Chain of Events
 

Today was the second day in a row that my chain came off. On two different bikes!
Yesterday my Trek and today my vintage Fuji. I wonder if the hot sticky weather is
making me sloppy in my shifting. I know the chains on both bikes are relatively new.
Think it is time for some routine maintenance and perhaps a slight adjustment on
the rear derailleur on the Fuji.

I don't think the hot weather has anything to do with it. The derailleur limit screws need adjusting.

J.

This is also a good time to check your chains for wear and elongation. I use Sheldon's formula -- if 24 half-links measure more than 12-1/16", indicating 0.5 percent elongation, it's time for a new chain.

Normally the chain comes off the front....if that's the case it might mean checking the front derailleur. I assume the chain is coming off when shifting from the larger gear to the smaller one and dropping towards the frame?

Your bike is talking to you. I hope you didnt dechain at a critical moment. Very strange you had probs with both bikes.

The newer bike, the Trek (2010) was in the front and was more of a jam between the derailleur and the cage.

The Fuji had the chain come off the rear and needs the outer limit moved in a bit to prevent a reoccurrence.

Generally, though I think there is a problem when shifting in too extreme a movement. That is, say the rear smallest gear and shifting to the lower gear in front and vice versa. Think I have never had the problem when shifting the front gears when the rear chain is in the middle. Both bikes are two chain wheel bikes; the older one a 12 speed and the newer one an 18 speed.

Went out today and had another chain slip off.
This was on the Trek. I had adjusted the Fuji but not the Trek therefore it was
telling me it, too wanted attention. Pesky things, derailleurs, very sensitive to the
slightest neglect, real or imagined. Amazing that they work as well as they do for
as much as they do. Just need a little tenderness from time to time.