I've been having issues for awhile now on my Felt road bike with the chain dropping off onto the BB shell when shifting down from big to small chain rings. Of course I tried all the usual stuff like low limit screw adjustment. Took it to the LBS, they adjusted it, even dropped the height of the FD just a hair to get it closer to the chain rings, all that stuff. The FD outer plate seems to be just slightly bent, which I thought was pushing the chain too far over, and I noticed that if the chain is about to drop and I trim the FD up, the chain gets onto the small ring fine.

So tonight I did a little close-up "slow-mo" inspection to try to figure out what's up. Well it seems like just before the chain drops past the top of the small ring (chain still half on big ring and just above the small ring), the chain is cranked over at such an angle that's pushing it past the small ring. But it's not the FD that's doing it! Through some trial and error, it seems like it only happens when the chain drops off at this one pin on the big ring, and only when there's a chain outer plate just above that pin. See below.

FD has been shifted down, chain is coming off the big ring, notice the chain outer plate just above that one pin:




Chain is deflecting sideways as it comes off the big ring:


As it drops lower, the chain is deflected so far that it's going past the small ring. At this point the chain is still hanging on the big ring by that one pin, with a chain outer plate link just above it:








It appears to only happen when it's just so that the FD pulls the chain off the big ring at that one point by that one pin, with a chain outer plate above it. If the chain has one of the inner plate links above it, doesn't drop. If the chain pulls off at any other point on the big ring, doesn't drop. Seems like it would almost never happen, but believe me it does, 5 times on my commute this morning.

In most cases I can backpedal a bit to get the chain to go on the small ring. However a couple times this morning it would not go on unless I trimmed the FD up, which then made the FD inner plate keep the chain from going too far left past the small ring.

Any ideas what it is about the chain or the big chain ring that is causing the chain to deflect so much at that one point that it's coming off?