Yet another math question

[Dave Bagwill]

I think we're in agreement. In my picture, I stated that I wanted to calculate the downward force on the top 'through the bridge'.

I can easily measure deflection at a given weight applied to the bridge. I'll attach a pic from ANZL forum.
But that really does not tell me much, right? If the top deflects at 25 lbs, well ok - how do I know if that is good or bad? Siminoff suggests taking apart the perfect guitar and measuring the deflection at various loads on the bridge, and building all your tops to match that deflection.

So, if the perfect top deflects .1mm at 25 lbs psi, your other tops would be braced to match that.

My point, though is this: strings vary over a 60 lb..range of tension. A .1mm deflection with a set of 13-58's would not even show up using 11's. I would like to start with the strings and optimize the deflection for that string.

But I cannot measure the deflection without knowing what 'weight' that set of strings applies to the bridge. At that 'weight', I want the top to deflect; if it does not, I can shave braces or thin the perimeter until it does.

I'm sure I'm not being clear.

[ken cierp]

That's a good puzzle, I will be looking in on this thread. And look forward to seeing your solution.

[John Parchem]

Deflection testing is a way to determine the stiffness of the top as a system. From my understanding of the Gore\Gilet books you can use the stiffness value in another set of equations to determine the rotation of the bridge under the load of the strings. They use 2 degrees of rotation as a good measurement of a top that is not too stiff or not stiff enough. The rotation of the bridge will take into account the load of the strings, the height of the saddle and the stiffness of the top.

So to Herman's point it is rotation that you are concerned about in the real world, not the downward force.
So although the deflection test is testing a downward load, the result of that test gives stiffness that can be used to take in to account torque or the rotational force.

From a practical building perspective. If you had a guitar that you really liked and had a deflection test measurement you could use that knowledge to not only help replicate the guitar (as best as possible) but to understand how to change the required stiffness and thus a deflection to deal with different types of strings, 6 string to 12 string, longer guitar ...

[Dave Bagwill]

How does Gore measure the degree of torque - you mentioned 2 degrees I think?

And guys, I get the torque thing. But I also know that the area around the bridge is going to also deflect, and that can be measured as well as torque.

I'll continue to pursue it. Thanks for the input!

[John Parchem]

This is just me going back to my physics and the use of force vectors. I am not sure I am correct. Anyway ... At the termination point of the string the bridge is pulled both up (away from the guitar) and pulled back toward the nut. Those two forces will pretty much equal the load of the strings. the relationship will be the cosine break angle times the string load. My guess is that the downward force on the saddle will be equal to the upward force at the termination point.

[ken cierp]

I am sorry but this " ... use 2 degrees of rotation as a good measurement of a top that is not too stiff or not stiff enough" to me is a magic mystery potion --- is that what Martin, or McPherson or Lowden or whoever use as their base line regardless of brace pattern?

I think Siminoff has it right, the correct top rotation/deflection has a direct correlation with string tension, if the strings do not move the top at tension that's not good if they move it too much that "might" be bad (overdrive). This by the way was pointed out to me by Bob Taylor as well, I think you are on a good path Dave -- as Bob pointed out to me incorrect match up of strings with sound board physical response is a key factor and first thing to check if a well made guitar does not sound too good. Teeter also touches on this concept.

I suggest that you try and contact Siminoff, he has corresponded with me in the past.

[John Parchem]

Ken, there is not that much difference from "if the strings do not move the top at tension that's not good if they move it too much that "might" be back (overdrive)" and rotate 2 degree. If you were reading Siminoff what is too much? what is enough? The 2 degrees was an empirical finding from their experience and factory guitars that they thought sounded good. How is that magical. The rotation of the bridge in Gore's example and in Siminoff example both properly related to desired stiffness. Is it magical they they tried to quantify the rotation to a target for clarity.