The low limit screw on my front derailleur constantly needs to be adjusted. I'll make sure all three chainrings can be accessed before I leave my house, but sure enough, every time I approach the bottom of a hill and proceed to shift down to my small chainring, something has slipped and the derailleur cage will not retract near enough to the small chainring to engage it. I then have to stop, whip out my screwdriver, let the low limit screw out a few turns so the derailleur engages it, and then I continue riding, having completely lost my momentum and at that point cursing my bike. A mile and a hill later, I'll have to do this again.
Let me be clear, this happens several times every ride. Why does my limit screw need constant adjustment? I ride a 2004 Bianchi Volpe touring bike, which I put about 2500 miles on in the last three months.
If the screw itself is actually moving, you could put some threadlock on it. That will keep it from vibrating loose as you ride.
It's normal for the shifting to behave a little different on the stand than on the road, but it should be consistent once you set it.
You got me thinking, and I'm not so sure it's the limit screw itself coming out of thread. Whenever I have to adjust it, I'm not turning it in; I'm turning it out. I could see a screw coming out on its own, but threading itself deeper? I'm having doubts its the screw itself, and worried that if I use threadlock on it I'll have just compounded the problem.
Unless the derailleur is pressing against the screw, it "could" screw itself either direction. If it's loose, I'd think it would tend to go "down" regardless of whether that's "in" or "out". Limit screws thread all the way through the derailleur body, so they are not engaging more threads as they go in which would tend to prevent that.
But is the screw fairly loose and easy to turn? If not, maybe something else is going on. Maybe try putting a piece of tape over the screw that will hold it from turning, but which is easily removed to test.
Are you sure the it is the limit setting that is changing? Does your shifter adjustment remain fine and it just won't drop in to the small chainring? Maybe check to make sure your shifter cable isn't slipping or that the derailleur itself isn't slipping on the frame. But it does sound like a limit screw problem. good luck