For some reason we seem to get an escalation of complaints about the chain falling off to the inside when shifting. It could be that more people are out riding but sure as it's Strawberry season, the chain seems to fall of the front rings more in early summer then any other time of year.
There are many technical items that can cause this to happen but 9 times out of 10 the solution is very simple. But let's start by describing what riders are experiencing.
What we hear is that I am JRA (Just Riding Along -- just incase you've not heard that term before) and my chain came off. Of course we need more detail then this to diagnose the problem.
After some more questions we find out that the chain is coming off on the front chainrings when trying to shift into an easier gear while going up a relatively steep hill.
The chain acts like a spring when under heavy loads. The slower the crank's RPMs the worse the condition becomes.
Any sidewards movement of the chain will cause it to jump off the current gear. When down shifting to a smaller ring in the front, the chain will 100% of the time, spring right of the ring and down onto the bottom bracket.
Now you're stopped on the middle of a steep hill with a chain that's hanging to the ground. You may as well start walking to the top.
There is a slim chance the front derailleur is out of adjustment, but even a perfectly adjusted derailleur can cause the chain to fall off the front (I can make it happen on my bikes 100% of the time if I try).
The solution you ask? Get yourself setup in the front rings before you get on the hill.
If you see a big hill coming, don't wait until you're in the middle of the hill to shift into your easier gear in the front. Do it BEFORE the hill. You may also need to shift the rear into a harder gear (2 clicks is usually enough) to compensate for the easier gear in the front.
Now when you need to make the pedaling easier, you can use the rear shifting, which is much more tolerant of heavy chain loads, to do the shifting.
Now, let's say you thought you could make the hill in your current set up. But half way up you realize that that was a bad choice. Do you need to get off the bike and walk or is there a way to get the chain to shift and not fall off?
If you do find that you need to shift the front chainrings in the middle of a hill, just let up a little to let the chain shift into the small ring and then get back into it. You don't need to stop pedaling but try not to pedal hard for a second while the shift takes place.
This technique does take some practice. Once you master it though you're chain will not fall off the front unless you forget to let up.
While changing your shifting style on a hill may not 100% solve the problem, you will cut back by 90% or more the number of times your chain falls off by using this technique.
Give it a try on your next big hill. I think you'll be happy with the results.