I was riding and shifted under load--somehow the front derailleur got stuck in between the teeth of my big chainring, such that I could not spin the crank at all while on the big chainring, but luckily I could still bike if I shifted to the little chainring. Since I was in the middle of a long, speedy ride during which I needed my big chainring, I just moved my clamp-on front derailleur further away from the chainrings (i.e. closer to the saddle).

Searching the forums, I've found a few other people describing a bent or rotated FD, but I don't really understand how to bend my FD back into shape. Can someone walk me through the process? The "arch" of the FD cage appears to match that of the big chainring, but it's offset in a way that just lowering the FD would get the rear edge of the cage caught in the chainring while the front edge would still be ~2cm away from the teeth...to guide the eye, I've also uploaded a picture where I've overlaid circles of the same curvature over the FD cage and the chainring. Before this incident, the cage cleared the large chainring with a uniform spacing of ~2mm between the bottom of the cage and the teeth.

In the attachment topViewFront.jpg, the front of the top of the derailleur cage is warped. Is this part supposed to be flat/smooth? These are 105-5800 front derailleurs, if it matters.

Thanks!

Cage plate distortions are hard to discern from photos as cages are no longer straight and parallel to rings in all cases. But cage and ring edges relating to each other are far more easily observed. Unless you are running a tiny large ring the cage/der is bent down at the cage's back end toward the ring by a lot.

But this brings a question to my mind. Usually the cage bottom edge is roughly a couple/4mm above the large ring around the cage connection to it's links/der body. In your photo that point id much farther above the ring's teeth. Was this the case before any potential bending might have taken place?

If a frt der is badly positioned the chance for chain jamming or something else catching the cage's end is greater then when the cage is properly positioned vertically.

It might be that the cage was bent both up and out to a great degree that we see what you show. Usually though when this is the case the two cage plates are no longer parallel to each other in the vertical plane.

Again this just shows why forensic analysis is so hard with out the stuff in hand, or any past history known. Andy.

The front derailleur was initially clamped on much lower. Because the derailleur got bent in the middle of a ride where I needed access to both chainrings, I clamped the front derailleur much higher, as shown in my first post's photos.

Attached are some photos of the front derailleur in a lower position, around where it was at the time of the bending. First, a depiction of what the front derailleur's position was before getting bent. In this photo, the clamp has been totally loosened, and I'm just holding the cage what seems to be a proper position. Based on this photo, I don't think the cage's arch has been affected much since it still matches the chainring profile pretty well.

And second, here is the front derailleur at the same height, but with the clamp totally tightened on the seat tube. Tightening the clamp forces the cage to rotate to this skewed position where the rear of the cage is catching in the chainring's teeth while the front of the cage has too much clearance.

Fortunately, my bike was purchased fairly recently from Nashbar, and they agreed to send me a free replacement FD! But I'd still like to hear from people who've fixed this problem how they did it.

It's a simple 2 step process which you have figured out already.

1) Throw bent FD in the trash.
2) Replace it with a new one that's not bent.

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