apodysophilic

When I am shifting the gears (either up or down) on the cassette, all of a sudden it feels like something is completely disengaged and I feel no resistance on the pedals, but then it fixes itself and shifts the chain. Obviously, when it happens I lose a lot of momentum, especially when I am climbing.

This happens to the same cogs that are in the middle of the cassette while shifting down or up *most* of the time.

Can anyone tell me what's wrong and what the solution is? Hopefully, I used the right terminology to explain the problem.

Thanks!

twinquad

Sounds like worn cogs to me. Have a close look at them; the teeth should be symmetric. If it looks like they're sloped more on one edge than the other, then that's most likely the problem. Unfortunately the only remedy is to replace the cassette. Replace the chain while you're at it, since the most likely cause of cog wear is a stretched chain.

Before you start buying parts, though, probably should have the LBS check it to confirm that this is the problem.

Peedtm

Just recently had the same problem. If you stop pedaling before it engages, you'll see your chain is sitting on top of the teeth of the cog instead of engaging them.
+1 to the last post, or maybe just a rear derailer adjustment.

Bill Kapaun

Also check your chain for stiff link(s)
Just turn the crank backwards and observe the RDER pulleys. You'll see the cage "twitch" noticeably when a stiff link passes through.