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What is their purpose?

It seems to me that their business, since the forward ones are usually under or near under a bridge wing, is to help prevent forward bridge rotation.

So it kind of seems counterproductive to me to have a peak in their middle, tapering or scalloping where it butts the x. Seems like it would make more sense to keep it talker at the brace end and tapering or scalloping it towards the end as it gets farther from the brace.

Whatcha think?

Taller at the brace side would make it much much stiffer. I hit that joint to increase flexibility in the top (I got that from the John Mayes DVD) and end up with a small mild peak. It's also triangulated, so there's not much weight there at all.

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Pat

I agree. I think their original purpose was to prevent cracks in that area, but they can provide useful stiffness as well, especially if you notch them into the X.

I've also seen some builders using a single wider flat graft on each side, rather than two taller braces, which seems good for the crack catching purpose. And with the height tapering down evenly from the X toward the perimeter, a little bit of useful stiffness as well.

Another option for stiffening effect is to leave them tall, except for right at the ends (near the X and perimeter), to give hinge type motion. Not useful from a "minimum total stiffness" standpoint, but on large guitars, you might want higher total stiffness than necessary, to get the resonant frequencies up, and use the hinge structure there to give the bridge more freedom of motion.

The way they're commonly done with a peak in the center and tapered down toward the X and perimeter, their presence probably doesn't change the resonant frequencies much. A little stiffness, and a little mass to cancel it out. Would be interesting to test a guitar with them, and then chisel them out and see if there's any difference.

Ed: Not sure if you are aware of Kent Everret, but he has a DVD on voicing tops where he shows doing exactly what you are saying. He also does the graduating of the braces with a sanding disc in a drill press. It is a decent DVD and well worth a look if you can get a hold of a copy.
Tom

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

The "finger" brace is a Martin invention and they were first used in the five fan brace configuration in early Martin Spanish style guitars C.1845.(Fig. 1)
When Martin went to the "X" brace they seemed to disappear in favor of the "outboard" fan braces still beside the "X"(Fig. 2)
The outboard fans soon disappear and the classic "finger" brace is back
but this time on the treble side only which I think reveals the true purpose of the finger brace which is to increase the cross grain stiffness in the top (enhance the treble) in that area.(Fig 3)
By around 1865 single "fingers" are on both sides of the "X".

Fig. 1

Fig. 2

Fig 3

I'm willing to venture that where you put that peak would be undetectable to the human ear. However, I think, that in order to look maximum cool, the peak should be 1" from the X. I had a Morgan. Great guitar. You might have made the neck. It had finger patches. 1/8" tall by 3/8" wide. I'm not sure how thick the top was but it never showed any signs of distortion and fundamentally sounded great. I agree with David. X grain stiffness and also X grain reinforcement is their duty. Some players, me, use that are as a percussion spot. I just want something there to keep it from cracking.

David LaPlante,
I went crazy looking at those bracing diagrams. Then googled a bit and found this.

http://www.metmuseum.org/exhibitions/li ... ly-guitars

It is almost like you have a connection to this cool exhibit, book and presentation.
The 1.5 hour video is absolutely fascinating!

Dave

Maybe the thinking of having the peak away from the X was to avoid creating a stress riser where the braces meet? I'm not sure, just thinking out loud...

That is incredible!

Bob