When I shift to the smallest cog, it's suddenly too loose 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.
Check your cassette is on properly. The smallest rear sprocket spinning around sounds like it is not on the freehub and spinning freely between the frame and the freehub.

There is a lockring on the outermost of the cassette which fastens the cassette to the freehub on the wheel. The cassette has notches machined out of it to match the freehub body and allow engagement.

If the lockring comes loose - the smallest cog may pop-off the freehub and not be engaged at all. You should be able to see this but you will also have a wobbling cassette.