Post For Tim Baker ( Sinew)

[Springbuck]

Tim.    "
" To prevent failure at weak points a cable must be of uniform diameter it's entire length. But the wood is tapered in width and/or thickness. Intuition is throwing a shower of red flags. Pondering the problems and solutions now"

My (limited) experience with cable backs tells me that it doesn't matter that the cable back is uniform diameter.  A cable of uniform diameter and strength, pulled from both ends, must stretch uniformly.  A wooden bow core, as you mentioned, must taper in thickness and/or width.    Some minor adjustments to tiller must be made, but barring a restrained or anchored cable, tiller can still happen. 

For reference, imagine we back a bow with a zero-stretch backing like a steel strap.  Terrible performance, of course, but the bow could still be tillered to desired bend, ya?    So, why wouldn't that be true of a back that stretches uniformly?

I also just realized that one could easily sinew back over a linen cloth cover (as on the hollow-limb bow from my previous post).  Just gotta cut a strip of linen from the cloth diagonally, so it still creates a tight span laterally, but offers less longitudinal resistance.

[Springbuck]

Tim, I dig the idea myself, but let me see if I understand the actual technical problem.   Ideally, we use half the belly thickness, a spacer, and a sinew back, to equal the energy storage potential of a solid limb, but with much less mass.

The issues I see are that whatever the "spacer" material is, it must allow the glue to anchor the sinew to its surface.  Without the "spacer" anchored to both backing and belly, you essentially have an irregular, flat cable and string bridges.    So, we are looking for a spacer much lighter than wood, but strong enough to apply sinew directly to so the sinew acts as if applied to a regular wood core, e.g. not shifting or sliding around, and maintaining its position and function opposite its corresponding belly segment.


The shear forces don't really worry me, here, except that any version if this I imagine has the likely problem of the middle layer being too soft to hold the backing and belly correctly relative to each other.

[Tim Baker]

PatM:

Sinew stretches about 8% according to the tests here, which also gives it's elastic modulus/stiffness:

https://journals.openedition.org/ceroart/5152?lang=en

In a sinew backing the volume of glue is small compared to the sinew, so it should have a relatively smaller effect on the stiffness of the two together.

Here Dick Baugh measures the stiffness of sinew:

http://www.primitiveways.com/secrets_of_sinew.html


Springbuck:

A back is as stiff as it's least-stiff area. If the cable is not uniform in diameter the narrow less-stiff areas would seem to represent the net stiffness of the back, the thicker areas being partially dead weight. The thinner areas will be stretched farther than thicker areas, possibly to failure. --  Even though the thicker area are harder to stretch I need to determine if they actually are storing less energy per mass than the thinner areas. Intuition isn't dependable.

The spacer doesn't have to be especially light. This test bow's spacer is sinew-heavy yet draw weight rose 400%  without quite doubling in mass.

Yes,  if  two slats of wood they must rigidly be keep from slipping in relation to each other.
But this might not be absolutely the case if the back is sinew:

Image a semi-rigid spacer, rigid enough to prevent the narrow outer-limb sinew from stretching dangerously more than the wider gripward sinew, but flexible enough to not fail when drawn from large reflex to a full draw
That's essentially the case with this bamboo/cotton-glue/sinew test bow. Surely not optimum, but worth noting.

But argue back.

Tim

[PatM]

 Is the glue fraction actually that small in a typical backing?
 
 Still not sure why you want a spacer but are averse to a wood space.r

[willie]

willie:  To prevent failure at weak points a cable must be of uniform diameter it's entire length. But the wood is tapered in width and/or thickness. Intuition is throwing a shower of red flags.

My (limited) experience with cable backs tells me that it doesn't matter that the cable back is uniform diameter.  A cable of uniform diameter and strength, pulled from both ends, must stretch uniformly.  A wooden bow core, as you mentioned, must taper in thickness and/or width.    Some minor adjustments to tiller must be made, but barring a restrained or anchored cable, tiller can still happen. 

I believe the adjustments have to planned for. I have some glue curing on one of Springbucks cable prototypes, hope to post the bow in a few days.

BTW, Spring buck . "A cable of uniform diameter and strength, pulled from both ends, must stretch uniformly." seems like reasonable assertion, but I have seen industrial examples that don't work that way. Enough for now, but if it seems to be relevant later in the discussion, I can describe the example, but have no explanation why it happens.

The thinner areas will be stretched farther than thicker areas, possibly to failure. --  Even though the thicker area are harder to stretch I need to determine if they actually are storing less energy per mass than the thinner areas. Intuition isn't dependable.

  Can I make an educated guess? If you spliced three equal length lines end to end, such that each outer third was made from 3/4 inch line and the middle third was 3/8 in dia, and stretched it to failure, (assuming that the line parted in the middle of the smaller dia. line), what would be the reaction? I think smaller dia. ends of the center third would react much more violently than the outer thirds, much snap back, especially if it is a stretchy materiel. It stretched further and stored more energy per mass. If the energy storage was proportional to the mass, the snapback would be uniform for the entire spliced line.

Tim, you have  made the claim a few times that doubling thickness of your stave yielded approx 400% increase. Is that the same as 4 times the stiffness? I thought that a doubling of thickness was supposed to increase stiffness 8 times in a uniform materiel. Different people figure percentages different ways. One common method is ...the difference between loads divided by the original load.

[PatM]

Part of the uniform cable problem is that it carries unnecessary mass out at the tips.  A lot of these experimental designs look promising and then they get dragged down by all the unforeseen extras.

 Springbuck mentions all sorts of lashing and bracing with extra strands which all has to be put in motion.

[bradsmith2010]

 (=)any chrono results,, 

[PatM]

Probably waiting for the touch up sinew to cure.

[Yellarwoodfellar]

What if one were to use praying mantis cocoons as the bridge? This things are tough as nails and light and flexible all in one! Just an ideer.

[Tim Baker]

PatM:

According to tests at this site hide glue stretches about the same % before breaking as sinew.
https://journals.openedition.org/ceroart/5152?lang=en


"Is the glue fraction actually that small in a typical backing?"

Before a bundle of sinew is set in place the excess glue is stripped out and off of it, a relatively small amount of glue, left.
 
 " Still not sure why you want a spacer but are averse to a wood space."

Adding a wood spacer would be like starting with a thicker belly to begin with. 

willie:

" Tim, you have  made the claim a few times that doubling thickness of your stave yielded approx 400% increase. Is that the same as 4 times the stiffness? I thought that a doubling of thickness was supposed to increase stiffness 8 times in a uniform materiel."

If you double the thickness of wood then stiffness increases 8 times, but if you double wood's thickness using sinew instead of wood it increases about 4 times, sinew being less stiff than wood.

[PatM]

PatM:

According to tests at this site hide glue stretches about the same % before breaking as sinew.
https://journals.openedition.org/ceroart/5152?lang=en


"Is the glue fraction actually that small in a typical backing?"

Before a bundle of sinew is set in place the excess glue is stripped out and off of it, a relatively small amount of glue, left.
 
 " Still not sure why you want a spacer but are averse to a wood space."

Adding a wood spacer would be like starting with a thicker belly to begin with.

  You  still need to test the mixture of the two.  Composite materials are known for "the whole being greater than the sum of the parts". I realize they are both the same material but the molecular structure is not.   You wouldn't replace a tendon with a medical 'gummy worm" even if you dried it first.

 However if they did actually have the same tensile strength after longer curing then the glue wouldn't give on so many sinewed bows.

 Even if most of the glue is stripped out or squeezed out it still amounts  to a fair additional mass amount.

 The point about the wood core was the assumption that reflex would be glued between it and the belly material.  It wouldn't be just a thicker belly then.

[Tim Baker]

PatM:

 " You  still need to test the mixture of the two.  Composite materials are known for "the whole being greater than the sum of the parts... "
 
For the fun of the argument let's say a sinew cable was created, the sinew strands prevented from slipping past each other not by twisting or glue, but by a massless magic force. I think we're agreeing that a same mass cable made of hide glue would stretch roughly as far without breaking as the sinew cable, and have roughly the same stiffness. But you suspect that by combining them one or both will then have substantially different properties. I'd bet the difference, if any, would be too small to matter, but it's valid question. Seems impossible to prove though, because a same mass sinew cable can't be constructed: glue would void the test, and twisting affects both the mass per length of the cable and its stiffness. 
 
" However if they did actually have the same tensile strength after longer curing then the glue wouldn't give on so many sinewed bows."

Different batches of hide glue vary in tensile strength and % of stretch by more than double. 
https://journals.openedition.org/ceroart/5152?lang=en

If glue in a sinew backing fails sometimes it make sense that it's a weaker versions of it.

A useful footnote: store-bought gelatin makes exceptionally strong glue
 
" Even if most of the glue is stripped out or squeezed out it still amounts  to a fair additional mass amount.

I just weighed an ounce of dry sinew and an ounce of dry powdered hide glue. The sinew volume seems maybe 50% greater than the glue, but hard to tell for certain. (used powdered glue to more closely match the spaces between strands of hide glue)  But lets figure sinew has 100% greater volume. In that case, even if most of the glue is squeezed free of the sinew a somewhat significant mass amount could still remain. A lot would depend on the diameter of the sinew strands of course. The finer the strands the more glue would be incorporated. Which leads to this question: what is ideal stand girth for strongest per mass sinew backings?

" The point about the wood core was the assumption that reflex would be glued between it and the belly material.  It wouldn't be just a thicker belly then. "

Please explain this fuller, I don't fully understand.  

This has been good line of discussion.

[PatM]

PatM:

 " You  still need to test the mixture of the two.  Composite materials are known for "the whole being greater than the sum of the parts... "
 
For the fun of the argument let's say a sinew cable was created, the sinew strands prevented from slipping past each other not by twisting or glue, but by a massless magic force. I think we're agreeing that a same mass cable made of hide glue would stretch roughly as far without breaking as the sinew cable, and have roughly the same stiffness. But you suspect that by combining them one or both will then have substantially different properties. I'd bet the difference, if any, would be too small to matter, but it's valid question. Seems impossible to prove though, because a same mass sinew cable can't be constructed: glue would void the test, and twisting affects both the mass per length of the cable and its stiffness.
 
" However if they did actually have the same tensile strength after longer curing then the glue wouldn't give on so many sinewed bows."

Different batches of hide glue vary in tensile strength and % of stretch by more than double.
https://journals.openedition.org/ceroart/5152?lang=en

If glue in a sinew backing fails sometimes it make sense that it's a weaker versions of it.

A useful footnote: store-bought gelatin makes exceptionally strong glue
 
" Even if most of the glue is stripped out or squeezed out it still amounts  to a fair additional mass amount.

I just weighed an ounce of dry sinew and an ounce of dry powdered hide glue. The sinew volume seems maybe 50% greater than the glue, but hard to tell for certain. (used powdered glue to more closely match the spaces between strands of hide glue)  But lets figure sinew has 100% greater volume. In that case, even if most of the glue is squeezed free of the sinew a somewhat significant mass amount could still remain. A lot would depend on the diameter of the sinew strands of course. The finer the strands the more glue would be incorporated. Which leads to this question: what is ideal stand girth for strongest per mass sinew backings?

" The point about the wood core was the assumption that reflex would be glued between it and the belly material.  It wouldn't be just a thicker belly then. "

Please explain this fuller, I don't fully understand. 

This has been good line of discussion.

 A  glueless sinew cable is easy to find. It's called a dried whole tendon.

 Not sure what the final moisture content was in the tested samples linked to but I'm skeptical that the glue was at full strength after what seems a rather brief cure.

 Maybe weigh a slim tendon rather than trying to estimate a pounded fluffed up strand of sinew.

 I no longer pound sinew to process it.

 I meant that the core and belly would have perry reflex themselves rather than just being a single thicker piece.

[Tim Baker]

A photo of the bow was posted on the PaleoPlanet site. It's the 11th posting down on this thread:

https://www.tapatalk.com/groups/paleoplanet69529/how-much-can-a-sinew-backed-bow-be-reflexed-withou-t66178.html  

The next version will be longer, have far more reflex, a  rectangular-section sinew back, elevated a bit more than on this first test version.

[BowEd]

Tim...Most times after it's done and cured fully with the normal squeezing out of glue from sinew before applying it on the bow the glue ends up weighing about 1/3 of the total sinew glue matrix applied on the bows.The 30 to 40 percent glue mass weight was the ideal weight to achieve.This can be a little variable as to how thick the glue is too when using,but in general a little thicker glue used while sinewing than used while sizing.I seem to remember Pat and me had a discussion about this on here in the past.