Primitive Archer
Main Discussion Area => Bows => Topic started by: Woody roberts on October 26, 2021, 10:59:54 am
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I’m not sure how to ask the question but here goes. All woods being individual notwithstanding is it better to make a bow wider and thinner or vice versa for a target draw weight. I realize soft woods will be thicker than harder woods at a given draw weight.
For instance would a hickory pyramid bow 2” x 1/2” out of the fades be better than the same bow 1-3/4 x 9/16” out of the fades.
My suspicion is the thicker bow may shoot faster but the thinner bow may last longer. I could be wrong, I often am.
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I think it depends on the characteristics of the wood. Dense woods like osage can be narrower and thinner but woods like hickory and maple work better a bit wider and thinner.
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My suspicion is the thicker bow may shoot faster but the thinner bow may last longer.
That is pretty accurate.
Limb thickness determines the strain/stress in the limb wood. The thicker it is the more stress the wood sees for any given amount of bend forced on it. Width determines the total draw weight because it determines the volume/mass of wood working.
The way I look at it is each piece of wood has a maximum stress level it can withstand before failing (typically by taking set in most hardwoods). This maximum stress determines the maximum thickness the limb can be for any particular bow design. At that maximum thickness every inch of width will develop a certain amount of draw weight. Width is then determined by what weight you want the bow to have.
Maximum efficiency for any given design is achieved when you take the wood right up to its stress limits, but not beyond. This equates to a thicker/narrower bow overall but if you go too far it will take set and lose performance.
Since wood is not predictably consistent in its properties like steel and other structural materials you either have to test and measure each piece individually or use experience and rules of thumb to guide how wide you make your bow and then refine it during tillering.
Mark
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This question may need a separate thread. I like to work my limbs very close to finished thickness before doing much if any bending. For hickory I start with 1/2” . Hackberry,walnut etc I start at 9/16 to 5/8”
I’ve only made 3 Osage bows and haven’t worked out a starting thickness yet.
Do these starting measurements sound about right or should I adjust?
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I basically don't measure thickness. I do start out about 5/8" to 3/4" and go down from there as I get the bow bending.
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Thicker is quicker.
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Do these starting measurements sound about right or should I adjust?
That depends on the bow design details (length, draw length, weight, width, style, etc.). A recurve will use different thicknesses than a flat bow, which will be different than an R/D design. Stiff handle, stiff tips, etc. will also affect it.
FWIW (given my small number of bows), I have never had one end up thicker than about 0.425"-0.435" thick. These are mostly pyramid bows with little reflex, ~67" long and around 40#@28" made from hard maple or red oak.
Mark
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Chapter 4 TBB vol4 (?) chapter on design and performance.
And yes thickers quicker :)
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Mark is right if I understand this correctly. So if you have a 50# Osage that is 7/8 wide and 7/8 thick will it be quicker than 1-1/2 wide by 3/8 thick? Of course the ends will taper from mid limb. . Arvin
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The assumption that soft woods will be thicker is wrong as density does not necessarily correlate with elasticity.
The general assumption is that a thicker narrower limb will have less mass and therefore be quicker. Badger often chimes in about a bow(s?) he made years ago that we’re wider thinner but lighter (less mass) than narrow limbed bows. I don’t think he measured the performance of those bows.
The issue with going too narrow is that you risk needing increased thickness for a specific draw weight but that increased thickness will be under a higher strain and more likely to take set. There is probably a sweet spot for low set/strain and low mass determined by a bows cross section. If your bow isn’t taking set late in the tillering process you have room to remove width for more mass reduction.
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I think the density will determine the size in general. The mass weight will be close no matter the material if it’s all the same material. For example bamboo cores in a composite bow will maybe mass weight lighter if the glue does not add to mush weight. So my simple composite bamboo backed bow came out lighter because of thicker bamboo than normal. I also trapped the rind because it’s more dense. The mass weight came out 3-4 oz lighter than a normal Osage selfbow. That one is fast! Arvin
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Correcto Arvin.That's my observations also.It'll happen putting horn on the belly too,but then it is'nt a self bow any more.
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Ryan, if it isn't taking set would it be necessary to narrow or would it be like the wider bows you just mentioned? Do you have to go extremely wide and thin for the benefits?
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Allyn, to be honest I’m not sure. I think we’d have to build a number of bows with the same or zero set with differing thicknesses. I am wondering if there are two advantages to wide thin bows. The first being that thin limbs will be under less strain and are less likely to take set. The second being that a higher percentage of working wood is closer to the surface with less non working wood in the middle/neutral plane of the limb possibly leading to lower mass. We often talk about the outer surface of the limb doing a majority of the work but there is some gradient of stretch between the surface and neutral plane. My thinking is that thin limbs make that gradient very “steep” if that makes sense.
I should add Steve’s thinking (if I remember correctly) that even in low set bows there is the possibility of damaged wood that isn’t contributing to work but adds mass. Making a bow extra wide may help avoid this due to very low strain. The big issue is no one wants to make 3” wide bows. (lol)
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Lol yes, my first bow attempt was 3" wide at the fades and I didn't realize at the time how much harder wood removal is at that width. It made me never want to try it again.
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Years ago I made a 2.5” wide poplar bow and even that was tough being a soft wood. I need a custom 3” wide spokeshave for the super wide ones.
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I am wondering if there are two advantages to wide thin bows. The first being that thin limbs will be under less strain and are less likely to take set. The second being that a higher percentage of working wood is closer to the surface with less non working wood in the middle/neutral plane of the limb possibly leading to lower mass.
In a rectangular cross section limb the percentage thickness that is contributing the most work is always the same. The gradient of stress/strain is a straight line in the rectangular cross section, from zero on the neutral axis to maximum at the surface of the limb.
I am going to tinker a bit with your idea about the wider limb having more wood at a higher strain level, there could be something to that.
Mark
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I should add Steve’s thinking (if I remember correctly) that even in low set bows there is the possibility of damaged wood that isn’t contributing to work but adds mass. Making a bow extra wide may help avoid this due to very low strain. The big issue is no one wants to make 3” wide bows. (lol)
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I’ll make them wide if it brings up the performance. That Osage bow that holds the fifty pound broadhead record was 2-3/8 wide at the fades but it was as a pyramid close to perfect diminishing mass. Where the mass is that’s the key. The less of it the more efficient the bow. Then set comes into play. Dead wood is dead mass. If your building replica bows none of this matters. If your building for speed and cast it all matters. Arvin
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Mark, I get that the relative percentage is the same. My mind goes to the surface area of wood that is doing work. My thinking is probably flawed but it seems like for a narrow/thick limb, there is more wood between the surfaces doing the work than in a wide/think. Saying it out loud I’m starting to think it doesn’t make sense.
Edit: I guess I’m thinking in absolute terms where if we say the back of the bow is 1/32” that’s doing the work, that would be a greater percentage of wood in a wide thin limb compared to a thick narrow limb.
Arvin, I appreciate your thoughts and real world experience. That width in Osage would equate to a 3+” wide for most other woods is my thinking.
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Marc was the bow probably over built on the inner limbs ? I would say yes but some how it worked. Mass on the inners must not be as critical.
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Marc was the bow probably over built on the inner limbs ? I would say yes but some how it worked. Mass on the inners must not be as critical.
It probably was overbuilt for osage, but the weight penalty is minimal near the handle because the inner limbs hardly move. All the losses happen farther out where the speeds and distance moved are higher.
Mark
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People say overbuilt a lot on here but in Arvin's case it clearly wasn't seeing how well the bow performed. It seems like if someone makes a bow that takes no set it's considered exceptional wood or tillering or both. On the other hand if it was made extra wide people say it's overbuilt. I'm starting to lean towards the idea of it doesn't matter how wide it is as long as it shoots hard.
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Could have been overbuilt in the inner limbs without being so in the mid/outer limbs. I recall one of Arvin’s record breaking Osage bows did break in the mid/outer limb but he can correct me on that.
If you subscribe to Klopsteg’s ideas on a convex limb width taper for an even thickness bow with even strain along the limb and circular tiller, pyramid bows would most often be overbuilt in the inner and outer limb and underbuilt mid limb where the straight taper would match a convex taper causing an area of higher strain. I’ve been wondering lately if this is a pattern we see in pyramid bow breaks. Still working on my conceptualization of this.
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If you subscribe to Klopsteg’s ideas on a convex limb width taper for an even thickness bow with even strain along the limb and circular tiller, pyramid bows would most often be overbuilt in the inner and outer limb and underbuilt mid limb where the straight taper would match a convex taper causing an area of higher strain.
Any links to Klopsteg's stuff?
Regarding convex limb width taper, David Dewey's spreadsheet outputs this shape for a constant thickness limb that tapers to a point at the nock, which is the classic theoretical pyramid design. If you analyze a straight taper pyramid on the spreadsheet it does indeed give higher strains in the mid-limb area. This correlated well with a straight taper maple pyramid bow I did last year (designed on the spreadsheet) where much of the set happened in the same mid-limb area.
My current bow project is a 3 lam red oak bow using the convex shape directly from Dewey's spreadsheet. It is tillered and ready for some test shooting but I have been swamped with a house move and setting up my shop before winter arrives tomorrow afternoon so it remains untested at this point.
Picture of the back shape after being cut on the bandsaw:
(https://i.imgur.com/qXeLwvC.jpg)
Drawn to near full draw on the tree:
(https://i.imgur.com/dFHOxfr.jpg)
Mark
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Mark, I don’t think there’s anything online yet but I think archerylibrary.com is working on uploading archery the technical side chapters. I’ll PM you.
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I found this a while ago and downloaded some of Hickman's work:
https://www.archerylibrary.com/books/hickman/archery-the-technical-side/
Mark
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Just found this and thought it might be of interest.
The first bow displayed is an Iroquois War Bow. Very wide limbs. Ash and draw weight of 80#.
Very powerful and impressive bow not considered suited to hunting because of over penetration on creatures such as deer.
A true war bow intended to penetrate heavy clothing and light armor at long range.
https://www.youtube.com/watch?v=5CWCSpAlick
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Bottom limb looks hinged? It looks like it could lose quite some weight off those mid/outer limbs ;)
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That video crammed a lot of questionable facts in a fairly short period. Delicate squirrels being one of the most glaring. ;D
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I haven't watched the video yet, but "over penetration"???
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Bottom limb looks hinged? It looks like it could lose quite some weight off those mid/outer limbs ;)
Zing! It does look like a hinge, but is where the underlay starts that I used to add material to those skinny levers. The spreadsheet says I don't really need them but I was being conservative with my first try of this style. When I cut the back profile those levers looked so narrow and delicate, but they are rock solid on the tree so what do I know?
Mark
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Maybe Mike was talking about the video.
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Maybe Mike was talking about the video.
After watching the video I think you are correct. No harm done either way, mine looks like there is a hinge on the left limb as well...
Mark
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That video had to be a joke.
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That video had to be a joke.
I think so to. Or not real knowledgeable.
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I am wondering if there are two advantages to wide thin bows. The first being that thin limbs will be under less strain and are less likely to take set. The second being that a higher percentage of working wood is closer to the surface with less non working wood in the middle/neutral plane of the limb possibly leading to lower mass.
In a rectangular cross section limb the percentage thickness that is contributing the most work is always the same. The gradient of stress/strain is a straight line in the rectangular cross section, from zero on the neutral axis to maximum at the surface of the limb.
I am going to tinker a bit with your idea about the wider limb having more wood at a higher strain level, there could be something to that.
Mark
Interesting points. I can guess but not sure. Can you explain "the straight line gradient" in ordinary language, please?
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Can you explain "the straight line gradient" in ordinary language, please?
It's really a graphical thing and so much easier to understand with a diagram of what is going on:
(https://www.bu.edu/moss/files/2015/03/bending_linearstress.jpg)
The stress through the thickness of the limb goes linearly from the maximum at the surface to zero at the neutral axis. This is a diagram of a portion of a beam in bending, like looking at the side of a bow while the limb is bent. The bow limbs would be oriented horizontally as the diagram is drawn. In this diagram the top surface would be the belly (in compression) and the bottom surface the back (in tension).
HTH,
Mark
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I haven't watched the video yet, but "over penetration"???
in a culture where more work was expended on arrow making than bow making, loosing a good arrow on a shoot through was painful.
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Can you explain "the straight line gradient" in ordinary language, please?
It's really a graphical thing and so much easier to understand with a diagram of what is going on:
Thanks. That's what I thought you meant. Then, why do they say that the top 10% of back does most of the (tension) work?
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A pass through is more likely to preserve an arrow for retrieval than one getting slapped by trees as the animal flees and then falls down on it.
If you are worried about your arrow when hunting, you will likely miss.
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I haven't watched the video yet, but "over penetration"???
in a culture where more work was expended on arrow making than bow making, loosing a good arrow on a shoot through was painful.
This is true, but I agree with Pat. I would think a full passthrough would be the easiest to find and the least likely to break. I am trying to think of a scenario where this would not be true, and can't come up with one.
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I would say not getting an animal because you didn't get a pass through would be more painful, and it would probably leave with your arrow as well so that's a double whoopsie
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Thanks. That's what I thought you meant. Then, why do they say that the top 10% of back does most of the (tension) work?
This is hard to explain in a short forum entry of text. Attached is a picture of the basic calculations to help illustrate. You may have to zoom in on the picture to see it clearly.
The basic idea is that the area under the curve (in this case the triangle) gives the total force generated. This allows us to calculate what portion of the total force is generated by different layers in the limb.
(https://i.imgur.com/sxksbqz.jpg)
The picture shows the same triangle representation, representing the tension side of the limb in this case. I added dimensions to illustrate:
t = the distance from the neutral axis to the outer surface of the limb
F = the maximum stress on the outer surface of the limb
Say we split the thickness t into half, representing the outer half of the tension side of the limb and the inner half. Call the outer portion Area1 and the inner portion Area2. We can easily see that Area1 is much bigger than Area2. The calculations show that Area1 is 3/4 of the total area and Area2 is 1/4, which means that the outer half is generating 3/4 of the total force.
This calculation can be carried out for any slice of the limb thickness to show how much of the total force that particular layer is generating. If you work it backwards you can calculate out that the outer 29.3% of the thickness carries 50% of the total load. For a rectangular section limb the triangle depth t is half the total limb thickness. This means 50% of the work in that limb is being done by a layer that is 14.65% of the total limb thickness on the back and belly surfaces.
To put this into real numbers, say your limb is 0.5" thick. 50% of all the work is done by 0.073" thick layers on the belly and back surfaces. The other 0.354" in the core is doing the other 50%, which means the core isn't doing much overall.
Clear as mud?
Mark
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Clear as mud?
Clear as a bell. But you are saying the same thing.
I am too lazy to do the calculation myself.
What is the percentage of work done by the top 10%?
It is possible that the bow stave is not quite homogeneous
and the top layer selected is quite stronger in tension
than the layers below, as in the case of bamboo.
It is also possible that the neutral plane is not quite in the middle,
as wood is usually much stronger in tension than in compression.
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Clear as a bell. But you are saying the same thing.
The same as what?
I am too lazy to do the calculation myself.
What is the percentage of work done by the top 10%?
The outer 10% (of total thickness) does 36% of the work.
It is possible that the bow stave is not quite homogeneous
and the top layer selected is quite stronger in tension
than the layers below, as in the case of bamboo.
Yes, that is correct. The above analysis assumes a homogeneous material. It can be applied to non-homogeneous materials with adjustments but that gets more complicated.
It is also possible that the neutral plane is not quite in the middle,
as wood is usually much stronger in tension than in compression.
The ultimate strength doesn't influence the neutral axis but the stiffness does. The above analysis assumes the stiffness is the same in tension and compression, which may not be the case. This was discussed here a while back and there was no conclusive answer as to where the usual bow woods fell on this question. The analysis method is still valid regardless of where the neutral axis is located, I just kept it symmetrical for simplicity and because that is a common situation on bows.
Mark
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I haven't watched the video yet, but "over penetration"???
in a culture where more work was expended on arrow making than bow making, loosing a good arrow on a shoot through was painful.
This is true, but I agree with Pat. I would think a full passthrough would be the easiest to find and the least likely to break. I am trying to think of a scenario where this would not be true, and can't come up with one.
I guess one could speculate that placing an arrow entirely in the body of a deer with nothing poking out wither side would be the safest arrow preserving shot. You would likely be a non-hunter to come up with that though. ;)
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I haven't watched the video yet, but "over penetration"???
in a culture where more work was expended on arrow making than bow making, loosing a good arrow on a shoot through was painful.
This is true, but I agree with Pat. I would think a full passthrough would be the easiest to find and the least likely to break. I am trying to think of a scenario where this would not be true, and can't come up with one.
I guess one could speculate that placing an arrow entirely in the body of a deer with nothing poking out wither side would be the safest arrow preserving shot. You would likely be a non-hunter to come up with that though. ;)
I think the maker of the video says that an arrow stuck deep in the body yet protuding and bobbing around as the deer tried to flee caused more internal damage resulting in the deer stopping sooner and dying quickly.
As a few threads I've read on deer surviving for days or weeks after a through and through wound suggests he might have something there.
Seems like few of the members who post of hunting with native American type bows use bows of greater than 45-50 pound pull. The video was remarking on the power of the 80+ pound Iroquois war bows.
I've read of some Northern tribes wearing body armor of padded layers of elk hide. I suppose the greater penetration was necessary for use in war time.
Alaskan tribes often wore armor made from chinese coins gotten in trade linked together by woven leather thongs. They also used very heavy draw horn bows in wartime.
PS
Saxton Pope found the best performing Indian bow he tested to be a 70# Yaqui bow made of unbacked Osage Orange.
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Ed ashby has don't extensive test on bow and arrow performance for hunting and he what he found was people assume the arrow cuts more if it stays inside but it doesn't, it's just false speculation. Same reason if you have a stab wound they tell you NOT to pull the object out to reduce bleeding until you can get help...just because a guy in a video claims something to be true doesn't mean it is, you need data to back up a hypothesis.
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Ed ashby has don't extensive test on bow and arrow performance for hunting and he what he found was people assume the arrow cuts more if it stays inside but it doesn't, it's just false speculation. Same reason if you have a stab wound they tell you NOT to pull the object out to reduce bleeding until you can get help...just because a guy in a video claims something to be true doesn't mean it is, you need data to back up a hypothesis.
I believe he was speculating on the reasoning of the Indians of the period, right or wrong, rather than stating it as a fact, though it seems to be his personal opinion as well.
He also pointed out the difficulty in getting adequate accuracy with that type of warbow due to stability at full draw. He found his aim was shaky at full draw with his replica. Not everyone can get good accuracy with that heavy a bow.
Modern bow hunting regulations suggest a minimum of 40# draw weight at 28 inches which suggests this is considered adequate for deer at most eastern woods hunting ranges. 80# would be better only if you can get good consistent accuracy under hunting conditions.
The longer war bow would be less forgiving in snap shooting and shots at running game or firing on the move according to his testing.
The Comanche preferred a very short bow with short draw for its manuverability, especially on horseback, and rapidity of fire rather than maximum power.
Those indians prefering longer bows seldom used horses in war.
The long warbows of Indians in Florida could shoot through two layers of Spanish chain mail armor, so they definitely were very powerful bows.
On the other hand some east coast tribes often used very light bows around 25# draw in war, depending on pinpoint close range accuracy rather than power, Shooting armored men in the throat, though they did have more powerful bows for hunting. The arrow heads they used were tiny compared to others I've seen.
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He is also shooting European style.