Ghost shifting under load

[JamesE]

The rear derailleur of my tourer's just developed an annoying habit of ghost shifting in very specific circumstances - only from 4th to 3rd cog and only when I'm stomping on the pedals going uphill. Per Sheldon Brown I tried greasing the cable guide under the bottom bracket but that hasn't made any difference. What should I try next? New cables? Derailleur alignment? Sacrificing a goat?

[Vorpal]

I would clean everything and run a little lubricant down the cable outer. If that didn't do the trick, my next step would be to replace the cable.

[cycleruk]

Are you sure it is actually shifting? It's not just jumping due to wear on the sprocket?
Does it go back to the original sprocket when you reduce the pressure?
Check for a bent sprocket tooth.
Check the chain for wear or damaged link plate.
Check that the derailleur hanger has not been bent. (Looking from the back of the bike the derailleur should be vertical in relationship to the wheel spindle.)
Check the sprockets are tight and have not worked loose.
Check that the wheel hub isn't loose or needs adjustment. (pull and push on the wheel rim to see if it moves) .

Some bikes have a slot on the rear cable stop (on the chainstay). This allows the removal of last section of cable outer which can then be slid along exposing the inner at that part. You can then inspect and clean this section of cable otherwise you have to undo the cable from the derailleur to clean it.

[coast 2 coast]

If it's the frame flexing under load, no amount of grease and cleaning is going to solve it.

[JamesE]

Yes, definitely shifting, and not returning to the original sprocket when I ease off. I fitted a new chain recently so maybe the quicklink is stiff/bent? Can't be a flexy frame as it’s only just developed the problem after more than a year’s riding and about 5,000 miles… worst case, it could have developed a crack, I suppose…

[JohnW]

If it's the frame flexing under load, no amount of grease and cleaning is going to solve it.

That's a point..............I've never had this happen on the rear changer myself, but it's not unknown on the front changer - and I've certainly had that.

[Vorpal]

Yes, definitely shifting, and not returning to the original sprocket when I ease off. I fitted a new chain recently so maybe the quicklink is stiff/bent? Can't be a flexy frame as it’s only just developed the problem after more than a year’s riding and about 5,000 miles… worst case, it could have developed a crack, I suppose…

If you replaced the chain and not the sprockets, that might be your problem. Probably a worn spot on the 4th sprocket. The chain and spockets wore together, so it wasn't a problem with the old chain. Do you still have the old chain? Can you put it back on and see if the problem goes away?

[Dave W]

As above it's probably one or two cogs that you tend to use the most and so have worn them slightly more. Might need a new cassette.

[JamesE]

As above it's probably one or two cogs that you tend to use the most and so have worn them slightly more. Might need a new cassette.

Why would that cause upshifting as opposed to chain skip, though?

[Valbrona]

I fitted a new chain recently so maybe the quicklink is stiff/bent? …

Nothing to do with the quicklink. You were a bit late in replacing the chain. A chain wear checker is a good investment. If you were even later replacing the chain, it would be skipping over all of the sprockets, and not just a couple, ie. the most used.

Put your old chain back on and it won't skip.

[JamesE]

Nothing to do with the quicklink. You were a bit late in replacing the chain. A chain wear checker is a good investment. If you were even later replacing the chain, it would be skipping over all of the sprockets, and not just a couple, ie. the most used.

Put your old chain back on and it won't skip.

It's not skipping. It's shifting. I have a chain wear checker, and the old chain was not excessively worn.

[531colin]

Is it possible you put the new chain the wrong side of that curious metal tab thing that sticks out of one side of the jockey cage?
In the general run of events, the first 3 things to check are the cable.......

[willcee]

sacrifice the goat!!! will

[james01]

If this occurs only under heavy pedal loading, and assuming the frame isn't flexing unduly, the most likely culprit is that the cogs are being slightly misaligned under pressure. This is a common phenomenon with freewheel blocks where bearing wear in the block combined perhaps with a bit of rear axle slack can produce lots of sideways movement at the cog teeth. If you've got a cassette it's worth checking for a loose retainer ring. Even if it's tight, check that the cogs sit solidly without movement, it's not unheard of for incorrect spacers allowing play even when the ring is fully tightened.

[Valbrona]

I fitted a new chain recently so maybe the quicklink is stiff/bent? …

Nothing to do with the quicklink. You were a bit late in replacing the chain. A chain wear checker is a good investment. If you were even later replacing the chain, it would be skipping over all of the sprockets, and not just a couple, ie. the most used.

Put your old chain back on and it won't skip.

I am still with a worn sprocket or two analysis. I use a Rohloff Caliber II chain checker and in my opinion its accuracy is dubious. I've replaced chains in what I thought was good time, only for the new chains to skip. And a symptom of a worn sprocket or two can be phantom upshifts. At what point did you replace? Well before the checker sits on the chain, a bit before the checker sits on the chain, or when the checker did sit on the chain, because that, I am guessing, is too late.

(Some Shimano pattern chains are directional. Meaning, you can put them on back to front/upside down, depending on what way you want to look at it).