Primitive Archer
Main Discussion Area => Bows => Topic started by: Jan de Bogenman on January 02, 2018, 09:55:18 am
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Spent a few weeks on making selfbows with a serious concave or hollow back. To see if it is possible to reduce mass and to gain speed. Given a concave tape measure, one would think so.
But wooden bows rarely work as you might think, so first let's see if it's possible with a serious cavity at all.
So far I have made three Hollow Back Designs (?) And they are not broken....yet.
The first has a cavity of about 3 mm, the third 6 mm.
One outcome is clear: Tillering such a bow is a thrill! With this thin lines on the back, you expect to hear that small "tsjick" or big "krack" every moment.
I will post some specifications later on.
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First attempt: a hickory flight selfbow.
Lines are taken from the example described by Dan Perry in his article in, yes, Primitive Archer Magazine. (Thanks!)
60"ntn, 35#@24". Cavity 3 mm (0.12"). Width 1.5"@ fades 0.24"@ noks. Set 1.5".
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Second try. A more conservative design. And in terms of dimensions and draw weight a copy of another, fairly new bow.
Too little material for conclusions yet. But compared to the twin brother, the hollow bow weighs about 10% less and is about 10 fps faster. But as said, I think it's not time for conclusions yet. Much is in the details, especially in speedtests.
Cavity 4 mm (.16") Both ash piramid selfbows, 70"ntn. 40lb@28". Width at fades 48mm(1.9"), at nocks 9 mm(0.35").
Out of curiosity about the results, and if she would hold anyway, I was somewhat hasty during tillering . Set is therefore somewhat larger than expected, but with about 1.5 inches it is still within the limits, I think.
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Hi Jan, this is a very interesting design. I never thought you could violate the back to this extent and get away with it! The tiller is looking good as well. How does the bow draw? Is there any kind of let off when it flattens out? I am interested in trying this design as well once you have gathered some more results from it. Awesome that you are trying it for sure.
Cheers,
Phil
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another question, is the belly flat or convex?
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Work in progress: A selfbow made from the inner side of split elm.
Cavity about 6 mm.
A relatively short bow, length is 61". Widest point 5 cm (2"), nocks 6 mm (0.24").
Current state: about 40lb @ 25 ". I am aiming for 26"to fit my flight arrows.
Maybe it's time to draw one conclusion: the design is stronger than I expected!
And I think a deeper cavity and sharper edges are possible.
Sorry for the last picture, I was shaking a little.
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The grain of the back isn't violated, it's much like a board bow. But I gather it's very challenging to tiller.
But the principle, I think, is very sound and worth while investigating.
Jan refers to a metal tape measure: if you hold it concave, it takes more force to buckle than when you hold it upside down with the back convex.
The easiest way to make such a bow (and test the merits of the design) is from split bamboo poles, which are naturally concave/convex depending on whether you take the inside or the outside as the back.
I'll let Jan do the hard work first ;D
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Hello Phillip,
I did not take any measurements for a draw curve yet. But I do not feel a "sweeter" draw. Given some experiments with bamboo and pvc I'd rather expect the draw to be rather "sturdy" and will not flatten out.......untill very sudden collaps.
I suppose the flattening out is thwarted by the concaving Poisson effect.
First build some more, than one for overdraw!!
The belly is a little convex with rounded edges. I am not sure yet this is the best design. I just follow my intuition and experience on this part. I need an engineer!
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Quite an investigation, and some nice bows,too.
I suppose the flattening out is thwarted by the concaving Poisson effect.
That would be interesting to measure, just how much shape changing is going on. One would hesitate to ask a guy to leave a bow at full draw long enough to take measurements, though.
Just curious what you might be asking an engineer? You might get more questions than answers from such a
conversation.
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@Joachim. Actually the Ash and Hickory bows are board bows and the grain was violated moderatily in this staves. I have enough confidence in this woods for such a challange. The elm bow is made of a split log. And there are two little flattened knots in the high ridge on the back!!
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Coming to think of it the design again it is very similar of that of a turkey feather or any bird feather for that matter. Two main spars on the back and the belly slightly rounded. That design is very efficient for being light weight and carrying high cyclical loads. In a feather there is a central groove but the middle is not as widely hollowed out as in your design. You are carrying all the tension on the two side spars. I think if properly executed you can very nicely balance the load on the back and the belly and avoid set. I have made a "feather bow" with a central groove on the back hollowed out into the spars but did not carry it all the way to the tip like you have, but that bow stayed perfectly flat and took no set. I had it thinned out a lot into the tip though. I think the way you do it is correct where you gently run out the hollow part into the tip just like in a feather.
I am looking forward to see the finished products!
Cheers,
Phil
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Wow, made arrows for years and never saw feathers that way! Very interesting and a good example.
And Wille, thanks. Maybe an engineer and a nice 3D modelling program could come up with some quick answers on a optimum shape. And how many load the material takes on what place, compaired to a more flat design. And yes, very great chance of a windy discussion! But experimenting and guessing is nice too.
I have tried to see a change in the cross section while moving and holding the ash bow. Little to see. But thats the same with pvc tube and bamboo slats. They even tend to tighten, or close, it seems.
Will try to do some measurements.
Jan
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Jan, nice to see you here as well! :BB
Great experiment if you ask me! The design scares the $#*^ out of me but somehow you make it work! Keep us posted on the elm version, I think you are on to something here.
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3d modeling sounds like fun, but I guess optimization would be a matter for working through the inevitable "trade-offs" that seem to be a part of every design process.
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when looking at the pictures of the drawn bow from above on the tillering tree, it rather reminds me of a crowned back. But instead, the high portions are on the sides, not the center. Theory suggests that crowned or trapped-back designs take less set (at the expense of risking tension failure) because they move the neutral plane towards the belly. So maybe the advantage isn't any different from a trapped back, unless the concave back flattens out during the draw.
As for grain violations in European ash: I've had a bow violently explode on me two days ago during tillering of a crowned back, on a knot that was violated ever so slightly. scared the hell out of me. (--)
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Ive got no business in such a post like this cause i like a very simple self bow.that said i really like where you guys are going with this.if it werent for tinkering and technology we wouldnt be typing and posting about what we like to do.
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I think it's a neat idea but I have no idea how it is staying together :) :) :) Are the edges of the back one ring?
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Joachim, its about mass in the first place. I suppose that if a trapped back is more economicaly in mass per strenght, all tape measures would have had that shape.
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Ive got no business in such a post like this cause i like a very simple self bow.that said i really like where you guys are going with this.if it werent for tinkering and technology we wouldnt be typing and posting about what we like to do.
Haha thats exactly how I feel about bowmaking myself. For 15 years now I am making bows and a simple selfbow of some local wood, made with simple handtools, gives me most satisfaction. But this hollow back design was on my mind for some time somehow and was triggered bij the hollow belly bows. Dont know yet if it's worth all the effort. But I think it can be, and it is big fun already. And like all wooden bows, big chance it has been done already, sometime, somewhere!
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I think it's a neat idea but I have no idea how it is staying together :) :) :) Are the edges of the back one ring?
It is hard to understand what you see happening on the tillerboard. I expected it would explode long before half of the draw length.
For the ash bow, the cross section is about 15% higher than in a flat limb with the same width. Due to the shape there is less material, but this material is further away from the neutral plane. So maybe tension is a little less.....I guess!
And other forces are working too. If you bend a half bamboo pipe this way, it will suddenly flatten out and split, not break. Now, bamboo is a wonderfull material with different characteristics than wood, but maybe something alike happens in this bows.
In the ash and hickory bows, the back does not follow the grain for 100%. In the elm, the ridges follow one ring. But has two little flattened knots on the ridges. The staves were originally selected for 'normal' flatbows.
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I like the looks of this design. I also like the fact that you have removed mass from the back. Wood is 3 to 4 times as strong in tension as compression, so the back is the right place to remove wood.
I am sure the woods that have interlocking grain are better suited for your design. A limb that tapers has the grain running off the edges of the back at a small angle. The edges are bearing almost all the tension strain in your design, and with hickory or elm, can probably survive.
The tiller looks very good to me.
Here is an extreme experiment I did with a miniature bow to leave the middle of the back doing the work. The strip down the middle has no grain runout until near the tips. It worked very well, but I have not scaled up the experiment.
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DC, I think it like a decrowned bow. as long as the grain lines run parallel you should be OK.
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Jim and Pat, thanks for your input.
I agree that the grain is probably more important than in a 'normal' limb. But I did not want to try this with material that differs from my other bows, to be able to compare.
Nice experiment too Jim. Not everything can be scaled proportionally, but this should work I think. Bamboo slats can do great work here too?
Thinking of it, maybe worth trying two narrow slats off bamboo along the sides.
Jan
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Jim
am looking forward to seeing the up-sized model when it happens. getting just the "right" amount of backing might be telling.
all tape measures would have had that shape.
Jan, tape measures are designed to be bent over backwards and rolled up inside the case, too.
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Willie, very true indeedindeed!
Maybe try to store my bow that way.
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I once made an Osage that had that concave naturally in the back, the only problem is that the rails on either side do all the work unless you leave the belly thicker to off set the concave. It worked fine for a while but after several hundred shots the rails couldn't take the strain and lifted a splinter, since then if I have one with the concave back [Hollow] I leave it thicker on the belly so the limb is the same thickness all the way across. Guess that would kind of negate what you are trying to do by reducing mass. :-\ Looks cool , hope it last ;) :)
Pappy
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Just thinking about your bow design. In theory ( mine ) it should be a durable design. The "area" of the back has more square CM's than a standard tillered back. Like a standard 1 1/2" back to a 2" back, more area in a wider back. If you could measure it somehow following the curves - up and down and flat. Would be interesting to compare the area to a standard bow back of equal pull. Then like you are doing compare the durability.
I like it.
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Years ago i had the exact same experience as pappy, it was an black locust flatbow. It shot very good unless the edges gave in and splintered.
I also feel that the reduction of weight in the reduced width of the back works as in a trapped design. Nevertheless perhaps the "tape measure effect" could play a role. Seems quite difficult to find out. though.
Maybe you should start to make samples of crossections (a short length), bend and measure how they behave (weight, bend strength, poissoning etc.). So you could test multiple cross sections without having to build alot of bows...
Very intersting topic, thanks for sharing an doing the work :)
gian-luca
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Interesting for sure. But the tabe measure bends to a point then brakes over when to much tape is is let out. I think it will work unless over strained. I expected it to split down the center. Arvin
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I dunno think this is a HLD bow, because there are not the advantages like on a hollow belly. But that groove in the back is mass saving for sure.
I believe it is no good idea to groove out the back and violate the tension fibers, better search for a natural concave back stave.
The work you invested in that concave back would be much more useful in an concave belly (and you get the same amount of mass saving).
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because there are not the advantages like on a hollow belly
Simson
As I understand the physics, your hld has the belly curl out, and Jans has the back curl in. Shouldn't both have the same advantage?
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Turn the stave around and work with the natural curvature of the rings to form the back so no grain violation needs to occur. The outer surface of the stave now becomes the belly. Steve
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because there are not the advantages like on a hollow belly
Simson
As I understand the physics, your hld has the belly curl out, and Jans has the back curl in. Shouldn't both have the same advantage?
No Willie,
that's a totally different thing. Here is info: http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/
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LongbowSteve. Exactly, the elm stave above was turned upside down. The ridges follow one ring.
Simson, very nice to hear from you on this topic. Love your HLD and other bows. The HLD triggered me to try this idea. So thanks!
I did not make a hollow belly yet, so I can not compare. But some thoughts...I believe, and you know, that a hollow belly saves mass and makes for a sweeter draw, if done properly. On the other hand, I think, the hollow back can enable more mass reduction, but will not have the sweet draw. And it is in greater danger.
this conclusion/thought/hope came after some testing with bamboo and pvc pipes and aluminium u profiles. When the belly is the round side, these half pipes carry much more load, but collapse very sudden. Upside down, they almost carry the same, untill the point where the ridges tend to break away. Maybe if one is able to avoid that, both are equally strong. My hope is to get beyond that point.
Logically, with hollow back, the edges are under great tension indeed. And I am not certain at all they will last. Some reports here make me very nervous!!
It's great fun testing anyway, Hope to post some more results soon.
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That would be interesting to measure, just how much shape changing is going on. One would hesitate to ask a guy to leave a bow at full draw long enough to take measurements, though.
Well what's a good experiment without some measurements? And is their any bowmaking without pain?
I have put the ash bow to the test and that was a little to much. But all for bow science!
Did some shape measurements before breaking. The draw curve however...
Some results:
- Very little shapechanging at full draw. Checked it several times and took my time. A more accurate vernier caliper is required. The small differences noticed, do not seem to indicate any flattening, but that the opposite is happening. That would match the bamboo test.
- The bow did not collapse completely! It was still braced. There is a safety aspect in here?
- Tension in the high edges is higher than I am used to....wood selection is more important....surprise!
So far for this ash bow. It has shot a few dozen arrows at full draw, set was within (my normal) limits. Mass was 15,7 oz for a 71" x 2 " pyramid flatbow.
The break can very well be due to the design, but certainly also to the bad, brown, brittle spot in the wood.
For me enough reason to try some more.
A better stave is selected and ready for the tillerboard!
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certainly looks to me like some rot in the wood at that location! I think this break might even be reparable and you might still have a working bow after all is said and done.
Cheers,
Phil
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(-P
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Jan, I don't want to affect you. Hope you don't mind, here are some thoughts from me:
1. Concave back is more mass saving than concave belly. > Not true! Mass saving is exact the same amount.
2. The cross section you have built is not able to spread/flatten out because of the geometry. In cross secction the thickest part is at the crown and tapers out to the sides. To avoid exessive stress in that ridges you have to allow them to flatten out. Or in other words work or load is not only in direction tip to tip, but also from ridge to ridge.
3. Wood selection: yes, only choose the best staves without imperfections.
4. If you have thick walled boo: Make your hollow back experiments with this stuff (split in half, inside is back). Look at Bhutanese bows!
5. Try a 'normal' HLD to get the idea and experience what happens when the the bow limbs flatten out.
Sorry for your breakage, in this case rotten wood.
Thanks for posting and experimentating.
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Simson, thanks for your thoughts. All critical, but helpful contributions are welcome.
Yours is sure stuff to think about!
As for the bamboo bow. I made a few of them that way. Great fun! A good bow in very little time and tillering by scraping on the back side.
The third bow on this webpage is a take-down of mine. http://zelfbogenmaken.nl/?page_id=487 (http://zelfbogenmaken.nl/?page_id=487). (Hope you like the website).
And I have two questions.
I dont hope to affect you.... All your bows look awsome, but maybe, somewhere you have a HLD bow that can be put to a test?? Somewhere, forgotten, in a corner?
Could be very interesting to pull it backwards and to see if there is any difference?
Maybe I should repair this Ash bow and do the same.
Second, can you give me an indication of how much the flattening out, in a good working HLD will be?
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No Willie,
that's a totally different thing. Here is info: http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/
Thanks for the link Simson, Interesting and nicely made examples. I am curious if your recent testing of HLD designs has changed your thoughts since the article was wrote.
From the article.....
"The depth of the limb (measured over the hollow) compared to the width should be in the ratio of about 1:3."
Is that still your preference?
Jan,
if I understand correctly, your primarily measurements indicate the limb gets narrower and thicker when drawn?
that would be consistent with a poisson effect explaning the HLD advantage.
Interesting discussion, hopefully some force draw info could have been measured :(, as it would be revealing to visually compare the curve shape of various designs.
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I've built bows belly out meaning using the belly of the stave for the back. They worked and had good cast. To be uniform it needs to be a clean stave though. Arvin
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Holy Cow! Thats a novel new idea. I admire your creativity and courage in working on it.
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Awesome work Jan, and great discussion. Now to add something to this: Would it be possible to hollow the back in the same way as Simon does with his bellies? So that the ridges are thinner than the middle so the bow can flatten out during the draw and thereby spread the tension forces from the ridges to the middle and the sides. Will probably be pretty hard because there is a point just before flattening where all the stresses are on the two ridges but if anyone can do it it is you jan!
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Leon, thanks, but to much honour really!! Actually, since you make this great looking, and performing, HLD bows, I had a little hope you could help me out on this one. If there is anybody.....(-:
Meanwhile, I am doing some little tests, and I have to warn all those who try thin edges on a hollow back...be very carefull!! Hope to post some results soon. (And to get some sleep!)
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For me enough reason to try some more.
maybe a gague sort of like this will help you not hold the bow at full draw so long?
Was the bow repairable?
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Check out the old bows on this page. ;)
http://www.bow-international.com/features/traditional/bows-over-the-borders/
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It seems to me, that you wouldnt want a bow with a curved belly or back to flatten out. It seems to me that would cause it to split down the middle and try to become 2 bows rather than one. It would be likely to tear in two unxer tension, sending cracks up the spine of the limb.
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It seems to me, that you wouldnt want a bow with a curved belly or back to flatten out.
if it bends a couple of different ways, can store more energy?
I ripped a piece of plastic pipe and tested the ripping in a spine tester. crown up bent more than crown down, (with the same weight). Doesn't this mean energy storage can be improved with a better or different crossection shape?
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@sleek, I think you are correct with your assumption, except that you would never bend the bow so much that it would split lengthwise in the centre. I have watched a 4 part series on how to make a River Cane quiver on Youtube and it was very interesting when the presenter bent the river cane halves (crown out) that make up the spines of the quiver for the bottom part. They split lengthwise in the centre after having been bent so both ends were up and formed the rounded bottom. It would represent an extreme draw on a HLD bow and you can nicely see how the river cane half flattens out when bent just before it splits.
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Here is the knowlege i am going on to form my opinion. With heat straightening osage, when i am taking out twist, if i concetrate on to small a spot, i have opened up cracks simply by twisting the wood to correct it by simply clamping it to a board flat. These are usually where the twist is in one spot on the limb. I dont know how it will play out when spread out ober the entire limb length, but with a curve flattening on the entire limb, it seems it would still be likely to split and all at once and at full draw. As a safety measure ud wrap sinew in a few place up the limb to hold it together shoukd that happen. And it may not, but there us a point where it will, whether the curve of the hollow must be deaper, bow must be shorter, draw longer and weight hevuer, idk, this is all new, but there is a limit. Id reccomend finding it to learn more if anyone had a mind to.
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I took some more measurements on the half pipe. when bent in the hollowed belly direction, I can see the flattening effect. But when bent in the hollow back direction, the cross section shape does not change.
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I took some more measurements on the half pipe. when bent in the hollowed belly direction, I can see the flattening effect. But when bent in the hollow back direction, the cross section shape does not change.
That is interesting. Especially because id expect the hollow back to want to curl up rather than flatten out.
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A little test. Two hollow back bows. With a thin thread around the limb. 3 persons where asked to feel the tension in the threads. 2 of them do not care about bowmaking and that is good for once!
Unonimously conlusion: the tension decreases when the limbs bend.
This seems to indicate that the limbs try to curle up. Probably forced by the Poisson effect.
When bend severely, the limbs will be forced to flatten out (Poisson is loosing the battle) but that point is beyond the draw lengths of these bows. Anyway the first reaction is to curl up.
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Check out the old bows on this page. ;)
http://www.bow-international.com/features/traditional/bows-over-the-borders/
GREAT!! Love the Scotties! And there really is nothing new under this sun.
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Anyway the first reaction is to curl up.
I expected the same and with an easy bend, it seemed it was curling up some, but with a severe bend, it seemed to be right back where it was when straight. Not enough movement to be conclusive, and my test was less than optimal, measuring a half round of plastic pipe with not so precise calipers. One thing that that was apparent was lots more change in width when the hollow was to the belly.
It will be interesting to see how these bows shoot. And how much they weigh. Have you read about the mass principle? Badger wrote it in TBB4, and it can be a practical indicator of efficiency.