Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on September 18, 2012, 09:24:42 pm
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For those of you with any interest in this aspect of wood bowmaking I thought I would start a thread on the various tests we might do on bows and wood to satisfy questions we might have regarding performance, use of different woods, and just anything else your imagination can come up with regarding bows.
A good example would be how much is the back stretching as oppposed to the belly compressing, or how much of the increase in draw weight of a bow is due to the wood compressing or just string angle. Tim Baker used to do wood bending tests on bows, he also used to add weight to the tips to see how much it affected performance. Anything you can think of please put it out there. Maybe we can figure out a test to satisfy the question. I have done a lot of my own testing and have to admit that proably 90% I would have to call non conclusive results for one reason or another usually relating to control over the test.
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There are two major impediments to this I can think of. First, as you mentioned, it's difficult to establish a control. Second, and more insidious, is that it's hard to establish a significant sample size from which to extrapolate results. It's easier when you're just doing bending tests with common woods because you can get ahold of 30-40 samples of similar enough wood and perform a test to get accurate measurements. But for more complicated issues of design, constructing such a large sample of *identical* bows is tough. And without the kind of rigor, you really don't know if what you're seeing is a real result or your own influence over the test.
It's a tough one. I am definitely interested in this problem though.
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Steve I believe that I read something some old fool cooked up called
"The Mass Principle" and believe it or not, it works ::) ::).
Seems like a feller that smart could figure out a method to test what you are after ;).
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I'm interested in hysterisis.
I have a plan to make a tri-lam with an end grain core....
Skateboards are made with end grain cores and somebody I was talking to said that it was for faster 'return to straight' speed.
I read a thread on PP that got me thinking about this one.
Steve what are your thoughts on this idea?
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Doesn't sound very scientific and I may be missing the point but i just make my "standard bow"
My standard is 60 ntn and 1/34 wide to 2 inch wide. I make the limbs a standard width for a certain length.
Then once the bow is on the board I just try to follow the no set mantra set out by your very clever self.
I will know that a "better" bow feels like in the hand with its weight and how it looses an arrow in comparion to my better "feeling" bows. This either works to a degree or like a previous ash bow it took too much set early on and rather than just lighten it up i will keep pushing it to see where it starts to fail. This will then let me know what the wood likes for the next one. If the bow has poor cast I look at the tips first then go though all the advice given by you lot. It does mean making my "standard" each time but it tells me a lot about the wood. That and if it makes it i get a bow out of it that i can give away or keep.
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Mike hysterisis is one of the few tests that I really felt gave me some valuable data. I had a hard time accepting the published data on how much hysterisis we could expect from a wood bow. I actually did a bit more testing that what I wrote about in the no set tillering thread. The concept of the test was to try and isolate the losses that occur just from bending the wood. So starting very early in the tillering process I not only measured the weight at various stages and how much it changed, I also took force draw curves and measured the bows efficiency with controlled chrono shots. The results of these tests were very dramatic and very conclusive even if they may have lacked some degree of precision. Wood has almost no hysterisis or at least very small until it starts to develop some memory of being bent. The amount of set a bow takes or does not take does not tell the whole story with hysterisis as wood can break down enough to where the back of the bow can actually pull it back into shape even though the compression side has been badly damaged. Monitoring the efficiency and stored energy throughout the process of tillering gives us a pretty dramatic and sometimes disheartening look at what we might be doing to the wood.
As for using the end grain in the core, I think this would be an excellent thing to test as cores are often debated. The common logic is that cores only seperate the compression and tension working part of the limb so the lighter the better. I have usually gone with this logic but have seen some good examples of purple heart cores and ipe cored bows performing very well., We often hear that most woods are 3 or 4 times stronger in tension than compression. Stronger usually means resists bending. The way I see it is that the neutral plane would find it self much closer to the tension side of a bow so we might have three times as much wood in compression as we do in tension kind of equalling them out. I suspect that bending wood in an edge grain layout would add quite a bit to the compression strength and move the neutral plance closer to the belly possibly allowing for a lower mass limb. At any rate I think it would be a doable test and worthwhile. Now we just have to decide how to do it.
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I do love threads like this. Cant wait to see what you guys come up with so I can unashamedly leech of the information.
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I haven't got time for a longer reply at the moment but just to say i'll be gluing up the end grain core bow today/tomorrow. I suppose i'll have to make an identical bow with a standard core/no core for a 'direct' comparison.
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Awesome thread! I am new to bow building...I am a high school physics teacher and am exploring a completely different approach to analyzing and predicting breaking points of bows...one thing that many people have already noticed is width of bows...well, it turns out, that a bow limb acts like a cantilever beam (for fixed handle bows--not working the d bows yet) so I looked up some engineering equations...again, something you all know but I had to find out--cross-sectional area is by far the biggest indicator of breaking. Now people on this board also know that depth, from back to belly, is a bigger driver than width. That comes from the equation for the moment of inertia of a beam...1/8 of an inch removed from the belly can have DRAMATIC impact on draw strength!
But you all know that, I am learning that stuff. I am currently developing a MS Excel tool to help predict breaking points...it's been a challenge! I haven't written a calculus statement in ages!
Another interesting thing I've read on these board--tension/ tensile/compression of a wood. Well, there's a thing called Young's Modulus or Young's Elasticity--you can look 'em up per type of wood. Hickory is 1.4-1.56 Giga-Pascals....
Since I have no experience with making a bow other than breaking my first 4 (I'm on # 5 now) I have to go the physics teacher/ engineer geek route...sorry :-[
There's another interesting thing...a limb that bends like a parabola or one that bends like an ellipse--very very different!
I'll keep ya posted on how that spreadsheet is coming along...
Cheers!
Brian
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I'll just throw this one out there.......Limb harmonics.
I'm very interested in limb harmonics. I wonder if there is a way to test how limb design, tiller profile and/or the properties of wood effect limb harmonics, and if there is the perfect harmonic to match a specific design.
By minimizing the dampening of a harmonic you are necessarily minimizing hysteresis right? It seems like all the things that dampen a harmonic (internal friction, too much tip mass etc) are the things that make a bow slower.
I'd to learn more about the relationship between harmonics, hysteresis, wood proprieties and limb/tiller design.
Another idea I like about harmonics is that it can be intuitive. Its not something that has to be determined just by computation. You might be able to 'feel' and even 'hear' the difference between harmonics in different bows, and that can inform our design.
Gabe
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By the way, here's a great article by Dick Baugh on hysteresis.
http://www.primitiveways.com/Bow_and_Arrow_Efficiency.pdf
Gabe
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By the way, here's a great article by Dick Baugh on hysteresis.
http://www.primitiveways.com/Bow_and_Arrow_Efficiency.pdf
Gabe
Interesting article. I didn't read it all but I didn't see any allowances made for air friction or the force of gravity on the limb when measuring the recoil.
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Yeah he discussed both, but not in detail.
"If there is air resistance,internal friction in the spring" (bow limb), "immersion in molasses or anything else you can dream of that tends to damp the motion then amplitude of the back and forth motion will get smaller and smaller with time."
"Depending on the stiffness of the sample and how heavy the weight, gravity will deflect the tip downward by a certain amount after the weight is attached to the tip. If the weight added is small then deflection is small and the tip oscillates up and down rapidly. If the weight added is relatively large then deflection is large and the tip oscillates up and down more slowly. All we need to know in order to calculate the time taken for one is how far gravity acting on the added weight deflects the tip."
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Woah some serious physics talk going on in here!
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The physics teacher, as well as Gabe, both touched on the subject of tiller shape and it's relationship to limb design. Is there a quantifiable difference in performance moving from limbs that bend circular vs limbs that bend with more of an ellipse. If so, does it matter where along the limb, on elliptical tiller, the bend begins. For example, longer bows with the bend starting outboard vs molly style bows with the bend starting in board and fading to the levers. For bows of equal length and design, is one profile advantageous to the other.
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That article by Dick Baugh is what inspired me to test out the no set tillering theory. Aside from going to designs such as Mark St Louis in known for which really have to be disected on their own, My findings are similar to Dicks but I feel I tested it out in a more applicable manner.
Using a shooting machine and fast flight strings for uniform testing purposes I find with rare exceptions a self bow will exceed about 176 fps. These are bows with some reflex, either flipped tips or slight d/r. Backed bows will rarely exceed 186 fps. I consider anything around 170 fps for a self bow or 180 fps for a backed bow to be excelent performance.
As for the Hysterisis tests done with a working bow. I can't attribute 100% of the gains on a no set bow to removing hysterisis but I know it is a large portion of it. I can seldom get a normal sized bow past about 23 or 24" draw without at least some breakdown of the wood. On one occassion I was able to get one out to 26 3/4" before the first hint of weight loss took place, this particular bow was my personnal fastest at 192 fps, after a slight overdraw the speed was reduced to a respectable 184 at 28" draw 10 grains per pound. This particular bow was backed.
As for various designs, I can't really see a lot of difference. Some designs will slightly favor heavier arrows while others might favor lighter arrows but when properly executed and backset being about the same most designs are very close. Always a bonus for skinny tips.
My testing shows a dramatic pattern where the tiller shape of the bow should match the front view of the bow. And both of these factors need to be decided on based on a reasonable mass figure. In other words, a red cedar deep section elb 66" long would not have the same tiller shape as a 66" long bamboo backed ipe bow. When the density of wood is so great that a bow cannot be made any narrower then we have to start pushing the bend on the tiller out away from the fades to justify the mass.
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Steve - "When the density of wood is so great that a bow cannot be made any narrower then we have to start pushing the bend on the tiller out away from the fades to justify the mass."
That right there is a helluva statement. Really interesting.
So, is the 'ideal' amount of bending limb a function of wood density? I mean, taking an idea to its extreme:
If a bow wood was extremely light, but also had extremely high elasticity, would 3 inches of bending limb and 23 inch narrow static levers be faster than a bow with 13 inches of bending limb and 13 inch static levers? I know there are other things going on here than mass placement.... just thinking on e-paper :)
Gabe
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Very cool conversation here! I love that I can totally roll this into my physics classes :D
On the ellipse versus parabola: My guess is that there is a quantifiable difference...but that is what I am trying to determine in my little spreadsheet tool! I'll keep ya posted.
On hysteresis...cross-sectional area would definitely have an impact on hysteresis. If energy loss is due to heat, and heat is caused by molecules moving against each other causing friction, then more molecules would equal more friction, right? Maybe...if I were a guessing man, I'd guess that histeresis is closely correlated with bow mass...
This is an excellent discussion, by geek brain is loving it!
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I think the development of hysteresis is more related to elasticity than mass.
Steve - "Wood has almost no hysterisis or at least very small until it starts to develop some memory of being bent."
And memory of being bent is a function of elasticity.
I could be wrong about this.
Gabe
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Gabe, I agree with you. If a wood has more elasticity it can be built at lower mass. Most woods are pretty close in elasticity. I don't know the numbers by heart but I do know that both yew and osage excel in being more elastic. Plum and ocean spray are also good examples of exceptional woods. Mass is proably the easiest thing for us to measure so working with averages it can give us at least a guideline until we get to know a specific type of wood.
If we took a piece of ipe at .9sg and built a bow with the same design using say maple with a .6 sg. we would have to make the maple bow about 50% wider and the same thickness if the elasticity were the same. If we tested the wood and found one to be more elastic than the other we could make that bow narrower and deeper with lower mass. I based the mass theory on the results I got from avrage woods. Primarily I was using hickory, osage, maple, oak, ipe and yew for the bulk of the bows tested with a good smattering of other types of wood I was able to get ahold of.
As for the example you gave earlier. about one with long stiff limbs and another with more working limb would be hard to call because it wouldn't be possible with wood unless it was extremely wide but. Shorter working areas tend to be more efficient because they don't vibrate. If the limbs were very light and bending right next to the fade you would have the best of both worlds so it probably would be faster.
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Hi Steve! I have done some thinking and reading since I posted about this sort of thing on the other site.
First, you were right about raw strength not illuminating how the back of the bow behaves during the draw and loose. Turns out, a tension break in wood is in the category of a "brittle" rupture like glass and ice--there is no stretch before failure.
What this means is that I was off the mark in thinking that narrowing the back would make the back work more. You were right that it seems only to reduce unneeded weight in the limbs--a useful outcome.
It also means, that the belly is always doing all the work of being a spring. That may figure on that other subject of whether a belly should be flat.
On the subject of an end grain core, the Forest Service Wood Products Laboratory data shows that hickory, hornbeam and locust (the only woods I took a quick look at) are about 5 times stronger in compression parallel to the grain as 90 degrees to the grain. (Yew is about 4 times stronger parallel to the grain.
Another strength factor I have pretty much ignored is the shear strength parallel to the grain. Don't know if there is some yield there before rupture. We don't find bows failing in shear very often, but if there is some distortion short of rupture, that could also be part of the energy storage equation.
Nice thread.
Jim Davis
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Thanks Jim, good topic on the shear forces. I am with you on that. I feel they are more important than we give them credit for. I have had several yew bows fail in shear over the years. I think shear forces help to explain why laminated wood bows will usually outshoot self bows. Lately I have been rethinking the concept of cores being just a seperator and the lighter the better. They may have more effect on sheer and the way the belly compresses than we give it credit for.
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About why narrow tips/nocks are fast..
Was just thinking that the aerodynamics is also better on narrow nocks. Don't really know how fast the nocks move when you shoot.
I actually think that contributes more why narrow nocks are faster on a bow than the lowweight explanation!
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TBod, the air resisitance doesn't really account for much loss. The tips are going less than 1/2 the speed of the arrow and most of that speed is gained toward the end of the shot. But no doubt it does acount for some.
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Ok, but the air resistance during acceleration increases exponentially. Maybe I'll do some tests not totally convinced. Thanks!
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Badger
Thank you for sharing your knowledge !
I am glad you have the words to explain these things to others !
Your knowledge confirms many of the things I feel and believe to be true but don't have the words to express or any means to prove!
Your work will fuel many generations to come !
Thank you !!
Guy
When you get bored with what is going on in the bow consider the arrow ,just a stick ,or is it ?