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Hi all,

I’m about to re-top a dreadnought I build and am considering experimenting with carbon fibre reinforced bracing along the way.

Who here has done this on a steel string and what were your methods, i.e. bracing style and pattern (lattice, X-brace etc) and how thick did you make the top?

My layman’s understanding of the X brace is it gives the body lateral and longitudinal strength while distributing the forces from the string tension to effectively four corners of the guitar.

Smallman weakens the lower bout by thinning the soundboard to 1mm, and I also guess the lattice has no real strength laterally or longitudinally, which is why he adds in the massive ply structure. This also is designed to minimise vibration of the rest of the guitar, focusing as much string vibrational energy on the section of the top where the lattice is.

What changes are necessary with the added tension of steel strings in terms of bracing? Does a Smallman-style lattice work for steel strings?

Given we know a lot more now and have access to better technology than Gibson and Martin did back in the day is it reasonable to suppose you could take the theory of old bracing patterns still in use today and apply modern knowledge and materials.

For example, given that sound is spherical, what if the lower legs of an x-brace were curved, forming a perfect circle with the lower bout. The bridge would be placed in the centre of this and it could be lattice braced and you now have effectively a speaker. This is much the same a resophonic guitar I suppose but the strings would be anchored in the middle. Of course you still have the issue of the huge tension pulling sideways.

Where on the lattice is the smallman bridge placed?

These are just the musings of someone who has built three guitars in his shed and knows nothing about anything. Insight and experience gratefully received nonetheless!


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I do think you have a lot of research to do.
For the level of detail and the number of questions you're asking I sincerely recommend you read Trevor Gore's "The Book" (includes a smallman type guitar plan)
I'm heading to the shower, but if you look at Mari Proulx's site he builds dreads with CF laminations in the braces and has construction pages. http://www.proulxguitars.com/buildup/build5.htm
And google "images of smallman guitar bracing"

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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.

I can only talk about X-braced SS since that is what I use carbon fiber with. Colin is right about Mario, he has been doing CF in his braces for some time. I looked to him for advice when I first started using it and I followed his recommendations.

I laminate .022 unidirectional CF pretty much like Mario shows on his web-site. These days I believe he only uses the CF in the x-brace. I tend to use it in most of the top bracing and even the back braces.

Although you can use somewhat smaller bracing and still achieve the same strength/stiffness with CF, I use it primarily for the resistance from top fluctuations caused by RH changes

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Joe Beaver
Maker of Sawdust

Trevor Gore has (literally) written the book on this stuff. CF tow reinforcement can go under and over the braces to significantly stiffen them for very little weight gain. It can be done with any bracing patterns including lattice, X-bracing or falcate. You can get a start on this topic in various internet forums (lots on it at the Australian and New Zealand Luthier's Forum) - and then invest in the Gore & Gilet books if you really want to join the cult.

I believe Mr. Proulx uses CF primarily to limit long-term plastic deformation of the top, rather than as a method to reduced mass in the bracing system.

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For the times they are a changin'

- Bob Dylan

Using tow is a entirely different proposition. I could be wrong but I believe Trevor uses it as a bracing element, not for brace stabilization, I know..., it is a fine line. The difference is my braces work structurally without the CF, I suspect his are engineered with the CF being a structural element.

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Joe Beaver
Maker of Sawdust

If you look closely at Mario’s top bracing (at least the images on his site) he scallops his x’s down very low. Next to the bridge plate. I couldn’t imagine doing that without the carbon fiber.

Since I started using CF in the braces I stay at 1/4" wide for the x but I do lower the brace almost 1/6". They end up pretty much at the same firmness, just a little lighter.

In the blog I see he is using CF in many of the top braces, I do remember he said a few years ago he is doing just the x-brace mostly now, but that may have changed.

I haven't kept up Mario's procedure but, if he does it I bet its good.

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Joe Beaver
Maker of Sawdust

Wood under constant loading will deform and take a permanent set...what is referred to as plastic deformation. I suspect Mr. Proulx's background as an engineer suggested that adding a material with significant resistance to this sort of permanent deformation (e.g., CF epoxy unidirectional sheet) could reduce both the rate and degree of plastic deformation in the composite structure. I also suspect that the extra material engineers add to wooden structures to resist permanent deformations over time might be reduced with that type of composite construction, given that traditional guitar structures are vastly overbuilt in terms of strength, even when just stiff enough to do the job.

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For the times they are a changin'

- Bob Dylan

Designing a custom bracing scheme and getting it right first time is not the easiest thing to do. You need a pretty comprehensive understanding of the workings of a guitar and how the static and dynamic loads on a guitar influence the bracing you might want to use.

I linked to this piece I wrote on lattice bracing not very long ago, but anyway, here it is again. You need to carefully consider the material properties of the wood you use (by measuring the piece you are about to use) so that you can size the parts appropriately. Just following linear dimensions off someone's plan will not guarantee the sort of result you'd like. For example, it makes little sense to use a balsa lattice (as per Smallman) in a steel string guitar, because by the time you have enough balsa in there to take the shear stress without failing, it will weigh more than if you had used spruce for the lattice.


Yup.

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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

Thankfully my wife has just ordered me a book for my birthday by some guy called Trevor something-or-other which I’m told will fill in my gaps in knowledge

Thanks for the link, Trevor. I have seriously been looking for a scientific explanation of how guitars work for ages and so am incredibly excited about getting stuck into your book!

I’ve just come out of a two year speaker build / journey which will hopefully have given me some foundational knowledge that can also be applied to the theory within your book.

Mario’s idea of laminating the braves with CF is an interesting one by the way.

Cheers, Tom




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Look what the postman brought

Buckle up. It’s a deep read. If you are not a scientist (and I am not), I recommend trying to read the design book first, but not trying super hard to understand the granular level of his proofs, if that makes sense. You can get lost in the math if you try to understand every little bit of it. But the overall points of the design book drive what is outlined in the build book, so it is a good idea to try to glean the big picture from the design book before getting into the build book.

The build book is an awesome guide to some great ways of doing the tasks needed to build a guitar. I put it up there with my favorites for that.