Hi guys,

I took my bike out a while ago and on the road home (20-25 minutes) my bike chain popped off three times as soon as I got up onto the pedals and gave any amount of forward acceleration (probably would have happened a lot, lot more if I'd kept doing it but I decided to stay on the seat for the rest of the journey seen as my kneecaps aren't fond of smashing off the bars every time the chain comes off).

After getting my bike home and having a quick check I noticed that the chain seems to be really slack, but after turning the cranks I've noticed that it's only slack during one point of rotation of the sprocket (e.g. chain appears tighter when the left crank is high and loose when right one is high). My initial thought was that the sprocket had broke and malformed for some reason but it appears fine, so maybe the issue is a lateral one? I have three tiny spacers between the cranks and the frame and I've noticed they are very slightly loose, could this maybe be causing some sideways movement that's causing the sprocket to sit at a funny angle? The rear sprocket also appears to have some lateral movement which would maybe be causing the chain to flick off.

What a nightmare, has this happened to anyone else or does anyone think they know what the issue is?

Cheers,

Zen

In general the chain has to be loose to be able to come off. So more chain tension could help. But when the latter is not constand during a full crank rotation, it depends on the amount of the difference. If the tight position is as tight as you are wiling to have it and the loose position is still to loose, then you can not solve the problem with more chain tension.

In most cases the chain tension is not fully constant. This is caused by tolerances in the production of sprockets, chains and drivers. And also by worn out center holes of the sprocket or similar problems like K.Wong mentioned. I had problems with a ovalized sprocket. In this case it was caused by a small crack at the area around the center hole of the sprocket. You might have a close look at the sprocket to check if everything is fine here.

I could imagine that your chainline isn't good. Maybe you can adjust the axial position of your sprocket by putting more (or less) spacer between the bearing and the sprocket. The chain should be as straight as possible, just in line when you look from the rear end of the bike along the driver to the sprocket...