Pyramid vs Flatbow

[loon]

what is the reasoning behind making  the straight limbed bows more eliptical?

thinner wood can bend more without taking set

[willie]

Loon,
any portion of any bow limb should be thin enough to prevent set, but I guess what I am asking is why should I want the outer limbs of a flat bow to be thin enough to make them bend more than the inner?

[Pappy]

What bubby said is what I have found to be true on stave bows, not sure on board bows, never built one of them.
 Pappy

[Badger]

Loon,
any portion of any bow limb should be thin enough to prevent set, but I guess what I am asking is why should I want the outer limbs of a flat bow to be thin enough to make them bend more than the inner?

  Loon thats a good question, the reason falls into what I call tiller logic. If the outer mid limb is just as wide as the inner limb (parallel) it would need to bend a little more to be under the same stress.

[Eric Garza]

Steve's right. When you bend a board that has the same cross sectional dimensions (let's say a 1 x 4) across it's entire length, more bend will be focused at the board's center (or the fulcrum over which it's being bent). To compensate for this, you need to tiller segments of the board that are further from the fulcrum of you want them to bend more -- and be strained equally to -- wood nearer the fulcrum. For bows, the fulcrum over which the bow is being bent is your hand holding the handle, so outer limbs need to be tillered, either along their width or thickness, to make them bend more and handle more strain.

[willie]

agreed that the further out on the limb, the thinner or narrower it needs to be, but this does not necessitate that the bend radius should increase to keep the strain consistent. On a pyramid bow, the width narrows, and with a more parallel width limb, the thickness tapers.

I guess I just cannot see how the common advice to prefer circular or more elliptical is helpful. Strain is dictated by thickness, and an equally strained crossection will bend more if it is narrower, and less if it is wider.

I have found that closely monitoring where along the limb, that the set begins to show, is much more telling than aiming for a particular shape of  bend.

Nothing wrong with the circular tiller of the pyramid, and it seems the same shape (without the non-working handle section) is also commonly advised for the more parallel width ELB type.

Is there something about  energy transfer or arrow speed that makes one tiller shape more preferable over another?

[George Tsoukalas]

I look at it this way...on a pyramid bow, more of the bending has to take place closer to the handle where it is thickest to minimize set. If a pyramid bow is tillered elliptically there will be more set towards the tips.
Jawge

[Eric Garza]

I sometimes wonder if people's advocacy of circular vs elliptical tiller gets muddled by semantics. When I see a bow with a rigid handle and non-bending tips the overall tiller looks elliptical. I posted pics and a description of a pyramid bow not too long ago and would call my tiller of that bow elliptical, not circular.

For bows that bend through the handle, I tend to tiller them so that although the handle does bend, it doesn't bend as much as the limbs do. This, combined again with non-bending tips, means the tiller again ends up elliptical, not circular.

I can't say that I've ever seen a bow that was tillered so its bend pattern is actually circular. Maybe someone can link to one?

[SLIMBOB]

I think Steves point is an interesting one, and I dont disagree with it, but I will add this, if a bow with parallel limbs to midlimb, has a bend radius that is the same from handle to midlimb, then it's thickness will be greatest near the handle.  Thicker limbs will not bend as far as thinner limbs without taking set.  Widen the limb near the handle and it will be thinner and can bend at the same radius without taking set.  Do that and you have a pyramid.
Also, I agree with Eric.  Circular is rarely right as a tiller shape in my opinion.

[Badger]

  Another way to think of it. Thickness determines how far something can bend and width determines how far it will bend.

[willie]

I sometimes wonder if people's advocacy of circular vs elliptical tiller gets muddled by semantics.........and would call my tiller of that bow elliptical, not circular.

could well be the case. so to qualify my questions a little better, I take "elliptical tiller" to mean a limb that is stiffer in the inner working portions and comes around into a tighter radius the further out you go, (or a limb that bends more near the handle and less towards the tips). Whether or not these limbs are separated by a non-working handle is not being considered. So on the other hand, a bow with two circle of arcs, separated by a non-working handle,  being similar in appearance to an ellipse, I would still consider to have a circular tilller. 

In other words, has a progressively bent limb  been shown to be better for certain applications, whether it is a bend thru the handle bow with a stiffer center, (or a holmie type with stiffer outers), or is it all in how well you tiller the curve you choose without damaging the wood?

Maybe someone can link to one?

 please see   http://www.primitivearcher.com/smf/index.php/topic,58175.msg805298.html#msg805298

[SLIMBOB]

Willie, I will take a stab at your question as I have considered the question myself.  If you tiller to a circular shape, then a bow will take set evenly along it's length.  Set on the inner limbs is costly to performance.  Keeping them slightly elliptical (less bend inner limb) remedies that issue.

[willie]

Slim
thanks for taking a stab.It made me go back a read Baker's "Mantra" for avoiding set on the inner limbs. I suppose the penalty to performance comes from loss of early draw weight that comes with string follow.

[mikekeswick]

agreed that the further out on the limb, the thinner or narrower it needs to be, but this does not necessitate that the bend radius should increase to keep the strain consistent. On a pyramid bow, the width narrows, and with a more parallel width limb, the thickness tapers.

I guess I just cannot see how the common advice to prefer circular or more elliptical is helpful. Strain is dictated by thickness, and an equally strained crossection will bend more if it is narrower, and less if it is wider.

I have found that closely monitoring where along the limb, that the set begins to show, is much more telling than aiming for a particular shape of  bend.

Nothing wrong with the circular tiller of the pyramid, and it seems the same shape (without the non-working handle section) is also commonly advised for the more parallel width ELB type.

Is there something about  energy transfer or arrow speed that makes one tiller shape more preferable over another?

To figure all this out you simply need to watch the set. The wood will most certainly try to tell you that a parallel width bow needs to get thinner as you move out to the tips and vice versas with a pyramid.
Make some simple test bows to prove it.
A pyramid is strained more evenly and scientifically is the better bow. The real World can be different but the theory is correct.

[George Tsoukalas]

See P 32-33  TBB 2 for a good discussion of pyramid vs parallel tiller by Tim Baker. Jawge