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?