I was out, only about 5 miles into a ride this afternoon. Came to a stop sign, and happened to throw my chain over the granny gear while downshifting to stop, which is strange as I definitely though I had the stops set properly and had enver had that kind of problem before. I move over to the side of the road to let a truck pass me, spin the chain back onto the granny gear and take off. Pedal a few times, then reach down to shift down a few gears....maybe 3, the rear dearailer rattles a lot more than usual and I hear a couple of taps as something hits the ground. Uh-oh.

Stop and realize that I somehow lost the lower sprocket on my rear derailer. I was about to find the cog, but not the bolt that was holding it on or the cups that went on the outside, and had to walk all the way back home with the chain sagging. I'd put about....30-40 miles on this bike since getting it, used. Before taking it out, I checked/cleaned everything on the rear derailer, so I know that the bolt should have been properly tightened. My only guess, though, is that it somehow came unscrewed - there's no partial bolt still screwed in, and I'm assuming the threads didn't strip. This really surprises me - I don't backpedal much, so I don't know what would have caused it to loosen like that, and at this particular time. I'd definitely been rougher with the shifting before (when I was trying the bike out before purchace and after tuning).....

Anyone have any imput on why this might have happened? I'll be sure to ask the guys at the LBS when I go to pick up replacements for the parts I lost (hopefully), but wanted some ideas from other people.

New derailers have bolts loctited in.Never had a properly tightened one come loose tho.Suspect is operator error....

It's not really new.....~87 105's. I suppose it probably was my error, but what would that error have been? I'm positive I tightened both bolts when I was cleaning/lubing the derailer.

Maybe not enough.

Should they be absolutely as tight as I can possibly get them then? I don't want to tighten to the point that I freeze the cogs?

I'm not trying to say that this wasn't my fault - I've no one else to blame it on than myself, just trying to figure out specifically what I did wrong. When I tighten a bolt, I'm not particularly soft with it....generally tighten as much as I can so long as movement isn't sluggish where it is I've tightened. I've worked on the derailer of my mountain bike doing the same things, and put 150 plus miles on with no such problem. Should I be doing something to the bolts when I tighten to make sure they stick?

AH... that's what happened. You actually can't bind the pulleys no matter how hard you tighten the bolts. The bolts pinch the pulley's axle tight between the cage-plates. Then the pulleys are slightly narrower than the axle, so they actually spin freely no matter how tight the bolts are. The upper pulley on Shimano is actually 1mm narrower than the axle so that it floats back and forth. Tighten those bolts to 25-35 in-lb of torque with some loctite.