An Unusual New Bow Design

Tim Baker
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Tim Baker
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Joined: 6:22 AM - May 23, 2005

11:56 PM - Jun 14, 2018 #1

This design evolved from the bottom bow illustrated on page 97 of TBB-3. 
First step: Don't bother with a rigid bottom portion; let both portions bend and store energy.
The page 97 version, with the cord attached to the recurve, allowed conflicting cord angles between the two limbs.
So let the cord wind up on a smaller-diameter side portion of the recurve base itself
And shape the recurve to create an n-shaped f/d curve, not possible with a conventional fixed recurve. .
 
So we end up with:
Two limbs, one above the other, separated by 6 or so inches.
Each limb long, wide, and thick enough such that tip movement of 3" stores the same energy of a conventional limb.
The bow string rotates a short whittled branch, a narrow portion of which is recurve-shaped in order to generate an n-shaped f/d curve.
Beside and attached to the recurve-shape portion is a smaller circular portion; a cord runs from it to the tip of the top limb; that circular portion is of a diameter which will draw the two limbs together, as the bow string rotates the recurve. 
Cord tension keeps the recuve in place in its recess in the bottom limb.
 
For the moment, ignore friction where the branch axle rides in the lower bow limb, and how the other end of the branch is supported, and the necessary fiddling.
 
Three questions:
 
--  Is there any reason it wouldn't work well, store far more energy that a conventional bow, and have related arrow speed?
--  Is it paleo-legal? Would a pre-metal age bowmaker reasonably make such a bow? Whittling the recurve would take a lot of time and attention.
--  Is the cam shape of the recurve too troubling due to it's similarity to cams on a compound bow? This is a case of parallel evolution--the porpoise didn't use the shark as a model. 
 
 Comments, criticism, suggestions welcomed. 
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Tim Baker
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Tim Baker
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Joined: 6:22 AM - May 23, 2005

9:21 PM - Jun 16, 2018 #2

An early brainstorming model. Even with recurves the allowed hinging angles wouldn't permit long-enough draw lengths. 
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Steilpassfaenger
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Steilpassfaenger
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Joined: 9:07 PM - Oct 20, 2009

3:39 PM - Jun 17, 2018 #3

That's a tough one.
 
1. In my opinion a bow of this design could have a high dry fire speed.
 
2. It is paleo-illegal. People back then would have been far more concerned with hunting, gathering and not getting eaten by mosquitos. But this design idea could be from for the Middle ages, or about 2000 BC Asia. Pulleys and stuff like that were in use. One problem would have and will be to get the circular portions light enough and the drilling holes durable enough for frequent use. Most troubling are the hinges. When making a weapon, it is always good to have as few moving parts as possible.
 
3. What is a compound bow?
 
Tim, I like were you are going and it encourages me to try new things. It seems you are looking for new ways to store more energy.
 
Matthias
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Tim Baker
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Tim Baker
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Joined: 6:22 AM - May 23, 2005

7:45 AM - Jun 18, 2018 #4

Steilpassfaenger:
 
Agreed, and if the rotating recurve is given the proper shape the bow should cast a 10gpp arrow well over 200fps, the present holy-grail performance target of natural-materials bows. But there are a few if's between here and there. One: the recurves are not fixed rigidly to the limbs, so during the draw each is free to proceed at slightly different rates should left or right limb tension vary, reducing energy stored, and causing the arrow nock to rise or fall, throwing the arrow release path off. This can likely be remedied by adjusting connecting-cord tension or cord placement at the top limb.
 
The goal here is to devise an exceptionally high-performance bow of all natural materials which could reasonable have been imagined and made by pre-civilization bowmakers, defined as pre metal-age and pre cultures having multi-hundred-population towns. That cut-off point should be about 7,000 years ago. There are no pulleys on this design, no drilled holes or such; it's all very pre-civilization, still, its highly unlikely that true hunter-gatherer people would have made such a bow. So it's borderline Paleo to my taste.
 
The above-pictured early-brainstorming model is way primitive in construction, simpler than the innovative Inuit bows, for example, so could reasonably have been made by h/g's, so I used up much of a ream of paper drawing different versions of it, but even with f/d-fattening recurve shapes and receptive cavities for their feet, there was no way to make the geometry allow a 28" draw. Maybe someone else will be able to.
 
Possible benefits of the present design: 
It should store 50% or so more energy than a conventional design.
The big energy robbers are hysteresis and vibration.
The double limbs tips will travel 3" each instead of 14", traveling slower in the process, for less vibration as they are brought still.
Conventional limbs use up half of their bending capacity just being braced, the stored energy generated not available to the arrow. The recurve shape here will allow the limbs to be under low strain at brace, rise to high poundage quickly, little wood strain wasted. To the extent hysteresis rises with wood strain this design should be unusually efficient in this area.
 
Comments, criticism, suggestions welcomed. 
 
The simple primitive hinging design that seemed to have potential:
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