Primitive Archer
Main Discussion Area => Bows => Flight Bows => Topic started by: Badger on October 18, 2014, 02:29:18 pm
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The goal here is measure something that is invisible, to create a picture of something that cannot be photographed using evidence based on a series of tests. My personal goal is to do the work in the backroom so to speak, testing, crunching numbers, developing tests etc. that can be easily converted to workbench techniques requiring nothing more than good work habits and some understanding of theory.
I am looking at ways to identify and minimize losses in efficiency of wood bows due to hysteresis. What we will not deal with here is bow design, excess mass, strings or any other factor that reduces efficiency. We are only dealing with hysteresis.
For some time my hypothesis on hysteresis has been counter to many of the experts who have chosen to simply accept that hysteresis is inherent in wood and that wood can never measure up to synthetic materials. I refuse to accept this and have significant evidence that the great majority of hysteresis present in wood bows is due to damage done to the bow during the tillering and shooting in process. Thousands of bows built and countless of hours of seemingly fruitless testing have in recent years started to produce undeniable patterns that have led me to a series of tests that could verify my suspicions.
The only way I can figure to make this post of any value to anyone is to explain the nature and logic behind the various test procedures.
Measuring stored energy in a bow:
With a fiberglass bow this pretty straightforward we draw the bow back slowly and measure the draw force in 1” increments, we might even slowly lower the bow back down re-measuring each increment to see if it recorded the same on the return stroke. We total up all these measurements for a stored energy value of inch pounds; we divide that by 12 and convert it to foot pounds of stored energy.
Now on a wood bow it is not so simple, we often do use the same procedure but we get a false reading because of hysteresis. To understand hysteresis in this context we need to think of a bow that was just unstrung, let’s say you took a measurement within 1 second of unstringing and found you had 2” of set, you took another measurement 2 seconds later and it was now 1 ¾”, 10 seconds later it might be 1 ½” now 10 min later it might go to 1 ¼” and after 2 hours may settle in at 1” set. After a few days it may even pick up another ¼”. The limbs never stopped moving they just slowed down dramatically as time went by.
What I have essentially done is to find a way to take this measurement into the bow while it is strung and actually into the shot itself using performance based evidence and comparisons as proof of theory instead of a tape measure.
My method of measuring a wood bows stored energy involves using the virtual mass of the bow. Depending on the length and design of a bow most bows will have a virtual mass of about 200 grains. I will explain virtual mass here and why it is so important to these calculations.
Virtual mass is the difference in the amount of weight an arrow would have to be to shoot at exactly the same speed as a tested arrow and be 100% efficient. An example would go like this, a stored energy measurement shows a bow stores 50# of potential energy. We shoot a 500 grain arrow through a chrono and find it is shooting at 188 fps. This represents about 39ft#’s of energy. If the bow shot a 640 grain arrow at 188fps it would represent about 50 ft#’s and be 100% efficient. I subtract 500grains from 640 grains and I get a virtual mass of 140 grains. If no hysteresis is present this 140 grains will stay relatively consistent regardless of the arrow weight I shoot and how fast the arrow is going. If hysteresis is present the faster the arrow goes (lighter arrows) the higher calculation I will have for virtual mass.
Because hysteresis is a bi-product of speed by measuring speed of various arrow weights I can track the presence and effects of it fairly accurately.
Back to measuring stored energy in a wood bow. Now that we understand virtual mass I will explain how I use it to more accurately weight the potential energy with hysteresis present. I do my first shot through the chrono with as heavy an arrow as possible. Because this arrow is moving so much slower I will minimize the hysteresis in the shot. The arrow I chose is 4,000 grains. Most of us understand that heavier arrows are more efficient, by going to an extreme here my 200 grains of virtual mass only account for about 5% of the total energy in the shot so I should produce a shot reflecting about 95% of my stored energy, if I have 5% hysteresis it will reflect about 90% of my stored energy.
As I continue to shoot lighter and lighter arrows though the chrono and calculating virtual mass after each shot I will see a pattern develop. If my virtual mass stays relatively consistent I know I have a bow with low hysteresis, if the virtual mass continues to climb with each lighter arrow I will be able to calculate my actual losses due to hysteresis in real time speeds.
Just having this knowledge does me little good if I can’t do anything about it, it does let me know how much room for improvement I have. This is where no set tillering comes in.
No set tillering is simply a method of monitoring the condition of the wood throughout the tillering process. Very easy, a little tedious at first but low tech. If the wood is starting to break down during the tillering process the first indication of this will be a weakening of the woods resistance to bending. An easy way to track this is to carefully measure the weight of the bow at a specific draw length early in the building process. I usually take my first measurement at around 14” draw. Lets say I get a reading of 20#@14”. Each 1” further I draw the bow I will always go back to my 14” measurement to see if it has changed. If it has changed it means the wood is starting to breakdown, if it has not changed I can feel safe to continue drawing further 1” at a time rechecking after each additional draw. Depending on how your tillering is advancing you can make decisions on how you want to continue, you may want to get a little more limb working in some areas or you may want to start narrowing areas as you approach final draw. The condition of the wood will steer you in the right direction as long as you are monitoring it.
Some guys have a natural gift and do exactly this without even being aware of it, some of us don’t have that gift so a little knowledge can be one more tool in our tool box. If you successfully manage to bring a bow out to full draw with no loss of weight you will be amazed at the performance and how close it can compare with a well made class bow of similar design. You may also decide that you might want to start making the working parts of your bow considerably wider once you realize how much you are loosing. Once I started doing this I started making my bellies flatter and less rounded, I also increased the width of my bows considerably and surprisingly enough I added no mass! Dead wood is dead weight!
I know I left some things out so feel free to ask any questions.
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Can I draw from that that you have found more 'overbuilt' (wider/thinner) bows are showing less hysterisis and you conclude this is due to them being less strained overall or just that they have been less strained during (careful!) tillering allowing you to get to full draw with less set.
Is hystersis equivalent to internal friction between the wood cells.
hys·ter·e·sis (hst-rss)
n. pl. hys·ter·e·ses (-sz)
The lagging of an effect behind its cause, as when the change in magnetism of a body lags behind changes in the magnetic field.
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Great post, Steve.
I understand some bits more, above all in calculating the determing factors.
But I don't understand the sense of shooting heavy- light arrows for to check hysteresis in the working parts of the bow.
Hysteresis is what happens in the bow, so wouldn't it be enough to draw the bow on the tiller board, the way you call positive tiller?
This part of your post is very clear, I do it the same way and tell it again and again when it comes to tiller the bows in my workshops. No set tillering is the key to a great bow.
Actually I'm experimenting with woods grown in a height of about 1000m in very shady places, there are some elms, mountain- maples, white beams and rowans. Woods grown there have to stand heavy storms, they seem to be more dense. I guess density means less hysteresis.
Michael
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Michael, the thing about the extremely heavy arrow all the way down to extremely light arrows just demostrates that the hysterisis is just a product of speed. Instead of the set being the 1 or 2" that we see it might be going 7" or 8" into the drawn bow but recovers too fast to be seen but not fast enough to avoid detection with certain methods of tracking. A very heavy arrow shooting at 70 fps enjoys a lot of the recovery from overstrained limbs where a very light arrow traveling at 220 fps leaves the bow before the recovery takes place. Kind of hard to imagine but working with a virtual mass product durring the testing it clearly demonstrates this.
The best way to clearly demonstrate this would be with one of the new testing machines that with one draw and let down it gives an accurate force draw curve. I am certain that if I had one of these testing machines that it would show slightly lower return stroke energy the faster I let it return.
I used the unbraced profile of the bow because at this point the slowness of the limbs recovery becomes much more pronounced but it still clearly starts to demonstrate that the closer you get to the unbraced profile the slower the recovery is. It might take 2 or 3 days for the limbs to completely stop recovering in the final 1/16" of an inch. In the first micro seconds after unbracing the recovery is much much faster but still slower than the arrow leaving the bow.
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Interesting post Steve. I enjoy your willingness to share your thoughts on performance.
I agree with you about wood/FG bows. A well made wood bow is every bit a match of a FG bow.
I also agree with your low set tillering. My bows have taken much less set in the last couple of years, because I tiller differently, and pay more attention to it during the process. I've also noticed that slightly wider and flatter limbed bows will tiller with less set, and I've been designing accordingly. FG bows are made with wide, flat and thin limbs, and there's a reason for that.
I also pay much more attention to where my bow's limbs are taking set. In the past, I've had decently tillered bows, but they've taken a lot of set in a small area of the limb. I want the set to be much more even the entire length of the limb, and then there's less of it. It's basic bow making I know, but not so easy to pull off in real life. I've been paying much more attention to set, where it's happening, and when. It will tell you a lot about your tillering, if you listen. One other thing I've noticed over the years... it seems wood will willingly bend to around 24" with little ill effect. Once you go past this point in tillering is where set starts to take place.
As far as hysteresis goes, I know some woods are better than others. I believe cherry has very low hysteresis, but it's not so easy to get a bow from it. I think the amount of hysteresis is fixed in the material, and it's our job as bowyers to minimize its effect on performance. I also don't think the hysteresis changes when we make a bow. It's inherent in the material, and in designing a bow, we can minimize its effect if we design accordingly. JMO. :P
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I'm still skeptical about this line of reasoning.
I think records are still set with bows that are "shot-in" but not "shot out".
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The biggest thing here I was kind of excited about was that I feel like for the first time I am able to actually evaluate a process with some accuracy simply by comparing measurements we have been using for years. We have never been able to identify with any accuracy real efficiencies of bows because we couldn't quantify the hysterisis factor. On bows with very low hysterisis all the measurements worked out pretty good, but great designs that were suffering from hysterisis in the wood gave faulty results if evaluating the designs, mass, efficiency etc as individual componets. I plan to play with this a little more.
It is hard trying to explain the concept of the process just because so few people use those concepts when measuring things, a lot of the process explanation gets watered down and drug out because it is neccessary to explain all the steps in order for it to make any sense.
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Badger, I'm not even sure whether you are talking about hysteresis or limb vibration. Is there any way you could post some diagrams or pictures?
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I don't think anyone knows what I am talking about. The vibration would go more with the virtual mass, the hysterisis is more just internal losses within the wood. I don't know how you could illustrate it. For the most part it is useless information to about 99.99999% of the population. I find it cool only because I have been chasing it for so long.
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Badger, I've been mulling over what you're talking about and I think there may be 2 effects that you'd be actually measuring. The first is the reaction speed of the wood. Basically, the energy stored can only flow through the limb to the string and then into the arrow. There is a minmum amount of time for the energy to flow through the path. As you lighten the arrow, you eventually reach the minimum reaction time for the 3 materials ( bow, string, arrow) and by default the max affective arrow speed. The other internal energy loss due to the internal "friction" of the materials is also measured. I think this internal energy loss is the true hysteresis.
I do think there are some interesting variations of your experiment with different types of arrow material, string material, and species of wood that could lead to an extrapolation for your mass principle, to give a starting point for the limb width. But it would require that the reaction speed of the wood (by species or SG) be known. It would be nice if it could be SG related, but I suspect it is a combination of SG and basic wood structure ( ring porous, diffuse porous, etc).
Enigineering hat on - a taguchi analysis structured experiment could shed some light on this if the under-lying variables are understood.
Ken
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Ken, reaction speed is kind of a false term, everything less than 100% is hysterisis. The more you slow down the limbs with heavy arrows the less you will experience. You will also naturally loose some power to vibration. The idea here is to be able to isolate one from the other by using virtual mass and a better starting point.
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I believe that you are on to at least a good part of the story and will be watching and enjoying your posts here
I do feel the wood types will change they-er pick up point for Hysteresis at different poundages
and most wood really does not like to be much over 50 lbs as well as it does under 50lbs
sorry if that makes no sense this backwoods boy ain't so good with words but does have a good "feel" for the things going on in his bow
Guy
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I don't think anyone knows what I am talking about. The vibration would go more with the virtual mass, the hysterisis is more just internal losses within the wood. I don't know how you could illustrate it. For the most part it is useless information to about 99.99999% of the population. I find it cool only because I have been chasing it for so long.
I don't think that's true, Steve. I think lots of people, and some right here on this forum, understand what you're talking about. I think hysteresis is inherent in wood, and different species will have different values. However, I don't think it has as great a significance as you give it. The last piece of the puzzle perhaps, but a much smaller piece than you think IMHO. You say you've been chasing this for years, but do you think perhaps you're chasing your tail a bit? And I don't mean that sarcastically, or in a facetious manner. I believe a well designed and well built bow (with no set, and low mass) and a small diameter string is far more important.
Perhaps the only way to see any meaningful results, would be to build a bunch of bows EXACTLY the same from different wood, and then test their performance. Same design, same draw weight, same mass, etc. Eliminate all other factors, and see what happens. And isn't that what takes place every year on the salt flats anyway? Even then, wood is not that consistent, even from the same piece. I'm just wondering if the Asians, who were shooting arrows 1000 yards many years ago knew about, or much less cared about, hysteresis?
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Adam, when I say I don't think anyone understands I am not implicating a lack of ability on anyones part other than my own for not properly communicating. I feel a certain test protocal does clearly demonstrate that a high level of hysterisis is not inherant in wood. We have plenty of example made by most of us here on PA that clearly show the difference between a high hysterisis bow and a low one.
Chaseing my Tail, yep I will own that one. I do tend to get hung up on things. I feel satisfied that I kind of cracked the code on this one though so don't plan to chase my tail here much more.
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Agree with Adam.
I just can't see it being the great difference maker.
The pictures of guys from the past show them shooting bows that had to be loaded with hysteresis to the max.
They just drew them farther to offset that and make the wood pay for it by contributing more total stored energy.
Take your record setting bow and draw it 2-3 inches farther. ;)
There are no prizes for" freshest bow" at the end of the day.
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Pat, when I comes to flight shooting their really are prizes for the freshest bow of the day. The old bows you mentioned that must be loaded with hysterisis. Not so! That is the biggest place they had us beat! They definitely had it under control. You cannot get high dry fire speeds from bow with high hysterisis no matter how far you draw them.
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I disagree. You can see the pictures of the bows being drawn. They have to be in that zone.
One of the bows that set a record was crafted from a shortened broken York longbow. Hard to break a bow without stressing it to that point.
Some of the bows also shot record type distances for years or set records after being shot in damper conditions and then shot again in dry conditions.
These guys probably never heard the word.
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First thing first. We need a graph of whatever that is being measured. Is it two-dimensional, three-dimensional, or even four-dimensinal?
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SCP, laying it out is where I feel like I failed. I was trying to condense it the best I could because it was too lengthy as it is. Each componet being measured requires almost a page of explanation.
Example. The first step is measuring the force draw curve of the bow. We all do this the same way. The fact is that if a bow with no hysterisis or very low hysterisis is measured it is pretty accurate, but if a bow has hysterisis it is a false measurement because we can't measure a force draw fast enough to keep up with the hysterisis factor. So all we get is a measure of how much work we are doing, it has very little relation to how much potential energy we actually have. This is the starting point for all the testing so if we don't find a more accurate method here everything downhill from this measurement is junk. Not having gone to school for any of this stuff I don't have the right terms to use. But starting at this point in the begaining if we don't identify the actual force going back into the arrow it will get lumped in with the virtual mass and thow everything else off.
Pat, the bows you are talking about doing so well are indeed low in hysterisis. The builder has no need to know anything about hysterisis if he knows what a good bow feels like. Successful flight shooters in the past were very aware of avoiding breaking down their bows. What I am talking about here in my thread is only useful for developing strategies to build bows, it just offers a way to prove certain theories that have been floating around for the past few hundred years.
The current record holding bows in the 50# class were all shooting in the high 220's maybe low 230's. We have bows now that are shooting in the high 250's and low 260's. Only because more attention was paid to hysterisis. Once I get the proper arrows to match thse bows I can back that claim up. The 50# records should all be over 400 yards , at present none of them are.
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I really doubt they coddled them like that. They shot them until they broke down to the point of not being competitive.
The goal was maximal distance, nothing else.
I think you mentioned the bow that you gave the girl which set a record was well shot in.
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Pat, actually I would like to see the term shot in become a thing of the past. If we get lucky and build one well inside its stress limits we can shoot it all we want and it never changes. My broadhead record holder I have been shooting for 5 or 6 years now, every year I have to scrape a pound or two off of it as it seasons and it keeps gaining speed. I don't likee the term a fast bow is 90% broke, I use to believe that. Now I figure a good bow is no where close to broke. My crude backyard testing shows that the 80% strain recomended by quite a few physics guys is actually overstrained because the set starts happening before that.
I see bows right here at PA all the time that would do very well in flight shooting.
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I was enjoying this till you started using math. ??? :P
Wayne
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But they were shot in.
The current record is still Harry Drake's by my reckoning and that bow dropped about 28 pounds while setting the record.
I'm still wondering why people are defiantly avoiding making similar bows to the style that worked far better back then.
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Pat, is that a regular flight bow or is it the yew bow he used in the complex composite? I would like to know more about that bow if you have information
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It was apparently just a medium length Yew recurve around 60 inches long. Very narrow and thick and pulling 87 pounds according to Dan Perry.
Before unidirectional glass came into use Drake made bows with toxhorn bellies and woven glass backings which showed set after use.
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That might not have qualified for the present primitive classes because of the arrow rests and overdraws. I wish I could find more info on that bow. I rememeber Dan saying that Harry felt that the Yew on that bow was the best he had ever seen. I would have been interesting to see how the arrows performed as the bow was being shot in. I think that bow shot 603 yards. Don Brown currently holds the unlimited complex composite record with 619 yards, not sure if Harry built that bow or Don built it. They never listed the bowyers only the shooters.
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They were both built by Harry. The yew bow shot 544 according to most sources.
However many of the old time bows were not shot with an overdraw. that seemed to be a later feature.
The picture of Murray Yantis shooting well over 400 yards(I think 466) shows no evidence of an overdraw and he has the long arrow cranked past his jaw.
PS Are you including sinew backed and simple backed composites in this discussion?
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As long as they are natural materials. I know they had several guys shooting bows over 400 yards in the 60 and 70 pound range. I am not sure if they were all natural or not. I really don't think it would have made a lot of difference.
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Steve,
Your opening post on this thread is excellent! I always viewed hysteresis as a fixed loss that I had no control over, so I never bothered giving it much more thought. But you have put together a really novel idea, to use hysteresis and virtual mass as a tool to explain the health of the materials in the bow. The challenge is being able to easily measure it. I wish I had a draw board that would measure the force draw curve of a bow in as little as a few seconds. Wouldn't that be nice to have?
Alan
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Allen, It would be nice. That was the reason I chose to use an extemely heavy arrow and just measure the speed instead of plotting the curve. Plotting the curve doesn't pick up on much of the hysterisis. The part I avoided going into here involves juggling a few percentage points at the start and finding where the virtual mass seems to either hold at the most stable point or change in a more linear fashion. Using the heavy arrow to figure stored energy I am forced to estimate somewhat the efficiency but because of the weight I am only working around a couple of percentage points.
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Did you know this guy's tests?
http://goldenhordebows.blogspot.com/2013/07/hysteresis-tests-part-2-wooden-and.html?q=draw+return
he did some videos too(bit chaotic):
http://www.youtube.com/watch?v=HcSJTObuMV0
With his tests in mind I would conclude time has a much more impact on hysteresis than I thought.
Michael
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I've been reading this Steve and I just don't know. Truth is I've never given hysteresis much thought and more or less equating it to internal friction. I've always viewed wood bows in the same sense as Ken wrote with it working as a function of reaction speed and this reaction speed being influenced by several factors, moisture content being one.
In your tests did you consider cell memory?
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Mark, moisture is a big factor in hysterisis, one of the biggest. Some bows have considerable set yet very low hysterisis. The main thing I look for are ways I can just recognise something at the workbench by feel or behavior. The technical testing hopefully gives me a better idea what I will be looking for. Far too much trouble to be considered a routine test.
The cell memory id kind of interesting, picking up old bows that have set for several years it seems like some of them have "heeled" up a bit. Could very well be moisture levels dropping, not sure at this point but something else to look at.
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But if a bow has set can it still be fresh? Seems like a bit of a contradiction.
Wood definitely 'heals" to the degree that the cells also creep back to an expanded position over time. Not to the degree that they are in original condition but if you have a great performing bow it makes sense that you can rest it and quickly string it up and shoot it before is has time to sag again.
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Pat, I have seen bows with amost no set be loaded with broken wood cells and bows with 2" of set feel fresh as a daisy. You can feel it by pushing backward on some bows. On a real badly broken down bow I suspect that the tension wood is just pulling it back into shape. Not all set is the same.
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If a particular species has more hysteresis than others wouldn't it have gained a reputation as sluggish by now, at least in the flight bow world?
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Pat, I have seen bows with amost no set be loaded with broken wood cells and bows with 2" of set feel fresh as a daisy. You can feel it by pushing backward on some bows. On a real badly broken down bow I suspect that the tension wood is just pulling it back into shape. Not all set is the same.
I know that but why do people espouse a no-set tillering method if it doesn't give a reliable indication of what is going on within the stave?
I know you measure weight but many people go by visual indications of set.
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DC, in the flightbow world hickory, osage and yew are about all I see. Hickory excels in a dry climate. One of the reasons I came up with the posted formula was to help myself identify woods that had better flight potential. ost woods are a lot more similar than they are different if properly dried and seasoned. Cherry and black locust are two woods that should excel if designed properly. A lot of woods that tend to chrysal are also low in hysterisis.
Pat, The no set tillering method is fairly reliable it tends to err on the side of caution. Some woods can take set and still be relatively low in hysterisis, other woods can take very little and the hysterisis will go through the roof. No set tillering will not seperate between these two if you don't apply a sense of feel to floor tillering along with it. I would say it is about 80% reliable though if I just took a stab at the odds.
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Steve,
I dodged the hysterisis bullet for some years now but this topic is great fun.
Back when I was on the university studying wood and natural fiber technologies I ran some tests on 127 different yew heartwood samples to check out their quality for bowmaking. I used European mountain Yew and Pacific yew from Orting Washington.
I was searching for signs of the perfect bow wood and how to separate the good from the bad and the worst. I took X-Ray pictures, tested them for their mechanical properties and examined them under the microscope.
Hysterisis was the last thing on my mind but as I was reading this post of yours, the gears in my head started moving.
Some bow woods have a predetermined amount of hysterisis or better called inner friction due to their cellular structure; even zero set tillering can’t change it, but will reduce the amount of inner friction, for some time. The more you shoot the bow the higher the inner friction will become. For example Black Locust has great potential but will be worn out after some time.
Ash and Elm work great at temperatures below freezing and in very dry conditions but will also have higher inner friction due to their cellular structure and low compression strength.
Osage, Hickory and Yew have one thing in common, great cellular structure for saving and releasing energy, while having great compression strength.
What I learned from checking out all the different yew samples is that looks can be deceiving. Even the best looking yew samples with tight rings where not the best for making bows. Most of them showed more inner friction than anticipated.
The density of those samples, the ratio of early wood and late wood and even the color could have fooled any bowyer.
My goal was to clearly say which stave is good for making great flight bows and which staves are good for normal hunting bows. But in this point I clearly failed. Only one out of 50 different yew staves really had the potential of having an outstanding bow hidden inside. But only from the looks of it, there was no way I would have known.
As I was writing on the start of the post, some bow woods have predetermined inner friction but what I learned from the yew samples was that the differences between samples from the same species could be even greater than the interspecies differences.
At first I was really disappointed with my results. Before making the tests I already knew how good bow wood looks like and after the tests I was none the wiser. But now after some years I am really satisfied with the outcome of my studies. I wanted to find some kind of rule to go after when choosing wood for different tasks but found out that there are still some surprises Mother Nature has for us when making a bow.
Steve, you are trying to prove the influence of hysterisis and inner friction on the performance of wood bows which is in my eyes a very important task. If you have bows that are out of the ordinary, extra fast or for their design and wood quality too slow you want to know the reasons for their performance. Was it exceptional craftsmanship? The best bow wood? The particular design? In some cases the questions are still unanswered and this is where your hysterisis measuring method comes to use.
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Mat, those were some great insights. I was particularly interested in you rcomments about yew and they agree with my personnal experience. I have some beautiful yew bows at home which came out nearly perfect but just don't perform at the level I would have suspected they would. In all honesty and not being humble I cannot attribute any exceptional results I may stumble on to now and then on my exceptional craftsmanship. I don't rate myself very high in craftsmanship. I try to stick to certain rules I have found that tend to give me better performance.
I find a certain amount of hysterisis is ineveitable but it is much lower than we have previously thought and more avoidable. Strong evidence suggests it increases as the wood fatigues and this process starts much earlier in the bending process than I had previously expected. Testing hysterisis by bending the wood over a radius as opposed to just bending the wood with no protection will give a good insight in to how quickly it can start to set in. Decreasing the size of the radius we bend the wood over durring a test I feel is a good way to measure.