Author Topic: Bend radius and compression loading of osage  (Read 887 times)

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Online sleek

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Bend radius and compression loading of osage
« on: September 11, 2017, 01:21:17 pm »
I am working on very stressed designs and I am to a point where I want to know how tight a radius osage can bend before set starts to take place. Of course what poundage it is also is a factor in that. So there must be a golden ratio between bend radius and draw weight.  This will tell me how short a working limb can be at what draw weight and allow me to compensate with deflex of a known angle to get longer draw lengths of shorter bows.
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Online sleek

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Re: Bend radius and compression loading of osage
« Reply #1 on: September 11, 2017, 01:22:04 pm »
I may do tests at 1.5 inches wide to set a standard.
Tread softly and carry a bent stick.

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Online Del the cat

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Re: Bend radius and compression loading of osage
« Reply #2 on: September 11, 2017, 01:42:07 pm »
Depends on the thickness, a thin shaving can roll up very tight.
Del
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Online sleek

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Re: Bend radius and compression loading of osage
« Reply #3 on: September 11, 2017, 02:10:56 pm »
Draw weight will certainly be a factor. I think I have a billet with no match. It will be sacrificed for testing.
Tread softly and carry a bent stick.

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Offline willie

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Re: Bend radius and compression loading of osage
« Reply #4 on: September 11, 2017, 05:13:48 pm »
draw weight and thickness go hand in hand.

something like this could be used like the tilering gizmo to read the curves
Code: [Select]
https://www.amazon.com/iGaging-Electronic-Digital-SnapDepth-Fractions/dp/B0063LMXJ6/ref=pd_sbs_328_1?_encoding=UTF8&psc=1&refRID=W089PQK05FMBB13WRPC3

Online Badger

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Re: Bend radius and compression loading of osage
« Reply #5 on: September 11, 2017, 06:07:01 pm »
  Thickness has a lot more to do with radius than draw weight. Ideally any weight bow whether 150# or 20# would be under the same strain. The width of the bow controls the weight of the bow. 2 bows the exact same length could have totally different style bends in the limbs and be different thicknesses.

  Sleek the answer to your question is you need to learn how to monitor the condition of your wood as you tiller the bow. ( No set tillering) You can find just the right amount of strain for the piece of wood you are dealing with. Some guys do this by feel and don't even realize they are doing it. However you do it you do need to learn how to monitor the wood. I give some directions on that in the no set tillering thread.

Online sleek

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Re: Bend radius and compression loading of osage
« Reply #6 on: September 11, 2017, 07:52:42 pm »
Steve, your process is certainly one I need to learn. Its only short coming is that you unfortunately find yourself making the draw weight to the bow rather than the bow to draw weight as most of us would like to do.  Perhpas that shortcoming is more mine that your process to be fair.

I am hoping to find a way to get to make the bow the draw weight and length I want and know the dimensions it needs to be rather than work dimensions and see what they will give me. My next bow will be a 50 pounder at 28. Its to be 50 inches when strung. I have a stave just shy of 2" wide and 56" long. How do i get this to give me my desired draw and keep set down while keeping string tension up?
Tread softly and carry a bent stick.

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Offline mikekeswick

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Re: Bend radius and compression loading of osage
« Reply #7 on: September 11, 2017, 10:08:35 pm »
How long is a piece of string? ;)
Badger just told you how to do it!
Any given wood has an 'ideal' thickness. Adjust width for draw weight you require.

Offline Pappy

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Re: Bend radius and compression loading of osage
« Reply #8 on: September 11, 2017, 11:57:48 pm »
Sorry to say Sleek but I have never found a standard formula for any piece of wood. :)
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Offline Dances with squirrels

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Re: Bend radius and compression loading of osage
« Reply #9 on: September 12, 2017, 01:52:36 am »
You guys are way over-simplifying it by saying 'any wood has an ideal thickness', and it's not true as stated.

Online Badger

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Re: Bend radius and compression loading of osage
« Reply #10 on: September 12, 2017, 04:27:19 am »
Steve, your process is certainly one I need to learn. Its only short coming is that you unfortunately find yourself making the draw weight to the bow rather than the bow to draw weight as most of us would like to do.  Perhpas that shortcoming is more mine that your process to be fair.

I am hoping to find a way to get to make the bow the draw weight and length I want and know the dimensions it needs to be rather than work dimensions and see what they will give me. My next bow will be a 50 pounder at 28. Its to be 50 inches when strung. I have a stave just shy of 2" wide and 56" long. How do i get this to give me my desired draw and keep set down while keeping string tension up?

   Sleek, it is a reality based method. You pull the bow to full target weight every time you pull it. If it won't take it you find out early and lower your draw weight. You can easily build bows to target draw weight without breaking them even if they are way past their elastic limits. They will still be dependable bows yet they won't perform as well. Just the opposite from what you are thinking, it is probably the easiest way to hit your target weight.

Online Badger

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Re: Bend radius and compression loading of osage
« Reply #11 on: September 12, 2017, 04:30:08 am »
You guys are way over-simplifying it by saying 'any wood has an ideal thickness', and it's not true as stated.

  How else would you say it, every length, weight, design etc will end up at different thicknesses depending on the radius of the bend you are using. I imagine you have a plus or minus factor of a few percentage points either way where it will still work just as well but there will still always be an optimum thickness for any design.

Offline Dances with squirrels

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Re: Bend radius and compression loading of osage
« Reply #12 on: September 12, 2017, 05:01:40 am »
Well that's a little better, and kind of what I was getting at, Steve. With your quantifiers and "etc", it leaves THE perfect thickness harder to nail down. Because in reality, and by your own admittance, there are a variety of "perfect thicknesses for any wood".

Offline Dances with squirrels

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Re: Bend radius and compression loading of osage
« Reply #13 on: September 12, 2017, 05:09:37 am »
Additionally, the percentage or plus or minus factor you mentioned is much greater in some woods than others...

....not to mention the rather sizable differences of ability possible within a single wood species that could/should cause a bowyer to alter the thickness to width ratio relative to draw weight and other factors.

Just don't want us to paint ourselves into a corner here.

Online Badger

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Re: Bend radius and compression loading of osage
« Reply #14 on: September 12, 2017, 05:33:03 am »
  I doubt we ever find the perfect thickness unless we just get lucky. I think it boils down to defining to ourselves what is acceptable to us. If you go past the limits of elasticity performance starts falling off gradually at first and then rapidly, if you never reach the limits of elasticity you end up with more mass than you need and it will start to slow you down. You have more tolerance on the too much mass side than you do on the overstress side of the curve. I think the idea is as a bowyer is to refine your skills so you get better at honing in on that ideal area we are looking for. Years ago it was published that about 10% hysteresis is just inherent to wood, we know better now. If you stay within the elastic limits wood has probably closer to 1% hysteresis. If we monitor the wood closely we can keep that number down in the 3% or 4% range. So practically I agree with you that we do have a range, but ideally there are methods that can hone in on that ideal number.