Rookie mechanic here...
I have a 2010 Trek 1.1 C road bike (58) that is less than one year old. From day one I've had a consistent problem -- the chain rubs the front derailer and makes lots of noise when I shift to gears 6, 7 or 8 on the rear cassette. I could tell the noise was coming from the chain rubbing the front derailer just by watching/listening with the bike on a rack.
What I did about it:
1. With the chain shifted onto the largest front chain ring I cranked the pedals and confirmed visually that the chain begins to rub when I shift to 6, 7 and 8 on the rear cassette. I did.
2. I adjusted the L screw on the front derailer to bring it outward a bit so it was just barely not touching the chain anymore, then shifted to 7 and 8 and observed no more rubbing or noise.
3. I pedaledt he bike around my neighborhood for several minutes and shifted constantly through all 8 gears leaving the front on 2. No problems or noise.
Now the front derailer does not respond when I try to shift onto the smaller chainring. I see the cable go totally limp, practically flapping in the wind underneath the down tube. The derailer doesn't move.
What did I mess up when I adjusted the L screw on the front derailer?