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I've just been adjusting my front derailleur, and found that the upper limit screw was severely impeding my shifting. My trouble getting into the big ring has been steadily growing, so it seems like the limit screw must have drifted lower over time. I didn't think that was possible on a static component, but evidently it is. Is this unusual?
I'm using a Shimano 105 group set.
They absolutely can vibrate loose. Derailleurs vary in how good they are at resisting it. Ones with no plastic retention element and where the screws turn with almost no resistance are the worst about it, which includes many old ones as well as some very low-end ones. Many old derailleurs have also used springs under each screw shoulder to prevent loosening.
In a shop context the best practice is use one drop of wicking low-strength loctite when you find a derailleur you're adjusting has no resistance at the screws. The wicking type lets you apply it after adjustment. This mostly applies to derailleurs where the screws are going into a simple metal piece, or where there was once a plastic retention component but it's already lost or broken/degraded. Adding the loctite to the more normal situation of a derailleur where the plastic is intact is bad because it's both not necessary and loctite can crack plastics. At home if you don't want to buy something special for the purpose, pull the screw completely and apply candle wax to the threads.
As Nathan and Weiwen point out - it is possible for the limit screws to drift over time. They have offered great solutions for preventing the drift.
Is it common? Not in my experience. I've seen it happen but what usually happens is the derailleur gets banged around while parking the bike or in a crash shifting its position relative to the cluster or chain ring. Front derailleurs can - if attached with a clamping mechanism - rotate on the seat post. Make sure the clamp is appropriately tight.
On a newer bike it is not uncommon for things to wear in and change as the bike gains miles - cables, chain, spokes etc. - requiring some adjustment.
In your case you are having trouble getting into the large chain ring which is usually a pull movement - the cable pulls the front derailleur up pushing the chain onto the large ring. Cables can stretch (especially on a newer bike) causing slack making changing the range of motion so that it might stop short of the limit screw.
In less common situations the inner skid plate can wear causing the need to adjust or in extreme cases replace the derailleur. In looking at the 105 front derailleur manual there is a removable skid plate. The manual (for the FD-5801 FD-R7000 versions of the 105) indicates it can be replaced if worn.
It's good to look at all the parts of the system when troubleshooting.
I suspect that it would be very unusual for a limit screw to drift. It is true that if bolts loosen, vibration is one possible cause, and bicycles are inevitably subject to vibrations unless they're track bikes (but if they are, then they have many fewer screws and bolts). Nevertheless, I have only had one limit screw (the B-tension screw on a Shimano R8000 rear derailleur) loosen with riding over all of my bikes. The mechanic I asked at the bike store said that he had not heard of this happening. This is why I think that it would be very unusual.
If you are absolutely sure that your limit screw is repeatedly loosening, then the obvious fix would be low-strength loctite. This is an adhesive that can prevent bolts and screws from loosening; the blue formulation is relatively low-strength and would be the version of choice here. Many bike stores probably have a bottle, so you can ask nicely if they can loctite your limit screw down.
Yes, I've recently had limit screws drifting a lot over time.
The cure is to apply some thread lock. If you have to apply thread lock to the front derailleur limit screws, I'd recommend applying it with the bike turned upside down, so the thread lock won't run down into the derailleur's pivots.
I'm no bike mechanic, but I write from recent experience with my own bike.
I actually have this problem with my DA-7900 front derailleur. The low limit screw will be noticably looser every 700-800 miles or so. Never had it happen on any previous bikes/derailleurs, so it took a couple times of the chain falling off when shifting to the small ring before I realized what was going on. I readjust about once a month now and it keeps it where I need it, and every time I do I remind myself to get some loctite for it but always forget until the next time I'm adjusting it :-D
