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Made a quick and dirty deflection testing jig for x braces that snaps onto my top deflection jig and uses the same weight.






I wanted to test what actual differences carbon fibre tow made in the stiffness of the brace.

What I found was that with one strand of 3k on each side, stiffness went up 10-12%, and with two strands, stiffness went up around 30% for only a ~5% weight gain.

As well, I checked for cold creep by leaving the weight on overnight, and there was only one or two thou difference on the dial.

So it seems to be pretty effective, FWIW…


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Thanks for that. The principle is known of course, but it's nice to see a figure.
Be interesting to see what weight reduction could be had with a reduction in brace height to the same stiffness, but IIRC this is talked about in the Book.

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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.

you should also do that just using the spruce and see if the plys of spruce add to the strength

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

One has to admire your scientific rigor in conducting this experiment. But, with all due respect, your results don't predict a better sounding guitar. There is no way to quantify "better" since this is a subjective attribute. For me it is magical thinking to draw a straight line between higher stiffness/weight measurements and a "better" sounding guitar. Again with all due respect, the conclusion of your experiment would be to make 2 identical guitars with the graphite laminated braces the only variable and getting both guitars into the hands of at least 50 experienced players to log their impressions.

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Stay with the happy people.
--Reynolds Large

I guess I missed the part where he said it made a "Better" sounding guitar. My take away was that the carbon fibre added stiffness without adding a lot of additional weight. Mario Proulx and a few other makers have been making carbon fibre laminated braces for a number of years. I think Ed was mostly showing his test rig and reporting his results - all good information.
In the past, the Larson brothers made laminated braces using rosewood and spruce, and their guitars are generally well regarded. They may have done it to help hold the arching as much as to add stiffness.
Laminating braces adds a bit more complexity to constructing a guitar, and a person has to decide for themselves if it is worth the effort.
Some instruments benefit from very light and stiff tops (lutes and ouds) while for others it is less critical. It might be interesting to build a lute with carbon fibre laminated braces.

Mario uses CF in a slightly different fashion, IIRC. He laminates it vertically in the middle of the brace, rather than horizontally I-beam fashion like I was poking at. His purpose for it is to protect against cold creep. That’s IIRC.

That's a cool set up. Thanks for posting it.

Do you know if the carbon fiber has any effect one way or the other on the strength of the glue bond to the top? The wood-to-wood contact area of the brace footprints looks to be reduced by about one third by the carbon fiber. Maybe it doesn't matter?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

No idea as of yet as I’ve not glued any to any tops. The above is my total experience with CF, lol.

However, I can surmise…The CF tow is very thin and very flat, and can be smushed even flatter.

From what I understand, you’re meant to use epoxy. So you lay down the tow not on the brace, but on the top, smush it nice and flat, and then before the epoxy sets up, you clamp the braces down on top of it so it’s all one go…I’m sure there’s more talk about it in the books.

Sorry about that. I misunderstood exactly what I was looking at in the photo of the braces with the carbon fiber. I looked it up in Gore/Gilet and I get it now.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

All good!

Probably doesn't matter since the carbon is so thin and epoxy is gap filling anyway. But just so everyone knows, there is a trick if you ever need to spread carbon tow extremely thin (up to about 1cm wide for 3k). Lay the strip on a piece of glass or plastic and wet it with alcohol until there's a bit of a puddle around it, and then gently tap it with your fingers to move the fibers around. The strip will hold its new shape after drying, so you can also make curved strips that will lay down nicely without any wrestling in the epoxy.

Neat!

It seems to me that a guitar top works best when there is a sort of 'balance' between the stiffness of the braces and that of the top plate. There are various ways to check this, such as by deflection mapping, either by 'feel' or measurements, or by 'tap tones', listening for 'clarity' and sustain, or by Chladni patterns, and probably others too. In any case, the usual way to achieve this is to shape the braces to work with the top, since the density and stiffness of the top can vary from point to point in ways that are hard to measure.

CF doesn't carve very well...

One of my students got interested in using CF to optimize his top some time back, and we made up a few samples with vertical laminations to check out. The carbon layers were thin, and didn't add much stiffness, and we realized that the braces would be hard to work with and 'tune' so we dropped the idea.

The brace setup that the OP shows is the same basic one that Smallman uses. It's a sort of I-beam, where the 'flanges' are CF and the 'web' is wood: balsa for Smallman and spruce here. The stiffness of the brace is determined largely by the cross section and material properties of the flanges, with the web only being there to maintain the proper spacing of the flanges. With that sort of brace you can't change the height without losing the CF flange, so there is no (easy) method of fine tuning. All you can do is either add mass or restrict the area of the top, both of which work against what you're trying to do with that brace construction, getting the highest ratio of top Area/mass with sufficient stiffness.

If you allowed the top to act as one of the "flanges" you could glue the brace on with HHG or Titebond. If you added the tow to the other side after carving and tuning the braces it might make construction easier, if making the final result a little less predictable.
Playing with carbon fibre is icky-sticky and hard on HSS tooling, but does offer some interesting possibilities.

When I first saw the pics, I thought it was a laminated brace. Now after thinking about it, and with what Alan chipped in, a thought came to mind. If one was to adjust the brace for the extra stiffness and leave the top flat, then cut a groove down the length of the brace for the carbon tow, then you could still cut down the sides of the brace to fine tune. Just a thought.

Brent

My hope is to be able to do a reasonable calculation of how much the CF will affect the main monopole resonance, adjust the braces accordingly, then add the CF caps. Hopefully it is reasonably predictable.