Dean's article "Tillering the Organic Bow" set me upon a quest to revisit what I thought I knew about bow mechanics. The negative tiller aspect in particular directly contradicted what I'd read and the whole basic premise behind the positive tiller paradigm. I resolved the issue eventually by assuming Dean simply used the negative tiller aspect as a literary device. Specifically, IF we choose to build a bow essentally upside down, what is called a "symetrical bow," which is in effect a bow with a shorter upper limb, THEN negative tiller would be the only way to balance the forces at work equally between the upper and lower limbs. He made a perfectly valid point, but used a method I could not initially accept. This is different in my view than advocating the "symetrical" design AND negative tiller. Rather, I think it exposes the so called "symetrical" design as being doubly wrong, at least in the sense that the upper limb gets a free ride at the expense of the lower, both in terms of the geometry and physics.
My view is that no bow can be made perfectly symetrical, as we must grasp the bow string at a point above the fulcrum of the bow hand. One simply can't shoot an arrow thru his bow hand. This is why bows have always been made with positive tiller, it offsets this inherent asymetry. One could just as easily make the lower limb a little bit shorter, putting the arrow pass nearer the dimensional center and perhaps more importantly the nock point of the arrow at or nearly at the center of the bow string. This would mitigate the need for positive tiller. If you go the other way, by putting the arrow pass say 2" above center in the so called symetrical bow, then you need MORE positive tiller to compensate. That's why a three under shooter can use a bow with less positive, or even tiller. He grasps the string lower on the bow, closer to center dimensionally, closer to the fulcum of the bow hand. In effect, he shortens the lower limb. So, shorter lower limb needs less positive tiller.
For glass bows, or longer bows where we have plenty of extra work capacity in the material it's not really as important. But for really short bows where we tax the material nearly to it's limits, it can make a difference. That is where I think or assume Dean focuses most of his efforts, at perfecting the design of short, heavy weight hunting bows. In the so called symetrical scenario, by having the upper limb be shorter (by grasping the string necessarily above the fulcrum) AND weaker (positive tiller) we stress it more than the lower. The lower gets a free ride, benefitting from the extra lenght in the handle, below the arrow pass.
It's a simple matter to see the asymetry in the so called symetrical bow. Just look at where the nock point is on the string, and compare that to a bow where the arrow pass is closer to dimensional center.
I've made a butt load of bows, and I honestly cannot say just how much offset, from none to 2" is "best," or that it really makes a heck of a lot of difference. But the science, and our experience from antiquity, sides with using a little geometry AND a little positive tiller. That's why most of the bows you see are made with the arrow pass at 1" or 1 1/2" above center, and relatively few where the arrow pass is 2" above which would be the truly symetrical 1/2 the 4" handle. It doesn't sound like much, but remember it is doubled, putting the arrow pass 1" to 2" closer to dimensional center, respectively, than the truly symetrical design.
I regret that internet discussions have left folks feeling as they do, that others have assumed some guru status and look down upon them, condemn them as being a disgrace or whatever. Knowing what I do, and who I do, I honestly don't think that was the intention, and no ill will was intended. I know I get pretty rough with my language in some of these discussions, and sincerely regret it later.
Topic: Symetry vs asymetry (Read 3532 times)
