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AVCase design Arvin bow build
mmattockx:
--- Quote from: bradsmith2010 on March 07, 2023, 03:45:09 pm ---you may have to re adjust the profile after you see what it does at full draw,, right now you are just guessing
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Which profile are you talking about here? The point of Alan's work is to not be guessing. If Arvin gets the thickness and width accurate to Alan's numbers then the tiller will not be a guess, it will be very close to what was predicted (assuming the wood properties are consistent along the length of the bow, which is a reasonable assumption). It is relatively easy to calculate the width and thickness to give you the tiller you want, the big questions are what the draw weight will be and how much set it gets because those are determined by the modulus of elasticity and the strain limit of the particular piece of wood and they all vary a bit.
The thickness numbers need to change with the reduced reflex because it changes how much strain the limbs see, which changes the draw weight.
For those saying they couldn't do this with a stave, this method does take very clean, straight grained wood because you are using rectangular limb sections and can't chase a ring on the back. I've done it with boards and lams cut from boards. The lams work the best in terms of being able to pick the best grain for the back and for the ease of achieving the desired thickness and taper rate.
Just to show that Alan and Arvin are not guessing, here is a red oak lam bow I designed using some of the same software Alan uses. I glued it up, cut the width profile and exercised it out to full draw on the tree with no tillering done (which is a good thing, because I'm still a hack at tillering by eye). The bend was not a matter of luck or guessing, it is the result of using math to predict the behaviour of limbs as structural members. These are essentially the same calculations used to design buildings, bridges and other structures, just applied to bows instead.
Mark
bradsmith2010:
the red oak lam bow is not the same as an osage stave,,
so dont think that is a good comparison,,
you wont know what kind of set the bow is going to take until you shoot it,, at given draw,,
you dont know what the profile will look like till you shoot the bow,, because its gonna change,,it may not change much,, but it will change,,and when the bow goes to a more humid place,, and has a reasonble moisture content it will take more set,, and unstrung profile will be different again,,as well as the tiller,,
its not a straight grain lam bow,, clearly,
if the limbs are out of tiller you will have to adjust it,,it wont matter what the numbers say,,,or how closly you measused,, if the wood shifts,, which it usually does,, depending on the finger pressure of the shooter,,,drawing the bow on a tree has a different effect than somone actually shooting the bow,,
I think the bow will be very close,, im just saying it will probably have to be adjusted after it is shot in,,or the moisture content changes,,
math connot predict what the bow wood will do under the strain of full draw,,,if the shooter has a longer hold,,, the wood may take more set,, or if the bow is left strung all day during hunting it will effect the tiller and set the bow takes,, if the bow is over draw at some time,, it may take more set,, one limb may take more set than the other after shooting,, these are not predictable,, the math is a great start,,
the red oak bow looks great,,,how many times had the bow been shot when photos were taken,,
George Tsoukalas:
Arvin, is it 5% on the surface or all throughout? I keep checking as I build. Jawge
avcase:
--- Quote from: bradsmith2010 on March 08, 2023, 12:04:43 pm --- math connot predict what the bow wood will do under the strain of full draw,,,if the shooter has a longer hold,,, the wood may take more set,, or if the bow is left strung all day during hunting it will effect the tiller and set the bow takes,, if the bow is over draw at some time,, it may take more set,, one limb may take more set than the other after shooting,, these are not predictable,, the math is a great start,,
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The reason I am very interested in this study is that it provides an opportunity to learn more about the variables and changes in the materials that occur when building a real bow from a stave. For example, differences in the model compared to the real bow may give an indication to how much non-visible set occurs and how much effect it has. In this case, the bending stresses should exceed what I have measured in osage test samples that I have tested, but Arvin routinely achieves this with other bows he has built so I just ran with it.
Alan
Selfbowman:
Jawge I think all throughout. It’s 4-5 years old. Up date . The bow came out at 35@28. Well I’ll say!🤠🤠 I think this is combination of several things . The bow weighs 20 oz. That’s s less than my normal 50# 67” bows by a couple oz. so this wood is not as dense. The engineer may been using a different wood model. And Arvin was close but not spot on . The back of was as flat as a glass bow. No knots. It had more early ring than I liked. It took 1” set pretty much throughout the limb. So that tell me that Alan and I did our work pretty good but we over estimated the wood. Fun experiment and I will continue the try a few more. I think the next one I will try to duplicate my wide pyramid in a real rectangular limb design and see how it comes out. If great then I will have pieces of the stave to send Alan with measurements and do this experiment in reverse . Having the computer check Arvin.🤠🤠🤠
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