Help! Chain caught between frame/ring underneath BB...
 

I was going up an ascent when I went to shift from the small ring to the middle ring and the derailler failed to move the chain. Instead, even though it was a full "pull" it just "trimmmed." So i dropped the derailler back down to the small ring and then as I went to shift back up the chain got wedged between the fram and the small ring underneath the bottom bracket. I have no idea how it got there.

How does this happen? (I need to know so i can fix it and make sure it doesn't happen again) It got wedged so badly that it completely bent/twisted the chain link and scratched the heck out of my frame.

This is definitely very annoying. Chances are your front derailer limit screws are adjusted wrong. Check out Sheldon Brown's guide to derailer adjustment: http://sheldonbrown.com/derailer-adjustment.html

Thanks. I thought it might be that but it didn't drop off on top, only underneath. That's why I'm a bit confused by it and not sure that's the only cause.

When you say it didn't drop off on top, only underneath... do you mean that the part of the chain on the *top* of the chainring stayed on, while the part on the *bottom* of the chainring fell off?

If so, that's really strange! The only thing I can think of is a bent chainring...

It was like this:

The chain on top of the chainring stayed on the chainring and underneath the chainring the chain got sucked in between the frame and the chainring.

Sort of like this:
http://www.mtbbritain.co.uk/images/it_sucks.jpg

I see two possibilities now: bent chainring, or slack chain.

If the chainring is bent, the chain may slip off the bottom of the chainring, while the correctly-adjusted front derailer prevents the chain from falling off the top. Check if there's more than one or two mm of wobble in that chainring (you may be able to bend it back, see Sheldon's site for tips).

The other possibility is that the chain has too much slack in it (perhaps it is too long, or the rear derailer does not have enough capacity to take up the slack). The part of the chain between the bottom of the chainring and the bottom pulley of the rear derailer is not under stress from pedaling, so if the derailer spring doesn't pull on it enough, the chain may go slack and it can simply "bounce" off of the bottom of the chainring. To check this, try shifting between the gear combination where you had a problem, and a larger chainring. Feel the tension in the bottom of the chain by hand. Does the rear derailer maintain adequate tension even with the small chainring?

Any chance you backpedaled, even just a tiny bit, while the chain was shifting back and forth? That could explain it staying on the top of the chainring while derailing underneath. If you then pedaled forward again, it could have pulled the chain taut on the bottom and jammed it as you describe. If not that, beats me- that's all I can think of at the moment.

Hmmm. I don't think so, but it is possible.

I think it's also possible that the rear derailler's not tight enough. i know it's not a too-long-chain as I took off links when I bought it to make sure it was the right length. The chainring isn't bent (even after this) as far as I can tell. Took the chain off and looked at it and it looks fine.

Hmmm. Possibly a rusty derailer spring? Or maybe the pivots just need a drop of oil?