Main Discussion Area > Bows
Poisson Effect Versus Neutral Plane - A Theory
tom sawyer:
Simon, its doing the opposite. The edges curved down towards the belly. If it was cutting a corner, it would have lagged behind like a standard Poisson Effect does. And yes, I shouldn't use that terminology, but how would I describe the idea that a non-symmetric cross-section will change shape in order to become more symmetrical with respect to a plane down the middle of the limb? Its an idea that is tough for me to express any other way.
Marvin, yes the NP is going to be right in the center of the limb. Measure from back tobelly at any spot on a selfbow limb, the halfway point is where the NP lies. Connect all the dots and you get a line across the limb tht runs roughly in the center of mass. Connect all the lines and you have a plane, although If the cross-section is symmetrical, then line is straight. If the cross-sectoin is assymmetrical, the line isn't straight. I'm theorizing that a non-symmetrical cross-section tries to bend into more of a symmetrical position when bent. If you remove mass, you have to measure again and draw a new NP line.
duffontap:
--- Quote from: tom sawyer on June 07, 2007, 04:03:08 pm --- A neutral plane (NP) is that imaginary line where half the limb mass is on one side, half on the other.
--- End quote ---
'Neutral plane' does not mean 'middle line,' or center of mass. It means wood that is neither compression wood, nor tension wood but is 'neutral.' For the neutral plane to fall on the geographical center of the bow, or the mass center you would need:
1. 100% homogeneous material.
2. 100% rectangular section.
Since 0% of bows fit that category, the definition you give above has a corresponding 0% application. I'm only harping on this because you are proposing a confusing and misleading definition. 'Neutral' wood is where it is--that is going to vary with section, substance, etc.
I agree that you can say it is very close to center on self bows with rectangular sections but that is not a definition of 'neutral plane.'
J. D. Duff
--- Quote ---
--- End quote ---
tom sawyer:
It does equate to center of mass of a selfbow, because it takes x amount of wood to store a given amount of energy in tension or compression. And equal amounts of energy is stored in tension and compression in a selfbow.
Just because a cross-section isn't perfectly rectangular, doesn't mean there is no neutral plane. There very well is a place in the interior of every limb that is under neither tension nor compression, and where shear is greatest. Measure the length of the back of an unstrung bow, and the belly. The distances should be the same, assuming not a lot of set. Now measure the length of the back and belly at full draw. The back is longer, and the belly is the same distance shorter. Stretching equals compressing.
tom sawyer:
Take any cross-sectoin, and draw a line across that give you euqal mass of wood on either side. That is one way to estimate the position of the neutral plane. But if you put dots along with center in between edges, the line won't be straight but stil you'll have equal amounts of wood on either side. My contention, is that this is more like where the actual neutral plane lies, since each little area of the back is working against its corresponding place on the belly. Sure there's some evening out, but not that much. If there was, then the crowned area of a limb wouldn't be under more stress since it would get shared around. Each little area is its own entity, which makes the neutral plane funky shaped. I simply think the explanation of Tim's second observation is explained by thinking of the NP this way, and that there are forces working to restore some degree of symmetry to the cross section and by extension, to the NP.
SimonUK:
Here's what I mean by the edges of the back trying to 'cut the corner' of the bent v shaped bow:
http://s166.photobucket.com/albums/u118/simon2468/?action=view¤t=scan1.jpg
The edges of the back can reduce the amount they have to stretch by trying to get nearer the string. Obviously there is a slight conflict of interest because as you say, it also wants to get skinnier so there must be some tension across the back from left to right.
I guess the net forces at work make frowning the least stressful position for the back to be in.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version