Chain drop under load

Question

I have a problem. Today I was riding (flat-out) up hill, and went to shift up a gear on the rear, which caused the chain to drop off the front chain-ring (was in the big-ring, so dropped to the outside). (For reference, road bike with Shimano 105, 52/39 and 25/12)

Throughout the ride it dropped a few more times - generally under some form of load. I noticed that it wasn't always dropping off the big chain ring. Occasionally when taking off from lights (in the little ring), when I stood up to get going, the chain would fall off the front chain ring, to the inside.

Is it possible this is anything other than incorrect limit screws? Maybe something out of:

• Chain problem (stiff link? But looks OK)

• Chain-ring worn (this is on a ~4 year old road bike, maybe 25,000km without chaning front chain rings, but they don't have the 'shark-tooth' pattern)

• Bent hanger?

Edit: Here is a look at the chainring

Answer 1

Edit: @Qwirk: If you look at your photo, where the chain is resting on the small chain ring, do you see the gaps between chain and teeth? that is a key indicator of either extensive chain wear, extensive chain ring wear, or both. In this case, based on the appearance of the upper chain ring, I'm going to say both.

Those are the first 3 options I'd look at, as well.

Have you replaced your chain recently? If so, the new chain may not be meshing well with the old chain rings.

To figure out which, first check the chain for wear.

If it doesn't show any serious chain wear, compare the chain rings on your bike to a set of new chain rings. It doesn't have to be shark toothed to e worn, but that is most common. Hooking, or simply teeth that are significantly shortened will cause similar issues.

Check as well for bent or missing teeth in the chain rings. This shouldn't be the case because it's happening in more than one gear, but it could still be. Also, check the rear cassette for missing or bent teeth, for the same reason.

Last, if the rings do not appear worn, you should replace the chain, (or soak it and re-lube it at a minimum). This will eliminate the possibility of a burr or a sticking point in the chain that is more subtle than a stiff link.

Oh, and a bent hanger would have to be pretty visible to cause this kind of activity, but it's still worth checking.

Hope that helps.

Answer 2

The first thing to try, of course, is adjusting the front derailer, but I suspect that it's a bad chain or the front rings or rear cluster are getting worn. (As the front rings get worn there's a tendency to increase the range of the limit screws to "encourage" shifting. If the rear cluster develops a "hook" then it will cause the chain to jump violently as you shift.) Could also be something (eg bent hanger) causing the rear derailer to grab.

(Note that a sprocket (front or rear) is worn out long before it has visible "hook".)

If it were happening on only one side it would be possible that it was a chain line problem -- wheel not centered or frame bent. But not likely with the problem in both directions.

[And if you haven't changed the chain in 25,000Km then it's is definitely worn out, and the cluster and rings are probably shot too.]