Author Topic: Hysterisis and performance  (Read 25905 times)

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Offline Badger

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Hysterisis and performance
« on: October 18, 2014, 02:29:18 pm »
        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.

mikekeswick

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Re: Hysterisis and performance
« Reply #1 on: October 19, 2014, 04:47:44 am »
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.

Offline redhawk55

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Re: Hysterisis and performance
« Reply #2 on: October 19, 2014, 06:30:08 am »
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
..........the way of underdoing.............

Offline Badger

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Re: Hysterisis and performance
« Reply #3 on: October 19, 2014, 10:10:23 am »
      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.

Offline adb

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Re: Hysterisis and performance
« Reply #4 on: October 19, 2014, 10:34:31 am »
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
« Last Edit: October 19, 2014, 11:22:00 am by adb »

Offline PatM

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Re: Hysterisis and performance
« Reply #5 on: October 19, 2014, 08:02:07 pm »
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".
 

Offline Badger

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Re: Hysterisis and performance
« Reply #6 on: October 20, 2014, 04:08:23 pm »
  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.

Offline scp

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Re: Hysterisis and performance
« Reply #7 on: October 20, 2014, 06:03:04 pm »
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?

Offline Badger

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Re: Hysterisis and performance
« Reply #8 on: October 20, 2014, 06:10:47 pm »
    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.

Offline KS51

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Re: Hysterisis and performance
« Reply #9 on: October 20, 2014, 11:39:41 pm »
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

Offline Badger

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Re: Hysterisis and performance
« Reply #10 on: October 21, 2014, 03:07:35 am »
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.

Offline Buckeye Guy

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Re: Hysterisis and performance
« Reply #11 on: October 21, 2014, 08:31:50 am »
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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Offline adb

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Re: Hysterisis and performance
« Reply #12 on: October 21, 2014, 12:38:17 pm »
    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?


Offline Badger

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Re: Hysterisis and performance
« Reply #13 on: October 21, 2014, 01:12:06 pm »
     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.

Offline PatM

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Re: Hysterisis and performance
« Reply #14 on: October 21, 2014, 01:35:12 pm »
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.