Post For Tim Baker ( Sinew)

[sleek]

It will be fun to see if it cant. But with the compression force of sinew pressing down on the cork as the bow bends, and the sponge like structure of cork. Id bet it will hold fine.

[sleek]

I think pure cork, not the shreeded processed stuff it what is needed.

[Stick Bender]

Thanks for posting this thread  I spent most of my work day thinking about your design it really passed the time , but the thoughts in the end were, wouldn't you gain the same performance increase  by increasing the reflex & stratigicly placing & crowning the sinew like Marc did in his 64 in. Maple recurve a while back ?  Or  a combination of  light lams in the mix increasing limb thickness ?  If the goal is to max the sinew tension with out mass gain it seems the above would accomplish that also ? Well I had 500 Mi. To think on it , Just my thoughts I'm not saying that defenitivly  I'm just asking ?

[Tim Baker]

PatM:

Sinew is much less stiff than wood and stretches several time farther, so making bow thickness double by using sinew instead of wood raises draw weight by about 4 times instead of 8, and does so safely, where wood would break.

Yes, glue-soaked cotton is as heavy or heavier than bow wood. It's likely not the perfect elevator. one of it's purposes was to save sinew. Elevating the sinew was the goal, and being essentially neutral in tension or compression, it did that without the complications a wood core would bring--using a wood core would be essentially the same as starting with a thicker bow.

Mark:

Yes, balsa, or similar, might be better. Sheer forces develop in the limb; if back and belly can slip and relieve the strain it seems this would create large hysteresis and reduce stored energy [more study needed], if so then the trick will be to find a material stiff enough to resist slippage/deformation but be as light as possible.


I recently whipped some hide glue into a foamy froth; when it dried it was about .10 specific gravity and quite rigid. Some versions of that might be the ideal elevator. Experiments needed. 

George:

The sinew on Inuit cable bows and similar was not highly elevated and not of large enough diameter to raise draw weight as in this case. And possibly the sheer-slippage situation too. Their ingenuity went simply into getting a working bow at all, from no apparent makings. If they'd had decent wood they likely would't have bothered with sinew. 

gfugal:

Yes, once made silk cable-backed bows, the cable elevated with the nodes. A couple of problems with that: Since the cable is uniform in diameter it stretches most near the grip, essentially dead weight at the outer limb. This also meant that energy was mostly stored in the near grip sinew.  
 
DC: Cork and related is well worth trying. It might surrender to the sheer forces, but only one way to know.

Stick bender:

Crowning the sinew means that only the top of the crown is highly stressed, sinew below that having progressively more mass per work done. And the hardest working crown-top portion would be a relatively small amount of sinew, unable to store near the energy of a rectangular section of equally high sinew. Typically such designs only gain a few extra pounds over the wood itself, the maid point of the sinew being to keep the bow from breaking. 

But this is all in-progress thinking.

Tim 

[upstatenybowyer]

"Crowning the sinew means that only the top of the crown is highly stressed, sinew below that having progressively more mass per work done. And the hardest working crown-top portion would be a relatively small amount of sinew, unable to store near the energy of a rectangular section of equally high sinew. Typically such designs only gain a few extra pounds over the wood itself, the maid point of the sinew being to keep the bow from breaking."

I can attest to this, as I recently finished up a short, high-crowned osage bow that got a narrow coarse of sinew along the top of the crown. It added about 5 lbs. of draw weight, helps hold the profile, and allows me to draw the bow farther. All of these are fantastic benefits, but it didn't increase performance in a way this thread is seeking to discover.

Cool thread by the way. Great to have you around Tim!

[PatM]

PatM:

Sinew is much less stiff than wood and stretches several time farther, so making bow thickness double by using sinew instead of wood raises draw weight by about 4 times instead of 8, and does so safely, where wood would break.

Tim

   Do you factor in that sinew and glue is not quite the same as dry sinew?   I don't think a sinew glue matrix actually has the stretch  numbers that people throw around for sinew.

 I'm not clear if you glued the cotton on in reflex or not.

[sleek]

Ok, here is my thought process. ( when I say that, be rrady to go so far out the box, you wont even know what way is up, it gets crazy out here ).

You need something light weight as a core. For that in aviation we use honeycomb. It has no strength in any direction but it resist compression well. But natural honecomb is not an option. So a corigation, like cardboard perhaps, using split bamboo shoots side by side width wise.

Incase thats a no go, the point is you need something that is more a filler than a solid rigid structure. So i thought about your whipped hide glue. I know if it were cooked it can become more solid. What about frying some rawhide, then soaking it in hide glue to make is stronger, then using that as a core?

This is where it gets real fruity... Sweet young thai coconuts have a very spongy shell. Use a knife a spin the coconut on a lathe, and peel off a long strip of this spongey husk, lay flat and let it dry. I bet that would work. If not, bananna peels, orange peel, watermelon rind, a split piece of sugar cane, palm leaf stem.... I cant think of everything, but thats a direction to look.

[willie]

The sinew on Inuit cable bows and similar was not highly elevated and not of large enough diameter to raise draw weight as in this case. And possibly the sheer-slippage situation too. Their ingenuity went simply into getting a working bow at all, from no apparent makings. If they'd had decent wood they likely would't have bothered with sinew.
Tim 

Tim,  there are certainly some examples of crudely made Inuit cable bows, but I hope you are not overlooking some of the more similar finely made cable bows found further south. Perhaps a thread investigating some better examples would be worth persuing at another time?

[Tim Baker]

PatM:

Yes, hide glue won't stretch as far as sinew. The small-diameter bends on some Asiatic composites sometimes show the whitish shatter of the over-stretched surface glue. But no need for the sinew to be stretched that far here, and hide glue is the only game in town anyway.

Mild reflex when the cotton was applied, taken to 5" for the sinewing. But this is all proceeding by braile at this point; lots of trial and error bows needed before ideal ratios and materials come in to clear view. It might have been just semi-luck that the ratios and materials used in this first test bow worked out so well. The law of averages pretty much insists that it can be improved on.

sleek:

Possibly the main quality the elevator should have is that it no add to belly compression load. mass maybe not at important. I'll give some of your suggested Thai coconuts a try... 

willie:

I've checked out the cable bows pretty well but likely haven't see all extremes of design. If you come across any of interest here please to let us know.

[willie]

1. Here is a drawing (from Murdoch) of a heavy sinew cable southern eskimo, the cable scales at 1/2" dia.
2. A photo some Haida bows, one with an elevated cable.
From https://anthro.amnh.org/anthropology/databases/common/image_dup.cfm?catno=16%20%20%2F%20%20%2033
3. A photo of a birch backed/compression wood spruce limb that failed in shear. It once had a rather heavy elevated cable. The 40" bow was pulling 70# @ 22" when it "slipped". It is 1.1 inches wide.

[sleek]

Tim, whats the ideal thickness of the core for you?

[sleek]

I GOT YOUR ANSWER!!!!! CHOLLA CACTUS!

Its a natural honey comb And its woody and super light weight. It will have some compressive resistance, which you dont want BUT.... if you cut it up int 1 inch or half inch long pieces, and butt them up against eachother, as the bow bends and sinew stretches, rather than add compression to the belly, the belly will force the cuts to open up and stretch the sinew more. If this wont do what you want, I dont know if what you want can be done. This stuff is the perfect core. Im sourcing some tomorrow to build one ASAP!

SORRY, Im excited 

[Tim Baker]

sleek;

The cotton/glue on the present bow is about 25% of finished bow thickness, the sinew about another 25%. I don't know yet if these ratios are ideal, and a lot depends on the bare bow's original thickness and draw weight.

willie: 

Thanks for the graphics. I've been pondering those and similar bows for 30 years--If you have TBB-1 page 110 shows early experiments. The Murdoch and Haida bows had similar limitations and problems. On the Murdoch bow the sinew is not elevated so it's capacity to do work was limited; as with typical whole-back sinewing, draw weight would rise just a few %. And since the sinew cable stretches more than the wood back the energy usually stored by resisting sheer would be lost. And the friction of the cable against the wood costs some energy. All of this together means a relatively inefficient bow compared to a conventionally sinewed wood bow, and possible a typical self bow too. The Haida bow has a quite low volume of sinew, and there's the no sheer-energy problem. Not an efficient bow, but at least it was a bow. And really, that all they were aiming for. If the sinew volume had been higher, and both elevated and rigidly secured to the belly, the draw weigh and energy storage could substantially rise. But, from reports, they didn't have quality wood to allow this. Otherwise they'd likely have made all-wood bows as in most of the rest of the world. 

Tim

[Stick Bender]

How thick of increase are you talking Tim ? I have a bunch of walnut veneer laying around that could easily laminated to any thickness or they sell it it in rolls up to 2 in. wide , unless I missed it what is your front view profile ? Plus your a big trouble maker my brain has been in over drive thinking about your project it's a refreshing thread 

[Stick Bender]

Sorry I posted the same time as you thought I was the only one that worked on bows in the middle of the night ... Lol but I still think you could come up with a combination of lighter laminated cores the whole idea is based on raising thickness with out raising mass ?