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So I am figuring out things for my next build and would like to try the whole tall/thin bracing deal. So with using the cube rule and figuring out how to get the same strength with taller bracing I am curious how thin you can actually go with the bracing. There must be a limit, as in don't go smaller than this or you won't have enough area to glue the braces but I am not certain what that limit is. So how thin do you go or do you think you can go?

Pictures would of course be more than welcome also!

Matt

When too thin, the brace can twist or flop sideways when stressed, at which point the cube rule goes out the window.

I use 1/4" and profile them into a soft triangular shape.

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Brock Poling
Columbus, Ohio
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Has anybody tried 3/16" bracing? Wouldn't lack of glue surface become an issue here even with a very high brace?

I'm not sure why you want to go any thinner than 1/4". You wouldn't be saving all that much weight. The total weight of all the braces is about 10% of the top weight. Most builders shape their braces into triangles or gothic arches which removes a lot of the brace mass without reducing stiffness. So, starting with a thinner brace wouldn't make that much difference.

Well I know that their isn't going to be a huge weight saving, its more for interest. I mean many now use 1/4 bracing and the weight saving between that and 5/16 or whatever it was that most used before wouldn't have been great either but every bit helps I suppose. On my last build I used 1/4 so am now thinking well how far can you go.

As well I am going to try deflection testing and generally just see how light I can get the top. As with everything I believe you could probably go too far and make something thats strong enough but with not enough weight. I think there is probably a happy medium where you get the best volume and tone but for the next I am looking to try and find one extreme of the scale (lightest top and bracing)

Matt

I use 1/4" and .270 for braces on SJ OM/000 models (Well for almost everything) Like Todd get much thinner the glue surface gets smaller, for really no benfit in weight or other aspects. Now I am talking SS and not classical, because have no idea about how thin they go as never built one. I do at times go .312 (5/16") but depends on the plate and if feel needed. Not saying can't go thinner, but question would be why on a particular build and to what benefit and do you end up spending a dollar to save a dime in gluing terms here.

Slight addition to the question (not really a highjack)
Do you all use the same width for the tone bars as well as the main braces?? If not, what width do we use there? Thanks.

Only ones I mess with as far as going .250, .270 or .312 are the X, and agan it depends on top and build. The finger braces and tone bars are .250. Now as far as the back plates I do use .270 or .312 on the 3 and 4 braces and .250 only on 1 and 2. As with the top it depends on what I feel is right for the 3 and 4. I do also at times do #'s 3 and 4 where they are wider and not tall and then go .270 or .312. I am not sure but think how Michael Payne shows in his plans, but haven't looked at them recently.

Not knowing how many builds you have or experience, I know I am sort old school way taught, But think if not have many builds, go with some good plans and basics until know what they are doing for your builds and then adjust the things one or two things at a time. Can't go wrong with basics until have things down pretty well to know what changes do. Payne's plans (OLF) are really good with good detail.

I go 1/4" on X braces and 0.200" on all other braces. I don't do tone bars - I do a lattice with interlocked joints.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Lute tops use tall narrow braces. Generally they use an aspect ratio of about 5:1; the braces are about five times as tall as they are wide. Lower braces are narrower. Since lutes were widely used over a long preiod of time, I take that as being more or less a practical structural limit. You might go taller, with a greater risk of the brace buckling or coming unglued, but it's hard to see what you'd gain at any rate.

One of the best explanations of the cube rule is on David Hurd's site http://www.ukuleles.com/Technology/bracemath1.html

Note that contrary to popular belief, the cube rule does NOT mean that you can shape the brace to a triangular profile, lose weight and not lose much stiffness.

A triangular brace of the same height and width as a rectangular one has half the weight but only a third of the stiffness.
You have to start with a taller crossection to save weight AND maintain stiffness with a triangular shape.

I like my braces to be 1/4" wide with the height varying according to the stiffness and strength of
individual pieces. I wouldn't recommend going much narrower than 1/4" since they can start flex
sideways under stress and the footprint may not provide sufficient gluing surface.

Regards,
Kevin Gallagher/Omega Guitars

Yep. That's what the physics books say too!

But... Say you start with 2 exactly same sized rectangular braces and start "Voicing" away...

You shave down the 1st brace to make a Triangle of the right stiffness... and you shave down the Top of a Rectangle to do the same thing.

And.. you finally end up with 2-braces the exact same stiffness (Let's just say 1/3 the stiffness of the original rectangle)

What you end up with is a Triangle that is still full height, but a Rectangle that is ~70% it's original height.

Now, the Rectangle is about 40% heavier for the same stiffness.

If, instead you removed that stiffness by shaving down the sides of the rectangle into a narrow rectangle... you would end up with a rectangle 1/3 the width and full height -- which would then be 1/6th less weight than the triangle brace....

But... say you went from a 0.5" x 0.250" triangle to a 0.5"" x .083" wide rectangle... Would you have enough glue line?

Thanks

John

Yes John what you say is quite right.
And reducing stiffness in voicing at selected positions along the brace is quite an important aspect. It is easy to make a top too stiff
In most cases you will be starting with a brace that is rectangular at the X intersection to allow for jointing and capping.
Changing the crossection as we move away from the intersection should be done with awareness of the true implications of that action rather than thinking of it as free weight reduction without loss of stiffness.

Thanks all for the responses so far!

I think I may have to get that book (left brain lutherie) as I think it would probably satisfy as well as stir up my curiousity quite a bit

Steve: by interlocked joints do you mean you cap the brace intersections of the lattice?

You need a good book on the strengths of material and I also like a tool makers handbook , these have all the formulas and will explain things so you won't have to over think things. Often many people hear a point of fact but don't understand it and assume things that are not true. Physics is Physics

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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

We made lutes for years in Williamsburg. They are ladder braced,though, except for a curved brace,and have a lot more braces than guitars.

Matti,
If I were you, and all the old guys here were telling me not to do something, and giving all the logic and facts of why, I would do it just on principle.

If you take everyone's advice and be safe, you'll never know for yourself, will you? And what does everyone know, it could be a brilliant guitar!

Don't be afraid to fail, it is only a guitar, you can always build another.

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I notch them just like the X brace joint. I do not cap them.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Natural selection tends to come about by finding , over time the best ways to do things . There are a few points brought out in this thread that are very pertinent . First is glue surface area, if you push the limit on that you will have nothing but trouble . The strength of the material , once that is exceeded you will have failure and lastly , You must be able to interpret and understand the physics involved.
When in doubt and you think you understand , make a small experiment . The shear forces of the top at the glue joint are important as that is what is holding the thing together . Cross sectional shape will influence the stiffness , so make some different cross sectional braces , figure the square inch of a standard brace and play with that in width and height to see what happens. While you can increase verticle stiffness with a shape you will loose gyration ( sideways strength ). This is one of the best ways to learn to see the influences of shape.
Keep a little log and make a standard test , with a defined span and weight , and measure the deflection. Once you understand these influences you will be well on your way.
One point that wasn't brought out is that forces will tend to follow euclidean lines ( sharp corners ) . You will want to radius all sharp lines to avoid force focus points.

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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

Isn't the asymtote of all this thinking an "I" beam? The height gets you the strength needed. The vertical section can be made very thin, and to stop it from twisting, you cap it with a thin plate top and bottom. The soundboard itself forms the bottom of the "I" beam, so you only need to make a "T" structure above. I'm going to try building a guitar with really thin (0.05" thick) "T" beams for bracing at some point and see how it works. I'm sure someone's already tried this - any ideas who? Got a link?

Cheers,
Dave F.

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Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

William Gilbert used a T beam structure for his bracing at one stage.
Unfortunately he glued it to the top upsidedown
http://www.schrammguitars.com/tbracing.html
But really, John Hall is on the money with his comments about glue line failure limiting how thin you can go on the braces.
A t beam arrangement would also not allow you to reduce stiffness selectively to voice the top.

Actually , while I totally agree about the glue surface area , Todd Stock had pointed that out a few posts ago. I have seen thinner braces but that was on a classical. I don't think you want to go thinner that 1/4 inch. There is a balance involved here. You can over loosen the top and at that point you will ruin the instrument. You also can ad a few different glue choices. I do use mostly tite bond , but you can add HHG and fish glue , white glue etc. One more point is gluing technique. Be clean and precise on gluing for best results. Bad glue joints do more harm than good.
The "headroom" of a guitar is that envelope that lets you have a range of playability before the tone and voice break up . If you build a guitar too light you can't play it hard . The tone will break down as you overload the top with the stress of the strings. There is much going on from the saddle. Too heavy of a guitar and it will be dead . I would rather over brace it than under brace as you can always lighten it but once the wood is off , it can't be put back on.

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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

If you look at the pics on David's site, you'll see that the T brace is in fact, glued such that it provides easy voicing options.

I've used I beam braces on my last several banduras and that definitely eliminates any tuning possibilities. Fortunately for me, I wouldn't know where to begin when it comes to tuning braces . I beaming the brace dropped the mass by about 25%

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