Fixit online!

Stop the drop!

Fixing chain-dropping with the front derailleur limit screws

• Problem: dropping the chain off the front chain rings

• Tools needed: small screwdriver

• Skill level: beginner 

Why have a technical page online?

Not everyone has the time or the desire to mess around with their bike and there is a lot to be said for having maintenance done by a qualified mechanic. But as a regular SFA rider, there is something very satisfying about knowing how your bike works. Feeling good about fixing your bike is one way of feeling good about riding it.

I’m not a trained mechanic, but doing my own maintenance over the years I’ve learnt a lot about the simple things that keep me and my bike on the road. So the aim here is to make these online articles as relevant and accessible as possible: no unnecessary or unexplained jargon, just clear tips about fixing the problems that any well-used bike will have from time to time. I’ll be using my own real-world, year-round bikes, which have common parts that I can fix myself rather than high-end exotica which are often best dealt with by an expert.

The question

Today’s question tackles a common problem: how can I stop my chain dropping off when I change front rings?

Remember that “heart in the mouth” moment when the chain has decided to jump off the chain rings, leaving you pedalling in thin air? Scary and potentially dangerous, fixing it can seem fiddly, even daunting. However, the solution to the problem is usually simple, needs no special tools, and can be done by anyone, even by the roadside, in just a few minutes.

So what’s the problem?

So what’s going wrong? When the chain drops, what is happening is simply that the front derailleur is pushing the chain too far one way or the other. There are several possible causes, but the most likely is that the front derailleur needs attention. Fortunately, derailleurs are designed to be adjusted easily, with two “limit screws” to stop the drop.

Limit screws: what are they, and which is which?

In this picture you can see that there are really only three parts that you need to remember for this job. This Shimano derailleur has a typical set-up with the 2 screws highlighted.

Highlighted in pink, you can see the inner, low limit screw: this stops the derailleur going too far inwards and dropping the chain from the small ring.

Highlighted in green, you can see the outer, high limit screw: this stops the derailleur going too far outwards and dropping the chain off the big ring.

You will see essentially the same thing on a Campagnolo derailleur, and SRAM often use the same setup. Sometimes, these screws are labelled L (low) or H (high), sometimes not. It doesn’t really matter: on my bikes, none of the screws are labelled.

So what do I do first?  

When adjusting these screws, the order doesn’t really matter, but I like to follow this sequence:

1.     Put the bike in the lowest gear (smallest ring at the front, biggest ring at the back) 

2.     Looking down at the front derailleur, notice the gap between the inside of the derailleur “cage” and the chain. The arrow on picture three shows where to look.

3.     Turn the pedals a few times and see what happens. The chain should not touch the inside of the derailleur cage, but the gap should be minimal, about 1 mm, or the thickness of a credit card. If this gap is too big, you’ll drop your chain when you shift down: if it’s too small, the chain will rub on the derailleur. This will make an irritating noise, and it might stop you reaching those lower gears and even damage your bike.

4.     With a small screwdriver, turn the inner “low” screw a quarter or half a turn one way or the other: clockwise will tighten the screw, anti-clockwise will loosen it. Note that:

a.     Ideally, use a good cross-head (“Phillips”) screwdriver, but at a pinch any small blade will do. 

b.     As you turn, you should actually see the derailleur cage move inwards as you tighten or outwards as you loosen the screw. So just turn one way or the other until you get that 1mm of space. 

c.     Usually, just a quarter or half a turn is enough, and it should be easy: if it won’t turn, or the derailleur doesn’t move, you have a different problem which might need specialist attention.  

5.     Now turn the pedals a few times to make sure there is no rubbing. If you see or hear any noise, or if the chain drops off, go back to step 4. 

6.     Now do the same for the big ring: put the bike in its highest gear (big ring at the front, smallest ring at the back) 

7.     Just as before, turn the pedals a few times to see what’s happening. If the chain drops off, or if there is more than 1mm between the chain and the outer side of the cage, it’s too much. So turn the other screw: again, you should actually see the derailleur move about, so just turn until you get the distance right. Picture four shows this gap between the chain and the outside of the cage on my bike. The gap is a bit big here, so the screw needs tightening. 

Once you are fairly happy, make some up and down shifts – half a dozen at least, to see how it’s working. You might have to go back and make some smaller adjustments as things “bed in”. 

If all seems OK, get on the bike and ride it a bit, shifting up and down. Putting some pressure on the pedals can make a difference, as can shifting in a riding position, so don’t be surprised if you have to go back to step 4. But if this is a roadside fix, remember that “good enough” is OK. A bit of rubbing is a lot safer than a big gap and the dropped chain that it will cause. Err on the side of caution so that you get home safely.

Yes, but what about…

Of course, I’ve left a lot out here. What if it still doesn’t work, for example?  What about derailleur alignment, cable tension or worn parts? What if I have a chain catcher, different chain rings, or electronic gears? These are all important, because a bike’s gears are actually a whole system: if one element is out of kilter, other bits can play up. It’s not always easy to know where the problem lies, either. But knowing how to adjust the limit screws might just avoid a nasty accident or get you home safely on a dark winter’s night.

Remember, also, that regularly cleaning your bike is one of the best ways of getting to know it and of keeping it running. This is doubly true of fiddly bits such as the derailleur screws. If they fill up with road gunk or even corrode, a simple job could turn into a nightmare. So keep it clean – especially in winter – and keep riding!