I use a single chainring up front and 8 cogs in the rear on my Mongoose. The big and little cogs definitely put a strain on the chain, but I've only had it come off once and that was when I hit a very bad bump and them immediately changed gears afterward. I have noticed that I have to spend more time getting the rear derailleur just right for it to operate smoothly and quietly, but it works. Preferably I'd like to get some sort of chain guard that mounts like an outer chainring and rig up some sort of clamp with an arm from the seat tube to keep the chain in place, but I'm still trying to figure that out. In the mean time I'll just keep an eye on it and try to be careful.

P.S. And try to avoid the gear combination I've done, which is a 42 in the front and a 12-23 in the rear. I built the bike as cheaply as possible, so I just used what I had lying around. Needless to say my legs are getting stronger (and more sore!)

Great work, John!

That sounds just about perfect for my own topography (but not low enough for trailering, but that's why I have another bike).