil or a light lube to see if it frees up. failing that, a replacement.