What causes my chain to do this?



It happened on my commute the other day. Good thing I already got to the other side of the road when it happened (Yes, I tried to beat the light! ).


Anything I should do to prevent this from happening again?

As always, thanks in advance.

Sounds like a case of Chain Suck. https://fagan.co.za/Bikes/Csuck/

Picture looks like you're in the small ring. And you were trying to make a light? Did you cross-chain and either a) have inadequate long arm rear derailleur to tension the chain? (because it's sagging in the pic) or b) or do you simply have a few links short? Those would be my first choices.

No, I am sure I wan't cross-chaining. I do have a long cage RD on the bike.

I did, just replaced the chain, though.

Possibly a tight link in the chain, where it was joined. I've seen this happen causing considerable damage to the bike.

Sorry, my bad. I meant a few links "long." not short. But the reason why I'm a bit confused is because the lower part of the picture shows a chain with curvature. There doesn't seem to be sufficient tension, even though you're displaying a picture with the small ring already sucking a link or two of extra chain. So where does the chain slack come from? Or that just an artifact of the partial photo?