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Poisson Effect Versus Neutral Plane - A Theory

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Pat B:
Damn, I'll be reading and rereading all night! ::)  Takes my pea brain a long time to absorb and comprehend. Give me a few days and I'll get back to you. ;D
   This is interesting and I actually think I understand ::) what you(or Tim) are saying...but I still have to think about it for a while.     Pat
   

tom sawyer:
Steve, I agree that rounding edges minimizes the consequences of the Poisson effect since there is less mass moving out there.  As for the bend transferring energy back, whenever you move something it has to be moved back so the Poisson bend going back uses a bit of energy.  If it didn't move at all, you wouldn't be wasting any energy.  I don't know if this is why your rounded-edge bows do better, it might also be better aerodynamics of the limb.

JD, a yew bow with sapwood back would be an exception.  And wood isn't perfectly homogeneous, but it is reasonably so.  Most bows have a fairly smooth thickness taper as judged by the feathering of rings on the belly.  If density were all over the place, you wouldn't see this.  And wood does respond differently to compresion and tension, but that doesn't mean it produces more spring power on the tension side than the compression side.  Elasticity is elasticity, whether you stretch or compress.  A metal spring is as hard to stretch as it is to compress, even though the metal might break in tension before compression.

Simon, you might be correct in your description but the back side wants to be skinnier so I would have predicted that the sides would still curl up.  In fact I would have thought they'd curl up farther since there's no belly wood preventing movement.  It seems to me that all the parts of the limb are trying to get to a place where there is eual mass on both the tension and compressoin sides.  Hence the idea of the NP wanting to be flat (in a 2-D sense).

marvin:
Lennie,

The NP refers to a "place" where the tension and compression forces are theoretically equal correct?

So if I take a perfectly rectangluar cross section your saying that the NP is physicaly/geographicly positioned dead center because I have the same amount of wood(mass) doing tension work as compression work?

If I remove material(mass) from either side of the original location of the nuetral plane then I have a situation where more wood(mass) is doing more tension or compression which would have to shift the physical/geographical location of the NP correct?

Looking for clarification before I jump in and reveal my ignorance further :)

SimonUK:

--- Quote ---Simon, you might be correct in your description but the back side wants to be skinnier so I would have predicted that the sides would still curl up.  In fact I would have thought they'd curl up farther since there's no belly wood preventing movement.  It seems to me that all the parts of the limb are trying to get to a place where there is eual mass on both the tension and compressoin sides.  Hence the idea of the NP wanting to be flat (in a 2-D sense).
--- End quote ---

I think it's wrong to say that the neutral plane 'wants' to be any particular shape. It is what it is, depending on what the wood immediately around it wants to do.

SimonUK:
Sorry, just to clarify what I mean on my first post. I was saying that the stretched back wants to be shorter - it does this, not by being skinnier, but by 'cutting the corner' on the curve you produce by bending the bow. i.e. it tries to form a less severe curve. But I see your point... generally things do get skinnier when stretched.

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