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Non-connecting X-brace opinions
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Author:  guitarjtb [ Sun Jan 14, 2024 10:33 am ]
Post subject:  Non-connecting X-brace opinions
This is nothing I've tried on a guitar top. Just a concept discussion . I have always made my X joint tight and added a thin spruce cap across the top. I have read posts from those who think a tight joint might hinder vibration of the top. How about if the 2 parts of the X-brace did not touch at all?
In the pictures, ignore the width of the brace examples. This is just 2 short scraps I picked up for the demonstration. They are 5/8" tall, which is in the ball park for real life braces. They can be shallower and still make the concept work. I sanded an arch in each brace, so that they would clear each other at the intersection. With this clearance, you can make the angle of intersection anything that is reasonable for guitar bracing. I just used the end of my belt sander to carve out the arches. You could use a smaller or larger radius sanding wheel and make it work, if that might add to the strength of the braces. Width of the braces can be modified and scalloping or tapering is still possible.
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Author:  Chris Pile [ Sun Jan 14, 2024 11:20 am ]
Post subject:  Re: Non-connecting X-brace opinions
I think you'd have to try it to evaluate the concept.
Author:  Clay S. [ Sun Jan 14, 2024 11:27 am ]
Post subject:  Re: Non-connecting X-brace opinions
The point at which you are weakening the brace is the point at which it needs the most strength. The "X" is placed where the top has a tendency to fold up, which is why the X brace system works so well (and the fact that braces are diagonally oriented to the grain of the top to reduce the stresses caused by the cross grain gluing of ladder brace systems).
Author:  guitarjtb [ Sun Jan 14, 2024 11:40 am ]
Post subject:  Re: Non-connecting X-brace opinions
Clay S. wrote:
The point at which you are weakening the brace is the point at which it needs the most strength. The "X" is placed where the top has a tendency to fold up, which is why the X brace system works so well (and the fact that braces are diagonally oriented to the grain of the top to reduce the stresses caused by the cross grain gluing of ladder brace systems).


As I stated, I use a tight fitting joint, with a spruce cap over the intersection. So, I agree with putting the strength were the downward stress is. Some folks advocated for a looser joint. This is about as loose as you can get. :)
Author:  guitarjtb [ Sun Jan 14, 2024 11:42 am ]
Post subject:  Re: Non-connecting X-brace opinions
Chris Pile wrote:
I think you'd have to try it to evaluate the concept.


I don't think I want to try it and wait 10 years to see if it works. I will leave that to others.
Author:  Clay S. [ Sun Jan 14, 2024 12:27 pm ]
Post subject:  Re: Non-connecting X-brace opinions
I believe Gibson once did an "X" with one continuous leg with the other "leg" being two short pieces butted against it.
Here Is a discussion from the MIMF archive:
https://www.mimf.com/library/Gibson_X-b ... -2008.html
Author:  Kbore [ Sun Jan 14, 2024 2:29 pm ]
Post subject:  Re: Non-connecting X-brace opinions
Interesting with regard to Martin's pre-tapered x-braces, which negate a tight fit on the upper part of the intersection. Does anyone know how the joint was implemented on the much-coveted pre-war Martins? I've heard the claims that a looser fitting X brace is desirable, but don't even know where to start to evaluate that claim.
Author:  Alan Carruth [ Sun Jan 14, 2024 4:08 pm ]
Post subject:  Re: Non-connecting X-brace opinions
I've long felt that the critical area for top structure is between the bridge and the sound hole. That area is handling bridge torque and is also under compression from the string load. Once it starts to buckle it can keep going, and you've lost the top. I don't think it's any accident that 'fan' bracing and 'X' bracing, which concentrate wood there, are the two most successful designs.

You need to watch out for the 'cube rule' here: the stiffness of the brace goes as the cube of the height. By making one member of the X half as tall on the 'top' side you've made it 1/8 as stiff there. Granted, the other member will carry most of that load, but it would probably work even better if there were a 'post' between them to maintain the spacing, and then you're back to having the high stiffness. Which is what the braces are for, after all.

The patch across the open part of the brace is there to keep it from spreading under the down load of the bridge torque. I've seen braces split crosswise in the center there because of the stress riser. Even the cloth patch that Martin uses works well, and some say it's even better than a wood one.

I've also seen those discontinuous braces peel up, again, killing the necessary stiffness.

At any rate, the area around the bridge doesn't bend much in vibration: the bridge takes care of that. The area to 'loosen up' if you want more sound is behind the bridge, just where people 'scallop' braces. Funny about that. ;)
Author:  Hesh [ Sun Jan 14, 2024 5:41 pm ]
Post subject:  Re: Non-connecting X-brace opinions
When I was building and having success with my stuff liking the resulting tone or starting to I was contacted by two very experienced Luthiers who used to participate here.

One who I will not name told me the story of the X-intersection patch and how this joint needed to be connected for largely what Al wrote above dealing with the loads.

The other experienced Luthier was at about 150 guitars at that point, over 400 now and he told me to make my X intersection no more than 1/2" high and even suggested that he's gone down to 3/8" high at the intersection and this is, importantly when using a spruce, rigid cap.

At about the same time in my loofieramic... development ;) Mario P. posted that a guitar to him is like a bellows for a fireplace and the top is supposed to pump air. He went on to describe this top as like a "spring" that can move back and forth returning to its neutral position because of how we construct them.

All these ideas converged in my head and a light bulb went off that I hope goes off here for others like it did for me. I now had a direction and a goal for my own guitars which of course I had before but now I believed I knew why. It all came together for me and I am not saying I understand all of this but I am saying that I had a goal and as I worked to this goal the tone I was rewarded with worked great for me and some others who play my guitars.

So long story short there is a big difference between uncoupling the X intersection which I would not do and building a joint that the top can flex at the intersection and the guitar top can pump like Mario's bellows. Hence the cloth patch which is one way to add some flex but still couple things. I always used caps though and never went to the patch but I did reduce the amount of wood at the intersection by lowering the height of where the X legs intersect. I believe that opened up things for me noticeably but this is again subjective, I have no data.

What's a guitar that can pump like a bellows sound like? Answer - a cannon if you need it to be but still very capable of subtile and soft tones.

With all this said I would patch or cap that intersection for sure.
Author:  SteveSmith [ Sun Jan 14, 2024 7:32 pm ]
Post subject:  Re: Non-connecting X-brace opinions
I've also lowered the height of my X brace to about 1/2'" and it's also still capped. I, and others, are really liking the tone I'm getting so something is working right although I can't say if it's the lighter X bracing.
Author:  bluescreek [ Mon Jan 15, 2024 6:15 am ]
Post subject:  Re: Non-connecting X-brace opinions
Since wood stiffness is cubed to the height you do loose some , I saw something lately some builders are doing used wedges in the joint . The key here in engineering terms is contiguous . Martin has used the tape method forever. I use it many do. As a repair guy I have seen very few of these get loose but many caps.
I agree , you should do it and see what you got. No true discovery was made by satus quo thinking. I think the siffness in your case would be added from the 1 braces. The curves will greatly reduce any stress riser. Keep us posted
Author:  Barry Daniels [ Mon Jan 15, 2024 10:01 am ]
Post subject:  Re: Non-connecting X-brace opinions
Martins are a poor example of the X brace joint in my opinion. To leave gaps in the joint and act as if a glue soaked fabric patch will fix it is just sloppy engineering. I have worked on Martins where this gap has caused a split to run thru a brace. My repair involved gluing the cracked brace and then installing tiny wedges of spruce in the joint gaps to try to bring some structural integrity back into the joint.
Author:  A.Hix [ Mon Jan 15, 2024 1:55 pm ]
Post subject:  Re: Non-connecting X-brace opinions
From a structural engineering perspective, this is an interesting concept. I think it would be well suited if the X braces were laminated, spruce with a hardwood or even carbon fiber center, which would almost eliminate the chance of either brace splitting, and the arched scallop would reduce stress risers.. These together would certainly not be any weaker than a tightly fitted lap joint. Hmm..
Author:  DennisK [ Mon Jan 15, 2024 2:13 pm ]
Post subject:  Re: Non-connecting X-brace opinions
I use tightly fitted and capped, since it has the best stiffness to weight ratio. If it's too stiff, reduce the height. Regardless of whether you do that or reduce stiffness by changing the structure, absolute stiffness must remain equally high or else it will cave in.

The only potential advantage I can see with the cloth cap is if it has enough tensile strength to resist cave-in, but remains flexible in compression/buckling. In that case you may be able to get some flexing on one half of the vibration cycle that wouldn't happen with a wood joint. Kevlar tow would be even better for that. Put glue on the ends of the fibers to stick them to the braces, but leave them dry in the middle so they have no resistance to compression at all. Carbon fiber would likely work just as well, but is more fragile so may get broken by hands blindly feeling around inside the box.
Author:  Barry Daniels [ Tue Jan 16, 2024 12:16 pm ]
Post subject:  Re: Non-connecting X-brace opinions
Tensile strength is resistance to being pulled apart. What you are talking about is bending strength which I think fabric has very little of.
Author:  Alan Carruth [ Tue Jan 16, 2024 4:06 pm ]
Post subject:  Re: Non-connecting X-brace opinions
The cap is there to keep the joint from opening up in tension, so tensile strength is what counts. What really counts is having a long enough glue line to keep the patch from coming up; the patch itself can be quite thin.
Author:  Honza [ Tue Jan 23, 2024 4:02 pm ]
Post subject:  Re: Non-connecting X-brace opinions
Ken Parker doesn't join his, though obviously that's on archtops. I don't remember reading anywhere about what his thinking is. Anyone know?

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Author:  Ken Nagy [ Tue Jan 23, 2024 7:16 pm ]
Post subject:  Re: Non-connecting X-brace opinions
From the beginning of Bracing the Top 1, of 8?

He thinks it's a good idea, and that is how he does it now.

His bracing and thicknessing is lighter than most.
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