Just a personal observation...

I'm running a 3x9 Shimano configuration and recently after a BB change-out I'm getting a much better shifting range . Shimano's PDF literature on the 9 speed front cranks and BB stated that a 126 length spindle was spec'd for my setup in order to establish the correct chainline. Well, after removing the stock BB I noticed the spindle length was only 124mm. Since I was unable to locate a new 126mm sealed cartridge BB I purchased a Shimano UN-55 with a 127mm spindle. After getting the front derailleur positioned and dialed in I"m now able to get up and down my rear cassette with only minimal rub (if any) at the extremes of the rear cassette regardless of the front chainring being used.

Those 3 extra mm's seem to have made a huge difference. I can now use a few more gears without having to hear the repetitive rub while needing to be in one of the more extreme gearing selections. Very Happy!

Good to hear. It is one of the things I suspect is an issue for many that want to fall back on the myth of crosschaining.

When you look at the chainline just relative to the front chainwheels and the rear cassette, does it look like the center line of the middle chainwheel falls on the centerline of the fifth rear cog?

To me that seems like the logical place to allow the chain to flex from the extremes, but the actual number of links between the large large and small small is different so maybe having the chainline hit a mm or so toward the big cog might be more ideal.

Don't know for sure, though I've read some articles about it, they mostly only wanted to consider how chainline is measured and only briefly mentioned the relationships of gears, chain, and shifting.

That seems like a minimal difference when you split it for each side. I'm surprised the small amount of change makes much of a difference. Glad the change allows you to use preferred gears.

I've experienced improved shifting on 9 speed triple Campy Daytona group by correcting the chain line. The custom bike built at a reputable shop on a Franklin Frames frame, had a too long Shimano cartridge BB. Luckily the correct Campagnolo sealed BBs for triples are readily available for good prices. The shift to the large chain ring really improved even though the chain ring is a replacement and doesn't have the shift improving pins like the original Campy ring.

I will give you that. I'm sure that spending some time to ensure the accuracy of the front der alignment had something to do with it too.

3 mm is minimal? The rear spacing on the cassette stack is 4.34 center to center each cog.

more likely that the front der. was out of alignment before you swapped BB's... it only takes a tiny bit to cause rubbing.

split that 3mm figure... 1.5mm per end of spindle.

It could happen, but I have had bikes where no amount of tuning would eliminate rub. Then the next time I replace the cable, no more rub. So now that is my SOP, if I can’t fix it, loosen everything and start over.