Front derailleur, or warped chainring?

I've finally figured out how to adjust my front derailleur, but I'm running into problems: If I'm in the lowest gear at the back, the chain rubs on the inside (frame side) of the front derailleur cage, no matter which chain ring I'm using (I'm using a triple). I expect a little rub in the big ring, and it doesn't worry me too much because that's not a gear combination I'm going to be using. The other two are a little weird. I've fixed the rub on the small ring. The middle ring is the weird one. If I loosen the lower limit screw any more, I'm worried about losing it on the road (and bad things would probably happen). Is it possible that this is caused by a warped chainring, especially since it doesn't appear to be rubbing constantly?
And is it really likely that the middle chain ring would warp without the large ring being affected?

Rear derailleur hanger could be bent. That would keep your chain from running true and will make adjusting your limit screws impossible.

Technically middle chainring/big rear cog is cross-chaining, even though I do it too. Does your front shifter let you trim? My Shimano R700 shifters do ... big push is a shift, little push is a trim. If I'm using the whole cassette in the middle chainring which I really shouldn't do, I need to trim at each end.

Did it not use to rub there and now it does? Is your FD clamp-on? It's possible for it to rotate on the frame. Mine did. Twice. Even though everyone swears that's impossible and if the clamp was loose it would slip down, not rotate. Mine did. Actually the symptom I was having was that it wouldn't go into the small ring, but I would expect rubbing could happen, too.

Limit screws aren't going to affect anything w/r/t the middle ring. Set those where they should be for your big and small rings, and do everything else with the cable adjustment.

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Originally Posted by OakLeaf

Technically middle chainring/big rear cog is cross-chaining, even though I do it too. Does your front shifter let you trim? My Shimano R700 shifters do ... big push is a shift, little push is a trim. If I'm using the whole cassette in the middle chainring which I really shouldn't do, I need to trim at each end.

Did it not use to rub there and now it does? Is your FD clamp-on? It's possible for it to rotate on the frame. Mine did. Twice. Even though everyone swears that's impossible and if the clamp was loose it would slip down, not rotate. Mine did. Actually the symptom I was having was that it wouldn't go into the small ring, but I would expect rubbing could happen, too.

Limit screws aren't going to affect anything w/r/t the middle ring. Set those where they should be for your big and small rings, and do everything else with the cable adjustment.

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Ah, that makes sense. I usually use one or two gears above for flat ground, but because of my muscle injury, I've had to move to lower gears. I haven't really noticed it rubbing, but that's because I haven't really used the gear. No idea if it'll let me trim. They're the low end shifters (2200). (There was a bit of cable adjustment to adjust the throw--now I can shift into the big ring!--so that may have played a role too.
I don't know what I'm doing well enough to play with cables, so I'll let the LBS take care of that.

Aw, c'mon, cable adjustments are WAY easier than limit screws! Just turn the barrel adjuster until it shifts right. If the cable is so loose that you've adjusted the barrel to the limit, then screw the barrel back down to the inner limit, get a pair of pliers and a wrench, loosen the cable clamp, pull the cable until it's just taut when the derailleur is in the small ring, then retighten the clamp, and take up any small amount of remaining slack with the barrel adjuster. Easy-peasy. :D