AVCase design Arvin bow build

[superdav95]

Very cool experiment.  I’d have to say the same as others here that the wood staves I have on hand wouldn’t allow for me to build a bow this way.  Should be interesting to see how it goes.  There seems to be a lot of inner bend on the outlined projected profile marked on the paper to my eye.  Maybe it’s an illusion just being on the paper and will look better when actual bow is bending.  I’m watching this. 

[avcase]

Alan do I need to correct those ends before I. Go farther?

Yes, because you said you wanted that much reflex in the beginning. Otherwise, there won’t be much point in matching it up visually to the braced and full draw side profile shapes. It needs a little more about 13” from the tip.

Having said that, I have no idea how the heat bending affects the wood properties. But this is a lower stress area so it may not affect things very much.

One more thought. Reflex is a killer for wood bows, so less reflex like you show here makes it easier.  It changes things. The limbs will need to be a little thicker.

Alan

[bradsmith2010]

so the numbers would need adjustment for the new profile,,?
just some thoughts,,
you wont know what your profile will be like till you string it and pull to target draw weight,,,, its gonna shift, if it was sinew backed it would hold closer, but self bow is a little trickier  ,,
a fiberglass bow will hold,, but this is a wood stave,,
when it gets to full draw it will probably shift more,,
the long string is one thing, but braced is a different pressure on the bow,
you may have to re adjust the profile after you see what it does at full draw,, right now you are just guessing,,and all the heat treating is making it dangeriosly low in moisture content,, most of the bows I blew up,, and measured the moisture content right in the middle of the stave were low in moisture content,,

[avcase]

Alan do I need to correct those ends before I. Go farther?

Yes, because you said you wanted that much reflex in the beginning. Otherwise, there won’t be much point in matching it up visually to the braced and full draw side profile shapes. It needs a little more about 13” from the tip.

Having said that, I have no idea how the heat bending affects the wood properties. But this is a lower stress area so it may not affect things very much.

One more thought. Reflex is a killer for wood bows, so less reflex like you show here makes it easier.  It changes things. The limbs will need to be a little thicker.

Alan

Arvin, I checked the impact of having the tips reflexed 2” like you have Vs. 2-3/4”, and the difference turns out to be very small. There is almost no impact to the braced shape and full draw shape. Bending stresses go down by 3% and you lose a little bit of stored energy. The limb needs about .007” increase in thickness to hit the same draw weight.  No cutting corners!  Hahaha!

[bradsmith2010]

what effect will difference in moisture content have,,? its a big piece of the puzzle,,

[mmattockx]

you may have to re adjust the profile after you see what it does at full draw,, right now you are just guessing

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]

      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 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.🤠🤠🤠

[Selfbowman]

It’s sure fun to back at it!

[sleek]

Hey Arvin, how wide and long is that bow?

[bradsmith2010]

thank you for posting,, very nice ,,

[simk]

Nice bows and very interesting cooperation Arvin and Avacase! A rectangular cross section might be the most efficient, as long as it matches the specific wood used. Curious about you being reverse checked Arvin 
cheers

[mmattockx]

the red oak lam bow is not the same as an osage stave,, 
so dont think that is a good comparison,,

I would say that wood is wood as long as you know the properties of the individual piece. As long as it is consistent along the length of the bow (which isn't always the case) it will always bend as predicted.

you wont know what kind of set the bow is going to take until you shoot it,, at given draw,,

Yes. As I said, the two things that are hard to predict really accurately are draw weight and set, because they depend on properties of each individual piece of wood. As long as the wood in the bow is consistent from end to end then I can predict the bend pretty accurately. If you have a knot or soft spot then it will throw that off.

drawing the bow on a tree has a different effect than somone actually shooting the bow

Yes it sure does, even if you set the bow to rock on the tree and pull exactly where the archer will.

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,,

This sounds about right. I figure the math will get you over 95% of the way home, but wood always has a few quirks that may require attention.

the red oak bow looks great,,,how many times had the bow been shot when photos were taken,,

It was finished in the one picture, so it would have been shot hundreds of times by that point.

The engineer may been using a different wood model.

To be 30% off in draw weight means the modulus of elasticity was a fair bit different than what Alan assumed it was. This is one of the challenges of doing this with wood. If I want a bow to be spot on for weight I will do a bend test sample off that specific piece of wood in order to be as close as possible. Limb thickness also plays a major role, but you would have to be off a fair amount to lose that much weight. If you were careful with removing material and measuring with calipers you should have been close enough to be in the ballpark.

A rectangular cross section might be the most efficient, as long as it matches the specific wood used.

Rectangular is absolutely the best cross section in terms of sharing the strain equally among all the wood in the limbs, but that isn't always the only consideration involved.


Mark