floundo

Been having an issue with my commuter bike going back to late last season (didn't ride it from late Oct-early April). I'm getting chain skipping but ONLY when on the small (34) chainring. And it seems to be only on the 3rd lowest gear/3rd largest cog in the cassette. Last year before I put it up for the winter, it was doing it only on the small chainring/largest cog. After reading around, I thought it might be a slightly out of alignment derailleur hanger. I did a thorough overhaul of it after a few rides at the beginning of April, including removing rear derailleur and checking with an alignment gauge, sure enough it was off a bit. While that fixed the problem on the large cog, it's now doing it on the 3rd. A little on the 4th too but not as bad. Starts with an intermittent click noise then the all too familiar awful grind noise before finally engaging, but then repeats the process until I shift up or down. Chain is fairly new and not even close to worn, just cleaned and lubed. My only other thought was worn cogs, but A) then wouldn't it be skipping on the large chainring/3rd cog combo as well? And B) looking at the cassette, seems to be no more wear on the 3rd/4th cog than any others. Adding in the fact that prior to straightening the derailleur hanger it was doing the same thing on the large cog, seems to be another issue. And cable tension was the last thing I thought to check, but seems fine to me, all other gear combos are shifting flawlessly, and wouldn't a cable tension issue usually manifest itself at either extreme, i.e. the large or out toward the small cogs?

Any ideas? I'm out of possibilities to check.

dsbrantjr

Is your cassette assembled correctly, with all of the spacers present and in the correct positions? You don't state which cassette you have, but many of them have spacers between some of the cogs, and the third/fourth cog is often the spot.

Wear is often tough to spot, do you use the 3rd and 4th cogs more than the others?

Bill Kapaun

When on the large ring, possibly the additional chain tension makes a difference????

IF that's a cog you use a lot, I'd consider it worn.

AnkleWork

Chain tension is highest in the smallest chainring, ceteris paribus. Most likely, your third and fourth cogs are worn out. Easiest to replace the whole cassette.