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Thanks Bob in winterpeg/gwn great white north.I will try one uke top using the falcate bracing,but will probably use either WRC or spruce for the laminates.I/.m assuming from your response that the kasha bracing pattern did not appeal to you.Regards from KC.

I'm having a hard time understanding the role of the carbon fiber tow. It is simply a series of fibers running the length of the braces held in place with epoxy it is not in the form of a woven sheet. Correct me if I am wrong but a woven sheet makes more sense. I cannot understand it's function or worth as simply stranded along the braces length. When is the carbon fiber stressed? The wood of the braces are held in position by the epoxy. I suppose the CF could be viewed as rebar in concrete but concrete stands on it's own and is much too hard and heavy to to fracture without the internal support. CF has high tensile strength but where is that strength being applied here? The brace of a guitar is almost all wood secured in place by an adhesive. I find the bracing scheme interesting but I am not seeing the need for the CF.
Can someone clear this up for me?

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No one appreciates the very special genius of your conversation as the dog does.
Christopher Morley

Well woven sheet in this situation would not work, you need the fibres to run the length of the brace not across it.
I pull a strand from plainweave CF cloth to use for the brace.
By being fixed to the top and bottom of the wood brace, the CF is stressed when string tension attempts to bend the soundboard.
The CF adds some stiffness and most importantly prevents long term deformation under load which wood is susceptible to.

BTW rebar in concrete is used because concrete by itself has good compressive strength but poor tensile strength.

Jeff
I'm still not grasping this concept. The only thing keeping the cf from flexing is the epoxy. In a woven sheet form it would help fight up and down movement but in simple stand form it does no work. Only the epoxy holds the strands in place. The fiber is unlimited in lateral motion in strand form.
I admit I am not at all familiar in using cf for anything but none of this is making sense to me. The tow is limp like a lock of hair, yes. Until it is glued in place of course. A rod of cf is stiff and when glued into a neck adds its stiffness essentially making one component by fastening with epoxy. Using epoxy on strands of anything simply holds the stands in place. The stands will just help to hold the epoxy together if the epoxy were to fracture.
Just trying to understand the concept.

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No one appreciates the very special genius of your conversation as the dog does.
Christopher Morley

Orion , jeff did a great job explanining it to you as best as he could .May I suggest you read the gore/gilet book, and see the photos, and read the design book which explains it better than we can, unless Trev steps in, to give his interpretation of it.

The epoxy does more than just hold the strands in place. It is soaked up by the cf strand so that you end up with a composite material which has both compressive and tensile strength.
When you subject a beam such as a brace to bending, the top and botton surfaces of that brace end up in either tension or compression. Adding a stiff material such as the CF to those surfaces gives greater resistance to bending.

So the CF is porous? I didn't realize that and it would make all the difference. Being encapsulated in epoxy is what I was envisioning hence the reference to rebar and concrete.
I do plan on getting the books but of course those nasty bills keep getting in the way.
Thanks

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No one appreciates the very special genius of your conversation as the dog does.
Christopher Morley

The strand (or tow)I use (pulled from woven cloth ) is 3k which means it is composed of 3000 individual fibres.
It saturates with epoxy quite readily.

It's hard to answer questions about composites and composites structures succinctly without raising more questions than answers, but here's a gross simplification...

The idea behind using a composite material is that you have two types of material; the matrix and the reinforcement. One (the reinforcement) is often much more expensive and has a much higher Young's modulus (i.e. it is much stiffer) than the other. When combined together the composite takes on (more or less) the material properties of the two together, roughly in proportion to the ratio of the materials and their relative Young's moduli.

When fibres are used as reinforcement they obviously take tension quite easily and the properties of the reinforcing material are apparent in the performance of the composite. The "magic" in composite materials is that the matrix prevents the fibres from bending or buckling by providing continuous support to the fibres and so the composite "works" in compression much as it does in tension. Strength/stiffness in compression is usually limited by the matrix material's ability to keep the reinforcement in line to prevent buckling, while in tension it pulls itself straight and more fully realises the reinforcing materials mechanical properties.

Examples of composite materials are reinforced concrete, glass/carbon/kevlar reinforced polyester/epoxy and good ol' wood itself.

Designing structures out of composite materials is another thing, where the main idea is usually the efficient use of materials, using the least amount of material (lowest cost, lowest mass) to make the structure survive (sufficient stiffness/strength). Hence the reinforcing material is usually organised to be as far from the neutral axis of bending as possible so that it has greatest effect. Hence capping braces with carbon fibre, for example.

But this is a simplification and there are other things to consider and maybe prioritise in different ways depending on the outcomes that are sought, which leads to the different uses of the material by those who properly understand the uses and limitations and its pretty much random use by those who don't.

Plenty more here:
http://en.wikipedia.org/wiki/Composite_material

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Orion, the fibers are not porous, they are very small in diameter however which is why they feel flexible. The fibers themselves are actually stiffer than a piece of steel would be of the same diameter. The epoxys job is to bond the inividuals fibers together so that they work as a unit and not as individual fibers. So the more fibers you have running along the brace the stiffer it'll be along the fiber direction.
hope that helps.

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Jim Watts
http://jameswattsguitars.com

Perhaps another analogy would work....

You can make a brace by stacking several carefully cut pieces of wood and gluing them together... Say you start out with 1/8" thick pieces... They are thin and floppy.... Now, you glue another one on top of it.. It gets stiffer.. Then another and another and another and it gets successively stiffer and stiffer and stiffer... The glue by itself is weak - it's the structure that becomes strong when the glue knits it together....

The CF works the same way... Carbon fiber graphite is massively stiff... Many times stiffer than steel.... but the fibers are very small... The goal in gluing them together is to bond the stack of fibers all very closely together... Use the minimum amount of glue possible - and it makes a very, very, very stiff structure.... Because it's so massively stiff - we only use a teeny bit... The material used to cap these braces is somewhere between 0.005" and 0.010" thick.

The techniques being discussed are to help deal with gluing the stuff... If you aren't careful - it turns into a sticky hair ball... where the hair has the strength of steel... so if it tangles around your tools or hands - it can be nearly impossible to break the stuff.. and it's very very difficult to do anything with it once it's hardened... so you gotta get it right on the 1st try...

Does this explanation help?

Thanks

Unkabob, Do you have any pics of the Uku falcate? We would love to see them....

John

John, bob sent us a description of his uke falcate bracing idea in a previous response in this thread, alas there were no photos, But if he does have one .I would love to see it as well cheers ernie

It's interesting that especially the lower bout braces don't bridge the middle seam. This would seem potentially weaker structurally. I would think running braces fairly perpendicular to grain is important too.

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

Nicton said...."It's interesting that especially the lower bout braces don't bridge the middle seam. This would seem potentially weaker structurally. I would think running braces fairly perpendicular to grain is important too."

I actually think that this is one of the great advantages of the falcate system that it does not lock the plate up with a member going from one side to the other as the main cross brace does in Martin style X bracing. As for the centre seam being a potentially weaker structure the bridge plate has woven carbon fibre under it and is all but bullet proof as a composite structure in the area of highest stress ie under the bridge. I dont see that as an issue at all.
As far as I am concerned this is a totally proven advanced design that is totally amazing...I am glad that Trevor published the details of this bracing in his book instead of keeping it to himself.... More power to him and better guitars to us! I have made 3 and they are the best sounding guitars I have made for sure... .

Cheers

John

I'm wondering the possible effects of the same layout scheme minus the carbon fiber. The braces would have to be taller of course. Has anyone tried or have thoughts?

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No one appreciates the very special genius of your conversation as the dog does.
Christopher Morley

Orion, here is Trevor's answer to that: http://www.anzlf.com/viewtopic.php?f=33&t=3942

Hi Orion, I wondered that too, it was my question and post that is referred to above. I thought that it could be done sans CF (I was dead scared of the stuff at the start...) and that would make it all much easier but Trevors response made it all make sense and the great benefit of the resistance to cold creep is of course great for guitars to age gracefully:)
...So I overcame my fear of the CF process and got on with it....and of course I am glad that I did!

Cheers

John

hi Orion
look at the first pic in my post.

best

I'm still pretty new at this and I always worry about how strong my plate joint seam is, so I like having braces cross it, especially on that lower bout. I notice some luthiers put little triangular patches over it sometimes too. Perhaps this is just my insecurity, due in part to my primitive work space that has no climate control. I'm experimenting with different glues for a stronger plate joint and discovered that using my Smith's epoxy for tropical hardwoods did not work very well (I had to re glue it)... Anyways I am learning a lot here and appreciate the topic.

I was thinking while in the shower: Wouldn't it be great to publish a book called the "encyclopedia of bracing patterns", with pictures of every bracing style ever tried. Now that would be a great reference book.

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

I was wondering about rough brace dimensions and top thicknesses for this type of guitar.
Do they remain much the same as a traditionally braced top?

First time I saw this thread. I like the way the bracing looks and it concept behind the ability to move.
Do you think you could use a different material for the core of the braces such as rosewood and still get good results?

Spruce gives the highest strength to weight ratio hence it's use in airplanes. Rosewood is much heavier for the same strength and therefore would make your top heavier. Since the goal is to build the top as light as possible (yes, I know) the materials are selected accordingly.

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Steve Smith
"Music is what feelings sound like"

Good results depends on much more than just the bracing system, but a good bracing system is pretty much an essential ingredient. What I look for in a bracing system is efficiency, i.e. the right stiffness for least mass. As a bracing material, spruce will always come in ahead of rosewood on that measure. However, some spruce will bend into falcates, some won't, so I've used a number of different woods for falcates, but always with an eye on keeping the mass down.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks Steve & Trevor for the responses.
So do you feel that CF is essential to bracing with this design?