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maybe stronger isn't necessarily better, unless we want to use guitars for stepstools

In the past year I've played 7 or 8 pre-war gibson L-00s and IMO they are great sounding guitars that have held up very well 75 years or so. I've recently been able to weigh the tops of a couple of them dis-assembled (not because of x joint failure). They are amazingly light.

A few days ago I got a peek at a 1926 Martin O with the back removed. That little cloth patch was still sitting there as tight as ever. Same with the cloth side strips. There was one very minor side crack btw - not bad for 84 year old brw.
Walter

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Walter Lay
"It's taken me so long, but now that I know
I can see. All that I do or say, is all I ever will be"
- Billy Joe Shaver

Definitely, stronger (or more specifically stiffer) is not always better.
But that location in front of the bridge is one where failure does seem to occur and I do not think that extra stiffness there is going to be detrimental to top vibration.
I would not expect to see cloth patch "failure". Even saturated with glue it is likely to have enough flexibility to yeild with minor deformation. It's just that basically the other leg of the X has to carry most of the bending load

Walter, would you share these numbers? Thanks!

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Build log

Like Filippo wrote - it's in the wrong axis to provide the strength you're looking for. Gussets will resist "changes in angle" of the brace.



Runamuck already posted on this but just to back him up - Nowhere in this thread have I read about anyone saying you shouldn't do this or that, what I read was a discussion of facts. There's a lot of folklore and voodoo out here in luthiery land and we all benefit from a discussion of fact before we apply our own voodoo. e.g.:

Myth: Inlaying the cap is stronger
Fact: Inlaying the cap weakens the X brace.....but it looks better

Myth: Cloth is stronger than a wooden cap
Fact: It is not, but it can be "good enough" for your application.

Myth: Gussets improve the strength of the braces at the X
Fact: yes they do, but not in the axis we're most worried about in that area.

For someone making design decisions, they need to know the facts to make good decisions. e.g. You're making a very thin top guitar with beefed up bracing, you've got to know that a cloth reinforcement isn't as strong as wood because your braces will be carrying more of the load. What may have been good enough for a thicker top may not be good enough for a thin top etc....

In the case of the inlaid cap - doing so for looks makes perfect sense. If on the other hand you're doing it to improve the stiffness of the X in the "vertical" direction, you're going backward.

This thread is helpful.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I looked inside my Collings last night and it has nothing on the x brace joint. Not an argument, I just found it interesting.

One thing that people should understand about this is that a wood cap on the lap joint dramatically stiffens the top and that is easily provable. I mocked it up awhile back and you can see the results on the second page here viewtopic.php?f=10101&t=20270&hilit=cap+load

Whether this stiffness is a good thing depends on design. If you take a guitar like a Martin which is designed to be stiff enough with an open lap and then add the wood cap, I suspect you would get a top that was too stiff. If you are trying to maximize the strength/weight ratio, you are better off with the wood cap and a lower brace of equal stiffness.

As for failure, I didn't test it at the time but I suspect the typical cloth patch does next to nothing to increase stiffness and therefore there is essentially nothing to "fail" and that's why they don't. They flex enough to allow the lap to open so the only way they could fail is if a brick fell on the top and shattered the X.

But because of that allowed flex with the cloth patch, I bet you would see more splits in the X-brace, parallel to the top, starting at the lap. By not capping with wood, you create a serious stress riser at the corner of the lap cutout.

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I don't object to the so-called facts being discussed.
I object to the tone of the thread that says to a novice builder that if you don't cap the lap, and in a certain way, you can expect failure.
The truth is you can make your X in many different ways, and expect success.

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Aww now come on Dave, ur just mak'in that up, no one done any such thing. All of the methods discussed can work, that is acknowledged. I guess you could even get by a while with some string, a half hitch, and some flour glue, but the point is that some methods are just more reliable than others.

What really happened in this thread was that a few engineering 'facts' where presented in response to some statements that had been put forward based upon common yet incorrect assumptions. It was no more than that Dave and nobody got personal or bent up..well almost nobody.

This is an open forum mate, what you seen happen here is precisely what is meant to happen when things work well. Its the reason many of us are here because it is from sharing like this that we learn. Sure the gooey feel good post about how nice your guitar looks makes everybody feel nice, warm, and fuzzy, but the truth is its the edgy stuff, the posts that challenge and make you shift a little in your seat that bring forth the good oil and it flows to everyone.

I have been proven wrong here many times but remain ever grateful to those who so kindly took their time to set me straight...Jeff Highland is very good at that, thanks Jeff

Its all good..Cheers

Kim

Sorry Kim,
I value grace & art, originality & self thinking above engineered structure. I should have saved my rant for the proper discussion thread.

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Kim: Well said.
Tom

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

Oh man, I better quit now!

Filippo, how could you say such a thing?

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Hey, I resemble that remark! One thing we of the limp wrist learn is to argue for our ideas. We also learn the difference between argument and dogmatic foot-stomping.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

My opinion remains unchanged, but I do apologize if my remarks "killed the thread."
I mean to add a diverse opinion, with history as my basis, and a desire for younger builders to understand the wide range of practice that can bring success.
My aim is to stamp out the fear of failure in the younger builder.
I will recede...

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Alexandru requested:



the 2 tops (1930s gibson L-00s) were just about identical in weight - 220g including soundboard (.100"), braces and bridge.

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Walter Lay
"It's taken me so long, but now that I know
I can see. All that I do or say, is all I ever will be"
- Billy Joe Shaver

jeez, when I posted my innocent OP enquiry, I didn't expect the Spanish Inquisition ...

I have found all the responses extraordinarily illuminating, and am resolved to do my first X brace joint as follows : downward notch (ie facing the back ) cut out 2/3rds of the height. Upward facing notch , remove 1/3rd of the height. Cap the downward notch with a generous length of 1/16" spruce, feathered down at the ends, and then butt up two similar pieces to the cap along the upwardly notched brace, which will achieve nothing strength-wise, but which will pander to my dilettantish obsession with symmetry.

Does that sound like a reasonable MO to the experienced builders ?

No real need to bother with the 2/3- 1/3 approach, 1/2 each will work fine with the capped joint.

Not sure you qualify for the stake but we can surely bring out the tar and feathers for you.

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Build log

It may have been a reference to the famous Monty Python sketch: "Nobody expects the Spanish Inquisition!! <hideous laughter>

Bring out.... the comfy chair!

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Damon Wack
https://www.facebook.com/LindamonGuitars

Jeff, I am not so sure, and hey, what do I know, I haven't even built an acoustic guitar yet.

But it just seems to me that if you only need a thin glued on cap to counteract the downward force at the joint in the X brace, then you only need a thin "cap" on the other side (glued to the soundboard), and that the more continuous timber in the intersecting brace, the better. As I say, purely a gut feeling of the stresses and strains involved, and I could well be mistaken.

This gets back to my original OP query about whether one continuous brace would work , with the other brace abutted up. I am satisfied now that this is not the way to go, but what I am thinking now is that a 1/2 -1/2 crosslap is not the way to go either. In fact, it would seem to me (and this is just a gut feeling) that even going 2/3 - 1/3 is not enough. It should maybe be something like 3/4 - 1/4, given that the cap is going to add so much strength to the joint.

And yes, the comfy chair ....

This could go downhill in a hurry....we'd better let the serious folks continue with the discussion!!!!

Actually I did have a serious question on this - is there any possibility that the top itself acts as a cap on the brace with the open slot against the top? Maybe not as strong as a spruce cap with grain running in the same direction, but how strong does it need to be? Then by capping the gap facing away from the top, all bases are covered?

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Damon Wack
https://www.facebook.com/LindamonGuitars

Yes Lindamon that is exactly the correct way to look at it.
Murrmac, I just would not want to be handling a brace with 2/3 of more notched out of it, just gets a little fragile

The cross-grain glue joint at the bottom of the top brace and the top of the bottom brace - if it's a good, tight joint -is second in strength only to an edge to edge glue joint ( where long grain is glued to long grain). And since a tight glue joint is said to be just as strong as the wood itself (excluding end grain, butt joints), I don't see why your idea of 2/3 - 1/3 would provide more strength than 1/2 - 1/2.

BTW: I've really enjoyed this thread.

Why, when the glue joint is as strong as the wood itself?

80/20, 70/30, 90/10. What difference does it make? (Assuming the joint is good, of course.)