Primitive Archer
Main Discussion Area => Bows => Topic started by: upstatenybowyer on March 26, 2017, 12:05:36 pm
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There's no doubt in my mind that good tillering, without over stressing the wood, is an important (if not the most important) factor in keeping set to a minimum.
However, I was wondering how much a wood's compression strength plays a part beyond tillering.
What got me wondering about this was having made a bow out of plum (said to have good compression strength). I was amazed at how it took no set at all. I did take my time with it, and it was a low-stress design (ALB), but I can't help but wonder if it would have turned out that well if it was made from a different wood.
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It's mostly the elasticity of the wood in compression that's the key., not its straight up strength.
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how much a wood's compression strength plays a part
most of it, I suppose.
Minimizing set caused by less than optimal tillering, is always good practice, as you have noted.
Hard to say about a different wood on the plum bow, but you might have been able to design to a higher weight OK, or had the same draw weight with lighter limbs.
Watching when set appears during the tillering process, helps to adjust weight goals, or a switch to side tillering if you are not looking to increase weight
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Plum is magic.
I believe design, tillering, and material matter. Choosing the wrong design for the wood and the stave you have never helps. Strength and elasticity both matter, but over and over it seems people confuse the two.
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I have never gotten much into wood properties beyond just getting to know a wood. But I believe strength of compresion would be more reflected in how thick the limbs come out and how well it tillers out has more to do with elasticity.
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All great responses. You've given me plenty to ponder. Thanks guys.
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I just made a short oceanspray bow and a long yew bow. I was sensitive to overstressing like I always am and after shooting both had very low percentage set. Now I'm working a relatively short but wide vinemaple bow with 3" of reflex. I barely got to the short string and thing has lost an inch of reflex already. I can tell it will lose all of it, at least, by the time I get to full draw. Clearly an inferior wood, at least this particular stave.
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Also it isn't 'compression strength'....this term doesn't really describe anything. What we talk about is really resistance to compression. Ipe, b.locust etc have high resistance to compression, yew, juniper have low resistance to compression. Ipe/locust bows will be much thinner than same weight juniper/yew bows.
How far something will bend before being damaged is known as its elastic limit.
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Terminology is important in understanding what's going on. I'm going to do my best to explain what I know.
The strength of a material is a reference to how much stress a material can take before breaking. In the images below it's a reference to the failure region: the tensile strength (or compression strength) and/or fracture point. Like others have mentioned strength isn't really a good indicator of anything by its self for us bowyers.
resistance to compression/tension or as I say stiffness, has a couple of official terms: young's modulus or modulus of elasticity (MOE). In the images below its the initial slope. As bowyers we can think of it as how much the wood is going to resist bending. The higher the resistance/stiffness than the higher the poundage will be for that bend for a given amount of material. mikekeswick described it well. A stiffer wood will reach desired weight with less material than a less stiff wood. look at the second image. at a strain of 10 the first material is at a stress of 300 whereas the second is only about 150. This is the biggest factor in considering how strong our bows will be. The way we can effect this variable is by controlling the amount of material. We can have more material by either having the wood wider or thicker. However, there is a distinction between width and thickness. more of it in both cases will equal more poundage. However, more thickness will also increase the stress of the outer fibers whereas more width does not affect the stress the fibers undergo. So really the best way to meet your weight requirements is to ensure your bow is wide enough. Because of this distinction i think of manipulating the wood like this: the thickness determines how far it can bend and the width the poundage.
Now lets move on to the stresses.
In the images below we have strain and stress on the x and y axis. For the sake of applying it to us think of it as how far it can bend on the x axis and how much poundage it will give us on the y axis. a greater slope means that for less bend it gives us more poundage. This is good right? not necessarily because we also have to look at the elasticity of the material. a stiff material with low elasticity will give you high poundage for little bend but will break sooner. This describes a brittle material. High resistance to bending but easily breakable. There's also an important distinction between elasticity before breaking and the elasticity before set. These are indicated on the graph as the failure point described above and yield point. The yield point is the point the material will no longer return to its normal shape perfectly like it once did. It still can do work without breaking and still return a little bit, just not ideally like before. One it passes this point it becomes plastic. In our terminology we describe this as set. Set doesn't mean a broken bow, but it certainly doesn't mean an undamaged one either. Unfortunately, there is no real data out there on yield points (sometimes research is tested to failure and sometimes only to yield points but they generally treat them as the same so it's hard to distinguish what they actually mean). Therefore we mainly have to go off anecdotal experiences. Like your for example about plum. apparently plum is pretty elastic meaning it has a high yield point. If you want to get an idea for a woods elasticity using data you can take the failure point and divide it by the stiffness (MOE) and it will give you an idea. It's not perfect though because we have to use failure points instead of yield points.
Another important distinction needs to be made about compression vs tension. The same principles apply for both but they aren't the same values in wood. Generally wood is stiffer in tension then it is compression. Elasticity is a little less clear. I would say that wood more elastic in compression if you are referring to its elasticity before breaking, but if you are your referring to its elasticity before the yield point/set I believe it is weaker then its tension elasticity. A subject that I want to investigate further is whether wood will undergo set in tension or if it's only compression. It doesn't make sense that it would only be in compression yet that's what we see generally. So maybe its just that wood plastic region in tension is so small compared to compression.
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I just made a short oceanspray bow and a long yew bow. I was sensitive to overstressing like I always am and after shooting both had very low percentage set. Now I'm working a relatively short but wide vinemaple bow with 3" of reflex. I barely got to the short string and thing has lost an inch of reflex already. I can tell it will lose all of it, at least, by the time I get to full draw. Clearly an inferior wood, at least this particular stave.
Steve, one thing to consider with vine maple reflex is that the tension of the back often pulls a vine maple stave into reflex. I think that when you start tillering a vine maple stave and pulling out some of that tension induced reflex that you are not creating set in the typical sense. I don't think that the cells are breaking down in the belly yet. Not to say that a vine maple stave can't take set, it certainly can. I just think that losing some of that initial built in reflex isn't harming the bow the same way typical set does.
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I think along the lines of badger, also making sure the wood not green. Upstate I know you don't have a problem doing and making bows. I think that will teach the most. Can't put nature in a box, even if you have the 'perfect wood', it will vary in consistency leaving some frustrated because on paper it was 'supposed to be' something else. I know that some wood is stronger than others, but the piece in your hand is whatever it is and using your skills gained from experience and by how the piece feels and responds, build accordingly. That's the way I go about it.
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Greg
nice explanation for the technically oriented. I do believe that the lower image more closely represents most woods, and the upper, that shows the hump at the yield point, would be more appropriate for alloyed steel?
I do not know of any strain hardening woods, but would would be happy to be proven wrong on that. :)
I do find interesting, the differences between how different woods respond to strain in the elastic zone. Or in layman's terms, how different woods take set.
Dan Perry, in TBB4, mentions a "co-efficient of restitution" on p.164. Other interesting reading on that page, for those so inclined.
willie
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Yeah the graphs aren't of wood. They were some general ones with good labels that i pulled from the web. I'm going to have to get the rest of the Traditional Bowyers Bibles but i unfortunately always end up buying tools or suppies instead with my budgeted money.
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Weylin...good stuff. PM sent.
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one thing to consider with vine maple reflex is that the tension of the back often pulls a vine maple stave into reflex.
curious about the stretching out of the reflex, and wish to ask how it gets pulled in, to begin with.
Does the stave go into reflex right when it is first split? or does it pull in as it dries?
thanks
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willie,
Generally, if a tree grows straight up then any half when split would be similar in strength than the other half. If the tree doesn't grow straight and is leaning, or in the case of a horizontal branch, the wood density and characteristic are different from the upper half compared to the lower half--the tree creating differences in order for the tree or branch to have the strength to grow leaning. So there becomes a tension and compression side of the tree itself. So if you make bows from the two halves then theoretically you are now making bows from inherently tension or compression wood, not to be confused with the tension/compression sides of the stave you end up.
Its different for hardwoods and softwoods, exactly what happens, and you can read about it, but theoretically with vinemaple the skyward half, is inherently tensioned wood, and so it will, as it dries, curl, because it has been released from it compression half. It will be the better half for making an efficient bow.
I have not found this to be true with any wood that I've dealt with, yet. But I haven't worked alot of vinemaple. In the case of the stave I'm working now, the tree was growing straight up but at some point in its life steered slightly so that the "straight up" trunk had a natural bend that would yield a reflexed and deflexed half. After drying neither half of the log moved much in any direction.
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thank for the report on the stave you are tillering. Some of the compressionwood I have worked with is said to exhibit 10 times the shrinkage/expansion of normal wood, with varying moisture conditions. I do not know the particulars about tensionwood, but would not be suprised if something similar happens. In addition to leaning trees,I have read where trees exposed to prevailing wind are also said to create reactionwood.
I have also noticed where a straight log, when split into staves, will have staves that go into reflex for not so obvious reasons.
Perhaps there are some drying forces at work? due to differences between sap and heart, or the moisture contents of the sap and heart?
In any event, I have been more inclined lately, to work with a stave"just as it was formed in the tree", and to minimize cell damages/warping due to drying stresses. With compression wood, this means reducing the stave so that the reaction between the different types of wood in the stave are minimized.
I have seen, when splitting logs, where the staves take a shape of their own once released from each other, and see this reaction often with dried wood. I am curious if anyone has noticed this happening with green wood.