Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on June 09, 2014, 06:25:59 pm
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" A pyramid bow of equal thickness will always bend evenly regardless of starting width"
I honestly don't know the answer, a good friend of mine and prominent respected bowyer insists this is true. I usually but not always find myself tillering out the pyramides with a slight taper. My belief is that the closer I am to the right demensions the less tillering it needs. He does awknowkedge that not wide enough will take set or break so we are not really that far apart to begin with.
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Depending on the woods origin, i.e. stave or board. A stave could vary too much in other ways to hold a thickness tip to fade. A board? I cant see the thickness changing anymore than .020-.030" with most cuts.
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pretty much that's true, but nothing is absolute, I build a lot of pyramids and they all start out even, and they bend pretty even right off the bat but to get a great tiller I think they end up with just a slight taper, slight
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Because it's wood,nothing is always totally uniform.
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You know, I've read this so many times.
I think the catch is that it's not quite possible to have a perfectly triangular limb. It would have to end at zero width and the string would have to attach there at the very end. Of course that's impossible. Even on the narrowest tips the zero point is a few inches at least off the end of the bow.
At least that's how I understand it.
With extra width you get closer because the angles are steeper, so it's slightly closer to truly triangular. Instead of the zero point being maybe 5 inches off the end of the bow, you're 2 or so.
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Depends upon extremes of the limb's width and or length. A really wide pyramid limb 3" and 30"(2x limbs /bow, 8"handle and fades, 68" btwn nocks) long is and a constant limb thickness is going to be bending a lot more(too much for a good tiller) in the mid limb and the tips and not enough near the fades. It would need stock removal from the thickness, nearer the fades to get a circle of arc tiller.
A pyramid bow limb 1&3/8" wide x 30"long with a constant thickness is going to be a lot closer to a good tiller than the first example.
Always go by how the bow bends rather than what a theory states.
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True, it will bend evenly theoretically, Badger.
Problem is near handle wood that is wider has too bend more than the narrower outer wood to give a more circular tiller.
A bow with parallel limbs out to just past mid limb will have an elliptical tiller.
I think that prominent bowyer and I have disagreed a bit on that point.
Jawge
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I would make an assortment of limbs from PVC to prove the theory and from then on it's up to the differences in different woods.
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I can't say for sure on boards because I don't use them. I have never had a stave bow have a constant thickness. A pyramid is my favorite style and I make them whenever I have enough width.
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8) Watching this...
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It's been always true with me with board bows that have a straight profile. Laminates not so much, but maybe because I reflex them during glue up, and that might effect tiller. Or I also might do a small taper to the pyramid board bows and not realize it so much. With either bow I find fades are very important to keep the bow from bending off the handle.
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With extra width you get closer because the angles are steeper
True, the wider the fades start out, the better the tiller will be in my experience.
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I've built a lot of pyramid board bows, cut the thickness on a tablesaw constant even thickness, most of the time after I cleanup the saw marks I go straight to full brace, good bends, but, and it's a big butt, by the time I have it finished if I mic it it will have a slight taper, very little is perfectly uniform with natural materials, bub
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EZC has it :)
Watch the fades at all costs!
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I always figure with a stave bow [haven't done a board bow] that it gets me in the general vicinity of where I want to be,but not exact,never really measured if they have a slight taper or not,[probably do] :-\ I just remove wood until it bends like I want. :) I never really do much measuring of anything after a stave is floor tillered. :) So Steve I guess the answer is True/Flase/some what true/some what false. ;) ;D ;D ;D You know that seemes to be the case in a lot of this wood bow building stuff. ;) :)
Pappy
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How do we define a pyramid bow? And what does "bend evenly" mean? Is a pyramid bow any type of bow that has the limbs tapering in a straight line? Would an English longbow, tapering from 1¼" wide at the handle to ½" wide at the tips, be called a pyramid bow then? And what about 4" wide at the fade, tapering to ½" nocks in a straight line? Or 3" wide tapering to 2" wide nocks? Or how about 1" wide fades, tapering to 4" wide tips? That's (part of) a pyramid as well, right? Just an upside down pyramid. And how about a bow tapering from 2" wide fades to ½" wide tips, that is 90" long? How much of a pyramid can we actually achieve in a bow that will always have nocks wider than zero inches (the tip of an actual pyramid)?
And how about "bend evenly". A circular tiller is "evenly"? And an elliptical tiller is not? Depending on the width of the bow, an elliptical tiller can still be the correct tiller that spreads out the load as evenly as possible.
In my opinion, a "pyramid bow" is a stiff-handled bow that has a more or less uniform limb thickness, with a straight line width taper from fade to tip resulting in a circular tiller. I call the thickness "more or less uniform", since you preferably want the tips to be a little bit thicker in most cases, to provide enough rigidity with the string grooves filed in.
A bow tapering from 1½" wide fades to ½" wide tips does still meet the pyramid width taper rule, but it will not have a more or less uniform thickness if you want to achieve a circular tiller. You would have to decrease the thickness towards the tips, if you still want this circular tiller, which is what we do in ELBs. A bow tapering from 4" wide fades to ½" wide tips will be quite the contrary. It will bend too much near the tips and the fades will be too stiff. So you would need to reduce the thickness near the fades.
In my opinion, a sensible pyramid bow will be tapering from 2½" wide fades (maybe 2¼" works as well) to ½" wide tips. The thickness could then probably be machine made with a thickness planer/sander to about 3/8" for a 40-50 pound bow. Then you would just have to glue down a thicker handle, while you could use powertools for the width taper (circular saw or band saw) and thickness (planer/sander). Only minor tillering would be necessary to create a circular tiller then.
So does "a pyramid bow of equal thickness always bend evenly regardless of starting width"? No, it doesn't. I've given several weird bow designs where Badger's intuition would probably feel that there's something wrong in that statement. The starting width at the fades would have to be around 2½" wide in my opinion to create a sensible pyramid bow. With other widths the bow would not have an equal thickness if you're looking for a circular tiller.
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Good post Darksoul
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I'm with Bubby. I've built a lot of board pyramid style bows (Lams and stave bows) , and I do! mic all bows while in the tiller process about every 5". (just something I do, and keep logs from each bow on limb thickness). I have found after tillering that most will have a slight taper in thickness from fades to tip.
Knapper
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Good points, DarkSoul. I think the "bend evenly" means no flat spots. This is pretty obvious with bow limbs produced by machine (straight and flat with no waviness as seen from the side view) but not so obvious with character wood. The benefit of a pyramid design with consistent thickness is the way that it takes the guess work out of the tillering process.
I vote "true" as long as we are not talking about circular tiller vs elliptical tiller (or any other shape of tiller).
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In my opinion, a "pyramid bow" is a stiff-handled bow that has a more or less uniform limb thickness, with a straight line width taper from fade to tip resulting in a circular tiller.
I don't understand why it would have a circular tiller when its limbs are tapered. If limb thickness is uniform, shouldn't its limbs have uniform width to have a circular tiller?
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SCP, I see the logic behind that if you took a long straight thin piece of wood you could bend it into a perfect hoop. But the geometry changes because of an anchor point at the handle and the string angle pulling on it.
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If we include the stiff handle in the picture, the tiller just cannot be circular. I'm not sure what it means for one limb to have a circular tiller compared to an arc. In most cases, some length of the tips are usually not bending much.
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I just remembered that the pyramid tapering actually closely matches the relative burden of each portion of limb from the handle to the tip. That does mean that the bending part of the limb would have a relatively close to circular tiller. Sorry for the confusion.
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Dang! All this time I just thought I was making pyramid bows :o
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SCP - wood at the same thickness can bend only so far. If it's all the same thickness it should be bending the same amount.
Thickness determines the amount of bend acceptable by the wood.
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SCP - wood at the same thickness can bend only so far. If it's all the same thickness it should be bending the same amount.
Thickness determines the amount of bend acceptable by the wood.
Interesting point. In that case, I guess the variation in limb width does not matter in determining the side profile. So such variation in a pyramid bow is only to increase the efficiency, but not for side profile. Even though this sounds quite reasonable, I'm not certain that this is the case in reality. Do you have any actual test data?