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More question(s) about deflection testing
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22506		 Page 1 of 1
Author:  Christian Schmid [ Tue May 26, 2009 10:43 am ]
Post subject:  More question(s) about deflection testing
Hi all,
I'm gathering information about what to do better on my next guitars. I definitely want to use deflection testing when constructing the top, simply to have some data and "objective" comparison point for future builds.

I found this thread here http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=17956 on the topic and found it most enlightening about how to go about it, which jig to use, and even some numbers as a starting point.

However it seems to me that the in the thread above, only deflection testing of the top is covered, without any bracing. My understanding is that you can have the nicest top in the world, if you mess up the bracing, you mess up the guitar as well.

So do you also do some deflection testing with the braces on to arrive at that optimal balance of strength and lightness in a top? Is it essentially the same procedure, or do you use a heavier weight? And if I dare ask - what numbers do you use, e.g., what kind of deflection for a braced top is a nice starting point?

I understand that there's more to voicing the top than deflection testing, e.g. the shape of the braces, scalloping, and tapping until it makes a nice ping here and a nice pong there... and that you could arrive at the same deflection values using wider, heavier braces that are not so tall vs. slim, lighter, and taller braces... which makes me wonder whether deflection testing after bracing is so useful after all idunno

Any input would be greatly appreciated.

cheers, Christian
Author:  David Newton [ Tue May 26, 2009 11:35 am ]
Post subject:  Re: More question(s) about deflection testing
Eat Drink
I'll be lurking, me just do the Ping and Pong method.
Author:  Andy Zimmerman [ Tue May 26, 2009 11:41 am ]
Post subject:  Re: More question(s) about deflection testing
Deflection always brings out some controversy. But that is always good so you can get many perspectives. All I can say is what I do. I do deflect all of my tops prior to bracing and prior to cutting into a shape of a guitar. (Different width plates will change your numbers.) I have come up with a degree of defection that works for me in my entire way of building. I thickness my tops to a greater degree of deflection for smaller body guitars than larger body guitars. I believe in the larger vs smaller drum head concept. Most of us I am guessing do not defect test after the bracing is done. This is a bit difficult to do. Some of us actually put the tops in a mold to "tap tune" as we carve our braces. This was taught at Ervins class. As far a bracing goes, I like that tapered parabolic approach and don't scallop my bracing. From an engineering standpoint I never understood the scallop thing. Why have massive changes in stiffness across the length of a brace. (That is what the scallop does) I personally taper them so the stiffness gradually decreases from the X brace to the edges of the guitar. I use the tall and thin approach as well. Again an engineering principle. Making a brace fatter doesn't add much stiffness, but increases the weight. I would rather make them tall and thin and control the stiffness by changing the height.

Deflection clearly helps for me. It gives me a frame of reference to go with especially when using different top materials. Keep in mind I have a spreadsheet that keeps all of this data as well as bracing measurements etc. So it is only one piece of the entire thing. I also photograph all of my tops for reference as well.

Giving you numbers is actually worthless because everyone uses a different jig/weight etc. (But some numbers are in the above linked thread)
Author:  Alan Carruth [ Tue May 26, 2009 12:53 pm ]
Post subject:  Re: More question(s) about deflection testing
It does make sense to get numbers, so long as they are standardized ones. That is, if you know the Young's modulus of the top wood along and across the grain, then you are in a position to compare your top with any other top that anybody has used, so long as they measured the Young's moduli as well.

Everybody uses a different system of building, of course, and you will have to figure out what top deflection works best for your system. Still, if you're sticking with more-or-less 'standard' stuff, a normal size and shape with standard bracing, then the numbers that worked for me should come pretty close for you, too.

There do seem to be different schools of thought on top thickness. Some folks like to use a thin top and stiff bracing, while others (myself for one) tend to go a bit thicker on the top and use lighter bracing. There are good arguments for either approach, but, obviously, the measurements I use would not be appropriate for somebody who's going to go with heavier bracing anyway.

'Scalloped' bracing puts the stiffness of the top in different areas than 'tapered' bracing does. A tapered top will tend to be stiff in the center, and less stiff toward the edges. For various reasons this tends to favor higher frequencies and longer sustain. Scalloped bracing makes the top more flexible in the center, and stiffer between the center and the rim, so it tends to have more bass, a stronger attack, and less sutain. Either can give a 'good' sound, but the guitars will appeal to different players.

If you have some method of 'fine tuning' the top once you get the braces on, then there's little need to do deflection testing of the braces themselves. You might save a small amount of mass by picking the lightest, stiffest wood for the braces, and making them as narrow and tall as you think you can get away with. I'd bet it won't be much of a saving, though.
Author:  Christian Schmid [ Thu May 28, 2009 2:08 pm ]
Post subject:  Re: More question(s) about deflection testing
Thanks a lot for your perspectives.

Christian
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