Originally Posted by
dsbrantjr
Did something else happen to the bike? Was the bike dropped or did another bike fall against it? .... .
Take this to heart, and check everything but especially the inner limit screw setting, before simply correcting the trim.
Gear cables don't shrink, so if the RD is suddenly out of trim, especially if now trimmed too far in, it's almost certainly because the hanger (or RD, though less likely) is bent inward a bit, probably from an impact. You can correct the trim with the adjusting barrel, but since the entire RD is now farther in than before both limit screws will be off. Ignoring this can lead to over-shifting the chain or RD into the spokes, with nasty, expensive consequences.
The best test for the inner limit screw is to shift the RD as far in as possible by pulling the bare wire away from the frame like a bow string. Do this off the bike while pedaling slowly with the wheel off the floor. Adjust the limit to the tightest setting that still allows you to shift to low crisply from the adjacent sprocket.
Now set the trim on the second or third to outer sprocket (not the outermost), by putting the lever in the corresponding position and centering the RD so it runs quietly. Then fine tuning so you can shift in either direction smoothly. Finally, adjust the outer limit so you can shift to high from the next sprocket smoothly, and the chain runs quietly i high.