My bike is outfitted with the Sora FD, total POS that I should likely replace but wanted to see if any of you might know some trick I could use...
Here is the issue. Any time that I swap from the small to large crank ring, I get a chain rub on the FD. I took it to the LBS and they did some adjustments, moved the FD down a little, and put in a barrel adjuster next to the brifter so I could adjust it out on the fly.
Just about every time I swap I get the rub. I can adjust it out and the next time I switch gears it starts again. If I go down to the small and back up, w/o changing the rear gear it gives me about a 50/50% chance of not doing it, but if I change the rear gear at all it will rub on the FD on the tall gear.
My brifter is also Sora and is made to be double or triple. The LBS did some adjustment on it so that it skips over one of the stops, or something (not sure) but I only get one click going up, but still get two going down if I press the button lightly enough. At times I am able to adjust out the rub simply by pulling or pressing the lever or button, but not always. Even with the barrel adjuster I cannot find a happy place for it to be and not rub on the last three cogs of the (rear) sprocket.

My thoughts are that the Sora FD is just not stiff enough and is actually flexing causing the adjustment "issue". Another thought, and I don't even know if this is possible...but since the Sora brifter is made to be triple/double, is the spacing of the index actually a little off from the compact Tiagra chainring that I have put on?
I replace the Bontrager Sport cranks with them, the rub previous to this time was being caused by crank flex that I could actually see. Now, IDK.

Any suggestions or reading on how to fix this issue? Replace part(s), adjustment, w/e?
Thanks ahead of time for any help you could offer.

I feel your pain I have the same bike mine does the same. I have adjusted the FD so many times an the chain still rubs.

I don't think there's anything wrong with your derailleur. I didn't even realize that they needed to be stiff to resist flex (flex from what?).

It is not the flex that causes the rub and changing the FD is not going to solve your problem.
Make sure that the high and low limits are well adjusted, angle is correct and there is a proper distance between the FD and the chainring. With that being done, the chain rub will be remedied by using the trim feature of the STI shifters.

Check the manual on the Shimano website on how to use the trim feature of the STI shifters. I have done many adjustments on Sora groups and they have chain rub issues that can almost always be fixed by the trim feature.

https://bike.shimano.com/media/techdo...9830615004.pdf

Take your bike to a competent shop. After they fix it, ride it around the parking lot and see if it fixed, if not bring it back in. There is nothing wrong with the stiffness of your FD. I don't even know how that would even apply except that stiffness and flex are terms way overused and over marketed in the industry. It sounds like a stop screw problem and trim, but w/out looking at it I can't tell you for sure.

"Brifter"?