Bracing question

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Joe Williams
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Bracing question

Post by Joe Williams »

This is my first, I have a question about my second. This one uses an unusual bracing system which, I think is responsible, among other things, for a sweet but quiet, highs oriented tone. This is fine for me, but a friend wants one like it, but with a more conventional sound. His, then will have a more standard x brace system. My question is how much arch do I need in the x brace so that it won't go concave under full string pressure? IOW, how much deflection of the top, given a string break like you see in the picture, may I expect under full stress? I fully understand that a precise answer is not possible. I am perfectly happy with the most ballparky of ballpark answers.
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Jason Rodgers
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Re: Bracing question

Post by Jason Rodgers »

That's an interesting combination of styles there, Joe. Sort of Selmer meets Panormo. Maybe you can tell us a bit more about the "unusual" construction details so we can understand what sort of departure is necessary to reach your goals.

Also, out of curiosity, how have you braced/reinforced that square soundhole? Without some sort of cross-grain treatment, it is very likely for cracks in the top to develop at the corners.
-Ruining perfectly good wood, one day at a time.

Alan Carruth
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Re: Bracing question

Post by Alan Carruth »

One way to figure it out might be to take a clue from the regs for wooden aircraft construction. Wood cold creeps; moves over time and takes a set under a sustained load, no matter how small. The rule of thumb in wooden aircraft is that the initial deflection under maximum load should not be more than 1/3 the deflection you can live with.

On a guitar, I'd say you can live with top deflection so long as it doesn't go concave over a large area. If the dome goes flat when you first string it, then it might go concave over time. If you've still got 2/3 of the curve left, you're probably OK.

Joe Williams
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Re: Bracing question

Post by Joe Williams »

That's exactly what my question is though. Given an x brace of "normal" dimensions, say with struts 3/8 wide, 1/2 deep and about 15" long, the degree of string break over the bridge that you see here, very roughly, how much deflection should I expect under full pressure?

As for the one in the picture, there is a structure running the full length of the box and tying together the end blocks, which contacts the top at a small point right under the bridge, giving the bridge a very firm platform on which to sit and, I think, relieving the top of all pressure from any direction. There are then 3 very light cross braces ( 1/4 x 1/4), one below the bridge and one above and below the sound hole. For the next one, I plan to keep that structure, except for the point at which it contacts the top.

Trevor Gore
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Re: Bracing question

Post by Trevor Gore »

Joe, it's pretty much impossible to give more specific answers than those already proffered without knowing the material properties of the wood you're using, e.g. Young's modulus, which varies a lot from species to species and also from piece to piece within species. The variability within species can easily be a factor of 2. The stiffness is also very dependent on the precise geometry and layout of your parts. So basically you have two choices: get into the engineering of it and do the calculations or knock up a "prototype" and do some physical testing.

Steve Senseney
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Re: Bracing question

Post by Steve Senseney »

It is difficult to any exact answers.

I would mention that the biggest brace (in most steel string X-braced guitars) is the bridge. With the long bridge in your picture, and the inside braces you describe, you probably have a lot of support on that top.

Of course, you might have such a stiff top that it does not move very much. That is one of the balancing acts on instrument building. Keeping the design on the edge of failure, without failing.

Joe Williams
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Re: Bracing question

Post by Joe Williams »

I'm unable to make myself clear, apparently. The part of the interior brace in the guitar above which contacts the top won't be there in the next one. With regard to the top, it will be conventional bracing. I understand the fuzziness of the question, but I'm not looking for an exact answer. The top will be red Cedar, but for the degree of exactness I'm looking for, I doubt that the species of wood in the top is a relevant consideration. The braces will be a quite hard wood, in this case Hard Maple. I'm only curious whether 1/16 is reasonable, or should I allow for a quarter? I understand completely that expecting better specificity than that is hard. Thanks for the responses.

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Mark Swanson
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Re: Bracing question

Post by Mark Swanson »

There's no way you guitar will ever sound like anything "conventional" if you use hard maple. You need to use spruce for the braces.
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Barry Daniels
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Re: Bracing question

Post by Barry Daniels »

To go even further, if you want a "conventional sound", then you should build a "conventional guitar".
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Joe Williams
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Re: Bracing question

Post by Joe Williams »

Point taken on the wood for braces. As for the rest, I view this activity as play, not an exact science in much of anything else than structure and fret placement, so I may not be very well suited to this forum. Thanks all, for your time.

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Barry Daniels
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Re: Bracing question

Post by Barry Daniels »

1/4" to 5/16" wide spruce braces is pretty standard. Sorry if I came across a bit rigid or overly serious. But many of us have refined the craft to a high level and take these topics very seriously. It takes some adjustment to provide advise to someone new who may just be dabbling in the craft. But please don't leave. I am sure we can find a way to get you the information you seek.
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Joe Williams
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Re: Bracing question

Post by Joe Williams »

I appreciate the work of those creating fine Armani suits, as it were, but what I'm going for is more of an old jean jacket feel. This one, for instance I took to the local rock haven music store for a far better musician than I to play. His reaction was "This is a blues guitar man!", and then he beat the crap out of it for 20 min. In his hands, it did indeed sound like an old Robert Johnson recording. I love those unexpected things and would never try to make them uniform, except in playability. As for the next one, I'll just build 3/16 or so arch into the braces and see what happens. Should be more than enough, given the low angle of break over the bridge. Again, thanks for your time.

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Peter Wilcox
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Re: Bracing question

Post by Peter Wilcox »

Joe Williams wrote:Point taken on the wood for braces. As for the rest, I view this activity as play, not an exact science in much of anything else than structure and fret placement, so I may not be very well suited to this forum. Thanks all, for your time.
Hey Joe, where you goin'?

I too only build for the enjoyment of the process. Many if not most of the people here are professionals or semi-professionals who depend on conventional techniques. There is a lot of useful information here, and very helpful people who are glad to share their experience and knowledge. The library is a huge resource.

Sometimes those who think outside the box and go against the grain are treated with less respect, and that is as it should be. Conventional wisdom is 99.9% correct, but it's going after that 0.1% that makes it fun and interesting.

Hang in there. Build your guitar and see how it sounds.
Maybe I can't fix it, but I can fix it so no one can fix it

Jason Rodgers
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Re: Bracing question

Post by Jason Rodgers »

Since you mention Robert Johnson, bluesmen of that era played Stellas (and the like) that were flattops with floating bridges and tailpieces. They have a distinctive sound. If that's sort of the direction you want to go, then I would suggest looking up the bracing schemes and dimensions on those guitars. If I'm not mistaken, some of those models were built with ladder bracing. Finding out more about this construction will also inform you on the appropriate amount and type of arching necessary to avoid top collapse.
-Ruining perfectly good wood, one day at a time.

Joe Williams
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Re: Bracing question

Post by Joe Williams »

It's not necessarily a sound that I'm going for, but it's a sound that I'm happy to hear now and then. The point of the exercise, for me, is serendipity. I want some consistency in things like playability and construction durability. Otherwise, I want to be surprised. I knew this one was probably going to be fairly quiet, due to the type of bracing I did, but I was surprised at how "sweet" the tone is, and how much sustain it has. The friend who wants one needs more volume and bass, so I'll go with a different bracing. But I'm not interested in production work, I already had a career. I'll use whatever wood I find pretty and make them whatever shape and size strikes my fancy this time. I just finally learned to use a pipe bender, so no more need for molds. Like I said, it's all just play for me, much more art than science.

Joe Williams
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Re: Bracing question

Post by Joe Williams »

BTW, I don't feel that I have been treated disrespectfully. I'm just in a crowd here, as you pointed out, most of which is used to thinking in certain ways. That's ok, I can work around that.

Rodger Knox
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Re: Bracing question

Post by Rodger Knox »

This isn't exactly the answer you're looking for, but I think it's close. The top usually has a 15' to 20' radius dome, so that's about 1/8" if I understand what you're asking.
The top is usually braced in a radius dish that forces the dome. Some are built completely flat, but those have pinned bridges, so the top is pulled up behind the bridge and pushed down in front of the bridge by the torque on the bridge.
With the trapeze tailpiece, the force will all be downward, so you do need a bit of dome on the top, and this is usually expressed as a radius. I believe you've expressed this as the offset across the guitar, and 1/16" is a little flatter than usual and 1/4" is more curved than usual, but your guesses are certainly in the ballpark.
A man hears what he wants to hear, and disreguards the rest. Paul Simon

Kerry Werry
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Re: Bracing question

Post by Kerry Werry »

Interesting design.. I don't think I can offer much in the way of advise other than experiment and see what happens.. make a guess :-) see how stiff the top is once is is glued up and then make some determination on how much stiffness you need to add with the braces. I'm not sure how maple braces would make a guitar sound but I'd love to hear one.. do you think you would gain much by using maple? Sitka spruce has a very high strength to weight ration - good enough to build airplanes out of ..

I think Trevor outlined your options pretty well.. from my perspective experimentation is fun...

Hopefully you hang around and show the results of your efforts some sound clips as well :-)

Kerry

Jason Rodgers
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Re: Bracing question

Post by Jason Rodgers »

Sure, make your X-braces 1/4"-5/16" wide by 1/2"-5/8" tall, splay them 90-100 degrees, position the X under the ends of the thicker bridge segment, fill out other "finger" braces and "tone bars" as you see fit, and arch it as much as you think will do the trick. Maybe move your sound hole (window?) north about an inch or so to give the X some room. Oh, and then make the sound hole big enough to put your hand inside to shave down the braces if they turn out to be too big (the small sound hole and its location might also be contributing to the softer voice).
-Ruining perfectly good wood, one day at a time.

Joe Williams
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Re: Bracing question

Post by Joe Williams »

Thanks very much, I think about 1/8 of arch sounds sufficient. I'm not married to hardwood braces and I think the Spruce is good advise to take. I imagine the small hole is a contributor, and the first time it needs strings, I may enlarge it some and see. The hole(s) on the next one, btw, as per the wishes of the guy it's going to, will be the outline of a feather or 2, to go along with a bridge in the form of a steer skull.

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