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Big Braces - Skinny Tops
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Author:  David R White [ Sat Oct 27, 2007 4:08 am ]
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The third guitar I built(Mahogany/Cedar OM) was a real stinker. It had several big visual flaws, the neck was too thick and most importantly it sounded lousy. For a fleeting moment I thought it was good to get all my mistakes out on one guitar, but that thought didn't last too long.

It sounded lousy mainly becaue it was over braced, I had use, the Antes plan and not done enough shaving. I tried shaving the braces through the soundhole but it wasn't improving a lot so I did what any world class luthier would do - I took the ROS to that sucker like there was no tomorrow.

I did this with the bridge on and guitar strung up, so essentially took a nice big half ring around the outside and behind the bridge but didn't really touch it in front. By the time it started opening up you could see a big ridge where the sanding had taken place and I figured it would self destruct and I would chalk this up to learning.

In fact it became a pretty nice sounding guitar after a few weeks, I refinished it and played it quite a bit. The sound was a little forward - great for gritty blues or cutting a lead line through other instruments. After two years it showed no signs of structural issues due to the above. For an unrelated reason I recently decided to re-top it and after taking the top off found that the section behind the bridge was all around .045" or 1.1mm.

So the point of all this...I'm not one to jump to a lot of conclusions from one data point, and I'm not advocating .045" tops - but at least in this case I was able to build a nice sounding guitar with an extremely thin top that showed no signs of structural issues after a couple of years. This has me wanting to explore the idea of thinner tops and varying the thickness of tops from the tail to heel.

Anyone care to comment?
Author:  James Orr [ Sat Oct 27, 2007 4:18 am ]
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Wow, David. My current top is extremely thin after sanding, too, but I
think closer to .080". It's an Italian spruce top. Can you say more about
your braces? Dimensions? etc.
Author:  James Orr [ Sat Oct 27, 2007 5:30 am ]
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[QUOTE=Hesh1956]explore making the entire system which is the top as
well as the braces less massive but still strong enough to hold together
over time and use.[/QUOTE]

Good notes, Hesh. The way I'd altar it would be:

Explore making the entire system . . . with a goal in mind. I read a
lot about making the top as light and resonant as possible, but what I
would like to add is the importance of having a desired character of
sound in mind. The character of overtones, fundamentals, speed, note
separation, etc. I don't hear much discussion over shaping the specific
character of the guitar's sound, and I'm not sure if it's being overlooked
in discussion or if we just don't consider it at all because we're not
familiar with the vocabulary.

I am interested in hearing more about David's guitar here. I haven't
read much about tops being THAT thin and would like to know more
about the particular guitar.
Author:  grumpy [ Sat Oct 27, 2007 5:53 am ]
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Structurally, we can have a thin/weak top and brace it heavily, or we can take a stiff/thick top and brace it lightly, and achieve the same structural results.

But one will sound very differently than the other. This is where each of us can find "our" voice in the instrument.

And bracing lightly or heavily ca also be achieved via engineering a
brace system to act stiffer or less stiff, despite the size of the
braces themselves. In other words, anf for an example, 1/4" braces can still render a
heavily braced top if the pattern itself is a stiff one....
Author:  David R White [ Sat Oct 27, 2007 5:55 am ]
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explore making the entire system which is the top as well as the braces less massive but still strong enough to hold together over time and use.

...agree, but you can reduce weight by reducing the top or reducing the braces so what happens if you favour one over the other?

the importance of having a desired character of sound

...agree, I am trying to make each guitar favour a particular style of music - and I'm happy to have a different guitar for different styles of music.

James, some more details on the guitar - the top started at .105" and half of the top remained there. The X brace was .32x.75, stayed at .75 for about 1.5" from the X then took a straight slope down to .14 at the end. The LF braces were .25x.45 where they met the X and took a straight slope down to .1" at the end. Definitely more bracing than I am using now.
Author:  Alan Carruth [ Sat Oct 27, 2007 9:23 am ]
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I've been keeping track of top weights for several years. Basically, what I've found is that the top itself accounts for far more of the total than the braces. Usually a 'bare' top for a steel string will be in the range of 100-150 grams, all of the bracing will come to 25-40 grams and the bridge is usually 25-35 grams. Yup; the bridge can weigh as much as all of the other bracing.

From this it would seem that the place to reduce weight is the top, but that's not the whole story IMO. As Hesh said, it's probably more important to get an entire system that's in balance than it is to make any one part as light as you possibly can. And, as James Orr said, you need to have an objective in mind: you can't get anywhere until you know where you're going.

I tend to use a thicker top than most, (often starting at .140") and, by most standards, I should be making quiet guitars. In practice my instruments can hold their own quite well, thank you, because I do a lot of things that make the top more _effective_, and get some work out of the rest of the box too, particularly the back, in critical frequency ranges. All part of the 'systems' approach.

I also generally thin my tops out toward the tailblock, often by quite a lot. This is a much less critical area for strength than, say, the area between the bridge and soundhole, although obviously you can't overdo it either. You have to be aware that this sort of thinning has specific effects on the tone, and compensate for them in other ways if you are to maintian the balance you like. There's no free lunch.

The real way to save wieght on a top is to use wood that is less dense. Generally the lengthwise stiffness 'tracks' the density quite nicely: all of the top wood samples I've tested fall pretty close to the same line, regardless of species. If, for whatever reason, you want a light top, then pick one that has low density, and leave it a bit thicker. It will end up having the required stiffness at a lower weight. Again, there will be tonal consequences, and you need to think about all of them before you start, but that's part of knowing where you're going.
Author:  L. Presnall [ Sat Oct 27, 2007 11:05 am ]
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How thin have you guys gone with braces on OM-sized guitars? I'm tempted to try just under 1/4", leaving the height alone at 5/8" to 3/4" and just leaving the top a little thicker than I otherwise would have...only thing that scares me is the top/back not holding their shape under tension. (I hope I didn't hijack the thread...it's not too much off track of the subject here).
Author:  David R White [ Sat Oct 27, 2007 11:26 am ]
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Personally not below 1/4". I've never gone lower because I'm concerned about the gluing surface getting too small.
Author:  Alan Carruth [ Sat Oct 27, 2007 11:28 am ]
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The only thing I can think of that might give a reading on how narrow you _could_ go would be the bracing on lute tops, which was generally about five times as high as it was wide. The obvious limiting factor is the strength of the glue joint holding the brace to the top, and I expect that they skirted the practical limit. The fact that the proportions were pretty consistent suggests that.

This is not to say that X bracing 1/8" wide and 5/8" tall would be a good idea on a guitar. It's possible, for example, that the total cross sectional area matters as well as the height, so that you might need to replace a 1/4" by 5/8" brace with, say, a .20" by .78". OTOH, the slightly narrower and taller brace would be stiffer, and that might not be all to the good.

Ultimately, there is only one way to answer your question: somebody will have to build the guitar.
Author:  James Orr [ Sat Oct 27, 2007 1:27 pm ]
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Remember the stiffness to weight formula Ervin mentioned in his voicing
lecture made available in video on the OLF.   If you have a unit 1 wide x 3
tall, it's 8 times as stiff as 1x1. The stiffness to weight ration is cubed. If
you have 3 wide x 1 tall, it's only 3 times as stiff as 1x1.
Author:  L. Presnall [ Sat Oct 27, 2007 1:51 pm ]
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[QUOTE=Alan Carruth]
Ultimately, there is only one way to answer your question: somebody will have to build the guitar. [/QUOTE]

Alan, if only I had the TIME!
Author:  grumpy [ Sat Oct 27, 2007 2:16 pm ]
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If you have a unit 1 wide x 3
tall, it's 8 times as stiff as 1x1. The stiffness to weight ration is cubed. If you have 3 wide x 1 tall, it's only 3 times as stiff as 1x1.

Almost, but not quite.

Doubling the height of a member stiffens by the cube. That means a 1" wide by 2" tall member is 8 times stiffer than the 1x1, where a 2" wide by 1" tall is only twice as stiff as the 1x1.
Author:  KenH [ Sat Oct 27, 2007 2:33 pm ]
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Mario,


I have adapted a variation of your "Magic tone bar" into my latest few builds. I like the added volume and I believe (i may be wrong) that this will help keep the guitar from developing a belly over time.


I have read your web site on the bar, but I wondered if you could give a more detailed thought on why this works like it does?> Is the added mass , added strength, or rigidity the reason this seems to work so well? Do you thin your tops thinner on the ones where you use this bar?
Author:  grumpy [ Sat Oct 27, 2007 3:50 pm ]
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Loaded question. How about I just tell you what the stock market will do next month? <bg>

I believe it's a combination of the added mass and the added stiffness,  working together, and where each of those is located that makes it work(or not work). I say "or not work" because on some, it just plain doesn't.

No, I never thin my tops in anticipation of something like this, but if you have a 'thin' sounding guitar, that isn't overbuilt, this is a sure way to add depth to the tone.
Author:  Rod True [ Sat Oct 27, 2007 4:26 pm ]
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Hey Mario, I thought you only added the tone enhancer "if" the guitar needed it in your opinion.

Maybe I'm wrong (has happened many times before) but I thought I read on your website that you make the brace but don't put it in till after the guitar is all finished and "requires" it.

Oh and please, do tell what the stock market will do over the next month, don't worry though I won't hold you to it    
Author:  Andy Zimmerman [ Sat Oct 27, 2007 6:11 pm ]
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Hi
I personally like Thin tops with not so big braces. I measure everything on
each top from weight/ density /deflection etc. This give a means of
comparison if you like a sound you are getting or don't like a sound you are
getting. Look at your data and see a difference. For the sound I like for my
guitars on an OM, my typical stiffness Lutz spruce ends up around 0.085-
0.09 inches. But that is with my bracing etc etc etc.

Bottom line is guitars tend to be overbuilt. Keep good records so you don't
have to reinvent the wheel. I even photograph all my braced tops with a
ruler etc so I can look up the data as well as a photograph of the bracing
setup
Author:  joelThompson [ Sun Oct 28, 2007 12:05 am ]
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I am currently building my first olf om.
The palns are great but i found it was increadably overbraced.
I know this is just a guide but i took 2mm off the hight of the x brace and
thined the braces down considerably and i still think it is over braced
even after considerable shaping and scaloping.
It is a flamed redwood top and i have left it quite thick (2.8mm) after
advice from the forum.
It was quite livey before i glued the top on but it has dulled considerably
after gluing.
i am going to leave the bracing as they are for now and try thinning the
top around the edges to open it up but i do agree that guitars tend to be
over braced these days.

I was told by someone however that om's in particular tend to be heavily
braced is this true and if so is there any reason for this.

Joel.
Author:  James Orr [ Sun Oct 28, 2007 4:27 am ]
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Thanks for the correction, Mario. I was watching the video, trying to pull
it out, and couldn't quite get it fully. Thanks.
Author:  grumpy [ Sun Oct 28, 2007 4:27 am ]
Post subject: 
I thought you only added the tone enhancer "if" the guitar needed it in your opinion.

If the guitar needs it or not is usually determined by what the desired outcome is.  Not all will need, or want what it does. And it's great to have it in your back pocket for when a surprise comes along and one you thought didn't, does, ask for it.

As for thickness and tone, all of you who haven't, should build a few arched instruments(archtop guitars, mandolins, even a violin or three). There's a lot to be learned from them. And there's a very good reason why the center area of many of these is twice as thick as the re-curve area, and even that, is thicker than most of y'all are doing for entire guitar tops! Yet power isn't a issue with them, and neither is structural issues(the arching lends immense structure on its own)...

A banjo is the ultimate in thin top technology. But do we want guitars that sound like banjers? Hope not....
Author:  McCollum [ Sun Oct 28, 2007 4:36 am ]
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.060 and .080 tops, sorry but thats nuts!
Theres nothing to help produce tone, not to mention the future warranty issues! I don't care how much bracing you put in the top is going to distort over time.
Lance
Author:  James W B [ Sun Oct 28, 2007 4:52 am ]
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So how does one actually measure deflection accurately?        & nbsp;      James
Author:  grumpy [ Sun Oct 28, 2007 6:21 am ]
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.060 and .080 tops, sorry but thats nuts!

Theres nothing to help produce tone

 That's what I was trying to say(without actually saying it), but you know how sensitive everyone is around here <LOL>

Thanks for saying it.
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