Official Luthiers Forum!

I'm building my first steel string (normally build classicals), and I have a big stock of very nicely quartered Engelmann Spruce brace wood. The plans call for Sitka top bracing. If I used the Engelmann, how much would I need to increase the brace dimensions (if at all)? Since this is my first, I can't really brace by stiffness because I have nothing to compare to,
Thanks for any help,
Mike

Use the wood you have.
It may be more flexible!
It may be lighter in weight ,which could help with loudness & tone.

Mike

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Mike Collins

It depends on how stiff it is compared to average book values.

Say average engleman has a modulus of elasticity that's 15% softer than average sitka... Since stiffness goes with the cube of beam depth - making up 15% stiffness only requires something like 5% more thickness... Say your top bracing sitka is 16mm - engleman would only need 0.8mm more depth to be back at the same stiffness..

If you look at compare average values listed for the spruces, Engelmann has about 90% of the elastic strength of Sitka. If you have a large stock to choose from, you might do a simple test to find out which are the stiffer pieces and use them as you would Sitka. Most plans list brace sizes on the conservative side, so even a less stiff piece will usually work.

I would chose the sitka for a ss gtr..I spent many yrs in BC and englemann spruce comes in light, med , and hvy densities depending on where it/s grown on the west coast.. If your going to use e. spruce test it to see how stiff it is , and then factor the difference. and add that height to your bracing e.g. if the plans for sitka brace height is 12mm and your englemann brace is 25% weaker I would make my e. brace 15mm high. This is just an example, your experiments might dictate otherwise

If you look on the bright side, most plans are over built anyway, so if you use the Englemann and stick to the plan, win win.

Truckjohn is correct. Stiffness is proportional to the cube of the height. Adding 25% to the height will increase the stiffness by 95%. If you want to make the braces wider, then stiffness is directly proportional. In other words, if you want to increase the stiffness by 25%, increase the width by 25%.
I built guitars all through the 1980's with Engelmann bracing, and I made the braces the same dimension as a stock Martin or Gibson, or very slightly taller (no more than 5%). Those guitars have survived just fine.

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John