Official Luthiers Forum!

Hi again everyone.

As everyone on this board I am continually trying to improve my guitars, I'm trying to get the ultimate in stiffness to weight for my support structures, including braces.

I couldn't find a lot of info through the search function so here's my question:

What is your take on laminating carbon in between spruce for the x-braces and back braces?

I have read it improves humidity stability as well as stiffness. Sounds like a no-brainer win/win situation. Or is it? Is there a downside?

The downsides are that laminating is a pain, epoxy is a pain, and carving carbon fiber will tear up your tools.

I see no reason that it would improve humidity stability. Braces should already be perfectly stable straight grain. The bending force that causes the soundboard/back to swell and sink comes from the plate changing width, while the braces resist it on one side.

Carbon fiber should improve creep resistance, though. It springs back better than wood after years of continuous stress. But you can also get this by laminating with a higher strength wood. The closer you get to the wood's breaking point, the faster it creeps. This is why cedar and redwood are not good for braces, because their strength is lower than spruce. Even if you work them to the same stiffness, they'll creep faster. Purpleheart is a great wood for the center laminate of braces. Its Young's modulus is extremely high so it doesn't add too much weight, and strength is very high as well. Your braces will be shorter than usual to achieve the same stiffness, though less so than with carbon fiber, and you won't have to use epoxy to laminate, or tear up your chisel in the process of carving

There's another way to use carbon fiber, described in the Gore/Gilet books, where you put a layer of carbon fiber tow between the soundboard and brace, and then another layer on top of the brace, so it functions more like an I-beam. This gives the best stiffness to weight ratio, but has the drawback that you can't carve it after adding the top layer of carbon, so you have to calculate ahead of time exactly how tall the braces should be.

In practice, sitka and red spruce are already almost ideal bracing materials for steel string guitars. Good stiffness to weight ratio, good strength for creep resistance, good split resistance, low vibrational damping, easy to work.

If you want better stiffness to weight ratio, try a double top. Paul Woolson is a great guy to ask about that.

Double tops isn't really my thing. However, would just laminating the layer between the soundboard and the brace help? I like to be able to carve the tops of my bracing through the soundhole if necessary. I don't mind the extra work it'd bring, just as long as there are gains.

I used a laminated upper transverse brace which is a brace that I want to be strong and not necessarily "singing" vibrationally speaking.

Back when I started doing the lamination thing for the UTB using .020" CF weave scuffed up and laminated between very old Adi brace stock (all I ever used...) I did an experiment of sorts.

As we all know.... we can tap things holding them on a node and see how much they ring. Although subjective as hell the presence or absence of any ring at all is likely something that is not all that subjective. Although the laminated braces did ring they did not ring IMO even half as much as a solid wood brace.

With this in mind I proceeded to use CF laminations in the UTB only not wanting my other braces to become dead.

So Joost if you subscribe to the school of thought that braces contribute to the "tone" and "sound" of a fine wooden instrument it might be something to consider in terms of judicious use of CF were strength is desirable and muting the stinkin thing is not.

PS: We've had a number of the CF guitars in our shop every few months or so and I am VERY impressed.... One brand sounded so very good that I would have bought one but they sold out to a rape and pillage, usual suspect musical instrument f*ctory but before they did the things sounded incredible. They are also immune to RH shifts, can handle the end of the world when the water rises, and can also be used as a boat paddle. What more could you possible want....

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Ann Arbor Guitars

Probably not. Too close to the neutral axis. Use the vertical arrangement if you want to carve. But the weight savings is still going to be pretty insignificant. The braces are such a small percentage of the overall soundboard weight in the first place.

Another way to go for improved stiffness to weight ratio is thinner soundboard with interconnected bracing (lattice, or notch all the smaller braces into the X). This is a good hybrid X-fan-lattice pattern you can use, if you want to push it to the extreme:




The second soundhole braces are probably not necessary if you make the others a bit taller. The idea with this was 1) never leave the plate to fend for itself (all braces connect to eachother for continuous stiffness), and 2) don't leave any large unsupported spans of plate, which feel squishy and may resonate independently to cause wolf notes.

Soundboard thickness is .070" (1.8mm). Spruce and rosewood have about the same Young's modulus, so you should be able to use the same dimensions for spruce to get the same stiffness distribution, and lose around 90 grams in the process Total soundboard mass on this one is 300g, including the 24g bridge.

Note that all the smaller braces connect into the linings at around 1-1.5mm tall, rather than being carved down to zero like usual. I believe that's important with super thin tops. Perimeter stiffness normally comes primarily from the plate, but in this case that's not enough and leaves it sounding sort of hollow. I think I should have left them maybe .5mm taller, even. It still has a bit of that thin top sound, but overall is good.

The drawback with thin tops is that you can't do a flat mirror gloss finish. The braces cause impressions on the outside, and overall the whole thing is a little wavy, and if you try to sand it flat, the stiffness will be significantly changed and screw up the voicing. French polish still works, though

So how would you describe the sound of this guitar?

As I remember, Mario Proulx makes use of CF in his braces and has done for years.
And he does make some fine sounding guitars.
http://www.proulxguitars.com/buildup/build5.htm

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Remarkably normal. Medium sensitivity and good headroom, as expected for the soundboard mass. Good bass, fairly bright, just a touch of hollowness on the trebles. Did a quick recording here: https://soundcloud.com/user-587599889/short-clip-of-rose-guitar