Phantom shifts under load
 

Had this issue but unsure why.
bike shifts well no issues until i was climbing and then chain jumped
what is the issue?

Frame flex under high chain tension can allow "ghost shifting" as can a poorly adjusted rear derailleur that isn't centered under the selected cog.

My thoughts precisely.

In my case, it was a worn out cassette.

When I lived in San Francisco my bike ghost shifted on almost every hill if I didn't gear down. The frame flexed enough to cause the issue so going to lower gears and spinning was the only way up Lombard like streets!

Might check for excessive cable friction as well. Grime, corrosion, and wear in cable housings and guides might lead to shifting issues especially under load.

Are you sure it is actually “shifting” to another gear? Or does it just feel like the chain is jumping?

A worn out cassette will do that under load.

Any chance you put a new chain on recently?

My thoughts exactly. A new chain will pop on a worn cog. You could try replacing the cassette to see if it helps with the problem. But before you do that, just turn the barrel adjuster on your rear derailleur a couple of clicks. If it's jumping onto the next smaller cog, turn CCW, if it's jumping to the next larger, turn CW. You'll have to experiment a little; too much one way or the other, and it won't shift correctly on the flats.

the engineers have labored long to speed shifting , some times those features work against you ...


Eg; ramps on side of cogs can pick up the chain if index location is not spot on..

So typical shop 1st check ; Derailleur hanger alignment (some people buy the shop tool)




....

Bike is full Carbon Fiber 928 Bianchi not sure if flex is the issue..But mayne

It's not flex. That has little or no effect on rear shifting. I still think it's a worn cog, since it's only happening in 1 gear. To check for cog wear, I have an unridden 21T Shimano cog in my toolbox. I take the cluster apart and hold each of the cogs next to it to see how it compares to new. There was a time when you could buy individual cogs in lieu of replacing the entire cassette. I've gotten some from here: https://www.loosescrews.com/, but they stock stuff mainly for older bikes.

How many cogs in back? I have a Bridgestone Radac (Vitus copy) that was originally 7sp and I upgraded it to 8sp for a long time. Eventually went 9sp and ghost shifting appeared with the narrower cog spacing. I don't ride it much and haven't taken it back down to the more workable 8sp.

When i had this exact issue, it was the chain. It was time to be replaced. If you haven’t checked it yet, you should. If you have and it’s ok, then my next thought would be the cassette. I just know that i had that exact problem and it was the chain.

I recently had a similar problem - in my case it turned out to be the derailleur hanger at fault. It wasn't bent, but rather twisted around the vertical axis, so while eyeballing it from behind it looked straight, but when viewed from above (or below), upon closer inspection I found that the guide pulley was not centered in regards to the frontal side of the cog.

Was it shifting ok at one time?
If it was then we can rule out frame flex. This has been thrown up again and again as a first and most likely cause of poor shifting.
Mind you they may now claim the frame must be broken or how else could it be shifting badly.
First stop is cables and shifters spray with wd40. See what happens.
After that it comes down to miles ridden or crash damage.