Cedar bracing

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zendo
Myrtle
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Location: Brisbane, Queensland

Cedar bracing

Post by zendo » Mon Jan 19, 2015 11:52 am

I have a stock of cedar bracing and want to use it on classical guitars. Some tops are cedar but most spruce. I am going to use the cedar bracing anyway because it is there so there is no argument about using spruce which I believe is better (but which I don't have and I have quite a lot I'd cedar - quarter sawn from Stew Mac). Does anyone know how much more cedar I need to use than the spruce to obtain the same strength, despite perhaps losing some musical qualities? Thanks.

Jim watts
Blackwood
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Re: Cedar bracing

Post by Jim watts » Mon Jan 19, 2015 3:20 pm

Why not dimension a spruce brace to an approximate dimension that you would use, deflect it, then match the deflection with a cedar brace.
This should give a pretty good comparison of how your cedar compares to the spruce you have.

seeaxe
Blackwood
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Re: Cedar bracing

Post by seeaxe » Mon Jan 19, 2015 7:59 pm

Hi Zendo

+1 for Jim's comments, but if you don't have any spruce to compare with you would be a bit stuck.

Although you mention strength, stiffness is generally what we are looking for, I think.

In theory, you should be able to use the elastic modulus of the two timbers to calculate how much bigger it needs to be. For example the Wood database (http://www.wood-database.com/lumber-ide ... red-cedar/) says WRC has an elastic modulus of 7.66 GPa and Sitka Spruce is 11.030 GPa. Thus all other things being equal, a piece of WRC needs to be 1.44 times (11.03 divided by 7.66) wider (thicker) than a piece of spruce of the same depth (or height), to be as stiff.

If you wanted to keep the width (or thickness) of the brace the same you would need to make the cedar brace higher than a sitka brace. Its relatively easy to calculate what the extra height needs to be for a rectangular cross section, (actually the cube root of 1.44 or 1.13 times, for those moduli) but it gets more complicated once you take the shaping of the top of the brace into account.

If you know what species of cedar you have you could look up the data and recalculate from there. If it is actually equivalent strength you are looking for then you need to compare the modulus of rupture of the two species. The ratio is 1.35 according to the same website, so its ratio is quite similar.

This of course relates only to the brace itself. Once you have glued it to the soundboard it acts in composite with that material and the equivalence is harder to calculate, but not impossible. Which all sounds a bit tecological given the number of other factors that will affect things, such as whether the bits of wood at your disposal are anything like the ones the website is referring to.

Which I think brings us back to Jim's method. One could use these sorts of calcs as an estimate, then over build the brace a bit (10 to 20% maybe??) then see how it sounds and if necessary trim the braces down to get the sound you want?

Anyway, hope that helps. Good luck!

Cheers
Richard
Richard

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