Primitive Archer
Main Discussion Area => Bows => Topic started by: Jefficus on April 11, 2016, 12:24:43 pm
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I just want to make sure I'm thinking about this the right way. It's widely said that (draw length)''x2 is a good basis for total limb length. Realistically some bows are better off with more, and others can be great shooters with less. So here's my question: is this for sure about total limb length, or WORKING limb length? I've always assumed it meant total length, and had to do with string angle and distributing tension rather than visible bending, but I'm hesitating. I've only made two straight pyramid bows and they're both overbuilt 72'' long because I just wanted them to work haha. But I'd like to design better.
Basically, if making a bow for 28'' draw, and the limbs are each 28'' long from the fades, does every inch need to be bending or holding tension to some degree? or is it more about the length, since the tension is naturally distributed through the whole limb automatically? For that same hypothetical bow, say if it had flipped tips or a static recurve to it, that would reduce the inches of each limb that could do work, right? Because the last 3-6 inches are static. I know that the tips aren't really supposed to be doing much work anyway, and that usually the most happens mid-limb depending on the profile of the bow. But considering that the hypothetical bow would then only have 22-25'' per limb AVAILABLE for bending, would that still work? or do you design 28'' of limb from the fades to the start of the static portion? I feel like that's not right, because then how would recurves ever be shorter than longbows? I guess I just want some of you more experienced guys to put my mind at ease that I'm thinking about this the right way haha.
Somehow I feel like I wouldn't even think about this making a straight bow, I guess because leaving the tips stiffer in that situation, I assume they're just doing some work I can't see with my eyes. But for a truly static tip or recurve design, it makes me hesitate.
Thanks in advance for your input!
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Working limb, for me, is the limb from past the fade to the tip. So a typical 28" draw bow would figure this way. 28 x 2 = 56" + 4" grip + 2"(x2) fades = 64" ntn or 66" ttt.
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As far as statics, I build them as long as I would a flat bow. Adding statics to the end of a limb doesn't do anything but change the string angle and maybe add some tension at brace if done right. Adding them to a bow doesn't mean you can shorten the recipe because its a static. On a typical flat bow you leave 5-8" stiff on each limb tip. Bending that same portion into a recurve doesn't add that much stress as its already stiff and un- bending.
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Actually adding recurves stresses the limbs more because it bends the limbs farther when drawn.
Pearlie's got the working limb thing right. I usually figure draw length x2 + about 10% for t/t bow length.
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Personally, I rather err on the side of making a bow "too" long than too short. As long as you keep those tips low-mass, a few inches of extra length only means a sturdier bow. A too short bow means a bow in pieces, or the ever-so-frustrating chrysals showing up after the first layer of finish (gah!).
I don't really have much smart math to figure it out, I sort of look at the stave (crown, knots, "character"), the type of wood, and my desired draw weight and take it from there. Somewhere along, though, I figured that 65-67" ntn is a good length for a pyramid design of medium weight at 28" draw, medium quality wood (elm, ash, juniper). This for a stiff handle (8-10") and 6-8" of stiff tips. For an extra inch of draw length, I add 1,5 inch ntn. For an inch less draw, reduce ntn with 1,5 inch. Parallel limbs half the limb length generally means I can shorten the bow a bit, but then stacking might become an issue so I'll usually add a bit of reflex/recurve to the tips.
But it all depends. I sell bows almost exclusively to folk who shoot at targets on a lawn, not hunters (hunting with bow and arrow is illegal in Sweden). I can certainly see how pushing the wood a little to make the bow shorter is a good thing for a hunter. Also, when you sell bows to people it's sort of bad business to design bows on the limit of what the wood can take. A little redundancy is good.
I think this thing with making bows short is something a bit more common in America than over here. That's just my feeling about it. Maybe euros are more influences by English archery, with their 80"-ers, and Americans more influenced by native american archery? Not sure.
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On a typical flat bow you leave 5-8" stiff on each limb tip. Bending that same portion into a recurve doesn't add that much stress as its already stiff and un- bending.
This seems wrong if the recurve is an actual significant hook.
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Pat, I think Pearlie has it right. It is the overall amount of reflex behind the back of the bow that has the most effect. For my mass theory I add wood for reflex whether it is recurve or just reflex doesn't matter.
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Really? It still seems off intuitively and mathematically. Where's Joachim? :D
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Dont bring me down with all the math crap, Pat....M
Imagine a static full drawn, now un-curl the last 7". The amount of limb bending wont change. At least I dont think it will :)
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Dont bring me down with all the math crap, Pat....M
Imagine a static full drawn, now un-curl the last 7". The amount of limb bending wont change. At least I dont think it will :)
The amount of limb length won't but the amount of BEND in that length will change.
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Frodolph what did you mean by "Parallel limbs half the limb length"? Certainly you don't mean if I didn't taper the limbs then they could be 14'' long rather than 28'', right? haha
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I think he meant "Parallel limbs FOR half the limb length" but I may be wrong.
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I always went with over all length of the bow tip to tip. Never had a problem. I made many 62"ttt bows that pulled 28 at a hunting draw. But this conversation cant be had without accounting for width.
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Dont bring me down with all the math crap, Pat....M
Imagine a static full drawn, now un-curl the last 7". The amount of limb bending wont change. At least I dont think it will :)
The amount of limb length won't but the amount of BEND in that length will change.
I'd agree there. I think it is a small amount though, especially when comparing a heavily reflexed tip to a static tip. I think on average it would compare to drawing the same bow another inch farther when hooks are added.
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PD gave a good starting point for a rigid handle bow from an excellent stave.
I would add an inch or 2 for heavily crowned staves and perhaps for staves with several knots and other imperfections.
Bend in the handle bows can be made shorter by at least a few inches depending on the condition of the stave.
The more bows you make the more you develop and intuitive feel for length.
Jawge
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Dont bring me down with all the math crap, Pat....M
Imagine a static full drawn, now un-curl the last 7". The amount of limb bending wont change. At least I dont think it will :)
The amount of limb length won't but the amount of BEND in that length will change.
I'd agree there. I think it is a small amount though, especially when comparing a heavily reflexed tip to a static tip. I think on average it would compare to drawing the same bow another inch farther when hooks are added.
You could trace the curve of two same length bows and see what the difference is at brace. I think it's probably quite a bit more than an inch. That's why you hear that recurves increase set or potential for breakage.
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Perhaps. I might even have a few test bows. Not the same wood, but that is irrelevant.
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I try and keep it as simple as possible, double the draw and add the non working part, what ever that is
is If want to be overly safe. :)
Pappy
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I agree with Pat on the bow length per draw and the effect of the static. You definitely have to consider working length. TBB adds weight in the mass equation for static tips and that's built around working length with 8" handle/fade. Also the mass equation handles recurve and reflex as separate entities to contend with.
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You could also place a static on a grid and then draw the projected straightened limb tips out. Surely that must equate to the bow being braced several inches higher?
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I think the common bow length figure of 2 X draw plus 20% is just a good rule of thumb so bows don't stack and don't have to be too wide. Anything you do to a bow to increase the radius of a bending area means you need to make them wider and thinner. Most recurves are a bit shorter than the longbows but they are also usually wider.
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Just nit pickin' but I want to be sure. You mean "decrease" the radius, don't you. As in bend the bow more.
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The working limb equivalent to draw length rule is really just a starting suggestion to be on the safe side. I would have to say, just from my experience is that it is on the extremely safe side, of course this all depends on the piece of wood you are working with. So make the bow that you and the wood can agree on, if you want to be safe, then make the each limb equivalent to one draw length plus the handle section, just make your bow wider if you go short. Utilizing the mass principle will really help with this. Going shorter required you to go wider, in order to gain back the mass lost from shortening. But of course there is a limit.......
Eric
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Just nit pickin' but I want to be sure. You mean "decrease" the radius, don't you. As in bend the bow more.
Yep, I had it backasswards.
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Jefficus, Of course I didn't mean that if you make the limbs parallel instead of tapering pyramid-style you can cut required limb length in half! Haha! That would be an awesome rule of thumb, though! The absence of a comma in the sentence you quoted is kind important ;)
I simply meant that if I want to make a bow shorter than my standard 65-67" ntn for a 28" draw – let's say 62" – I make the limbs parallel about halfway from fades to the tips, then let them taper to narrow tips. Another way of putting it is that I add more wood, more mass, to make the bow take the extra strain of a shorter design. The draw back is that with that kind of wood placement you sort of have to make the tiller more elliptical, otherwise it'll take a lot of set right close to the fades. The elliptical tiller in combination with the bow being shorter increases stack, so I reflex/recurve the tips to counteract that. Hope that clarifies.
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Jefficus, this question, and the whole discussion, are why this is a rule of thumb, and not an exact rule. It's an interplay between length, leverage, limb to string angles, etc...and has to do with Badger's mass formula. It always needs tiny adjustments, often to width.
Have you read the TBB's? baker has some really good examples of how this works with his logical extreme Andaman/Holmegaard bow (which we are now more correctly calling a Mollegabet rather than Holmegaard).
So, imagine a pyramid bow, with limbs as long as draw length. You can make that bow shorter IF you make it wider. BUT, if you keep doing this, ever shorter and wider, at some point the shorter bow gives bad leverage, bad string angle, and thus stacks more and stores less energy. Likewise, you can go longer and narrower until you start to get efficiency losses due to limb length, weight, and string weight. Eventually you are under-straining the bow, unless you adjust tiller.
Now consider a Mollegabet, with wide, flat, bending inner limbs, and the stiff, narrow outer limbs, same thing; total limb= draw length. This style has very favorable leverage due to the long stiff tips, and will hold that string angle better than the pyramid, BUT, since only the inner limb is bending the risk of extra set needs to be offset by moving more mass (width) down closer to the handle. If total limb length stays the same, but you make the outer limb longer, you have to keep moving more of that mass to the inner limb. The string angle stays more favorable because of the lever, but again as you change the ratios you hit a law of diminishing returns where you can't eliminate enough tip weight in the outer limb, or add enough width to the inner limb to help.
Next, imagine that pyramid bow from before, that was too short. If you start adding a longer and longer handle, eventually, you regain your string angle and leverage, but it takes MORE added handle length to do this than it would take added LIMB length, because of where the bend is happening out at the tips. It's whip-tillered. You can still have problems with the string angle.
Baker's pecan bow was basically a stiff long middle, WIDE bending midlimbs, with long stiff tips. Almost a hybrid of the Sudbury bow with Mollegabet outer limbs, sort of. In his case, more total limb length let him get away with less bending limb length, and PUT THAT BEND WHERE IT DID THE MOST GOOD.
Now, this gets really too general, but here are some "rules" Bending enough limb LENGTH reduces strain, which reduces set. Bending in the right spots minimizes the EFFECTS of set. Bending enough limb WIDTH can compensate for bending less length, as far as set, but not as far as string angle. Bending bows too much close to the handle can induce high set. Bending bows out by the tips can reduce leverage. Bending anything anywhere too much is bad, and adding length can counter most of these problems within reason, but creates other problems.
So, the original limb length rule gives us a starting point: a decent balance between leverage and string angles, enough wood to bend, predictable set, and options about where to place limb mass and various bend radii, in several styles of essentially straight-limbed bows.
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"I'd agree there. I think it is a small amount though, especially when comparing a heavily reflexed tip to a static tip. I think on average it would compare to drawing the same bow another inch farther when hooks are added."
This is about how I see it. I started flipping tips on whitewood flatbows specifically to counter set they took. Like a bow would take an inch and a half of set, then I flip 6" of the tip, which was usually stiff anyway, forward 1.5-2". Usually that would bring draw weight up a tad, increase string tension at brace, but the bows usually would take on any more new set. You could easily over-do it, of course.
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I enjoyed reading this. Type of wood, width, profile, backed or unbacked, One last thing to consider. If you are making the bow for yourself and not concerned with a long life you can fudge this. I have a 26" draw and sometimes like making 48" wood bows. Even with a perfect stave and using a caliper, I would not expect the bow to last years. It's quite dry where I live and bows tend to break before they take too much set. Unless I'm using hickory or yew.