Height of balsa in lattice braced guitar

[escpg]

I have had this doubt for a while. Considering the strength of the epoxy-carbon fiber mix, does it really matter how high is the balsa wood inside the sandwich? I mean, epoxy-carbon fiber is so stiff, so whether I make a 3 mm or a 8 mm balsa core does it really matter? How is it possible to find the truth?

[Allen]

It's the height of the balsa that makes all the difference. And its huge!

If you use your example of 3mm vs 8 mm then the one with 8mm tall braces will be just a shade less than 19 times stiffer.

Math for this is 3x3x3=27 and 8x8x8=512 and then 512/27=18.962

How can you figure out what is right? I think if you build for the rest of you life you might start to get a handle on it.

[jeffhigh]

Not quite Allen
In this case the stiffness of the balsa is quite low so it is in effect just providing spacing between the carbon fibre at the top and bottom.
This is one case where you cannot just apply the cube rule
Assuming that you use the same amount of CF on both, the stiffness will only be roughly proportional to the height, not to the cube.
And then as Trevor points out, you run into shear strength problems with high balsa braces.

[Mike Thomas]

Jeff (or Trevor), if you have the balsa grain vertical, would that solve the shear problem? I have often thought, quite possibly wrongly, that the best orientation for the balsa would be with the grain vertical, because its function is to act as a spacer, and vertical grain balsa is excellent in compression. Also, it may be possible to use much lighter balsa with the grain oriented in this way, if minimising brace weight is an important part of the exercise.

[escpg]

I am pretty sure I read in Trevor's book that balsa needs to be used with horizontal grain. Although I don't remember the reason.
Instead I do not recall where in the book Trevor talks about bals shear problems. What is shear problem anyway?
I have to admit that in the only lattice braced guitar I built I did not pay attention to balsa grain orientation at all. I might be wrong, but I have this feeling like balsa is there just to provide a support for the epoxy-carbon fiber. It should be as neutral as possible. And of course as light as possible.

[Trevor Gore]

I am pretty sure I read in Trevor's book that balsa needs to be used with horizontal grain. Although I don't remember the reason.

The medullary rays seem offer better shear resistance (slipping of one plane through the thickness over the other).

Instead I do not recall where in the book Trevor talks about bals shear problems. What is shear problem anyway?

See Section 4.4.6 (Design).

If you use balsa in end grain orientation it just soaks up the epoxy, gaining mass. That's why I talk about using WRC, as balsa is pretty close to its shear limit and is very variable. Also see Fig 4.4-10.

[liam_fnq]

Trevor, do you know all the sections and figures off by heart or do you have to check before offering advice?

[liam_fnq]

Not quite Allen
In this case the stiffness of the balsa is quite low so it is in effect just providing spacing between the carbon fibre at the top and bottom.
This is one case where you cannot just apply the cube rule
Assuming that you use the same amount of CF on both, the stiffness will only be roughly proportional to the height, not to the cube.
And then as Trevor points out, you run into shear strength problems with high balsa braces.

Wow! I didn't know this.

Does this mean you could just go twice as wide instead of twice as high, reducing the shear problems?

[jeffhigh]

You could increase the stiffness by using more CF such as 6K tow instead of 3K. Just increasing the balsa width won't achieve much.

[liam_fnq]

So a CF capped balsa braces' stiffness doesn't vary proportionally to it's width?

[jeffhigh]

There is not much stiffness in the balsa itself, It gains it's stiffness from the amount of CF it is capped with.