Main Discussion Area > Bows

Poisson Effect Versus Neutral Plane - A Theory

<< < (12/17) > >>

shamus:

--- Quote ---If there is no net movement in the cross-sectoin, then there is less energy lost to returning this mass to its initial position.  Increased effiency would be the result.  And the two counter-acting forces, would presumably produce slightly greater stored energy in the limb.  Granted, these increases would probably be small.
--- End quote ---

I've been reading this over at PP and here. If this theory pans out, then the increases in efficiency would be small (as you said). My initial thoughts are that the increases would be so small so as to have no measurable affect on the efficiency of the bow whatosever. But I'm not saying for certain that is the case.

Fascinating theory, and I look foward to seeing Tim, Tom Sawyer, and Ken work this out. 

My basic working theory on bows is to go symetrical. If the back is flat, then the belly is flat. If the back is crowned, then the belly matches the crown, and so forth. Extremes in either way (high arched D bows, super flat bellies on crowned selfbows, etc) make me nervous.  Work with the wood and try not to impose.
 

NOMADIC PIRATE:
Agree whit all you guys said even if I didn't understand a word ::) (had to stop readyng on page 3 :o :o

Coo-wah-chobee:
 Wow ! Learn sometin' every day ! Didn't realize "herpatology an poisson" had anything ta do with neutral plane ! ;D ;D...bob

SimonUK:

--- Quote ---Simon, I see what you're saying, it could be that this is the explanation.  What would you call that?
--- End quote ---

Oh it's got to be the 'Sawyer effect' if it doesn't have a name already  :)

tom sawyer:
JD, hope your weekend was as nice as mine.  The wife and I went to Cloverdale on Saturday and shot the Trad Nationals, then went to the wild game potluck that evening.  Very nice time, saw a lot of nice people including Pappy from here at PA.  He was up to his usual, making snakey bows and helping anyone that stopped by his camp.  I beat the wife on the course, but she was the only one who hit the aerial target game they were running.

I remember now where I read about the info pertaining to this argument.  It was in Archery:  The Technical Side by Hickman and Klopsteg.  Unfortunately I only have a xerox copy and I can't locate it at the moment.  I did Google a site using "bend moment neutral plane":

http://hsc.csu.edu.au/engineering_studies/civil_structures/bending/bending_stress.html

I quote a section of the page:

"The geometric centre of a two-dimensional area is known as the centroid. In relation to beams the centroid will be the geometric centre of the cross-sectional shape of the beam. It is significant because the neutral plane, discussed above, will pass through the centroid of the beam. When considering a rectangular beam, the centroid is simply found by joining the diagonals but becomes more complex for composite shapes."

They are talking about a beam with a symmetrical cross-section, of an isotropic material that is supported on both ends and bent in the middle.  This is close to what we have with a bow limb.  You can argue that a piece of wood is not isotropic but I think it is reasonably so if the piece is clear and straight.  Consider that you can make a backed board bow from any orientation of wood (plain, rift or quartersawn) ands you don't have to orient a plainsawn board so the outer side is to the back.  To me that is proof that wood is a reasonably isotropic material.  In any case, mull this over.  I know how you guys are thinking, I used to believe this until I read otherwise.  Its confusing because you would think that "stronger in tension than compression" means it stores more energy in tension than compression.  Thats not the case as I hope I've shown now.

Simon, I have nothing to do with this stuff, other than bringing it up for consideration within the context of our application.  Tim made an interesting observation, and Ken described what was going on.  All I'm doing is passing it along because I think it is interesting.

Navigation

[0] Message Index

[#] Next page

[*] Previous page