Acoustic Guitar Construction

Does this guitar have a tail piece? If so you are correct as would be different as the strings do not terminate on the top. What is the string angle from the saddle to the tail? The smaller the angle the less vertical force.

johnparchem wrote:Does this guitar have a tail piece? If so you are correct as would be different as the strings do not terminate on the top. What is the string angle from the saddle to the tail? The smaller the angle the less vertical force.

If it is a bridge that has string holes your question is not relevant to the problem and you should refer to my earlier answer about a deflection test determining stiffness and determining a desired stiffness from a target rotation. Or use the jig that Ken referred to.

It is not irrelevant if you would consider that my hypothesis BEGINS with string tension and break angle. I want the string to control the top deflection, not the top determining what string to use.

What I need to do, I guess, is build one of Siminoff's little jigs for measuring torque - forget about deflection in terms of weight - the jig has tuning posts and a torque measuring device, so one can do the measurement, then use his larger torque machine (fig 11.17 etc in his book) and go that route.

Have not heard back from him yet.

Sorry my point is that on a strung up guitar the downward force is irrelevant. The total downward force is effectively zero.

A straight deflection test does have value in a few different ways.

The stiffness of a top, which can be measured with a deflection jig will also control how the top responds to a rotational force.

If you measure the stiffness of all of your tops, you can achieve a bit of constancy or if you feel a top is too stiff or too loose you have a stiffness number that you can work with neck guitar.

Taking into account the height of the strings from the top, the stiffness of the top and the pull of the strings you can determine about how much a top will rotate, or probably more relevant you can target a certain top stiffness for a target rotation.

John the downward apex force is irrelevant only if we choose to make it so -- you are correct it is a smaller number than the one Dave has been investigating but the idea of correlating that "variable weight/force" to string tension will be a very cool and very useful design and construction tool for testing sound board responsiveness. -- I totally get what he is trying to accomplish.

ken cierp wrote:John the downward apex force is irrelevant only if we choose to make it so -- you are correct it is a smaller number than the one Dave has been investigating but the idea of correlating that "variable weight/force" to string tension will be a very cool and very useful design and construction tool for testing sound board responsiveness. -- I totally get what he is trying to accomplish.

Sorry I was somewhat articulate. I think his end goals are very valuable. I got nervous when I saw the calculation resulted in a number that did not make sense, so I did a more detailed force vector analysis. I can see that the apex force is relevant, and I can even imagine that there is a correlation between that force and the force required to test the top in a deflection test. I just do not think the two forces are the same. One is the top deflecting from a single point the other is the top deflecting around two points (saddle and bridge pins).

I can not write the math to correlate the two. But I can imagine coming up with two separate equations.

One: starting with a target rotation and given the string load and the geometry of the guitar how stiff do I need to make the top.

Two: how much will a top at the above calculated stiffness deflect under a given load.

So using equation from one I can account different string tensions. Then using the equation from two I can calculate the target deflection

" The significant thing about the design of the contemporary guitar bridge is that not only is there a longitudinal force acting, as in the lute, but by virtue of the break angle of the strings as they pass over the saddle a vector force is created which actually pushes downward on the face."
PRINCIPLES OF GUITAR DYNAMICS AND DESIGN

by Ervin Somogyi

So there is something real, a downward force. that can be measured