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I know some of you use Carbon Fiber in your braces. The questions are what braces do you use it in? What benefits do you enjoy?

Image from LMI

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Joe Beaver
Maker of Sawdust

Joe: The man you need to talk to is Mario Proulx . Maybe he will see your post and jump into the pool.
Tom

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A person who has never made a mistake has never made anything!!!

Carbon fiber used this way will help prevent top distorsion over time. CF deforms itself a lot less than wood, thus helps the structure maintain a stady shape.

Mario Proulx, who is amongst the initiator of this practice, uses it pretty much on all top braces (correct me if I'm wrong here...). I also heard of classical guitar makers who use it on the braces of their fan brace system.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Thanks for the info. I do know some classical builders use it. Trevor Gore uses a modified version with CF Tow. not sure why he prefers tow by I trust him to do it right.

With all the talk of potato chip tops during humidity changes I was thinking CF should limit the movement of at least the bracing. Is that true?

Regarding Mario, he builds one heck of a guitar from what I hear. I would like to hear his and others opinions that have experience with it.

Size of the CF is one item I am interested in. .030 for x-bracing?

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Joe Beaver
Maker of Sawdust

0.022" for bracing. I tried thinner, and tried thicker; thinner didn't control the bracing enough, and thicker just added weight and didn't improve long-term stability.

I use to use it for most of the braces, but now only use it for the X braces most of the time(exceptions will always exist, of course).

Thank you Mario. I was planning to use 1/4" x 9/16" scaloped x-braces. Maybe with the CF I can go a little thinner and shorter. Say 6mm x 14mm?

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Joe Beaver
Maker of Sawdust

here is one I just completed with carbon fiber bracing inspired by Trevor Gore...the braces are .350 high at the center of the X and .250 wide, the lower braces are .310 and .275 respectively.. the guitar sounds fantastic and is louder than my other guitars. It seems I can go a little smaller on the braces , I am getting a little less than one degree of rotation at the saddle when the strings are unloaded so it may still be a little stiff....the bridge plate is also a laminate and is .080 thick ..

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MK5acoustics.com

Wow Lewis. That is an innovation bracing pattern. I really like what you say about the sound of it!!!

Did you use the CF tow? What did you laminate the bridge plate with?

Todd,

Yes, that makes sense. Sounds like if you are not careful you could over brace with CF. I was thinking it would help with top deflexion due to humidity changes. And maybe reduce the size of the bracing a little.

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Joe Beaver
Maker of Sawdust

braces have cf tow top and bottom..the bridgeplate is a lamination of bubinga, cf cloth, and bubinga
the braces are 2 piece adi spruce laminate bent on a form with a blanket

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MK5acoustics.com

That is a beautiful top.

I have been thinking about doing something like that but have been playing with the idea of bending the braces then laminating them together with several layers of CF tape.

I have never used CF but the tow thing scares me. Seems to me several layers of vertical CF tape epoxied together should work, but I've been wrong before.

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Joe Beaver
Maker of Sawdust

The CF does not make the brace appreciably stiffer and obviously not any lighter for the same dimensions; what CF is used for as pictured is to reduce the creep that wood under loading exhibits over time.

Correct on all counts, except for the stiffness part. CF does indeed make for a good bit stiffer brace of the same dimensions; it's not as stiff as it would be if we capped the brace with CF, but it is a good bit stiffer than just spruce. And as such, one must be careful not to overbrace, because it's easily done. The extra weight of the CF can be countered by using lighter spruce(Engelmann works great!) or even cedar.

Best not to go any narrower than 1/4", as you'll quickly run critically short on glue surface area.

Wes, that's one massively braced top! Neat pattern, but you've concentrated the vast majority of structure and mass right at the bridge area. As good as you think it sounds, there's a lot of room there to free-up the design much further. Also, by placing the CF in the middle layer of the bridge plate, it's not doing much of anything, other than adding weight. If you want to play around with CF and bridge plates, try some spruce or cedar between two layers of CF.

Also, best to avoid epoxy, as it will kill most of the long-term creep benefits. Best adhesive I've found is polyurethane glue.It's messy, but it won't creep with time, hot or cold. Epoxy will cold creep under load, and quickly when the temps get above 100f.

And what's this CF tow stuff?

It's carbon fiber that hasn't been weaved. Straight fibers. Used to design special configurations where strength is needed in generally one direction. CF square rods used for neck reinforcement is carbon fiber tow laid up so all the fibers run the length of the rod.

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

I just strung up my first two falcate braced guitars this week, a steel string and a nylon Blanca.
Very noticably louder and more responsive than conventional construction.
No Pics yet still some cosmetics to be attended to and tuning of body resonances etc

I just picked up some 12k tow off of e-bay. What count did you use? What epoxy did you use?

I understand that Gore recommends using CF top and bottom but it seems like that CF next to the plate (top of brace) wouldn't really get you all that much.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

So, tow is the fiber, without the epoxy matrix? How do y'all control the amount of epoxy, and do you try to replicate the high pressures/temperatures used in good quality CF laminates?

Mario,
The tow is appears to be carbon fiber cord. The first I saw of it was on a picture of a Trevor Gore guitar. Apparently he forms his curved braces and smashes down a tow cord on the top and bottom of the brace. I think they use expoxy to bond the strands together and to the brace.

It is an interesting process. Here are some pictures.

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Joe Beaver
Maker of Sawdust

A bettter picture.

Trevor Gore Top

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Joe Beaver
Maker of Sawdust

Grumpy asked:
"How do y'all control the amount of epoxy, and do you try to replicate the high pressures/temperatures used in good quality CF laminates?"

The short answer is; you don't. Using tow in the way Smallman and Gore do can still end up making a lighter structure for a given stiffness though. That's because CF really shines in tension; it's not much better in compression than the matrix it's in, if I understand it correctly, and is not all that good in shear. In the 'central web' brace that you use, most of the CF is in the compression and shear parts, and is not doing you as much good as it could in terms of adding stiffness. It does confer a lot of benefit in controlling cold creep, and there's nothing wrong with that.

Smallman makes what is, in essence, a sort of I-beam, with the 'flange' being the CF tow, and the 'web' being balsa. In this case, the limiting factor is probably the shearing failure point of the balsa web, which has to be wide enough to have sufficient cross-sectional area. With a wide web like that, the flange ends up thin. The fact that you've probably got more matrix in the flange than you'd like isn't all that much of a penalty, since there is not a lot of it in any case. You could use pre-preg for the flanges, of course, but you would lose some of the ability to vary the stiffness as you like along the brace (which is mostly done through the depth of the web anyway). Using tow also allows him to put on a lot of thin layers in the flanges, that cross at the intersections, thus tying the whole lattice together. In Smallman's tops, the lattice _is_ the structure: the top plate itself is pretty much just a membrane to fill in the spaces and move air.

The issues I see with this are that it's very hard to gain any exact control over the distribution of stiffness from point to point, and even harder to alter it once you're done. Smallman lays down the tow on the plate to start, builds the balsa lattice on that, shapes it, and then builds the tow on the top surface. Since the tow is so strong small variations in the amount from one place to another could make disproportionate differences in the stiffness distribution, and thus alter the way the top vibrates. You could conceivably add to, or sand away, tow on the upper surface of the lattice, but you can't get at the stuff that's in between the plate and the balsa.

One of the issues Smallman has had in the past (I have no recent information) was 'fine tuning' the sound once the instrument was together. If there is a 'wolf' of some sort about the only way to control it is to add mass in active areas of the resonances involved, to alter the pitch. Since the whole point of this structure is to reduce the mass of the top to the minimum, that goes against the purpose. From what I understand he used to (and maybe still does) simply rout the top off if it was taking too much added mass to get things under control, and build another. Other makers, if I understand correctly, fill in areas around the edge of the top with epoxy; essentially reducing the vibrating area, and once again, gaining control over problems at the cost of giving up some of the added efficiency of the lattice top.

I've often thought that it would be interesting to make the lattice 'in the air' so to speak, and test the stiffness and mass distribution of it either by modes or by deflection. You could eliminate hard and soft spots in the lattice by adding or removing tow on both surfaces of the webs, and then glue it to the plate. It might take some experimentation to find the optimum stiffness distribution, but once you did you would know how to do it again.

I'd need to re-read Trevor's book to get myself current on his use of tow, so I'm not going to comment on it now. I'm sure he'll be along to correct me if I'm wrong!

There are some sacrifices made in going each way.

With tow or capping with cloth, the quality of the composite is sacrificed in exchange for freedom of form. Form can matter a whole lot to the final result, though, and things like falcate bracing wouldn't be possible otherwise.

With CF 'plate' or pultruded rods, you get predictable composite properties but you're stuck with the form they come in. That doesn't leave you much to work with in terms of shape, straight cross-sections only, but the tweaking is a lot easier when you can depend on your material properties.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Should have read up on epoxy 'creep' sooner. I have been using it in my 5 part neck layups because I was trying to avoid yellow glue creep. Oh well.

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Joe Beaver
Maker of Sawdust

From a quick perusal, it seems that epoxy creep is more prevalent in fast set epoxies.
Anyhow you dont have much choice when using tow capping, polyurethane would just foam in this unconfined situation.

Whilst it is not spelled out in the book, the strands that Trevor specifies, pulled from plainweave cloth are in fact only 3K tow and this seems to be plenty for the Falcate design.

Jeff, I do use West System GFlex. They claim it is good stuff. Creep may not be a problem under my loads. I guess the biggest challenge is clamping the joint tight enough to make a clean line but not so tight as to starve it.

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Joe Beaver
Maker of Sawdust

I use 12k tow and system three epoxy,,,

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MK5acoustics.com

NOT one of my tops. It's a falcate top by John Buckham aka "Woodrat". Credit where it's due.

Here's one of mine: Gore falcate

Lots of stuff in this thread needing longish answers, but busy right now, so will get back later.

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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

Trevor

Got it, sorry for the mix up. I'll be looking for your post

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Joe Beaver
Maker of Sawdust