Official Luthiers Forum!

On I-beams, triangles, etc., I'd agree that the weight differences among any of the brace cross-sections is pretty small. If it were important, one could tune the I-beam shape by selectively narrowing the exposed-flange width, and if needed, shortening the web. Still, it's less linear than shaving a regular brace. With some inertia calcs, one could design the lengthwise-profile of the exposed-flange width, to match a known tapered or scalloped profile of a standard brace. Then fine tune from there.

The torsion properties ('J') of the braces are interesting, too. I'm not sure if one would want more J or less... more 'I' (for a given mass) is good for responsiveness, as we need a certain 'I' to resist the string torque, among other things. But it might be helpful to free up the higher modes, which would mean less J. John P, it would be interesting to see the change in higher mode freqs and amplitudes from micro-slitting the braces lengthwise (that would keep the same I of each brace, but drop its J by more than half).

I checked the J (polar moment of inertia) of the cross-sections posted earlier (which give equal 'I' for the top+brace combo):
Rectangle: 0.0033 in4
Triangle: 0.0037 in4
Parabola: 0.0034 in4
Some differences, but those numbers are all dwarfed by a 3" wide by 0.110" thick piece of top: J = 0.2478 in4
So locally, it appears that J is dominated by the top (near 100%), while 'I' is dominated by the brace (~80%).
Across the whole top, though, the joined X-brace will give torsion resistance simply from its X geometry and the 'I' of each brace... but that may be more a lower mode issue.

_________________
David Malicky

Actually, I think it would be linear. Shaving a regular brace is non-linear.

(not that it really matters in the big scheme but just saying....plus I had never thought of that before and you're giving me ideas)

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

What a beautiful thread, full of thought, beliefs and ideas, thank you all. Working on that which you may never figure out is an honor. T.

I have a friend with a 1964 Guild without back braces using the pressed back. His guitar sounds good. Volume is slightly on the low side.

Ed

I'll put it on the list but it'll be a while before I get there.