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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.

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  • Same goes for the pressure-adjustment screws on V brakes - I've seen bikes where they've fallen out completely.
    – Criggie
    Jan 25 at 5:27
  • 2
    The derailleur limit screws did rattle away from where I'd set them. These days, I'll put a drop of nail polish on them to efficiently glue them in place. Jan 25 at 9:37
  • are you sure it isn't the cable stretching?
    – tedder42
    Jan 25 at 23:31
  • Thanks everyone. @tedder42 I had just fitted a new cable when I found the limit screw was far too low, which I suspect was what had caused my initial problems in shifting. It's still bit of a mystery because the resistance on the screws is high, so all things considered drift seems unlikely. Maybe it was sabotage from a jealous rival? Jan 26 at 18:56
  • Update: It's possible that I'd misdiagnosed - 105 front derailleurs don't behave the same way when you move them with your hand as they do when you shift them using the cable: The upper portion of the mechanism can impede the motion of the whole because you are pushing it rather than allowing it to do the pulling. So what I thought was the limitation of a shifted screw might actually have been imposed by that effect. Feb 16 at 11:27

3 Answers 3

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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.

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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.

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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.

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