Dropping the chain can be happening for several reasons. It can be from a bike that has a bit of cable stretch (as with a new chain over time) or if the limit screws on the derailleur are not set properly and the chain either goes off on the frame side of the chain rings or the pedal side. If it happens, it is easily fixed by pulling forward on the derailleur and putting the chain back onto the smaller front ring. You might get some grease on your hands but that's why I wear black shorts! If it happens very often,take note of which side the chain drops to so you can get your bike shop to reset the limit screws. If it happens only rarely, it can be a bad shift and I think that happens to all of us at one time or another!
Check out this website for help on a dropped chain...not trying to hijack this thread!!
http://www.bicyclesouth.com/chain.htm




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Just as you need to shift before you run out of power, and lighten up on your pedal stroke so you're not putting a lot of torque on the chain when you shift, it's the same with clipping out. Know when you're starting to run out of steam, know the minimum cadence and maximum power where you're no longer able to keep the bike upright (by feel, not necessarily by gadgetry, although those are nice if you have them), and don't be so hung up on getting up the hill without walking that you don't clip out until it's too late.