The other day I was riding with the chain on the smallest cog in the back (and the middle chainring in the front) and suddenly something felt like it slipped and the chain was suddenly way too loose. It felt almost like the chain had fallen off, but it hadn't.
I realized that as long as I stay in the next-to-smallest cog in the back, or any other cog actually, things are totally fine. But as soon as I get it to shift to the smallest cog (which doesn't happen all the time, but that's another problem entirely), again it's like I'm suddenly in the lowest gear possible.
In that smallest cog, as I make a full rotation, about half is totally loose, and the other half feels a bit more normal, only to return to being way too loose again.
Sorry if this makes no sense--I'm happy to clarify. I can't seem to see what's causing this problem at all. If I get off my bike and turn the pedals myself in the smallest cog, it looks and feels totally normal.
Thanks for your help.