Primitive Archer
Main Discussion Area => Bows => Topic started by: Matt A on July 10, 2012, 04:07:24 pm
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I've been wondering this lately, is there certain woods that prefer a flat belly or round belly or is it only a performance/preference issue?
And is it lets say, all white woods need a flat or rounded belly or other woods like osage need a flat or rounded belly? If you get what i'm saying. ::)
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Hi Matt,
In general woods that have greater compression strength (e.g. ability resist compression of the belly cells) can tolerate a rounded belly better than woods with less compression strength. A more rounded belly allows you to design deeper narrower limbs. However a wide flat bow, with proper limb design can be as fast as a narrow limbed bow.
There may be some benefit to narrow rounded limb design in reducing hysteresis (internal friction) as well as vibration, but I would be a bit out of my league in answering that question specifically.
Gabe
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A wood that is weak in tension but comparatively strong in compression, like yew or juniper, should have a rounded belly. A wood that is strong in tension but comparatively weak in compression, like hickory, should have a flat belly.
"Comparatively" is the key word here. The best way to tell is with an experiment. Cut two sticks of the same wood in the same way, with one side flat and the other side rounded. Bend one with the rounded part as the back and the other with the rounded part as the belly. The stick that takes the least amount of set will tell you if the belly should be rounded or flat.
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Good compression woods include: yew, osage, cherry, maple, ipe, ash, apple, and jatoba.
Poor compression woods often include: hickory, oak, purpleheart, and walnut.
As you can see high density does not necessarily mean good compression strength.
These lists are based on my experience and research, others may have other opinions.
Gabe
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For what its worth every bow I build regardless of wood or style (except ELB) is flat bellied with pea sized radii on the edges. I like for my limbs to have shoulders. A touch of robustness never hurt a wooden bow of my own building.
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I like to and do make all my limbs to a shallow oval. Even wood s like hickory which I build bows from a lot seams to get by just find. I built bows from hickory with flat belly and as long as you keep the limbs to a low oval I can't see any diffenence in cast,speed,hand shock or even set and string follow.
Woods like Osage like rounded bellys again I can find no difference in a shallow oval even though it works good with a deeper oval. I've found any any woods work good with this a shalow oval. I can build my most of my bows to keeping a little reflex no matter the wood with a shallow oval belly. I reflex all my staves while green to have 2 to 3 incks of reflex before I start. Doing it like this
makes a smooth well rounded no sharp edges that just look better when your finished.
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I'm with Pearl. :)
Pappy
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We have been left without information by the Forest Products Research group. They give statistics for compression strength, but none for tension strength. The relationship between compression and tension is different for every wood species.
Good information above, generally, but most woods are 3 to 4 times as strong in tension as in compression. The known exceptions include, as noted above, yew and cherry--eastern red cedar too. (I've not seen statistics for the others noted above and have not found ash or maple to be stronger in compression.)
With careful tillering, we can get away with rectangular sections for all the other woods, but the belly is doing all the work. If the backs of bows made of these woods that are stronger in tension is narrowed by 30 percent or so, the back begins to share the work load. That makes it easier to avoid chrysals and frets. It also makes for slightly lighter limbs and better speed.
Hickory is about 3-1/2 times as strong in tension as compression. Elm seems about the same. Both do very well with a flat belly and a narrowed back--even a highly crowned back, as when made from a 2- to 3-inch sapling.
A very good suggestion by jackcrafty to test the shapes of belly and back for a given wood. I have done that with red cedar and the results very clearly showed that red cedar is much weaker in tension than in compression.
As I said earlier, careful tillering can result in a good bow with oval or flat belly, but with most woods, leaving the back full width makes the belly do all the work.
Jim Davis
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I agree with the above post.
Trapping of bows backs is underated in my opion. - it can make a real difference to set when used correctly.
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thanks for all the info! how about a real dense wood like HHB? would that more than likely require a flat belly
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The Wood Database http://www.wood-database.com/wood-identification/ (http://www.wood-database.com/wood-identification/) has some interesting information. I don't know what they call it, but if we divide the elastic strength with the crushing strength, we come up with a number that might be useful in making better bows. If we compare the numbers for several bow woods, we might be able to guess its meaning a little better.
pacific yew 166
mulberry 167
sassafras 170
osage orange 179
dogwood 199
black locust 202
pignut hickory 233
white oak 241
red oak 266
What do you think?
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Interesting site. Not sure what is meant by elastic strength, so also not sure if the ratio you suggest is useful to us.
The mechanical properties of wood are usually given in such terms as "modulus of elasticity," "modulus of rupture," "work to maximum load" and such.
Of most direct value to bowyers would be "compression strength parallel to grain" and "tensile strength parallel to grain." The first can be found at http://www.fpl.fs.fed.us/research/centers/woodanatomy/techsheets_display.php?geo_category_id=2&genus_commonname_criteria=c&sorting_rule=1a but the Forest Products Laboratory doesn't offer tensile strength data.
If we had compressive and tensile strength numbers, we could come up with a very useful ratio for every wood type. We could make the back-to-belly width ratio fit the tensile/compressive ratio and be off an running, or some other metaphor. :D
I found this tidbit for these woods (http://i33.photobucket.com/albums/d72/Reparrow/archery/woodtensilestrength.jpg)
Wish I could find that kind of data for all the other common woods. It is exactly what I would want.
Jim Davis
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I very slightly round my bellies so the tools work easier and then durring the final tillering my scraper usually flattens them out pretty well.
I don't find engineering specs very usefull for selecting bow woods. Strong in compression has nothing to do with good in compression. Many of the strongest compression woods are terrible about chrysaling. How strong something is refers strictly to how much it resists bending. The ratio of tension to compression probably has more effect on where the nuetral plane will fall. The amount of elasticity overall will determine how thick we can make it and how tightly we can bend it.
A bowyer has little interest in when a wood will fail, a bowyer concentrates his efforts only within the elastic limits of wood. Most woods fall somewhere around 1% plus or minus about .2%. This sounds small but a wood with 1.2% elasticity has almost 50% more elasticity that a wood with .75% elasticity.
The strength of something probably would interest us most as a ratio of mass weight in the wood or specific gravity. If a wood were not very strrong in tension or compression but was very dense and heavy it would not be good, however if a wood were light but only medium strong in might be a great wood.
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Interesting site. Not sure what is meant by elastic strength, so also not sure if the ratio you suggest is useful to us.
Terms are explained in the site.
I find it fascinating that the number 166(%) for pacific yew is rather close to the ELB width/thickness ration of 8/5 and the similar number for Sassafras might explain why certain oldtimers liked that wood.
It's just a heuristic hypothesis but the numbers might be useful in getting the starting ratio of width and thickness for each wood. I wonder, though rather extensive, how reliable the data in the site is.
Anyhow, I find it intersting that osage orange and mulberry have rather low numbers like yew and sassafras.
BTW Badger, I'm using elastic strengths, not rupture strengths. I'm still trying to incorporate the specific gravity into the hypothesis.
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This site is incredible !!!! From one end of the spectrum to the other. Sicentific comment to the most common sence simple comments. I love this site ...so cool!
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SCP, I use specific gravity as in the mass principle only because it works on averaging things out. Once we get to know specific woods we go a bit one way or the other.
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... Strong in compression has nothing to do with good in compression. Many of the strongest compression woods are terrible about chrysaling. ...
But strength in compression compared to the same wood's strength in tension does make a difference in how a bow should be designed.
A wood that is "terrible about chrysaling" is a wood that has been overpowered by a back that is much stronger in tension. We can relieve the belly of some of the work by reducing the amount of tension wood in the back.
Black locust is infamous for chrysaling. But I have not had one chrysal since I started narrowing the backs. Before that, I made a BL bow that had 5" of reflex when unstrung. It was a pyramid design (as all of mine are) and well tillered. It shot very nicely, but it was a maze of chrysals from tips to handle. It performed fine with the chrysals, because they were everywhere--no hinges.
Badger, I think you are probably correct about the neutral plane being farther from center because of the difference in compression and tension strengths. But that ends up meaning the belly does all the work.
jm $.02
Jim
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I actually spent some time in an engineering lab testing different materials until they failed. Different materials fail in different ways. Some do not show much sign of damage before failing and some get all ugly and scary before they fail. Point is, just because something might take a lot of abuse before failing doesn't mean it looks pretty doing it. Like Badger said, we are concerned with the elastic limit. That's the point where the material will still go back to it's original shape (or close to it) and before it starts to get ugly.
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If you tiller a bow with hand tools you will automatically have a slightly rounded belly more than likely. Hand tools function best with rounded bellies so the tool you use will determine the design. The bellies of my bows are slightly radiused or rounded because I tiller with hand tools. Jawge
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There seems to be quite a bit of confusion about what is desirable in bow wood. Compression strength has nothing to do with how good a wood will be for making bows. Some wood species have very high compression strength yet will chrysal readily. Bubinga for instance has very high compression strength but it will chrysal fairly easily if over stressed, Black Cherry is even worse for chrysalling. The most important property that a wood can have is elasticity. That will determine how well it will tolerate a round belly in a bow. A wood's compression/tension strength will determine how physically small a bow can be made for a given draw weight, generally speaking its mass will still be the same though.
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Badgers TOTALLY RIGHT also like someone said you can't build wood bows from spec's. Its not lams of wood and glass. DESIGNS not specs either. I 've went throuhgt the stage where I built all types of bows and designs. I have built many wooden bows both ways flat and rounded bellys with all woods both ways and keeping the limbs a flat ovel seams to be in between the scale.
I can't see a difference. If your build like D bow designs thats designs to the exstream. Then woods like OSAGE, YEW are far better with that design than say HICKORY or some other white woods which prefers a flater belly. But it dos'nt have to be straight flat with a starp edge.
I've found that the flat ovel limbs are in the middle of that scale. And don't have any affect on set or string follow as long as you don't make your limbs to rounded.. And the rounded limbs (FLAT OVAL) helps with chrystalizing on all woods.
Thought the years I come to build bows my way, my own design and center shot, flat ovel limbs. Produce a releiable, fast good cast, spine tolerant, little set little string follow hunting bow. I just start with a straight 2 inchs of reflex added stave. Any swists or slight curves you can take out in tillering. If the stave so out of line I have to use heat I wou'nt ues it. Matter fact I would'nt have cut and seasoned it in the first place. I know you'll say well we have to use what we can get. Thats fine makes you a better a building bows.
But you went through the trouble of getting that suckie stave why not just get or cut only good ones.
I'm not trying to get anyone to build your bows my way. Just stating the way I've came to build all my bows and what works for me.
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The perennial question is whether it is wood, design, or both. I'm still in the camp of design, within reason, of course. I would not say that a pine bow can shoot as well as a yew bow, no matter how well it is designed to fit its wood. But I do think a well designed oak bow would outperform a mediocre yew bow.
As for the compressive strength, its number alone does not mean much in bowery. It needs to be used in conjunction with the wood's elastic strength. Hence, I'm interested in the ratio of elastic/crushing strengths. The index might be useful in determining the width/thickness ratio of each wood. At least in the beginning stage of bow making. It also says that it would be dangerous to make round bellied longbow out of red oak (266%) without backing it. How far we can go with this simplistic heuristics is a matter to be tested empirically.
IMHO it would be silly to round the belly of wood with its index higher than 200%. That's about it, for now.
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I understand where your coming from.With a plain wooden bows not lams with the index higher than 200 % how wide ,leanth and poudage you want.People never figger in poundage but say where a HICKORY BOW flat back a 55#bow would'nt chystalize the straight edges a 75 # will. Most of all your tilling exsperance can take over design some what. With wooden bows straight or square edges chystalize far sooner than rounded edges no matter the design or wood type. Again tillering comes into play here again.
I say this ,you can take a bow with the right design for the wood and if your tilling sucks your bow sucks. But I can take a bad design bow for that wood and with good tillering make that bow exseptable. It may not be as good as a bow as it could be with the right design but you will make a exceptable bow.
I'D SAY IT'S BOTH THATS WHY A FLAT OVEL LIMB DESIGNED WELL TILLERED FALLS WELL IN BETWEEN DESIGN AND TYPES OF WOOD.
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It might be helpful to post some definitions of different properties of wood: here are mine wrt to bow building, please correct me if I'm wrong.
Compression Strength: The resistance of a material to breaking under compression, or the amount of force it requires to crush the belly cells of a bent bow.
Tension Strength: The maximum stress that a material (back of a bow) can withstand while being stretched before it fails.
Elasticity: The tendancy of wood to become deformed when a force is applied to it.
Gabe
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Gabe, tension and compression strengths are how much force it takes to bend, or how hard it resists bending.
Elasticity is how far you can bend it before deformation starts to take place. Plasticity is the area between elasticity and failure. We don't even want our bows going into the plastic range. Woods that tend to chrsal have a very short range between elastic and failure, the plastic range is very small.
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Thanks Steve, that's what I was trying to say, although you were more succinct.
So... a large plasticity range isn't necessarily a good thing then. Even if the bow doesn't fail, excessive deformation (set) damages performance.
So, like Marc said, elasticity is probably the most important feature in a desired bow wood. Because tension and compression strengths can mostly be addressed with limb design.
As an aside (in another thread) I think it would be really interesting to evaluate the relationship between the properties of wood (e.g. elasticity), limb design and hysteresis. There might be some useful info hidden in there. Maybe this has been done though.
Gabe
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Gabe, the study you mention would be a good one. The backyard laboratory work I have done kind of indicates that hysterisis goes up dramaticaly once we get into the plastic range. Woods like black cherry and locust for instance have very small plastic ranges and likewise have very low hysterisis compared to other woods. Some woods will allow us to work deep within the plastic ranges of the wood often still mainating a decent profile, invariably these bows will shoot like dogs inspite of impressive looking profiles. Osage can be a big offender in this one as well as some white woods like hickory and elm. Moisture aggaravates this greatly.
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Ok, so what would be an ideal design to reduce hysteresis in a wood like black cherry, with small plastic range and high compression to tension strength ratio.
Theoretically it would be very different from the ideal design to reduce hysteresis in a wood like Hickory with a large plastic range and a low compression to tension strength ratio.
This could be fun. Damn, not enough time. Steve, you'r retired, you should get on this :).
Gabe
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Black cherry keeps popping up so o will ask a question using that as the example ...aside from mc, damage (set) is caused primarily on the belly side as is chrystaling so to reduce both trapping the back on said wood with a semi ovald belly with a pyramid design would be a way to make a good performing bow???
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Black Cherry is stronger in compression than tension. Trapping the back would focus the tension forces to a smaller area, which would be a bad thing for cherry. Especially if cherry has a lower plasticity range. You'd rather trap the back on a wood that is strong in tension but weaker in compression.
Gabe
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So more of a flat bow to avoid Chrystals???
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On Gordon's yew thread he said he did a round belly to better focus the compression. Which makes sense. The is a lot of useful information on this thread. Kind of changing my thought on some woods and belly formation.