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Question about my top deflection test results
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24587		 Page 1 of 1
Author:  SteveSmith [ Sat Nov 14, 2009 1:04 pm ]
Post subject:  Question about my top deflection test results
I've just finished thicknessing and deflection testing 3 tops; I think they're where I want them but I'd sure like to hear some of your thoughts on these numbers.

My tester is not set up as well as it should be, here's a link: http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24464. My supports are 17" between centers with a 5 lb weight.

1. Sitka for a size 0: density 385 kg/m^3, Deflection 0.210" at 0.103" thick. Pretty floppy.

2. Sitka for an OM: density 539 kg/m^3, Deflection 0.140" at 0.105" thick. Stiff.

3. Adi for a Dred: density 437 kg/m^3, Deflection 0.140" at 0.114" thick. Very stiff.
Author:  Ti-Roux [ Sat Nov 14, 2009 1:37 pm ]
Post subject:  Re: Question about my top deflection test results
SteveSmith wrote:
1. Sitka for a size 0: density 385 kg/m^3

2. Sitka for an OM: density 539 kg/m^3


We can have that large difference of density between 2 pieces of the same species?
Author:  SteveSmith [ Sat Nov 14, 2009 1:49 pm ]
Post subject:  Re: Question about my top deflection test results
Those two pieces came from totally different sources and I've had them for maybe 7 or 8 years. The piece for the O has a lot of streaking and could by something else.
Author:  Jim Watts [ Sat Nov 14, 2009 2:07 pm ]
Post subject:  Re: Question about my top deflection test results
Steve, why do say # 3 is very stiff and #2 is only stiff?
According to the data you show #2 is your stiffest top.
Author:  SteveSmith [ Sat Nov 14, 2009 2:47 pm ]
Post subject:  Re: Question about my top deflection test results
Jim Watts wrote:
Steve, why do say # 3 is very stiff and #2 is only stiff?
According to the data you show #2 is your stiffest top.


I should have been more clear, I'm referring to the apparent cross-grain stiffness.
Author:  Alan Carruth [ Sat Nov 14, 2009 7:05 pm ]
Post subject:  Re: Question about my top deflection test results
Those densities seem to me to be near the extremes for Sitka, from the few that I've measured, but not out of all reason. Yes; ALL woods can vary widely in density.

I'll note that for all of the softwoods I've tested density is a reasonably strong predictor of Young's modulus along the grain. The stiffness, the ability to resist bending under load, will be proportional to the Young's modulus times the cube of the thickenss. You need to have a certain amount of stiffness to resist the bridge torque over time, and, while you can get it from the bracing, I like to have the top taking it's share of the load. Under the circumstances, I'd have made that low density top a bit thicker than the others, even though it's for a smaller guitar. This really illustrates the fact that you can't just go by the species and guitar size to determine top thickness: you've got to measure the stiffness in some way to really know, preferably before you get it thinned out.
Author:  SteveSmith [ Sat Nov 14, 2009 7:23 pm ]
Post subject:  Re: Question about my top deflection test results
The two pieces that I think are Sitka may be something else. I don't have enough experience yet to be sure. I bought them long before I ever built anything so I wouldn't put those numbers in anyone's database for sure.

Alan, thanks for responding. I thicknessed the O top based on what I thought might be a reasonable deflection but it just doesn't feel stiff enough lengthwise (as if I actually have a clue). I think I'll put this top aside and use a euro top I got in the Swap-a-palooza.

I read all I can find about this stuff and think I understand but there's no substitute for just getting in there and trying it out. duh
Author:  David Malicky [ Sat Nov 14, 2009 7:42 pm ]
Post subject:  Re: Question about my top deflection test results
Those densities are also near the extremes for my samples of Sitka brace stock. I have seen as low as 22 pcf (350 kg/m^3) -- that board was very flexy and actually seemed somewhat weaker than its low density would suggest. The highest I've seen is 35 pcf (560 kg/m^3), which felt more like VG fir.

Actually, we could consider ourselves lucky -- the range for balsa is 4 to 20 pcf. :shock:
Author:  Tom West [ Sun Nov 15, 2009 7:11 am ]
Post subject:  Re: Question about my top deflection test results
Steve: Wonder if the three tops were the same size IE 17x22 or whatever? If so it would seem that #2 sitka was much stiffer than the red?
Author:  SteveSmith [ Sun Nov 15, 2009 8:42 am ]
Post subject:  Re: Question about my top deflection test results
SteveSmith wrote:
Jim Watts wrote:
Steve, why do say # 3 is very stiff and #2 is only stiff?
According to the data you show #2 is your stiffest top.


I should have been more clear, I'm referring to the apparent cross-grain stiffness.

The width is very close on #2 and #3 and you are correct that #2 is easily the stiffest with the grain. However, #3 is by far the stiffest cross-grain.
Author:  Alan Carruth [ Sun Nov 15, 2009 3:52 pm ]
Post subject:  Re: Question about my top deflection test results
I've got Engelmann that runs from 335 kg/m^3 to about 510. My range for Sitka so far is 410 to 500.
Author:  truckjohn [ Mon Nov 16, 2009 10:50 am ]
Post subject:  Re: Question about my top deflection test results
Personally, I wouldn't count on the wood being anything other than what it was sold to you as.... There is a whole lot of wishful thinking when some folks get a wide-grained, stripey, and super hard Sitka top without particularly prominent med rays... Ooh Ooohh Maybe it's Adi!...

Nope.. Don't count on it! If they sold it as Sitka.. it is most likely Sitka... There really is that much variability!

On to the next thing...

Be very careful with your perception of "Stiffness" when samples are different thicknesses... because the "Cube Rule" is in full force here... A top that is 10% thicker will be 33% stiffer in deflection.... and you will notice this crossgrain quite a bit... which is probably accounting for some of your perception of "Stiffness"

You really need to calculate Moment of Elasticity X and Y numbers (With and across grain).. This helps correct for the Thickness of the sample...

If you search Al Carruth's posts, he goes into this in gory detail.

Thanks

John
Author:  SteveSmith [ Mon Nov 16, 2009 8:58 pm ]
Post subject:  Re: Question about my top deflection test results
John, you make some good points.

My biggest problem is I don't remember what the wood was sold as and at the time I was not labeling my wood as I do now. idunno

I suspect you're right about my perceived cross-grain stiffness. I need to remember that the "Cube Rule" relates to tops just as well as to braces and other structural members.

So it looks like that was a nice experiment but I now realize I have more work to do. Onward to Ex, Ey, and Eave.

Thanks all for your comments and suggestions.
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