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    Wood Prods

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    GodricSwin
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    Wood Prods

    Post by GodricSwin on Tue Mar 31, 2015 2:29 pm

    Hi All,
    I "carved" the beechwood down to about veneer thickness and it's still 'stiff as a board'.  That's okay because I've set it aside and a hazel stave is coming along nicely . . . I think; I'm nearly at the point of needing a tillering stick (has to be a milestone in the life of a crossbow builder).  I'm going to try, if it doesn't become too time consuming, to work in a wippe-like addition to it.  It will provide some measurements for when the time comes to make the one for the finished crossbow. 

    Anyway, Mac, I'm looking forward to what you have to say about prod width vs. thickness; I think it will be an issue very soon - if not already. 

    If anyone has experience with cutting new wood in the forest any thoughts would be much appreciated.
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    Re: Wood Prods

    Post by mac on Wed Apr 01, 2015 8:56 am

    To understand wooden prods, we need to understand bows in general.  I'm going to try to write some explanatory material in a few installments.  I will begin with non/wood examples, but I plan to address issues that are specific to wood along the way.

    Thickness (the front to back dimension) and width (the top to bottom dimension) or a bow are interrelated, but I will try to separate them through analogy and thought experiment so that their important points become clear.

    Let's imagine a "bow" made of some uniform material.  It's 18 inches long, 1 1/4 inches wide and 1/32 of an inch thick.  Perhaps we could imagine it as one of those springy aluminum rulers.   Let's imagine bending that bow/ruler around a cylinder that is one food in diameter.  In order to do that, the material on the "back" will have to stretch, and the material on the "belly" will have to compress.

    Because out model was rather thin, the amount that the back had to stretch and the belly had to compress were not very great.  When we allow it to unbend, it returns to its original shape and is flat.  Because it returned, and did not take a permanent bend, we could say that out model remained withing its "elastic limits". 

    Every material has different elastic limits.  If did the same experiment with "bows" of the same dimensions but different materials, we could expect different results.  If we made our bow out of glass, it would have broken before had bent very far at all.  If it were mild steel of soft brass, it would have bent around the cylinder, but only partly returned to straight.  

    Now let's imagine that our springy aluminum bow/ruler is twice as thick.  As we try to bend it around the one foot diameter cylinder, we will immediately notice that is puts up more of a fight.  This is because the material on the back now has to stretch twice as much and the material on the belly to compress twice as much as before.  Oddly enough, this does not make the bow merely twice as strong, but instead it makes it eight times as strong.  That's not really intuitive, but it is a fact.

    When we release our experimental bow from the curve, we will find that it did not return fully to its previously flat condition.  We thought we had a good thing going, but no....  It was too much tension or too much compression (or both) for the material to take.  For this particular "bow" we find that 1/32" is Ok but 1/16" will not work.  

    We could accept the 1/32" thick bow, but it's really a lot weaker than we were hoping for.  The 1/16" bow was more like the strength we were hoping for, but it could not stand the stresses. There are couple of paths we can take.  

    If we had a sort of "double wide" springy aluminum ruler that was 2 1/2 inches wide it would behave exactly like putting two normal 1 1/2 wide rulers next to one another.  Since the thickness would be the same 1/32", the stresses when we bent it around our 1 foot cylinder would be exactly like out earlier experiment.  However, the strength of our bow would be exactly doubled.  Two narrow rulers side by side is equal to one wide one.  If that comes out to be a strength we are willing to accept, and it is not too wide to fit in a tiller, we have solved that part of the problem.

    On the other hand, if that is still too wimpy or it's ridiculously wide, we have to take another path.  Somewhere between the 1/32" thickness that works fine, and the 1/16" thickness which is too high is an ideal thickness.   We have seen that the strength of our bow increases very rapidly with an increase in thickness, so this is a very tantalizing route.  Let's assume that we can "get away with" making it exactly midway between those thicknesses.  Lets say that a "bow" made of springy aluminum that is 3/64" thick passes our bending test and returns to straight.  Our new "bow" will be four times as string as our original one, but the same width.  If this fits within our design aspirations we have solved the problem again.

    Now let's say that our 3/64" thick bow is really just a bit weaker than we were hoping for.  We can employ what we learned earlier, and make it twice as wide.  This will double the strength, while at the same time keep the stresses to the same levels.   We now have bow that is eight times as strong as our original. 

    If we find that that bow is going to be too wide, we could make it (say) half again as wide, rather than twice as wide.  Our 3/64" x 1 7/8" would now be half again as strong as our 3/64" x 1 1/4" bow.  That's still six times as strong as our original one.

    That's it for now.  More to come.

    Mac
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    Re: Wood Prods

    Post by twedzel on Wed Apr 01, 2015 9:44 am

    Mac and I posted this at the same time, we have a lot of the same information but just presented differently.

    This is my short answer to when you asked Mac about prod width vs. thickness. Make your beechwood prod as wide as it can possibly be made, tiller it perfectly, and hope it works out.

    Now for the long answer. Picture in your mind a stone arch. The stones that make up the arch would need to be smaller on the bottom and wider on the top in order to fit together properly. This size difference between the top and bottoms of the stones will be greater the thicker or more exaggerated the arch. This is exactly what is happening to wood as it bends in a bow. The wood on the back is stretching under tension and the wood on the belly is compressing. The thicker the piece of wood the greater the amount of stretching and compressing it will be subjecting to. More elastic wood species like Osage Orange or Yew can be made into thicker bows. Less elastic wood species (like beechwood) will be subject to plastic deformation (permanent change of shape) or out right wood failure when subjected to the same forces.

    There is hope for less elastic wood. When a bow is made thicker its draw weight increases quadratically so double the thickness and you get 8 times the draw weight. It increases linearly when made wider. So double the width and double the draw weight. To make less elastic wood into bows you need to make them wider and thinner thus taking compressive/tension strain off of the wood while increasing its draw weight. How wide and how thin depends on the wood you have in hand. Longer bow limbs will also help take the strain off of the wood. You could potentially make a prod out of pine if you can make it long thin and wide like a buicks front end.

    Now here is the problem with all wood prods. Short heavy weight bows are the hardest to make. They put the greatest amount of strain on the wood and stretch the limits of what the material is capable of. Any imperfections in how evenly the bow takes the strain (within an limb and between the two limbs) will put correspondingly greater amounts of strain in the weak areas. Failure will be almost assured with any imperfections of bows that are at typical crossbow draw weights/ length. Even perfectly made prods will need to take more strain than most woods are capable of withstanding at reasonable widths. Most wood would need to made prohibitively wide in order to keep up. This is why most medieval prods are made from steel or from a wood, horn, and sinew composite.
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    Re: Wood Prods

    Post by mac on Thu Apr 02, 2015 7:23 am

    twedzel wrote:..... we have a lot of the same information but just presented differently.

    It never hurts to have a couple of different explanations of the same thing.  The more ways you can look at a thing, the more likely is that one of them will lead to the "Oh! I get it!" moment.

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    Re: Wood Prods

    Post by GodricSwin on Thu Apr 02, 2015 12:33 pm

    Thanks, guys!  I'm at the point where I'm getting it and trying to apply it to my bow - it was just what I needed.  I'm enjoying the process and working with the hazel and thinking about the rest of it.  I'm leaning towards that bow in Scotland - I like the style, looks like a "working man's" bow without a lot of decoration - good for a beginner's first go around.
    Again, I appreciate very much your explanations.
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    Re: Wood Prods

    Post by mac on Thu Apr 02, 2015 1:58 pm

    In my earlier post, I had us bending our model bows around a 1 foot diameter cylinder.  I did this because I wanted a situation where every part of our uniform thickness bow was strained to the same degree.  The cylinder insured that every part of the bow was bent the same amount as any other.  Every bit of the back had to stretch the same amount and every bit of the belly had to compress the same amount.   The catch is, that's not how such a bow would bend.

    If we take our model bow and try to bend it without wrapping around our cylinder, it will bend mostly in the middle.  This is because the limbs are acting as levers.  If we think about some point just to one side of the middle, we can see that it has the entire length of the limb acting on it like a lever. Thus, the middle is under a lot of force, and it bends a lot.   If we move out attention to a place half way out the limb, we see that the lever acting on this point is only half a limb long.  With only half the leverage to bend it, this place bends less.  Way out at the tip, there is no lever, and the tip does not bent at all.

    A bow that bends mostly in the middle has two problems.  The first is that we can only safely draw the bow until middle bends as much as it can without permanent deformation.  That will be a lot less than when we were bending it around a cylinder.  We would have to settle for a much shorter draw. 

    The other problem with that is this.  Only the middle of this bow is really working for us.  The limbs are not doing their share of the work, and in a sense are just going along for the ride. We really want as much of the bow as possible to bend as much as it is safe for it to do.  To make these under-worked ends of the limbs bend more we have to change their width, their thickness, or both.

    Lets think about changing the width first. The bending force that any point of the limb experiences is a function of the leverage of the limb "outboard" of that point. That leverage is a "linear" function of length. A point half way out the limb sees half the force that the middle of the bow sees. Likewise, at 3/4 of the way out the force is only 1/4 of that in the middle.  Right at the nock, there is (theoretically) no bending force at all.

    If we were to taper the width of the limbs in proportion to the bending load that they will experience, we could get the whole bow bending so that it would all lie on the same radius.  That is to say, it would be as though we were back with our original model, except that no 1 foot cylinder would be needed. As it happens, this is easy to do (theoretically).  If we start with the original thickness in the middle and taper the limb thicknesses out in straight lines till they come to points at the ends, we have done that. 

    Of course, that leaves us with no where to nock our string, so we would have to deviate from that ideal to make a real world bow.  At a minimum, we would have to make the tips wide enough to give the string some place to be. This is the basis of all modern prods that are cut out of plate metal.  The result has a bit more stiffness out at ends of the limbs than perfect efficiency calls for, but it's a pretty good compromise.  The other place where these modern prods differ from the theory is in the center.  Out theoretically equally stressed bow has a sort of point in the middle of its planform where the two limb tapers meet. The modern makers just round that off to make it fit in a tiller better, but this make little difference to performance.

    The other way we could make the limbs bend more would be to reduce the thickness while maintaining the width. This is the road less traveled. The resulting limbs would be heavy, because they would owe their strength to width rather than thickness.  Remember how doubling the width increases strength by two, but doubling the thickness makes strength go up by eight.  Width is the safe way to strength, but not the light way.

    The third way to make the limbs bend more is to reduce both the width and the thickness.  This is what is done on forged steel prods and on historical wooden prods. Unfortunately, it is here that we leave the land of simple solutions and easy models. Since both dimensions are going to be reduced, there is more than one way to solve the problem. What's more, the ideal solution for one material is not necessarily the ideal for another.

    Our ancestors have already worked out the solutions, and if we are making a steel prod, we could make it look like all the extant steel prods, and be confident that it will be close to ideal.  We are not so lucky if we are making a wooden prod, though.  Of the hundreds of thousands that must have existed, only a very small number have not ended up as firewood when their working life was over.

    More anon....

    Mac
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    Re: Wood Prods

    Post by Geezer on Thu Apr 02, 2015 5:05 pm

    Mac: One thing you might have mentioned about narrowing the outer-end of a metal prod... it's possible to get it so narrow that the nock-end twists/torques when you span the bow.  On release, the nock-end's unwinding throws the string away very nicely.  Darkwood had that problem on some of their first production steel prods.  Eventually they went for slightly thinner stock and broader ends.  Problem went away.  There's always something, dammit.
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    Re: Wood Prods

    Post by mac on Thu Apr 02, 2015 5:53 pm

    As Geezer rightly points out,  if the limb is too narrow before the nock, the bow might try to bend some other way than the one you hope.

    This is something I need to ponder.  I would never imagine making a metallic bow where the width before the nocks was less than twice the thickness.  On the other hand, yew wood bows routinely end up close to round at the ends. 

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    Re: Wood Prods

    Post by twedzel on Thu Apr 02, 2015 6:26 pm

    Wood bows have another consideration, set. This is where the wood has exceeded its elastic limits and is now in a state of plastic deformation (permanently deformed under strain). Set robs a wood bow of performance as it reduces the limbs capabilities to store energy. The nearer the set is taken to the center of the bow the more damaging it will be to the bows performance. So center and mid limb needs to be left wide enough that the bow will take little to no set at these locations. Outer limbs need to be made narrow as possible while still maintaining stability. This is especially important with recurves. However excess weight at the tips will also robs a bow of it performance so you need to balance weight vs stability.

    Mac had said that tapering or thinning the limb is the road less traveled this is actually pretty standard for wood bows. It is a fairly simple process to remove wood and check the bend. Its also a fairly simple process to remove too much wood and hinge your limbs (uneven strain in bending). Its tougher to do this with steel unless you are a lot more gifted than I am with both angle grinding and tillering. Width tapering alone is far easier than thickness tapering.
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    Re: Wood Prods

    Post by mac on Thu Apr 02, 2015 7:02 pm

    mac wrote:



    The other way we could make the limbs bend more would be to reduce the thickness while maintaining the width. This is the road less traveled.
    Writing is a difficult thing.  I knew that there was a chance that this sentence could be misread, but I used it anyway.

    I meant reducing only the thickness and keeping the width the same. That is to say a rectangular limb.  As such, it really is "less traveled"... almost untraveled.

    I may go back and make it clearer sometime later. 

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    Re: Wood Prods

    Post by twedzel on Thu Apr 02, 2015 9:28 pm

    Opps my bad, I read faster than I think... which is surprising considering how slowly I read.

    Tapered rectangular limbs would indeed give you the worst of both worlds. I can see why that road won't get much travel. But now that I think of it, It's fairly standard on compound bow limbs. But I know nothing about them... I figure the cams must need all the support they can get, anybody know more?
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    Re: Wood Prods

    Post by hullutiedemies on Sat Apr 04, 2015 3:54 am

    mac wrote:
    I meant reducing only the thickness and keeping the width the same. That is to say a rectangular limb.  As such, it really is "less traveled"... almost untraveled.

    You mean "Meare Heath"-tiller ?

    That is also shape of the original skåne bow-prod:


    More pictures on Swedish forum
    http://armborst.forum24.se/armborst-about42-0-asc-25.html
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    Re: Wood Prods

    Post by mac on Sat Apr 04, 2015 9:23 am

    hullutiedemies wrote:
    mac wrote:
    I meant reducing only the thickness and keeping the width the same. That is to say a rectangular limb.  As such, it really is "less traveled"... almost untraveled.

    You mean "Meare Heath"-tiller ?

    That is also shape of the original skåne bow-prod:


    More pictures on Swedish forum
    http://armborst.forum24.se/armborst-about42-0-asc-25.html
    You are right.  That does appear to by what's happening here.  I guess that means that this "road" was more "traveled" than I thought.

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    Re: Wood Prods

    Post by Geezer on Sat Apr 04, 2015 9:56 am

    I don't see why one couldn't do all or most of the taper on a wood prod via thickness rather than width.  In fact, most medieval steel prods I have seen had minimal taper in width... most of theirs seems to have been in thickness.  Certainly it's much easier to taper width in modern steel prods made from flat sheet, but I suspect if one had to hammer the stuff out of thick bar, it didn't much matter which way one tapered the outer ends.  Geezer.
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    Re: Wood Prods

    Post by twedzel on Sat Apr 04, 2015 12:54 pm

    Weight at the tips reduces arrow speed. Its like a piano metronome, put the weight on the end and it slows down put the weight at the base and it speeds up. There is much less force exerted at the tips so additional weight there is simply inefficient. Thickness taper reduces draw weight force at a ratio of 8-1 where as width taper reduces draw weight force 1-1. So you can remove much more material with a width taper as long as you are not destabilizing the tips by reducing too much width. But the efficiency of light tips is more pronounced with light arrows. Increase arrow weight and heavy tips can dump more of their energy into the arrow. Medieval bowyers would certainly know all this, so why make prods with little width taper? I don't know, probably it is more owing to industry and saving costs on mass producing prods rather than small gains in increasing efficiency on truly heavy weight prods throwing heavy armor piercing arrows. Light tips is more critical with lighter limb materials like wood or fiberglass throwing lighter arrows.
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    Re: Wood Prods

    Post by mac on Sat Apr 04, 2015 1:19 pm

    The most typical way to get a bow limb to bend and "work" evenly is to taper both the thickness (front to back) and the width (top to bottom). 

    Let's return to out original model of a "bow" made from a springy aluminum ruler.  It's 1 1/2" wide, 18" long, and 1/32" thick.  We want it to be capable of banding safely from flat to a condition where the ends are 1 foot apart. 

    We are going to taper both the width and the thickness, but let's consider the width first.  Let's taper the widths of the limbs from our starting thickness of 1 1/4" in the middle to 7/8" at the ends.  This taper can be a straight line like our previous example. 

    If we try to bend this model, we will find it does not bend in a nice circle like our ideally-tapered-constant-thickness model that I described the other day.  On the other hand, it does bend "better" than the untapered-constant-thickness model.  To make it come round to a circle, we will have to carefully remove thickness from the limbs.  I say "carefully" because a little change in thickness makes a big difference in stiffness.

    This is the "tillering" process.  If we were looking at making a real bow, rather than modeling it in our minds, we would need consider how this difference in thickness was produced.  A wood-bowyer would remove material with cutting tools and scrapers.  Books can be, and have been, written on this process. A steel bow smith would form up the shape plastically under the hammer until he thought that the shape was the one that his teaching and experience showed would work.

    Let's say that by the careful grinding away of thickness, we have our new model bending in a half circle.  While this achieves our stated objectives as set out above, it does not get every part of the limbs working to the same percentage of its capacity.  Remember, that the amount that a limb can safely bend is based on its thickness.  Our limbs are now thinner than they were, and they can be safely bent farther than before.  

    Our model could now be safely forced to bend into something more like part of an ellipse, but it won't do that by its self because we have only removed so much material as it takes to make it naturally bend to a half circle. To make it naturally bent elliptically, we would have to take away just a bit more thickness. 

    We now have out model bending naturally into a shallow ellipse with its tips coming to be 1 foot apart.   This is an evenly stressed thing of beauty, with every inch of its limbs contributing as much as any other.  Every place on this bow is equally close to its capacity. 

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    Re: Wood Prods

    Post by mac on Sat Apr 04, 2015 4:54 pm

    Now let's examine what it takes to make our model bow be a model crossbow.   There are two things that separate hand bows from crossbows.  The first symmetry, and second is strength.

    Let's begin by addressing symmetry.  A crossbow's bow/lath/prod is bound to the tiller.  As such, there must always be some tiller material between the bow and the bolt.  In an ideal universe, the string would travel along the top of the tiller and barely touch it.  In a real (traditional crossbow) world, this in not possible.  The string will have to press against the top of the tiller to some degree, and that will rob us of performance by turning stored energy into friction and throwing it away as heat. Still, we have to do what we can to minimize the amount of pressure the strings places on the top of the tiller. 

    If we take a "normal" symetrical sort of bow and bind it to the tiller at an angle, we can do a lot about the string pressure.  If the bow is attached at such an angle that the nocks are level with the top of the tiller when the string is drawn to the nut, we have reduced the string pressure in the early part of the release to virtually nothing.   In many traditional crossbows, this is practiclly the only thing which has been done to reduce the string friction.  Many early and "primitive" wood bows, as well as virtually all of the horn and sinew composites, are essentially symmetrical.

    Angling the bow helps reduce string friction as the string leaves the nut, but by the time the bow has returned to its brace position, the string is very noticeably and heavily pressing on the tiller.  The way to make progress against string friction late in the "power stroke" is by making an asymmetrical bow.

    In our earlier models with tapered limbe, I did not specify how the limbs were to be tapered.  Let's think about that now.  If, when tapering the width of a bow limb, we take all the material off the bottom, we have effectively moved the nocks up.  When the bow is bound in, the nocks will be closer to the level of the top of the tiller.  This will help some in reducing sting friction in general, and string friction late in the power stroke in particular.

    We could theoretically carry this further by making the upper edge of the bow concave and by so doing bring the nocks up even higher.  Thus the planform of the  bow would have a sort of "smile".  We see this to a greater or lesser degree in historical steel bows. Unfortunately, there are limits to how much of this sort of correction we can employ. The greater the "smile" the more force trying to twist the bow in its bindings.

    Lets imagine a symmetrical bow which is bound in at such an angle that its "draw plane" passes through the middle of the bow and the fingers of the nut.  That is to say, when the bow is drawn, there is nothing trying to twist or rotate the bow in its axis.  Thus, at full draw, the bow is quite stable in its bindings.  After the shot, when the string has come to rest at its brace height, there is a certain amount of deflection of the string from the draw plane.  This effectively tries to lift the bow tips up, and the result is a small twist around the bow's axis.  This twist wants to rotate the upper edge of the bow away from its seat in the tiller. The force is small, though, and causes little trouble.

    If we employ a smiley planform, we raise the nocks with respect to the top of the tiller.  This reduces the pressure of the string on the tiller and its resulting friction, but this gain comes at a cost.  At when the bow is drawn, there is a force trying to make the upper edge of the bow press harder into its seat than the lower edge.  The greater the smile in the bows planform, the greater the twisting force at the bow seat when the bow is drawn. 

    The effect of this twist on the bow seat will be different for metallic and organic bows.  Metallic bows are quite flat across the belly and so long as the bindings remain tight, the bow is unlikely to twist in its seat.  However, if it does twist, the wood of the seat is likely to become compressed by the edges of the bow and accuracy of the bow-tiller fit will decline.  This can lead to a vicious cycle, where the fit gets worse and the bindings get loose with increasing rapidity.

    In bows made of wood or other organic material, the problem is different.  The bellies of these tend not to be so flat, nor the corners so square those of metallic bows.  This makes it more likely that the bow will yield to these twisting forces and become damaged as it gets wrenched back and forth in its seat.  I think this is why we see less "smile" in wooden and composite bows than we do in steel ones.



    There is also a potential problem with this twisting force effecting the limbs themselves. If there is a significant twisting force on the limbs when the bow is spanned and the limbs are highly stressed,  there is a chance that the additional stress of the twist will exceed the bow's design capacity.   The limbs might deform permanently or even fail.   Even if the bow remains within its elastic limits, there will be an untwisting of the limbs which accompanies the release.  This may lead to osculations of the limb tips outside of the draw plane.  The results are likely to be reduced accuracy, unpleasant vibrations,  and increased wear an tear on many parts of the crossbow.



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    Re: Wood Prods

    Post by mac on Sun Apr 05, 2015 10:21 am

    This brings us to the other place where handbows and crossbows differ. That is strength. to be effective, crossbows must have higher draw weights than handbows.

    Let's see if we can examine  why that is without having to crunch too many numbers. A bow can only make use of the energy that we can store in it.  That energy comes from our muscular exertion and is stored in the bent limbs of the bow. We can put the energy in sort of all at once with a full body grunt like a hand bow archer, or we can put it in slowly by twiddling the handle of a craniquin for thirty seconds.  It's all the same. 

    The amount of energy stored in a bow can be approximated if you know the length of the "power stroke" and the draw weight. The power stroke is the distance from the brace height of the string to point where the string is fully drawn.  You can measure it in whatever length units you like.  The draw force can be thought of as the amount of weight you would have to hang from the string to pull it all the way back. You can use pounds or kilos.

    Here is the only math I hope to use here.  If you multiply the power stroke times the draw weight, and then divide by two, you get a reasonable first approximation of the energy you have stored in the bow. If you think in the units I do, your result will be in "inch pounds".  If we know how many "inch pounds" a bow stores, we can compare it in a crude way to any other bow for which we know the numbers. 

    If we were to take an English long bow as a benchmark of an effective medieval weapon, our calculations might look something like this.  A longbowman shoots an arrow of 30".  His string is braced at 6" from the belly.  Now we have the  "power stroke", and it is 24". At full draw, the "weight" is 100 pounds. If we multiply his 100 lb. draw weight by his 24" power stroke and then divide by 2, we come up with 1200 inch pounds.  That's the amount of energy he has stored up in that bow. If we want a bow that's a player in that field, we have to come up with something that stores a similar number of inch pounds. 

    The problem is, of course, that crossbows have much shorter power strokes than handbows.  Let's say that our crossbow is spanned with a belt hook and stirrup and it's power stroke is 8".  That's only a third as long as the longbow has. If we are going to get into the game, we will need something with a much higher draw weight than the longbow...  Perhaps three times.  Lets see what the math says.  Our 8" powerstroke times a 300 lb. draw weight divided by 2 gives us the 1200 inch pounds we need to have an roughly equivalent weapon.

    There are other combinations of lengths and weights that will give us our 1200 total, but for reasons I hope to get to later they all involve even shorter power strokes and proportionally higher draw weights.

    Even if we don't need to keep up with the Longbow, the principal is still the same. If we want to play at the range with the guys who shoot 28" arrows from 45lb bows, we will need a 150 lb crossbow. 

    Bows just don't scale down the way we would like them to.  If we were to take that 100# longbow and reduce everything proportionally to 1/3, hoping that we could just bind it to a tiller and have a crossbow,  we would find the draw weight disappointing.  The bow would still draw to the same beautiful shape, and the wood would be just a highly strained as the full sized bow, but the draw weight would be no where near the 300 lbs we were hoping for. If we try again with thicker wood and the same length and draw, it will either blow up or take a big permanent set...  A set... hmmm.  that should get us thinking.

    Let's return to the springy aluminum ruler models for a bit and see if we can understand something that will help us with wooden crossbows.

     
    Given our need to make strong bows, we run into a sort of dilemma.  The safe way to make strong bows is to make them wide, but they get absurdly wide long before they get really strong.  The efficient way to make strong bows is to make them thick, but thick bows can not flex as far as thin bows can. The historical solution to this with steel bows (and presumably wooden ones as well) is to design the crossbow around the idea of thick bow which does not have to bend very far.

    In the model bows I have described earlier, we have always started with a flat strip, and flexed it into the shape of a fully drawn bow. By the time out models were fully flexed, they were close to their limits.  If we think about actually putting a string on those models, we can see that just by flexing the bow to its brace height, we have used up almost half of our available flex. 

    In one or out earlier experiments  we made a bow model twice as thick in order to make it stronger.  We then tried wrapping it around our 1 foot diameter cylinder and found that it took a permanent bend. You may remember that we were disappointing because it was very strong, and that seemed very attractive.  Let's assume that the shape this model came to be after its permanent bend was nearly the shape of a bow that is braced but not drawn.
     
    Let's  assume that this partially curved shape, rather than a flat strip, as our starting shape. If we take this model and "tiller" it just like our most successful models above, we will find that we have a rather powerful bow.  It does not happen to go back to flat when we release it, but that's not important.  Starting out flat was an artificial constraint that we can abandon.

    In the language of bow making, we would say that our new model was "deflexed", or had "taken a set" or had "followed the string".  This is a condition much detested by the makers of hand bows, but we should not let that bother us.  We are obliged to work in a different scale than they, and what is a fault for them is a necessity for us.

    To make powerful crossbow, the bow must be thick; and to make it thick, it must be deflexed. 

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    Re: Wood Prods

    Post by GodricSwin on Mon Apr 06, 2015 2:09 pm

    Hi Mac, and all,
    I have to thank you and my thanks should include those of any who are thinking of wood prods; a lot of good, well-thought out explanations.  I hope this will inspire others to go for "natural" wood prods; I would like to hear of their experiences.

    My hazel stave is progressing, but it is progressing very slowly as I'm being very careful - all the while trying to take into consideration the information in your posts.  I can at this point with a lot of effort get the stave to bend about 5cm beyond its natural reflexed state.  I'm afraid to go much further until I take off a little more thickness.  After I've done that I will make some serious attempts at tillering.  Starting at about 10cm from both ends there are twists (both going in the same direction); I'm at a loss as to what to do with them - I'm hoping that when I steam in the deflex I can correct it.

    Continuing on . . .
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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 3:08 pm

    I did an image search for crossbows with wooden prods the other day to try to see if there was anyone out there making heavy ones.  For the most part, all I found were pretty light bows in the 50# to 100# range. 

    I did find this one from over on the Vikingsword  Ethnographic Arms and Armor forum. http://www.vikingsword.com/vb/showthread.php?t=8644  This guy's bow looks like it's more powerful than my 150 pounders, but it's hard to guess what it does draw.  One thing I think we can tell, though, is that the prod is deflexed.  He has shoved the bow onto his shoulder for convenience, and we can see how much the  the string has been deflected by that.  This suggests a relatively low initial draw weight, and that is consistent with a deflex. 



    A different thread on the same forum http://www.vikingsword.com/vb/showthread.php?t=3078  brings us three more  wooden African crossbows.  The first one has more deflex than I think is good for it, and this may be the result of it having been left strung in some collection. 





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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 3:13 pm

    Here's another Pigmy crossbow from Cameroon.



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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 3:38 pm

    It turns out that an image search for African crossbow is pretty fruitful.

    If turns up some butch looking bows as well as some butch looking archers.





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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 3:50 pm

    By contrast, the crossbow makers of Vietnam have taken a very different route.   They seem to have longer bow and what looks like a longer stroke as well.  The result is not unlike a short handbow on a stick.  They seem to have little or no deflex.



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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 4:23 pm

    Here is the famous 14th C. Islamic crossbow from Granada.    The bow is primarily of wood, but it has a relatively thin backing of what looks like sinew.


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    Re: Wood Prods

    Post by mac on Mon Apr 06, 2015 6:44 pm

    Here are pics of E.1939.65.sn in Glasgow.  The bow is yew wood.



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