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Originally Posted by redir
It sounds like it's sliding in into an argument of technical terms. I'm not an engineer but it sounds like the term 'stiffness' might be part of the engineers manual and that it is an actual property of in this case the top of the wood.
So if you measure the top for stiffness you get a number. Then no matter how you brace it you are not changing the number or the measured properties of the top. However of course you are also bracing the top which makes it stronger for the sake of handling over 100 pounds of string tension.
So if you suspend a floppy unbraced top across two supports at each end and put a 5 pound weight in the middle it will deflect say 1/2 inch for example. Now brace the top and do the same test. It only deflects .03 inches now so its 'stiffer' in the layman's sense of the word but the Young's Modulus of the top has not changed.
The engineers can correct me if I'm wrong.
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Young’s modulus, numerical constant, named for the 18th-century English physician and physicist Thomas Young, that describes the elastic properties of a solid undergoing tension or compression in only one direction, as in the case of a metal rod that after being stretched or compressed lengthwise returns to its original length. Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression. Sometimes referred to as the modulus of elasticity, Young’s modulus is equal to the longitudinal stress divided by the strain. Stress and strain may be described as follows in the case of a metal bar under tension.
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https://www.britannica.com/science/Youngs-modulus
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The bending stiffness (K) is the resistance of a member against bending deformation. It is a function of the Young's modulus E, the area moment of inertia I, of the beam cross-section about the axis of interest, length of the beam and beam boundary condition.
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https://en.wikipedia.org/wiki/Bendin...by%20a%20force.
The top material has its modulus of elasticity (Young's modulus), the bracing has its own. The top material has its own bending stiffness, the bracing has its own. When you join them - such as by gluing them together - the bending stiffness of the "assembly" is a combination of its parts.
As is often stated here, the area moment of inertia, and hence, bending stiffness, of a beam of rectangular cross section is proportional to the cube of its height and directly proportional to its width. That is, if you double the width of that "brace", it is twice as resistant to bending, but weighs twice as much. If you double its height, it is eight times as resistant to bending, but weighs twice as much.
Similarly, the bending stiffness - a function of the geometry of the "beam" - of a guitar top, for example, is heavily influenced by the thickness of the top. The top's modulus of elasticity - a property of the material - is not affected.
To be clear the "stiffness" we are discussing is its resistance to bending, aka "bending stiffness". This discussion has to do with mechanical structure.
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Originally Posted by Victory Pete
It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone.
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Victory Pete is asserting two things. First that tightening a truss rod increases the (bending) stiffness of a neck. Second, that the increase in stiffness changes the response of the guitar to increase its "volume, sustain and tone".
The second assertion has to do with changes in response based on assumed changes in mechanical structure.
I'm not going to attempt to dissuade him from those unsubstantiated assertions.