Official Luthiers Forum!

Hi Folks,
I need some help.
I am trying to learn how to carve back braces. I'm going to build a Drednaught model.
The plans I purchased from Stewmac are conflicting with itself. By this I mean it's printing one size in the log and a different size on the plans itself.
Also, the back braces from Cumpiano's book are different as well.
Would you folks help me with some dimensions for the four braces??

I need width for each brace and height for each.
Many thanks,
Walter

Try .250 wide by .625 tall. Taper them up to a nice, small rounded top. You didn't ask about back radius for the braces. Do you have a number on the plans? Should be in the neighborhood of a 25 foot radius.

Hi Hank,
Shouldn't the 2 lower braces be wider.??
Cumpiano's book indicates a 5/16 wide brace for the 2 upper braces. He then uses 3/4" for the 2 lower braces. Gee that seems very wide and heavy...doesn't it??
How does the width of the braces affect the sound??
Thanks, walter

Walter:
I have plans from Stewmac and they have the brace size written on it. Two at 5/16w"x5/8h" and 3/4w"x3/8h".
then as Hank said you put a nice round on the top for 5/16" and then carve the 3/4" ones after you glue them. Oh by the way my guitars have lots of volume with good sound.
Martin has been building them like that for ever. I use a back radius of 15 ft. Made my own bowl. And it works well.
So don't get worried all plans have small mistakes, just take your time and measure twice --cut or glue once. Hope this helps you out a little.

Thanks Ron,
I appreciate your help and advice!!!
Walter

Walter I do my top two like Hank and the bottom two are about 8mm wide and 16mm high, I like mine thin and tall...

I think it's just a matter of preference honestly....

-Paul-

_________________
-Paul-

Patriot Guitars

My back bracing is a bit different, but I use about 7.5mm wide and 16mm high for the x-braces. This one still needs a good cleanup!

Walter...those big, wide braces seen in the older Martins are overkill, IMHO. I have never used anything that wide on my backs (for 120 guitars) with no ill effects. Actually, the reduced mass helps lighten the back for more volume and tone production from the instrument.
Engineering-wise, the major strength comes from the vertical height, not the width of the beam. Check out an "I" beam for strength to weight (mass) ratio. A simple home test. Lay a 2x4 flat, between two blocks, on the wide side and check the flex. Now try it standing on the 1 1/2" side. Much stronger in that mode. Imagine how thick it would have to be when laid on the 4 inch side to get the same strenght. Probably close to a 4x4!    All that added mass will just suck up sound energy that you are working so hard to get out of the box and to the listener.
My original back braces were about 5/16" wide and 3/4" tall. I have worked them down over time to the current dimensions and like the results very much.

I agree with Hank. Mine are between .25" and .270" wide depending on how stiff the spruce is. 9/16" - 5/8" tall. I've been using Carpathian Spruce lately and really like it's weight to strength ratio.

_________________
tim...
http://www.mcknightguitars.com

Thanks Tim...
I appreciate your valuable input!!!
Walter

Thanks Hank!!
I'm glad I asked, I was about to use a 2x12'.
Less is making more sense.
Much appreciated!!
Walter

like Hank said Think of how the load is being applied to the braces. the vast majority of the load is in the vertical column therfor the effective thickness is the height of the brace. The width does increase the amount of area the stress is being carried on. That said I did a static load calc along time ago on an OM and found that at 5/8 tall x 1/4 wide would support a 125# per sq in. static load at the intersections of the brace to top. Less than that off center from the brace. I don't recall what section modulus I used for the brace in the calculation. I recall getting a value from a engineering friend that worked in the experimental aircraft industry. They use a lot of spruce struts. But never the less in terms of a guitar 5/8 x 1/4 is more than truly need.