Yet another math question

[Dave Bagwill]

But I am not interested in comparing top to top. As I said, I want to start with the string that is best for the purpose that I choose, and knowing what the deflection should be will allow me to measure it, and adjust the top accordingly.

Thanks for the help btw..

[Dave Bagwill]

A good article here: https://books.google.com/books?id=l5uIn ... ge&f=false

You can read almost the entire chapter freely. Lots of equations!

edit: you can scroll up and start with chapter 2. Also, hit the button for full-screen and the reading is much easier.

[John Parchem]

Maybe keep life simple. torque and displacement are both proportional to the force involved. So if you have a nominal, or reference displacement you are happy with and a reference set of strings, increase or decrease the target displacement by the percentage difference in the selected strings vs your reference set.

Example if your reference strings had 150 lbs of pressure but you wanted to build a guitar with strings at 165 lbs, decrease the amount of displacement you are looking for by 10%.

Lets say you know the guitar will almost always be DADGAD and using a string tension calculator you find that the tension will be 135 lbs increase the amount of displacement by 10%.

Get you reference displacement from a top you really like.

[Dave Bagwill]

Thanks John, worth thinking about!

[Dave Bagwill]

Here is the reply from Siminoff. The 2 scenarios he refers to are the drawings I sent him with my questions; I've attached them below.


David…

Thanks for the email…

Forgive me for being brief, but due to dealing with a lot of emails and requests of this kind, I have a limited amount of time that I can spend on each such request.

1) In "Scenario one" there is no "string break angle" to calculate the torque moment on the soundboard. Regardless of where the strings are attached to the bridge base, and what angle the strings make to the anchoring point, and as long as they are anchored to the bridge base, the critical component is the height of the saddle above the soundboard. The string break angle does not matter.

2) On movable bridge instruments, the "string break angle" is important and is used to calculate the down pressure on the soundboard.

3) There is no "deflection" on the center axis of the bridge in "Scenario one," but as you pointed out, there is a torque moment, and the torque or twisting of the soundboard creates a depression - or "deflection" in front of the bridge, and a raised portion or bulge behind the bridge. In essence, there is no downward pressure on the soundboard at the center axis of the bridge.

4) In "Scenario two" a weight of 34 pounds will deflect the soundboard, but the amount of deflection is entirely dependent on: the type of soundboard wood, the density of the wood, the grain-count of the wood, the thickness of the soundboard, the type of bracing, the location of the bracing, and other such structural issues.

Hope this helps…

Best,
…R

Thanks for building with us...


Siminoff Banjo and Mandolin Parts

PO Box 2992 - Atascadero, CA 93423

805.365.7111 - http://www.siminoff.net


Straight Up Strings; a paradigm shift in string technology. For more information visit: straightupstrings.com


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On Aug 6, 2015, at 10:50 AM, David Bagwill <deadedith@gmail.com> wrote:

> Mr. Siminoff - I would appreciate you taking a moment when/if you have one, and take a quick peek at the attached highly intensive graphic that I prepared using the Cray supercomputer in my basement. Well, not actually - the paint program in my PC.
> My question is stated on the jpeg.
> Thanks a lot - I've learned a lot from you.

[Dave Bagwill]

So - the torque testing jig Siminoff uses in the Luthier's Handbook figures 11.12 etc, seems to be the best bet after all,
He also shows another jig that will measure the torque of a set of strings (at a particular scale length etc.), so I can use that measurement, and still accomplish my goal, which was to adjust the top - but instead of deflection, I will apply torque to the bridge and discover if the top torques to the measured torque of the strings, or is too stiff or not stiff enough.
Where my method differs is, that he worked from measurements from a known top, while I will be working from measurements directly from the strings of choice, which should give me a greater measure of flexibility. We shall see.

I will be starting the build of the torque measuring jig tomorrow - it is not that difficult - but must wait for a few $ to roll in to purchase an inch-lb torque wrench. Where can I get one of those - HF perhaps?

Thanks for the great discussion guys.

[John Link]

It certainly was a great discussion. Thank you Dave for starting it, and persisting.