Primitive Archer
Main Discussion Area => Bows => Topic started by: Frodolf on September 17, 2016, 04:30:18 pm
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This time of year I always have a lot of issues with chrysals. I guess the average humidity is higher leading to lessened compressive strength in the wood. This regardless of what wood I'm working with. But there are a few things I've never fully understood with chrysals, and I was wondering if you guys could help me out.
First of all, I realize of course that chrysals come from the belly being over-strained, for various reasons. Many of the bows that have developed chrysals lately have been sort of high-crowned flat bows. But the strange thing is that the chrysals show up on the belly close the the edge of the limb, not under in the middle of the limb, right under the crown, where the bow is thickest and thus where more compressive strain occurs. Why the edge?
Secondly, I've never really found much of a correlation between chrysals and draw weight. Logically, a higher draw-weight means more compression strain and therefore higher likelihood of the bow chrysaling. But I've had several examples where this doesn't seem to apply. A year ago or so I had a (well-tillered) kid's bows chrysal but (well-tillered) 70#-ers come out nicely – made from twin staves! Two moths ago, I made a 50# hazel flat bow, 28" draw, 68" ntn, 1,75" wide at the fades, 3" of reflex at the start. Last week I made virtually an identical bow, but 2" wide to compensate for the slightly higher humidity, and a draw weight at 35#. That bow chrysalled. It's not like it was sahara dry in june and amazon wet i september, the increase in humidity isn't THAT great. Sure, different individual trees have different properties, and the chrysaling can probably be accredited to that, but the point is that I can't see a clear correlation between draw weight and chrysals even if I put all my years of bow making into it. Anyone else with similar experiences?
Thirdly, flat bellies are supposed to be stronger in compression, less likely to develop chrysals. But I can't find a clear correlation between flatter bellies and less chrysals. Lat winter I made two juniper ELB's. One at 40# and one at 75#. The 75#-er had a slightly rounded belly, the 40#-er had a flatter belly. The latter developed chrysals around a knot – the 75#-er (which had more knots too btw) turned out great and healthy. Again, different individual trees have different properties, but on average I've never been able to find much of a correlation here.
Sometimes the Idea pops up in my head that there is something wrong with the whole explanation about why bows sometimes develop chrysals. Or that I have misunderstood something vital. As if Baker and Hamm and the crew were withholding some awful truth, something conspiratorial, like chrysals develop more or less depending on what kind of shoes you wear. Or chrysals develop more the farther away you are from a beer. Or something.
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Why the edge?
good question, possibly Poisson effect? perhaps you could compare the belly "crown" or flatness at brace to full draw?
Or chrysals develop more the farther away you are from a beer. Or something.
LOL.
I have always had a suspicion that the stress distributions across an ovoid or elliptical cross section is not as simple as assumed in classical bending theory. Also consider that bowyers typically work with strain levels much closer to rupture than other crafts, where otherwise minor effects might make a bigger difference.
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well bottom line is the wood is overstrained,,
some wood seems more delicate than others,,
overbuilding a bit to compensate in difference or variation in woods could help,,
I work mostly with osage,,, and it rarely has an issue with chrysals,,
some of the woods your are working with may vary in quality quite a bit,,
and you might consider backing off a bit on how far you push your bow design,,??
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I have absolutely no answer to any of your questions. But for me, chrysals are more likely to appear the CLOSER I am standing to a beer. :o
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I believe with 0 facts to back it up, that compression stresses go deeper into the wood than tension. As the edges are thinner than the center, the edges are likely to fail first as there is less wood there.
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I hate chrysals with a passion, but if I do get 'em I want to see 'em ideally spread evenly along both limbs telling me that at least the tiller was even!
Del
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Brad, you're absolutely right. It's over-strained. But that's the weird thing because it doesn't seem to correlate very much with my designs, weight, draw length, width, or wood kind. The whole thing seems lottery-ish. THat's what's irritating.
Sleek, that's an interesting hypothesis. I think maybe it goes together with Willie's idea of some poisson effect going on. Maybe this is simply the effect from trying to make bows too wide from small diameter staves? Maybe the 35# hazel bow I mentioned would have made it if I had NOT made it wider than the 50# version, leaving the outermost parts of the crowned limbs so thin (maybe 50 % thickness of the thickest part)?
I'm interested in trying out the no set tillering technique. I've heard it mentioned here and there on this forum. I wonder it that technique would be a somewhat slower but safer technique to avoid chrysals?
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The only bow that I had that freted was a over built maple/sinew bow I thought it was rare & traced it to a minor flaw in tiller the funny thing it fretted in the upper mid limb but it stoped me dead in my tracks & started thinking about bow wood and design my first thoughts were maybe I will just stick with osage but with 20 plus white wood staves in my shop its not happening so I find this thread very interesting also wondering with heat tempering on white wood if that plays a significant diference I have a heat tempered sinew backed hickory bow that seems inpervious to fretting so far & it was under a lot more stress then the maple
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Stick Bender...That's because maple is crappy wood compared to hickory or osage....ha ha ha ha.Just kidding.It's got to be design for the wood along with good tiller the cure.I had a beautiful sinewed black cherry 64" TTT/1 and 5/8" wide chrysall on me after well over 2000 shots.It did have 4" of reflex though.So I just figure black cherry is fine for a bow wood but not in a stressful design.
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I would venture to say that with the high crown and flat belly that the belly when drawn would have a tendency to cup making the edges do much of the compression work hence the chrysals. Giving the bow a shallow D section may be the way to go in this case
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If a part of the limb bends too much in relation to the rest of it then you may get chrysals in that spot depending on the compression strength of the wood that you are using.
If the chrysals are spread out over a large portion of one and most likely both both then that's a design flaw. The limbs are too short, too long or the bow is too heavy for width and length.
I've had plenty of chrysals related to the first scenario but not the second though I've seen bows with chrysals spread out over large portions of the limb.
I usually leave high crowned bows a few inches longer than I usually do.
Jawge
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I agree with both Marc and George. I make only "pyramid" design bows. The limbs are one thickness from fade to tips. I haven't had a chrysal in years because I stay within practical limits for weight, thickness and length. If I have 45# bow that is 1-3/8 wide I know a 50# bow of the same design needs to be a little over 1-1/2 wide--same thickness and length.
Making a classical American flatbow, you are scraping and guessing all the way.
Jim Davis
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Tiller is central, of course. And I've had my share of chrysals due to bad tillering. That's why I nowadays make ELB's much stiffer in the handle section than I used to because I had several bows crash in that section. But then I've had bows chrysal where there really was no discernible flaw in the tiller. Where I spent a lot of time and care on the tiller. Bows of perfect balance, straight shooters. I can't quite blame my tillering, not always anyway.
Marc, that sounds plausible. I'll take that into consideration in designing bows in the future. It's an interesting conclusion. But still, no one else have experienced chrysals at the edge of the limb like that?
BTW, I realize it may sound like I don't do much else than make bows that chrysal :) It's not THAT bad. But I'd say that something like 1 in 15 bows develop some sort fret or chrysal. Maybe that's a high average?
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Well, Frodolf, the picture perfect tiller is not always the best tiller for that stave.
For example, knotted areas should remain just a little stiffer than the rest of the limb.
Also, deflexed and reflexed areas need to do their share of the bending load. All of which may seem like the tiller is off when it is not.
All things to keep in mind regarding chrysals.
Also, choose good, dry wood. Saplings are fine as long as you keep them longer.
Anyway, have fun.
Jawge
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Making a classical American flatbow, you are scraping and guessing all the way.
Jim Davis
Hardly. No matter what type of selfbow you make you're not cutting it out with a cnc machine.
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Making a classical American flatbow, you are scraping and guessing all the way.
Jim Davis
Hardly. No matter what type of selfbow you make you're not cutting it out with a cnc machine.
As a cnc machinist, challenge accepted. :D
Now, where is some perfect osage?
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i like the idea of making the belly slightly rounded for the high crown,, I need to keep that in mind,,
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Making a classical American flatbow, you are scraping and guessing all the way.
Jim Davis
Hardly. No matter what type of selfbow you make you're not cutting it out with a cnc machine.
No, but you could cut out a pyramid bow that way and it would be ready for the short string right off the machine. I work mine down to about 9/16" thick and they are usually very close to finish tiller. Usually just a matter of smoothing them up and fine tuning. The straight side taper and uniform thickness involve arithmetic ratios. A thickness taper involves exponential ratios--exponentially more skill and care required.
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It all depends on who you are, what you know and your personality type. Its just who we are and how we see things. Different brains approach tasks differently. I consider my tiller-eye quite accurate and guessing isn't a part of any equation. I've never made a pyramid bow and probably never will. Tillering is one of my favorite aspects of self bows. Nothing I like more than helping or teaching tillering.
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slightly rounded seems work for me, as I also find it a little easier to take the belly down that last little bit when it is rounded some. Has anyone experienced any demonstrable benefit to keeping it super flat?
btw, Thanks Mark for describing possion effect in plain english.
Frodolf, even though the summer humidity change is not all that drastic, could the edge of the bow be most affected by humidity, due to its thinness?
Jim- I have found that wood free from obvious defects can be remarkably uniform, and agree that consistent thickness (tapered or straight), is the most critical aspect of tiller longevity.
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Making a classical American flatbow, you are scraping and guessing all the way.
Jim Davis
Hardly. No matter what type of selfbow you make you're not cutting it out with a cnc machine.
No, but you could cut out a pyramid bow that way and it would be ready for the short string right off the machine. I work mine down to about 9/16" thick and they are usually very close to finish tiller. Usually just a matter of smoothing them up and fine tuning. The straight side taper and uniform thickness involve arithmetic ratios. A thickness taper involves exponential ratios--exponentially more skill and care required.
You could also do it with an American flatbow. A CNC machine has no trouble with math after it's programmed.
But I can tell you really love the process of bowmaking. ;)
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Actually, though I have probably made 200 bows since 1997, I like shooting much better!
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I'm with PEARL. Most of mine are closer to parallel than pyramid. I have no problem with either, but there is very little guessing on the thickness taper beyond roughing it out. The rest is a pretty methodical process.
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I know, George. That's what I mean by perfect tiller – each part of bow doing its part of the work, no matter if it's parallel or pyramid, wavy, snaky, knotty, etc. I'm pretty used to character staves and their challenges. My experience as a bow maker – which I would hardly call extensive, but thorough enough – has led me to start questioning the common explanations as to why bows fail in compression. Simply because the failures I've had do not often fit the common explanations. It's like the data that I've gathered over the years don't quite fit the theory, which makes me want to rewrite the theory. I don't know in what ways yet, though... Was sort of hoping I wasn't the only one with odd examples of compression failures that seem to deviate from the rules... :)
Willie, that's a good point! I've never thought about that but it makes sense.
I too agree that that there isn't much guessing involved in tillering. I always make sure to look at the bow in many different angles before removing wood. I often use pencil lines drawn with a ruler on the side of the limb to help my eyes see past too much character. But maybe "guessing" was just a poor choice of words? It's still not math based but based in one's ability to see shapes. That can appear to be guess work. I think the more experience you have the less guessing it becomes.
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The guessing I'm thinking of is in how much to scrape to correct what can be seen in the curvature of the limbs. It's a guess that becomes less uncertain with experience. But it's not a matter of making straight lines and even thickness. The latter takes less skill. That's all I was trying to say.
Those of you who make American flat bows and other bows with parallel limbs have to be more skilled than what is required to make a bow with uniform thickness.
Really hard for some of you to take a compliment, isn't it! ;)
Jim
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Hahaha! Point taken, Jim! :D