Official Luthiers Forum!

John, I would certainly agree that the x brace and top are acting in compression from the bridge load however, the rotational forces from the bridge are going to induce some tensile force in the edge of the brace furthest from the top plate, and the brace with a notch out of it here is weakened.

Well, I was pretty sure I had a good grasp of the forces involved here but to illustrate it, I mocked it up.

The piece glued across the X-brace equates to the bridge. The weight added is the string load. With no weight and no cap on the X, the lap joint was perfectly tight. As weight was added, a visible gap opened up in the lap joint on the side that would be capped.

With no cap, the brace deflected .020" when the weight was added.

With a cap, it deflected .010".

The X-brace is clearly in tension on the side away from the plate. A cap clearly makes the brace stiffer.


Whether that stiffness is desired is another question. Obviously there are thousands of guitars with cloth caps (that have almost no structural function) that are doing fine. A reasonable explanation is that the top and the uncut leg of the X on those guitars is sufficient to withstand the downward force created by the torsion on the bridge. The tone of those guitars my actually suffer if a wood cap was added because they may then be too stiff. But that's just a guess.

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http://www.chassonguitars.com

that is a beautiful set up. I love to see this kind of thinking. I am pressing your buttons not to get you to speak louder but to improve your argument.
You have to look at the stress and what is important . I think if you are using standard proven bracing patterns , so to speak , you don't need to reinvent the wheel. My experience in repair has shown that the top brace span isn't as much a benefit as we think . If you are pressing the limits of bracing , it may be something you want to explore.
I use a deflection test on my top, and that correlates to the braces I need. I want to keep my mass and deflection to a certain standard. This is the beauty of this as we each want to build the guitar that we the builder want to produce and market. If we all built the same , and wanted them to sound the same , why would you want to build.
Thanks for this useful and intelligent topic. I think we all should have learned a little something. Don't be afraid to do a paradigm shift in your thinking.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

In order to make your study more accurate you need to apply the braces to a "top" complete with braces plate bridge etc.
When in doubt try it out.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Adding a top would change the numbers but it wouldn't change the mechanics of what's going on at the lap joint. The structure would still be stiffer with a solid cap. If we are talking about a guitar that failed partly due to collpase north of the bridge, a cap would have helped to prevent that.

If we are talking about a guitar that is already structurally fine without the cap then the result is less clear. In other words, I'm not saying an X without a cap is doomed to fail. I'm just saying the top is stiffer in a structurally relevant way.

_________________
http://www.chassonguitars.com

And you could also look at it this way..... using a structural cap on the brace joint may allow you to use a slightly smaller and lighter brace section than you would have been able to use uncapped whilst maintaining the same deflection resistance.
As Kent said the op's guitar has partially failed in this location so adding a cap would be in order

The cap may help but the scalloping at the area of most rotational force is what may be causing more issues. Try making a true scale with the braces shaped Etc. The bridge , plate , top all work together and the braces will control the rotation. If the braces are compromised you will have a failure. I am saying that the cap may add some stiffness but is it needed on a properly braced guitar? CF Martin makes 250 guitars a day and doesn't use one. Gibson doesn't use them .
john

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hi guys

Many thanks to everyone who's tried to help/advise on this issue. It's really appreciated.

I've spent a couple of days thinking about it. I'm going to try some of the tips you've given before going for a top removal and see if the soundboard has stabilised. At the very least it's going to give me some valuable repair experience.

The owner's a good friend of mine. I think I'll just build him a new guitar which I'm sure will be better anyway - even if the top hadn't buckled.

thanks for your support

Mat.