Anyway this event made me think some more on CF, and wonder if 2 rods placed under the FB would have prevented the backbowing.

I am also wondering if long term issues with necks bowing up are actually created by repeated low humidity hits rather than cold creep from the string tension.

Had a coco bolllo fb go wonky on me(upbow) in high summer heat using titebong, the coco was abt 22 yrs old an bone dry. It was a big PITA to remove I replaced it with a 1yrold persimmon FB, looks fine now.

... When I last did the calculation, two 10mm by 3mm rods under the fretboard increased the total neck stiffness by ~9% iirc, which means you will still see ~90% of the deflection. ..."

I agree; 10% stiffer is a valid improvement and nothing to turn your back to. Consider that the vast majority of necks hold-up perfectly well without any CF at all, and that extra 10% adds a great deal of extra "insurance".

Besides, that 10% figure has to be accepted as a rough figure, at most, because of the natural variability of wood. So while an an average piece of neck wood(what wood(s) did you use for your figures, Trevor?)may only see a 10% gain in stiffness, and stiffer examples will see less improvement, the weaker examples, which are the ones that naturally need the most help, will see a lot more than 10%. And of course, we can always use steel, for even more stiffness, and to eliminate adjustable rods altogether.

Do you have any experience, and/or opinions about the D shaped CF rod referenced above?

See my post above

On classicals the ebony problems might be exacerbated by the wider and usually thicker fingerboards. I would guess that a thinner FB has less force to bend the neck. For example 5.5mm instead of 6.5-7mm thick.

I agree; 10% stiffer is a valid improvement and nothing to turn your back to. Consider that the vast majority of necks hold-up perfectly well without any CF at all, and that extra 10% adds a great deal of extra "insurance".

Besides, that 10% figure has to be accepted as a rough figure, at most, because of the natural variability of wood. So while an an average piece of neck wood(what wood(s) did you use for your figures, Trevor?)may only see a 10% gain in stiffness, and stiffer examples will see less improvement, the weaker examples, which are the ones that naturally need the most help, will see a lot more than 10%. And of course, we can always use steel, for even more stiffness, and to eliminate adjustable rods altogether.