Front derailleur rotates on frame

Back in the fall, my bike suddenly quit shifting into the small chainring (of a triple). I suspected a misaligned front derailleur, but was too afraid to touch it; a friend of mine agreed and re-positioned it. It worked fine then and whenever I checked it on the stand after washing, but I've been in Florida all winter without any need for it.

It shifted fine after I re-assembled the bike in Ohio. But the first time I actually needed the puppy gear, guess what. (Yeah, I made it up the hill, but I need that gear. :o) From what I can tell, it looks like the derailleur is again rotated counterclockwise on the frame, just slightly. But I'm a little afraid to touch it, again. :rolleyes:

What could have caused this the first time? Did it happen again because my friend didn't put a torque wrench on it when he fixed it in December? Can I assume that the specified torque for the seatpost clamp is safe for the front derailleur clamp as well, on a carbon frame? Do I need to just HTFU and move it back to where it eyeballs in proper alignment, being careful not to change the vertical placement?

Are you having problems with it now?

Is it severe? You should generally check the tail rotation when in the big ring small cog combo. In this gear, your chain should be pretty much centered in the cage. Judge this by standing near the rear wheel and looking at the lower horizontal piece of the cage. Note that this is independent of whether you're getting rubbing on the cage (farther towards the clamp) due to an incorrectly set upper limit.

If it has moved on its own, then it could be that it wasn't clamped correctly, but I would think that if it was loose then it would go too low rather than rotate because of downward tension from the cable.

If it's not moving but seems like your cable keeps losing tension (seems to be set right in terms of limits and clamp but you keep having issues getting in the big ring), then maybe your cable is at the end of its life.

The cable's keeping tension, the cable was replaced this winter (I could look up the mileage, but it's definitely under 1200), and IAE it's the small chainring it won't shift into, not the large one. There's adequate slack in the cable when I try to downshift, so it's not the cable hanging up someplace.

I don't know what you'd consider "severe." The cage is moving enough to rub against the chain, if that's what you mean. But not shifting is "severe" enough for me. :p

I really can't imagine the low limit screw would have moved on its own. Last time it was definitely the derailleur alignment - I don't know how it moved on its own or why it moved counterclockwise rather than down, but it did. So I'm guessing that's what it is again. Will check it in the gear combo you recommend as soon as it stops raining and I can go out to the garage where there's some light. :rolleyes: But I'd like to get it solved.

If you're certain the L limit is correct, then another reason it may not shift to the small ring is that your cable is too tight.

Check the tail end of the cage alignment in the big ring little cog combo.

Then check for chain rub when you're in the middle ring and large cog. If you get chain rub on the inner cage, loosen the cable.

If there's no rubbing in the middle ring, then shift to the small ring and large cog and check your lower limit. If that's ok and it's still slow to shift to the little ring, loosen the cable tension and try again.

Again, if the clamp is loose, I would expect there to be more than just a lateral shift going on. The cage would most likely also drop down lower due to downward tension from the cable coming up from the BB. Then you wouldn't be getting enough clearance over the big ring either.

okay, now that there's light...

First thing I tried was the L limit screw, and that fixed it. (Yes, I did try that back in December and that's definitely not what it was then.)

As I said before, the cable is and was plenty slack when shifted to the low position.

But that begs the question of how the screw spontaneously tightened itself. If anything, you'd think it would back out, and I really wouldn't expect that screw to move at all.

Also, just for grins and never mind why I didn't do this months ago, I put a torque wrench on the clamp. (A good CDI click-type torque wrench, same one we use to put the valve covers on the motorcycles, not a cheap beam wrench.) I set it to 50 in-lb (well within both Shimano's spec for the clamp and Cannondale's spec for the seatpost clamp), and it turned a full turn without clicking, at which point I quit tightening it any further.

Any input on (1) how the L screw could have tightened itself or (2) what the proper torque on the derailleur clamp should be?

There is probably a spec/manual page for Cannondale somewhere. At least all the warning stickers on my Specialized says to check the web manuals for torque specifications.