Primitive Archer
Main Discussion Area => Bows => Topic started by: Slackbunny on January 17, 2014, 12:07:56 pm
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When reading the thread with Simson's hollow limb bow, I had a moment of inspiration. But I didn't want to hijack, and I thought this topic might be good enough to merit its own thread
My thinking is that a hollow limb cross-section could be significantly more efficient than a standard cross-section bow because of its unique geometry.
Its like steel I-Beams. They have similar strength characteristics to solid steel beams, but with far less mass. This principle doesn't just apply to metals, its a geometry thing that should apply to all solid materials.
So in theory a hollow-limbed bow with a certain mass should pull heavier than a standard bow with the equivalent mass because its geometry gives it a better strength-weight ratio.
If my thinking is correct, then the hollow limb geometry would require a major adjustment to the mass principle in order for it to be accurate.
What are you thoughts?
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Getting out my slide ruler, pocket protector, and making popcorn. this should be interesting in a geeky sort of way!
OneBow
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Just ask simson to shoot the bow and tell us the mass and draw weight and tell you how it compares to a bow of the same weights in regular configuration. ::)
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All other things being as equal as wood can be, 2 bows of the same length, same mass, I am pretty sure the HLD design will have significantly stiffer limbs.
Grab yourself a sheet of printer paper. Let's consider this a simple cellulose analog to the wood in the bow. Whatever way you fold it, the mass stays the same. It does not take much effort to bend it into a U shape from top to bottom. Now bring the two edges together in a similar shape the HLD limbs would be. Put your thumbs in the bottom of the concave shape on top and bottom of the paper and try to bend that piece of paper in on itself, this concave shape is noticeably more stiff. A piece of paper probably isn't the greatest example, but its an easy example of why this would result in a stiffer limb.
For a better analog one could go out and find a suitable sapling for a break test. split it in half. Hollow one side out until its bending good, take the other side from the same tree and scrape it down until the mass is the same and preform a break test. If it goes as expected it should take more to break the HLD, or at least the HLD should be stiffer than the the flat backed sample.
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My thoughts are...that I cant shake the thought of all that stress concentrated in two narrow ridges. Im sure that material/stress simulations that would shed light on this exsist.
Simsons bows are thought provoking (in so many ways)....I am just pretty sure he could get the same effect with a none hollow, but more narrow same-mass design.
Cheers
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My thoughts are...that I cant shake the thought of all that stress concentrated in two narrow ridges. Im sure that material/stress simulations that would shed light on this exsist.
But the concave also provides a much greater surface area on the belly to distribute the stress. That may help to offset the effect that you are talking about.
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My thoughts are...that I cant shake the thought of all that stress concentrated in two narrow ridges. Im sure that material/stress simulations that would shed light on this exsist.
But the concave also provides a much greater surface area on the belly to distribute the stress. That may help to offset the effect that you are talking about.
It may....and it may not. The ridges will be farthest away from the back at all time and should take most, if not all the compressive stress (I say "should", because it being wood there will be a lateral flex too....the limb ridges will be pushed out as well as compressed. And I cant anticipate what effect that has). If anyone can show me a hollow limb design with a fret in the midline, I will bow my head and leave in silence;-)
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Simson should correct me on this if I am not correct, but I think I read that when the HLD is drawn the limbs will actually begin to flatten. The stresses are being distributed along the curved limb. It's sort of a dynamic distribution of stress that to me seems a little more complicated than simple compression/tension, but a process that seems to distribute those forces in a very acceptable manner.
Case in point being this bow of Simson's. If this design wasn't a good one, this bow wouldn't have worked. He took a piece of wood rotting from the inside out that few people would have done anything with other than throw it in the fireplace and turned it into an HLD bow.
http://www.primitivearcher.com/smf/index.php/topic,41217.0.html
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I tend to agree with you Wiley, I feel like it might slightly flatten out. I have no idea how it would relate to the mass principle. If I try to duplicate the action with my tape measure it just twistsithout bending and then kinks, not sure why it seems to be effective with wood.
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It will flatten out (that is what I mean by saying the ridges will be pressed outwards).
Simpson is an expert no doubt about that, and finding a bow inside that mess of half roten laburnum was good bowyering.
But its not enough to convince me that hollowlimb design is more effective that than other designs.
Cheers
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I think simson has proved that the design works when using a high crowned stave. In terms of mass I think that a rectangular cross section bow vs a HLD bow of the same mass would result in a stiffer limb in the HLD, If both were reduced to the same weight of pull the HLD limb bow would have less final mass. I imagine a lower mass bow of the same weight of pull would probably result in a higher efficiency, low hand shock, and shoot faster. I am not a bowyer but I soak up things that interest me like a sponge, so i'm just going on what i've read.
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Hmm, if it flattens out, then that might change things. Now instead of dealing with a single cross-section for the duration of the draw, we have a cross-section that is changing dynamically. That complicates things. We're leaving the realm of simple geometry and stress-strain and getting into the realms of calculus now.
The limbs flattening out may be the only reason the design works at all. What Holten said about the stress concentration in the ridges is probably true. But if the bow flattens out when it is at its maximum stresses, then all of the sudden we don't have that issue anymore, because we no longer have ridges, just a really wide flat bow.
It seems to me like an ideal design. It changes its cross-section to the ideal state at any given point in the draw and the release. It is narrow with ridges early on to give high initial draw weight storing more energy early on. Then as the stresses increase, it becomes wide and flat, a cross-section that we know deals better with greater tension. Then it narrows as it accelerates through the release, becoming more aerodynamic as it gains velocity. And it does all of this while seemingly being of lower mass than a comparable standard bow.
I don't know if all that is true, but that is how my mind is processing it at the moment. Feel free to bring me back to reality. ;)
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I don't think it actually flattens as much as it does attempt to flatten or possibly even turn itself inside out. It really is interesting, I can figure out why the steel tape will not duplicate the bend unless it is because it lacks tapering.
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While I suspect the slightly improved aerodynamics of the limb as it is released and re-shapes itself into a high crowned geometry would be very hard to quantify in any assessment of the performance of the bow, ...the rest made sense to me. The effectiveness of the improved aerodynamics would require a medium with a viscosity something like pudding before I imagine it would show up in the overall performance of the bow.
However; the bit about dynamic redistribution of the forces, both tension and compression, during the draw and release do sound like they would make significant physical difference to the performance of the bow.
Simpson - If you are monitoring this thread, it would be great to have any performance measurements you might have for this little bow. ...and if by any chance you had a similar stats bow made in one of the more traditional crossections, some comparables would be of interest.
OneBow
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Well it's a sort of concave form, yeah? It creates an arch (like a 'U')? Arches are among the strongest structures. If it bears a stress on it's top side (the opposite side of the hollowed side) it will divert the tensile stress, increasingly, towards the bottom of the arch's form. The more rounded the portion of the arch is, the more it will evade tensile stress. The straighter portion of the arch will take on that stress. Eggs are arches as well, in fact. That's why you can lay a heavy weight on the top of an egg without it breaking, yet if you lay it on the side, it will be crushed. So wouldn't it stand to reason that a hollowed limb bow like that would possess a heavier draw weight but be able to build up more 'potential energy'? I don't know honestly, I'm just going on my layman's understanding of physics and bow making.
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I agree with onebowwonder...I dont think improved aerodynamics is an issues worth exploring...any lateral deformation will be too slight to have any effect (I generaly dont belive air resistance on the back impacts much on effectivness in wooden bows).
Testing wether the hollow limb design is more effective should be rather easy. I would suspect that taking any relativly high crowned flatbow and gradually make the limbs more hollow while recording weight, draw weight and arrow speed, would reveal alot.
If you are right that the hollow limb design is more effective (and there by circumventing the mass principle) then that design should soon dominate competetive flight shooting. I remain sceptical until I see more evidence (Im still in awe of Simpsons work though).
Cheers
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Fist I admit I couldn't be bothered to read through all the posts so I may be talking out of turn.
A lot of the analysis is IMO flawed. We are not after stiffness with a bow we want flexibility.
Tubes, cylinders I beams etc give good stiffnes but have nasty failure modes.
An I beam may be great for a lever on the end of the bow, but I'd guess not for the limb.
I can get impressive failure modes without looking for less stability :o!
But hey what do I know I'm a cat :laugh:
Del
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It wouldn't circumvent the mass principle, it would just require a new parameter be taken into account. I would guess that the new parameter would be the average moment of inertia of the limbs.
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I think you folks need to stop the flapping and get to scraping if you want to know the answers to your questions !
But what does a worthless old nut know anyway.
Have fun
Guy
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It wouldn't circumvent the mass principle, it would just require a new parameter be taken into account. I would guess that the new parameter would be the average moment of inertia of the limbs.
Yeah....the "circumvent" note I regret now...please ignore it:-)
Cheers
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I think you folks need to stop the flapping and get to scraping if you want to know the answers to your questions !
But what does a worthless old nut know anyway.
Have fun
Guy
...but I'm ever so much better at flapping! :o :( :-\ ;)
OneBow
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First off, I haven't made an HLD bow yet and I'm certainly no physicist. This is just my conclusions, hypothesis, mental flapping or whatever that I have come to after much deliberation and contemplation of this design. 150,000 miles in the saddle every year leaves a lot of time to think. I believe that it works so well because the limb doesn't know its arched. If the basic principle on which a bow works is the opposing forces of tension and compression, then surely the HLD is mostly a wider flatbow thats arched. Meaning the entire outer surface is under tension, while the entire inside of the arc is under compression. This would put more wood to work than if it were simply a high crown flat belly design. I got out pen and paper to attempt to illustrate what I mean. The arrows in the neutral zone are representing the way I perceive the opposing forces of the different cross sections. If my thinking is correct, the ten percent of thickness that carries the tension and the ten percent that like wise carries the compression is drastically improved with the HLD. In effect, your taking an inch and a half wide sapling and getting the effect of a much wider flatbow. If you add in the structural rigidity added by the HLD, you are talking significant improvement over the old high crown flat belly design. Just my opinion of course. If there are holes in my explanation, please feel free to challenge it and enlighten me. Josh
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As a flight shooter you can be assured I am looking real hard at this. I am also cussing! Just when I think I ran out of things to try one more pops up, it just keeps happening over and over and over LOL.
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UUUh my!
That is an interesting discussion, always wanted to initiate this. I just have answered some questions in the elder HLD thread, don't want to do that here again.
Measurement: I will do that tomorrow and post it on the Elder HLD thread.
I have read all the replies, but haven't understood all. Have to translate some things with google ....
Please excuse me for now, I'm coming back tomorrow.
For Steve: I always thought of flight shooting. Here in Germany flight shooting isn't an item. Can you tell me what determines a bow as a flight bow. Too bad you are not here in Germany ...
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First off, I haven't made an HLD bow yet and I'm certainly no physicist. This is just my conclusions, hypothesis, mental flapping or whatever that I have come to after much deliberation and contemplation of this design. 150,000 miles in the saddle every year leaves a lot of time to think. I believe that it works so well because the limb doesn't know its arched. If the basic principle on which a bow works is the opposing forces of tension and compression, then surely the HLD is mostly a wider flatbow thats arched. Meaning the entire outer surface is under tension, while the entire inside of the arc is under compression. This would put more wood to work than if it were simply a high crown flat belly design. I got out pen and paper to attempt to illustrate what I mean. The arrows in the neutral zone are representing the way I perceive the opposing forces of the different cross sections. If my thinking is correct, the ten percent of thickness that carries the tension and the ten percent that like wise carries the compression is drastically improved with the HLD. In effect, your taking an inch and a half wide sapling and getting the effect of a much wider flatbow. If you add in the structural rigidity added by the HLD, you are talking significant improvement over the old high crown flat belly design. Just my opinion of course. If there are holes in my explanation, please feel free to challenge it and enlighten me. Josh
With all due respect, but the perception of the forces at work is wrong Gun Doc.
The force planes (tension, neutral and compressiv) are decided, not by the cross section of the limb, but by the direction of the overall bending force. If you tip over an I-beam (making the cross section like this: I--I) and bend it, then the upper flanges take tensile deformation and the lower will compress and collaps....the middle wont be stressed til the flange collaps happen. Im sorry I cant explain it better with my limited english:-(.
Cheers
Edit: God I sound like a twat:-(. I dont know for sure how the forces behave when bending a half pipe...I dont have the anwsers or knowledge to settle this in any way. And im sorry if I come across as impolite.
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Simon, I will just give you an opinion here. One of the biggest challenges to a flight bow builder is mainating efficiency with very light arrows while still storing sufficient energy. Longer bows with longer draws store more energy, shorter bows tend to be more efficient. This is where I may be wrong but I dont hink so. I believe that the slightly longer bows of around 62 to 64" will eventually prove out to be the best light arrow bows when made from all wood. The biggest single loss I have identified from efficiency comes from hysterisis. The best way I have found to reduce that is by reducing set or damage to the wood fibers that happen when we bend the wood. I can get my short bows very efficient but they just seem to lack the energy storage I need. I also believe that very light outer limbs with lowest possible string angles reduce momentum in the limbs that is exagerated by light arrows. Everything relates to giving the arrow more leverage to control the limbs with. A 200 grain arrow has very little mass to excert on the limbs durring acceleration so it needs all the help it can get from better geometry and lower mass limbs.
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This is one of the most thought provoking topics that I have read on this sight. Another way to do a test would be make a wide flat bow and bend it lengthwise over a pipe, with another half pipe over it as a form. You wouldn't have to make a full scale test just some thing to get a good idea. I might just have to try something like that. If you made the limbs thin enough. Wouldn't you get more of the flattening out of the limb. My mind is kind of hurting thinking about these things because I am not too into physics but this is really intriguing.
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This is interesting and i think gun docs picture makes sense but wood is not a manufactured product and any knot or twist,or wiggle in grain could affect performance.just my two cents -Hammertime
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OK - ...so I can't stop thinking about this idea of an HLD laminate bow. I realize this may be a distraction from the main topic here, so ignore me if need be, ...but if I don't write what I am thinking/dreaming about I might just burst. (...which would not be pretty!)
What would be the potential of building a long tapered half conical or cylindrical form to lay real thin flexible lams up on to build an HLD bow? You could select the wood species for the lams based on the suitability to withstand the compression and or tension strains anticipated. If the Lams are thin enough, getting them to wrap around a mold and glue-up in the HLD shape shouldn't be too tuff.
Hmmmm... Pondering... I guess you basically wind up with an engineered 'plywood' bow. ::) :-\ OK, now the idea sounds much less exciting after typing that last line. I can move on now. ;D
OneBow
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Some FG lam bows have been made in that design. I can't remember which. It will come to me...or not. :) Jawge
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My thoughts run along with Doc's. I just look at it from a different angle, that being: for any given width the two surfaces (one concave and one convex) with greatly increase surface area over the same width limb with flat surfaces. Increased surface area means increased distribution of load....either compression or tension.
I have no interest in "maximum Performance", meaning comparing one thing with another, but I really like this design because it's a new way of looking at a problem......and that's always good. I hope Simon's design kicks some serious butt.
rich
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Some FG lam bows have been made in that design. I can't remember which. It will come to me...or not. :) Jawge
Just the ACS design Jawge.
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Well put Rich.
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Thanks, Pat. Jawge
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Jawge, it was Ol Adcock and the ACS. He had such a minor amount of concave affect I don't think it would come close to qualify as a comparison. It turned out quite a few bows at that time were actually faster than his. I think he used it as a sales tool more than a real tecnique.
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Steve, I never actually saw definitively which direction the concave limb was facing. Was it cupped to the back or belly?
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Pat, it was concave and convex on the back, he used a press to slightly shape the limb, it really was more hype than anything as the concave was almost imperceptable if you weren't looking for it.
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I had a whole novel typed up, and then realized it was mostly speculation, so I decided to just sit back and watch the show instead :-X
Ah heck, who am I kidding! I tend to agree that increased surface area does not necessarily mean the load is distributed any better. A lot of that added surface area is just making a larger neutral plane. I would think all of the compression forces would be concentrated on the outside edges, and that any forces applied that would be enough to "flatten out" the arch must cause set-inducing damage to the wood along those edges.
Just a hypothosis ;)
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The most obvious simple test would be to simply saw out a bamboo pole into a pretillered shape then check out what it does. I realy don't know id anything is gained or not. Seems like the compression on the ridges would be excessive but he has successfully completed several of these now. I wouldn't be surprised if some primitive tribe hadn't been doing this for centuries allready.
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Compression force less on the edges and increased in the middle when drawn. Really messing with a slide rule and a piece of bamboo. As it flattens the pressure in the middle increase by a coefficient of about 10, until full draw where it drops to about 3. Initial force creates intense force and then the load is distributed to the flatter surface, thus evening tension across the flattened width of the limb......or something of that nature
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The most obvious simple test would be to simply saw out a bamboo pole into a pretillered shape then check out what it does. I realy don't know id anything is gained or not. Seems like the compression on the ridges would be excessive but he has successfully completed several of these now. I wouldn't be surprised if some primitive tribe hadn't been doing this for centuries allready.
That's waaaay too sensible ;)
BTW, regarding arrow mass. If you calculate the g force as an arrow is first loosed it's pretty huge, so there is plenty of inertia for the bow to work against at the start of the loose (or at the maximum accelleration point, which is presumably somewhere near the start of the travel).
Presumably the problems all occur after the arrow leaves the string, and the limb tips are having to stop?
So obviously ;) if the arrow has gone, it can't be the thing that's causing the problem >:D
Del
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First off i'll start by saying I am off to get one of these roughed out. I've got a perfect maple sapling that is just about dry now.
So to my mind the flattening of the limb is only going to be minimal , it has to be or else you would get a split sapling! I really don't think that the bow is trying to 'flatten out' during the draw. The poisson effect which goes on in every bow that is bent is actually trying to increase the belly concavity and make the back concave too. So maybe it's actually more about opposing forces resisting each other. The best cross section to minimise the poisson effect in a bow limb is a rounded belly and a flat back.
I can't wait to get this bow finished and shoot some throught the chrono. It will reveal the truth ;) I aim to make my HLD bow the same draw weight and length of my fastest straight stave bow and see how they compare.
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Mike, with the poisen effect I believe the developed ridges are facing away from the bend.
Del, the idea is to stop the limb tips before the arrow leaves the bow or at least slow them as much as possible.
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I have posted the measurements of the HLD Elder bow on the other thread!
For comparison here are some other elders of mine:
http://www.primitivearcher.com/smf/index.php/topic,35631.0.html?PHPSESSID=ojscmbeir9rl5rm6ccho63ej64
http://www.primitivearcher.com/smf/index.php/topic,37923.0.html
http://www.primitivearcher.com/smf/index.php/topic,38689.0.html
http://www.primitivearcher.com/smf/index.php/topic,38720.0.html
(http://i1252.photobucket.com/albums/hh576/orangesimson/1401%20elder%20HLD%20hemp%20handle/P1020300_zps98416dde.jpg)
First sketch is the bow unbraced with depth d1 and width w1 and groove g1
Second sketch shows the bow drawn with d2, w2, g2
What did happen?
d1>d2: you get a bow with a thinner limb, only a few millimeters make a great affair!
w1<w2: you get a bow with a wider flatter limb, the increase isn the last inches is lower than 'normal'.
g1>g2: you get a belly surface with compression force vertically (same as on every 'normal' bow) and additional tension force horizontally. Perhaps this effects preventing chrysals and lower stress. The wood cells have more room caused from zhe tension force for the vertically compression.
My thoughts for designing HLD:
- the walls must be thin enough to flatten out a bit when bow gets drawn
- the groove must be pronounced enough to get an decreasing limb thickness
- the Cross section should show a horizontally taper to prevent splitting. that means the wall is thickest in the middle and tapers out to the ridges
- there should be a vertically taper (of course!) to get the bow even bending. But this also must prevent the sudden 'snap-effect' as can be seen on metal ruler tapes
- the bow must be tillered in the right proportions to get all that things working. But when done alright, you have a bow with high early drawweight, lower increase in the last inches ( of course no 'let-off'), the graph of the fd - curve is in the first inches steeper (more energy is stored) and there will be less stress on the belly - always compared to a 'normal' bow.
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Fascinating Simson.
Somebody get that man a set of digital scales and a chronograph.
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Whatever this works as said or not (to me it "feels it should to some degree atleast)
there is another advantage here!
It lets you use smaller diameter branches and saplings.
Just a thought...
/Mikael
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The most obvious simple test would be to simply saw out a bamboo pole into a pretillered shape then check out what it does. I realy don't know id anything is gained or not. Seems like the compression on the ridges would be excessive but he has successfully completed several of these now. I wouldn't be surprised if some primitive tribe hadn't been doing this for centuries allready.
If I had the time, I would make several HLD's from pine to study the chrysals and mechanics of failure. I believe that would provide some insight on how the forces are distributed across the limb. Unfortunately, I'm already to far behind on making bows that are supposed to hold together to go making a bunch of sticks for the sole purpose of failing. Josh