Official Luthiers Forum!

Brace wood the lighter is better or what?

Is Engelmann good for or as brace wood, I have access to a large amount of Engelmann for brace wood. The only thing is this wood, the grain is a little wider than I usually use. I am building Nylon string 14 fret cross overs. My own design. Any advice would be appreciated. This wood is rather light in weight yet fairly strong in the deflection tests I have done

In engineering terms, the deflection varies as the inverse of the cube of the depth, so if we increased the test sample from 1/2" deep to 9/16" deep (only a 12.5%
Increase) the deflection should be nearly half, decreasing from 23mm to 12mm (calculated as 23/1.25x1.25x1.25). To check this, you could cut a piece 9/16" in grain direction x 1/2" x 20" long and hang the standard brick on it. A 12.5% increase in volume would increase the brace weight the same as increasing the specific gravity from 0.34 to 0.38 (calculated as 0.34x1.125). The end result is this new brace (but deeper) would be very nearly the same stiffness and weight as one from your local lumberyard englemann spruce. In fact, it would be quite a bit lighter brace than the Sitka brace I used in the past.

I thought this would make a great discussion for the forum.

Thanks

Blake.

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Blake Dixon
Kelowna B.C.

For a nylon string - I would want Light bracing anyway.... We aren't talking "Framing lumber" like on big body Steel strings....

Then.. Wide vs Narrow grain... Doesn't really correlate to anything useful structurally.... and it's going to be in the braces up under the soundboard anyway....

So why not go ahead and use it for bracing.

Thanks

What truckjohn said!
You can ship some of that this way !
I'd gladly pay you for some!
Mike

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Mike Collins

With spruce, stiffness tracks density in a linear fashion and it's usually very consistent across species, so that lighter spruce (like Englemann) is less stiff and heavier spruce (like red) is more stiff. If a piece of Englemann is 20% lighter, it will usually be about 20% less stiff at the same thickness. Number of grain lines per inch doesn't seem to matter... weight is a much better indicator. Keep in mind that grain runout will lessen stiffness too, so if your local lumberyard Englemann has a lot of runout it may not be ideal.

Taking measurements of weight and deflection with the wood you have is the best way to figure out which wood will work best for your design. You can use those measurements to get the maximum stiffness with minimal weight.

Thanks so much
Blake

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Blake Dixon
Kelowna B.C.

You also might also want to delegate the engl. spruce into 3 piles, light , med, and hard depending on their densities an future application.

True, but because of the cube rule and the fact that braces can vary in height, the less dense spruce braces that are made taller will have more stiffness for a given weight (or lighter weight and the same stiffness)
After all, it is the stiffness to weight ratio that we should be concerned with.
When I was building with Engelmann tops in the 1980's, I used Engelmann bracing almost exclusively.

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John

I have had great success with Engelmann bracing.... As John says, we can adjust braces so easily by simply adding or removing a wee bit to their height that very often, the lighter/weaker pieces actually prove superior.

Only so much energy that can be transferred to the top from the strings, so if one wants the top to move, reduce the weight. So as the others have pointed out, use the light stuff to make the top more responsive. That carries over to the brace on the outside of the top as well.
Tom

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A person who has never made a mistake has never made anything!!!

Keep in mind that 2/3 or more of the mass of the top is the top itself. If you save 10% of the weight of the bracing, that's 10% of 33%, which is 3% of the total top weight. Given that it's difficult to determine the actual Young's modulus of the wood any closer than about plus or minus 10%, that's well within the margin of error. Also, do you really know exactly how stiff the braces need to be? Most of the time we're not working that close to the line.

The relationship between Young's modulus along the grain and density in softwoods looks pretty linear in the range of densities we see. However, if you plot it out as a straight line it doesn't go through the origin: a piece of wood with zero density would not have a Young's modulus of zero. Since that pretty much has to be the case, the relationship has to be nonlinear; we're just far enough from the origin that we can ignore that without getting into too much trouble.

From my measurements, I'd expect a piece of softwood with a specific gravity of .3 to have a Young's modulus (E) along the grain of about 6000 MegaPascals, and a piece with a SpG of .5 to be somewhere near 16,000 MpA. About 2/3 of the samples I've tested fall within 10% of that line plus or minus. Thus going from a SpG of .4 to .32, about a 20% decrease, moves the E value from 11,000 to about 7000, an almost 60% drop. It's still worthwhile to use the lighter wood to save some weight, just not as much so as you'd think.