Primitive Archer
Main Discussion Area => Bows => Topic started by: gfugal on November 09, 2017, 11:28:59 am
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So I'm trying to have an open mind, but I'm not seeing how a rounded belly (other than rounded corners) is more beneficial than a flat bow? Enlighten me. From a physics standpoint, by rounding the belly your essentially doing the same thing as a crowned back. It causes the middle fibers to be further from the neutral plane and thus under more stress. Wouldn't it be better to evenly distribute the load between as many fibers as possible? In fact, If you have a crowned back wouldn't you want to hollow out the belly?
If the reason is aesthetic, then I'm okay with that. As long as it's in the realms of what the material can take then who cares right? but if you're going to tell me it's better mechanically I'm going to need you to convince me. Also, I heard that ELB were designed the way they were so they could get more bows out of the yew they had, so that's always another reason but that's not really an issue today as we aren't chopping all the yew trees for our military anymore.
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Sorry if this opens an old can of worms
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I am not sure most think it is more beneficial ,, it does work,, there are other designs that work as well,, and from what i know it really depends on the kind of wood as well,, Badger mentioned he made a english long bow for someone that set flight shooting record,, and I think that says alot,,, I dont make them so it is out of my area of expertise and have relied on the documentaion of others to make my opinion,, ok well a long time ago I made a semi rounded belly bow and shot a buffalo with it,,, but I think any well made bow would have worked..
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If everything is perfect and there is absolutely no misalignment in the limbs, a squared off corner would techniqually be better and distribute the load better, but this is not the case. Corners concentrate the energy and become areas that will splinter out, so by giving the corners a radius, it distributes the stresses over the radius and not at a difined point, and helps against splintering, in addition to a radius's look better than squared off corners....that's for glass bows.
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Yes the rectangular section is the 'best' for even distribution of strain but the rounded (subjective...how rounded?) belly section doesn't lose much to it. However when it comes to heavily ring porous woods like osage, ash, oaks etc a dead flat belly with a thickness taper leaves a wide section of earlywood on the belly and it is easy to make a series of hinges each time you go down through a latewood ring into the earlywood. A gently rounded belly eliminates this potential problem by feathering the rings termination.
There is also the poisson effect to take into consideration with extreme thin/wide cross sections. The effect being more strain on the upturned backs edges. Of course this isn't a big deal on wooden bows but something to consider.
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Yes the rectangular section is the 'best' for even distribution of strain but the rounded (subjective...how rounded?) belly section doesn't lose much to it. However when it comes to heavily ring porous woods like osage, ash, oaks etc a dead flat belly with a thickness taper leaves a wide section of earlywood on the belly and it is easy to make a series of hinges each time you go down through a latewood ring into the earlywood. A gently rounded belly eliminates this potential problem by feathering the rings termination.
There is also the poisson effect to take into consideration with extreme thin/wide cross sections. The effect being more strain on the upturned backs edges. Of course this isn't a big deal on wooden bows but something to consider.
The difference between growth rings is a valid point! and so is the corner rounding but that's not really what I'm wondering. I'm talking about like the deliberate crowning of the belly. such as depicted in this sketch (http://www.stavacademy.co.uk/mimir/images/archery23.gif)
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That was a interesting comment on poison effect but I'm wondering how much it really plays out on all wood bows that are well built maybe more so on hi stressed sinew bows or under built wood bows to be honest I never considered when making wooden bows but some thing to think about !
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I feel like I'm missing something. What's the poison effect?
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Basically in very simple explanation in a thin wide limb or a high stressed back the limbs warp on the outer edges it can cause failure ! Something that's not discussed in all wood bows often !
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The picture with rounded belly is most likely an older photo, my guess is that's from the time they made bows based off of the ELB design, in the time when the only good woods were yew, Osage and lemonwood.
The poissen effect just says that a material that is compressed will expand perpendicular to the applied force and contract when under tension. Since the back is under tension, the material borrows while the belly expands causing the upturned edges mikekeswick mentioned. Which would cause some concentration of forces.
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During the heyday of the rounded belly the woods used could take that shape. They were not limited to The "big three" either. Many tropical woods were available then and the British bowyers of the time used them all. Lancewood, Snakewood, Ruby Wood, Fustic etc.
Logically a fishing rod and a whip and trees themselves are round and bend a lot so there's no reason to think a bow can't just be made the same.
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those guys that wanted a bow to shoot further would just shoot a heavier bow,,
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I feel like I'm missing something. What's the poison effect?
if you bend a square pencil eraser, you can see cupping on the back that necessitates the need for the corner radius (on the back). Of course the opposite happens on the belly, and the belly tend to become more rounder.
I once heard an explanation about some belly wood being overly strained is an acceptable tradeoff from performance, if the strain on the back can be reduced. Makes more sense if your in a situation where your life depends on keeping a working bow.
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I have YET to make a wooden bow with a board-flat belly. All are fully radiused... ELB, flatbows, d/r, recurves, all of em. Generally, the lesser the wood, the wider and thinner and/or longer the limbs, and so by default... the more subtle the radius... but it's radiused.
Flat bellies just ain't done yet ;)
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I can't get a flat belly on me, so I put then on my bows ;D ;D ;D
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Stop eating the burgers to much poison effect :-K
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It's the economy, stupid.
No, really. The reasons yew longbows had a rounded belly, was out of economy. You can cut more staves out of a log this way. It's described in Bjurhager et al. 2013, Fig 5. https://sci-hub.cc/https://doi.org/10.1515/hf-2012-0151
Since yew can take that extra compression (especially in the juvenile wood at the center of a log), and since Yew was so prized that the Medieval English cut nearly all yew woods on the European mainland (at least where they could lay there hands upon), economical use of the wood was required.
The belly of ELBs is rounded despite the disadvantage it yields.
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That diagram is ludicrous!
The bow shape is wrong, those bows would bend sideways!
That log would probably give 4 bows, 6 if you were lucky, assuming it's roughly to scale.
Del
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I suspect they probably drew it like that to allow for the handle wood on a slightly deepened non-bending handle section. Imagine those shapes as a cross section of the handles, not the limbs. But I agree, I've never gotten that many staves from a yew log. They always seem to have issues I have to work around, and I tend to play it a little safer when sawing them out, leaving more wood around each one to allow for various design options, and layout and alignment options later. I usually get a few good staves/billets wide enough for ELB's or the occasional 'flatbow', and try to plan my cuts to allow for slicing some heartwood slats to be backed... if possible. There's not much waste after I get finished picking the meat off a yew log, but I'm not going to cut them into nothing but skinny staves that only allow for ELB's just so I get one or two more.
I don't radius or round bow bellies to ration my wood. I do it because I like them that way, to me they feel and appear a bit more refined, curvy, maintain the bow's shape and 'flow' from one tip, through the handle and dips, to the other, are more impervious to scrapes, dings and their effects, and better facilitate the shaping and tillering process, in my mind anyway, especially in character bows. I honestly don't know how I could make some of these bows with a rectangular cross section, even if I wanted to. If my radiused belly flatbows are a few fps slower, I wouldn't know or care or change them.
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Linking discussion to a different can of worms, "pyramid" bows can have a grow ring on the belly run from the fade all the way to the tip, because the limbs are the same thickness from fade to tip. Another advantage of the design--all late wood on the surface where the work is done.
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Linking discussion to a different can of worms, "pyramid" bows can have a grow ring on the belly run from the fade all the way to the tip, because the limbs are the same thickness from fade to tip. Another advantage of the design--all late wood on the surface where the work is done.
True, but that means you have to commit to larger weight drops per growth ring. ;)
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also if you are just working with a draw knife, the rounded belly is good for that,, and someone skilled could probably make the whole bow with the knife and not even need to sand it,,,please dont challenge me to do it,, just saying,, (R
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A drawknife also used to be used for tapering cedar shakes so it does a wide flat surface pretty well too. ;)
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Linking discussion to a different can of worms, "pyramid" bows can have a grow ring on the belly run from the fade all the way to the tip, because the limbs are the same thickness from fade to tip. Another advantage of the design--all late wood on the surface where the work is done.
True, but that means you have to commit to larger weight drops per growth ring. ;)
Actually, no. Weight in the pyramid bow is regulated by width.
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I think it would be a fun challenge for you experienced bowyer's to see if you can make a bow with nothing but a draw knife. I mean after you made close to a 100 or so what's it going to hurt right ;)
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I think it would be a fun challenge for you experienced bowyer's to see if you can make a bow with nothing but a draw knife. I mean after you made close to a 100 or so what's it going to hurt right ;)
It isn't difficult. Very easy to be real accurate with a properly sharp drawknife. Square the spine and you have a scraper too. :)
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Linking discussion to a different can of worms, "pyramid" bows can have a grow ring on the belly run from the fade all the way to the tip, because the limbs are the same thickness from fade to tip. Another advantage of the design--all late wood on the surface where the work is done.
True, but that means you have to commit to larger weight drops per growth ring. ;)
Actually, no. Weight in the pyramid bow is regulated by width.
You still have to choose a thickness.
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I think it would be a fun challenge for you experienced bowyer's to see if you can make a bow with nothing but a draw knife. I mean after you made close to a 100 or so what's it going to hurt right ;)
;D Oh thats easy! :BB