There should be some way to make a test rig from scrap spruce mimicking a top and loading it to failure. That would be very interesting.

I have never seen an X brace fail. For those that have what was the mechanism?

Put me down as not a fan of the glue rag.

I've seen a fair amount that look like this:

Put me down as not a fan of the glue rag.

I've seen a fair amount that look like this:

Is it really so bad anyhow if you get a crack at the x brace? (as long as it is not from normal service)
It is easily glued up.
Everything will break anyhow if it is hit hard enough, I don't really see failures here unless there has been some impact.
Sometimes it is better to have an easily repaired break than have the forces transferred further into the structure.

Is it really so bad anyhow if you get a crack at the x brace? (as long as it is not from normal service)
It is easily glued up.
Everything will break anyhow if it is hit hard enough, I don't really see failures here unless there has been some impact.
Sometimes it is better to have an easily repaired break than have the forces transferred further into the structure.

I have seen more cracked braces than failed caps.

When you notch that brace there will be a sharp corner so the joint should allow for glue. Not so tight that it springs the joint but a soft rub is fine so the glue can fill the joint and make it solid.

????

You have seen cracked braces starting at the intersection with intact caps? Seems like if the brace is cracked, the "cap" has failed almost by definition............

The sharp corner is the crux of this experiment .......

I'm all for the experiment but I'm not clear on the endpoints you are measuring or maybe I'm not clear on how you are going to measure them.

As I understand it, you are seeking to eliminate/reduce the stress riser from a notched brace. I see at least two questions you should be asking as well as asking how you will measure and define the results:

1) Is it better structurally? This, in and of itself, has several definitions but I think you are asking is it less prone to the brace splitting. This is a great opportunity to do controlled tests simulating the actual stresses involved on all the different joint types. This would also include a batch of failure tests from impact or other stresses not associated with normal playing.

2) What impact does the new joint have on stability (top geometry under load), stiffness, tone. These are the more interesting questions but much more difficult to define and test. The more common joint types have been show to be mostly strong enough for millions of guitars. This new joint may prove to be slightly better in terms of structure but create other detriments to sound or stability. The joint may be a win tonally but end up being a structural failure in a way we aren't anticipating. Or, hundreds of other scenarios.

This could balloon into experiments involving thousands of guitar bodies and mock ups. I'd tackle one question at a time and start with small experiments with specific measurements in mind.

I would imagine that besides direct impact, the next most common cause would be placing songbooks on top of the guitar and closing the case.

What I have seen it the thickness of the cap being .060 and that thin of a cap will flex and allow the sharp corner crack . If the cap is to thin or even to narrow from shaping its not any better or even less effective than the cloth .

I don't think the method you use is as important that you do use something to make the joint contiguous. When you notch that brace there will be a sharp corner so the joint should allow for glue. Not so tight that it springs the joint but a soft rub is fine so the glue can fill the joint and make it solid.

Hey David what’s your camera setup? I need to do that.
Thanks
Terry

David, these cracks in the X brace that have no symptoms other than seeing them when you push on the top, does the tone change when repaired? I would expect that a brace with a crack is several times less stiff but just as heavy as before (the rest of the brace hanging there but not contributing tho the stiffness) and would have a big impact on the tone (for better or worse). If pushing on the top makes it open enough to be visible then clearly the bridge rotation is able to move that area of the top more than it would when it is fixed which may also change the geometry in front and behind the bridge. I'm not saying that either the subtle change in geometry or tone should lead one to find a crack, but asking if the repair is obvious once the problem is discovered.