Chain Jump in largest gear

Today I was out riding and decided to sprint a little ways, so I shifted up into the never-really-used 52x12 combination (the highest gear on my bike) and as soon as I opened it up, the chain jumped off the big ring. I've determined that it's not a derailleur issue, but could be a bad chain, overly flexible BB, or some other crankset issue. One other noteworthy thing is that it only jumps when I'm pedaling downward with my left foot, opposite the crank.

Any ideas what the problem could be, and more importantly, how to fix it? I'm tempted to just take it to the LBS tonight or tomorrow and let them deal with it.

 

It's falling off to the left then... If your chainring has rarely been used then chainring wear isn't a factor. Worn chain? Use a chain wear indicator (or a quick rule of thumb is to look for pin-to-pin length much beyond 12"...getting close to +1/8" means time to replace...1/8" is roughly 1% elongation which means time to replace! or wear out your rings/cogs).

 

Perhaps the chain isn't tight enough... that usually shows in bigger gears (small cog)...

 

I doubt it is the chain or you would have seen a problem earlier. From your description it sounds like a bottom bracket issue.How old is your bike? There could be a little play in the bb (from past experience) if the BB is a ball bearing type. This rarely happens with a sealed bearing type BB. Put the pedal at the bottom of the stroke and stand on it. One side then the other. Listen for and feel if there is any movement.

 

The bike only has 160 miles on it so I doubt it's anything wear related, and the chain jumps to the right when pedaling with the left. I have no idea what type of BB the bike has.

 

Have you tried winding in the front mech hi-limit screw a fraction?

 

I have not, and I might try that.

 

Are you kidding me?!?!? Jumps to the RIGHT? And you haven't tried the limit screw? Please tell me you're kidding us and this is all a bad joke...