Chain is skipping on the cassette, and it occurs only when using the smaller chain ring. No problems when in the larger chain ring. Local bike shop changed out the derailleur, but problem persists. Cassette is fairly new, installed this year, if not mistaken. Chain wouldn't ordinarily need replacing yet. Any ideas on what might be causing this before replace cassette and chain and hope that does it?

And the bike shop didn't have any more ideas?

Chain wouldn't ordinarily need replacing? You probably need to qualify that more. Is it a new chain, only has a few 100, 1000 or 10,000 miles on it? Regardless, it's so easy just to measure your chain with a steel tape measure and find out for certain. Might take a third hand if you aren't use to doing it.

When I first took the bike to the shop for this problem, I figured it needed a new chain, but they measured and said it wasn't sufficiently worn to call for replacing it. They diagnosed the skipping and decided it needed a derailleur. Got the derailleur, rode it twice at 30-50 mile distances, and came back yesterday saying it's still acting up. The repair guy then adjusted things and suggested it might be ''air in the line'' whatever that is exactly, basically saying it might clear up, try again. Rode right after that, 30 miles, problem is the same if not worse. Problem occurs when going up hills that put me in the smaller chain ring. Which means 90% of the ride is fine, but it's a mess going up hills unless I really slow it down.

Can you clarify the "skipping"? Do you mean that the chain is riding up on the teeth of the cog, like it wants to shift, and then slotting back in to place? Or do you mean that you need to click up/down on the shifter twice before it'll shift? Is it only when shifting in one direction (up the cassette or down the cassette)? Does this happen on anywhere and everywhere on the cassette or only in a certain range of cogs?

Chain bounces from one row to the next on the cassette. Sometimes back and forth. Seems that it's more common on the middle rows. But I think it occurred on the highest row yesterday, so not sure if there's a pattern. I don't get in the smallest (ie, cross chaining) except when in the larger chain ring, when no problems occur.

The shifter is working, as far as I remember. So it will go into a new gear. Just won't always stay there.

Weird. Is the cassette lockring tight ie will the cogs wiggle a bit?

Not certain, would have to check that.

Went down to check things, and I notice that I can see the problem occur occasionally just manually pedaling with my hand. So even without riding or applying any real torque, it occasionally will skip from one cassette ring to the next. And even when it doesn't skip, I notice that it occasionally (not all the time) will sort of lift up the chain slightly and drop it back down on the cassette ring.

The culprit for a skipping chain is usually the over-worn chain you just discarded, which, in turn, wore the cassette down to the point where it no longer fits a new chain. This can happen surprisingly fast. The smaller ring applies more force to the chain because of the greater torque arm on it.

Yes, could be, although the current chain was working OK with the sprocket originally. If what you suggest is the case, wouldn't the new chain have trouble right away?

Yes, but I'm not ready to let go quite yet. Is it all the cogs or just the ones you use the most?

Not certain. I previously would've said those that I use the most, which are those in the middle, but I believe chain was also bouncing off the lowest gear the last time I rode it.

What happens when you adjust the barrell adjuster? Seems like the bike shop should’ve figured this out already to me whatever it is.

New rear? derailleur seems like one of the last things to try.

Chain wouldn't ''ordinarily'' need to be replaced... Did you check how much it's stretched with a tool? Never assume anything!

6000kms on my KMC and it has still not passed the 0.05 mark yet.

I always start with the "process of elimination".
visually inspect the rear derailleur...is the jockey/idle pulley cage straight and in proper alignment?
check chain stretch with a "check tool"
check cassette with a "check tool"
check rear derailleur hanger with alignment tool
check rear derailleur limit screws...if hanger is out of alignment you should have to reset the limit screws
check cable tension to assure correct derailleur alignment after all of the above have been checked and/or set
a skipping gear can be the result of a few things, usually a stretched chain not not always.

If the chain isn't worn, my bet would be on derailleur hangar being bent a bit.

Sy Reene beat me to it. I'd start with verifying the hanger's aligned.

There are two very different issues being discussed in this thread.

Skipping, i.e., the chain staying inline but riding over teeth on the cog(s). This happens when the cassette and chain are no longer engaging perfectly. Cause: the cassette is worn, the chain is worn, a.k.a. stretched, or both. Oh, this can happen with chainrings... that's just pretty rare.

Chattering, jumping from one cog to another. This has lots of causes. But chain and cassette wear are usually not the cause.
- Limit screws are usually not the cause either. That's a "set and forget" thing. Assuming they were adjusted correctly when the derailleur was installed, they can usually be left alone.
- Cable tension - quick and easy. Cables used to stretch more than they do today. But still - putting a twist in the barrel is an easy solution... sometimes.
- Cable and housing condition; when did you last re-cable? Dirty, corroded (sweat, etc) cables can add drag and fight with the derailleur's spring. A kink or very tight bend can add drag too - but you'd have seen that by now.
- Derailleur hanger alignment. It doesn't have to be out much. If you find that the derailleur is good on some cogs, not in others and adjusting the cable tension just moves that chattering around to adjacent cogs - that's a red flag for the derailleur alignment being off. If you don't have an alignment tool, have the shop do it. And yeah... they should have checked that already.

When I had issues with the chain dancing between several cogs it turned out to be my cable binding badly. One of the other symptoms was that it wasn't responsive to shifting to the next higher ratio cog on the rear and during the time it was taking to make one shift, I might have made several shifts of the lever so it's dance over a long period of time until it finally caught up.

New cable guide under the BB fixed the issue.

Though if your problem is similar to this, then I'd think that when you shift to lower gear ratios that you'd be having to use a lot of force on the lever.

I'll add that a dirty, corroded, binding cable often has some tell tale signs or sign - the derailleur will move the chain up the cassette (bigger cogs) and even feel normal, but be slow to drop it down - to smaller cogs. That can then get you to shift again... now the derailleur is one or two (or 1 1/2) cogs off... so, you shift the other direction, the chain's chattering and jumping, etc. You get the point.

A really dirty drivetrain can behave like that too. But there's no way you'd let this thread get to 19 posts and have the cause be a dirty bike... right? ;-)

There's a "check tool" for cassettes?!?! Tell me more please!

Try a different bike shop.

Bent derailler/mount?

here's a link
https://www.ebay.com/itm/12425019023...hoC70EQAvD_BwE

I'll be damned: https://www.kmcchain.com/en/teach/sprocker-checker
Interesting use of the word "digital" but I can see how this would be as effective as most chain checker tools.