Advanced X Bracing vs Rear X Bracing

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Fernando Esteves
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Advanced X Bracing vs Rear X Bracing

Post by Fernando Esteves »

Hello!
I have lots of doubt about the Advanced X Bracing.
So far, the info I found is that the old one is ~about 1"~ closer to the soundhole and scalloped.

My doubt is that if they are really better sounding, if so, what kind of string gauge can it handle? I read it is weaker than the newer one and the story of Clarence White putting .013" on his dread and it belling up.

Also, is possible to use the forward bracing and have the same resistence than the rear one?
Thanks
Amateur luthier from Brazil.
I'm here to learn!!!
Brent Tobin
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Re: Advanced X Bracing vs Rear X Bracing

Post by Brent Tobin »

It depends on what you design the guitar for. When Martin changed from gut strings to steel, the stings were very light. When some players started putting heavy gauge strings on them, problems showed up such as bellied tops. To stop guitars coming back for free warranty work, they moved the X brace back and started to taper the braces instead of scalloping. When you shift the X forward, you get a larger area of top vibrating. Scalloping can also make the top more responsive. So yes, IF you design the guitar properly and use the appropriate strings, a forward shifted top can be just as stable.
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Fernando Esteves
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Re: Advanced X Bracing vs Rear X Bracing

Post by Fernando Esteves »

Thanks.
Do you think .011" is good or need to be lighter?
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Tim Allen
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Re: Advanced X Bracing vs Rear X Bracing

Post by Tim Allen »

Overall, I agree with Brent, but I can offer a couple other things.

If you want a pre-war Martin sound, you should probably use advanced bracing. Otherwise, it's really up to you. Variations on X bracing seem to work similarly and work well. Regardless of the position of the braces, their shape, thickness, and the stiffness of the wood you made them from will be important. Top thickness and stiffness factor in.

As I understand it, light gauge strings were not generally available until the 1960s, but now they are what most people use. If you expect to use those or specify those for your guitar, that's another factor that goes into your design.

The main thing I want to say is that beginning guitar players are usually advised to build on the heavier side at first, and then lighten up as they get more experience. When I started out, I didn't follow that advice, and instead put together my own approach combining what I thought were the most plausible arguments on the Internet. My early guitars tended to belly up. Not good. I think my worst mistake was scalloping too deeply, but who knows.

If you use advanced bracing, you should build a little heavier. How much heavier? It depends on so many things. The main point is to build it, taking notes as you do it, and learn from the result. And have fun.
Alan Carruth
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Re: Advanced X Bracing vs Rear X Bracing

Post by Alan Carruth »

The bracing on the top is there to help counteract the torque produced by the fact that the strings are up above the plane of the top. If it were only a matter of tension; if the strings could somehow be down inside the wood of the top, a fairly thin piece of spruce by itself would be plenty to take the load. In direct tension along the grain spruce is good for a working load of about 3000 psi, if I remember what I've read in some of the airplane literature correctly.

Torque is a product of the string tension and the lever arm produced by the bridge and saddle; the higher the strings are off the top the greater the torque. Changing the break angle doesn't alter how much torque there is, but it does seem to affect the way the top deforms.

What this means in practice is that you can use fairly heavy string tension on a relatively light top IF you have a low bridge and saddle. Flamenco guitars use tops that are thinner and more lightly braced than Classicals, but the string tension is not all that much different. They get away with it by having the strings 7-8 mm off the top rather than 12-14mm. Obviously there are limits as to how low you can go before the guitar starts to look like 'Trigger' too soon.

This all feeds into the sound of the guitar, of course, and that's where all the questions really need to start: what sound are you trying to make?
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Fernando Esteves
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Re: Advanced X Bracing vs Rear X Bracing

Post by Fernando Esteves »

Thanks for the answer, guys.

Well, pre war Martins are told as the best guitar tone by 99% of players. I'm thinking in go that road, try to get as close as possible as a 30's Martin.

BTW: Just arrived your DVD, Alan. Does it work for both styles of X bracing? I didn't watch it yet
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Alan Carruth
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Re: Advanced X Bracing vs Rear X Bracing

Post by Alan Carruth »

The details will be different with asymmetric bracing; what's shown on the disc is double-X bracing, which is what I use these days. Asymmetric bracing will produce more asymmetric mode patterns, or require you to carve the bracing asymmetrically to compensate.

That's what Dana Bourgeois does in some cases: he'll scallop the bass side X brace but leave the treble side uniform in height. This reduces the asymmetry of the top stiffness along the two diagonals and produces better mode shapes. Better mode shapes tend to give 'clearer' trebles whether the bracing is symmetric or not, so the difference in sound is not because of 'stiffening up the treble side' per se; it's from the better balance of stiffness.

I went to the double X pattern because it didn't make much sense to use asymmetric bracing and then try to get the modes to be symmetric. The usual pattern produces a more 'traditional' sound, while symmetric bracing gives a more 'modern; timbre, or so I'm told. I used the Chladni method before I switched over on a lot of tops, and it helps.
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