Reducing brace dimensions

[Phil Walsh]

Hi everyone,

I've got five SS acoustic builds under my belt, and now starting number six. I've been following the C&N book pretty closely, but would now like to start lightening up the top bracing. Any tips, advice, or warnings regarding how much I can reduce the C&N brace dimensions?

Sorry if this has been posted before.

Thanks,
Phil

[Clay Schaeffer]

It depends on the size of the guitar and stiffness of the soundboard and bracing and the forces acting on it.. Someone who has a stronger engineering background may be able to give some ways to measure and calculate those things but I usually go by experience and intuition for the most part . Dimensions smaller than what factories commonly use are sometimes recommended, but if you build too light your instruments may not hold up over the long run.

[Doug Shaker]

Stiffness is directly proportional to the width of the brace. That is, if one brace is X wide and another is 0.5 X, then the first will be twice as stiff as the second.

However, stiffness it is proportional to the cube of the height. If one brace is X tall and another is 0.5 X tall, then the first brace will be eight times as stiff as the second.

I don't know the dimensions of the braces used in the N&C book, but you could try making them half as wide (removing half the stiffness), but 1.3 times as tall (putting back the same amount of stiffness). Those braces would be just as stiff, but weigh 0.65 as much as the original braces, resulting in greater top mobility.

[Mark Swanson]

Doug is right. He shows that you can make a bigger difference quicker by varying the height of the brace, and leave the width of the brace to an acceptable size. Most builders use 1/4" or 5/16". If you make the brace any thinner than that, you begin to suffer from a gluing surface that is too small, it's easy enough as it is to pop a brace off the top.

[Doug Shaker]

If you want to follow the Australian Science Maniac School (as I do), you can make the braces out of tiny I-beams: a layer of graphite fiber between brace and soundboard, a thin trapezoid of wood, and a strand of graphite on top, with epoxy used as the wood-to-graphite glue. Very light, very stiff.

Details in Trevor Gore's two-volume work. Well worth the read, in my opinion.

[Phil Walsh]

Thanks everyone. I think I'll go with the taller but thinner braces on this go-round, and maybe get a little more high-tech on number 7.

[Patrick Hanna]

This is a very interesting topic for me, one I have pondered for a long time. I'm not an engineer. I first started to think about this several years ago when looking at the narrower, taller, scalloped bracing that Bill Moll (who frequently contributed here in former years) was putting into his arch top guitars. It seemed logical to me that he was onto something--maybe a number of related effects. It seemed logical to me that a taller, narrower brace was as strong or stronger than a wider brace that was less tall. My mind then went to the next logical question: A reduced brace foot print would leave quite a lot more unbraced surface area on the inside of the top plate. Would this result in a more beneficial overall sound? I have no idea. If any of you folks can resolve that for me, I sure would appreciate it. But "sound" is very subjective. I will tell you that I corresponded with Mr. Moll and followed his lead when I built my arch top. It sounds great to me and to everyone who plays with me and I will never look back. But... you know.... it might have sounded equally good with conventional brace proportions. I just don't know. I probably won't ever know, because if I build another, I will most likely follow Mr. Moll's lead again.

[Randy Roberts]

Phil,
This link to an old thread in the library may help you visualize the thickness / stiffness thing. It has bits and pieces missing that were lost in the transition to the new site for the forum, but most everything is still there.

http://www.mimf.com/library/Randy_Rober ... -2010.html

[Michael Lewis]

Maintain a good sized gluing surface of your bracing, and make them narrow at the opposite side of the brace. Some very good sounding old Gibson guitars from the 40s have braces like this, sort of an isosceles triangle cross section. It's the easiest way to make the bracing lighter and stiffer, and doesn't cost anything.