Post For Tim Baker ( Sinew)

[willie]

And since the sinew cable stretches more than the wood back the energy usually stored by resisting sheer would be lost............ The Haida bow has a quite low volume of sinew, and there's the no sheer-energy problem. 

Tim, I am not familiar with the no sheer-energy problem. Shear seems to be touched on frequently in this thread, concerning both your design, and the traditional cable design. Are you suggesting that usable energy is stored in shearing forces? Perhaps you would be willing to expound a bit more about shear, or at least point to a resource that helps the bowyer understand how it needs to be a design consideration?

........... 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.

The northern Eskimos did not seem to have a good source of wood, and I agree that a cable might not be a very efficient way to store energy, my guess is that there is too much friction between strands (hysteresis) when retracting. But in spite of this inefficiency, it appears that a Haida yew paddlebow still benefited from the addition of a sinew cable, even in a wet environment. Maybe sinew has a lot more to teach us?

http://www.haidanation.ca/?p=1677
“Pacific yew is so closely tied with Haida tools and weapons that it shares its name with the paddle bow: hlG̲iid. In Haida Gwaii, these hunting tools are uniquely fashioned. The unusual Haida paddle bow is stripped of its sapwood. This design is thought to be well suited for canoe life, where waves and rain keep everything well soaked.

[bradsmith2010]

Tim the chrono results will be very interesting,, thanks for sharing B

[Tim Baker]

Stick Bender:

Thee in the morning when the world is asleep. A good time for serious pondering.

willie:

Take an 80lb bow at full draw and slice it in two along it's neutral plane. It's still at full draw, and essentially looks the same as before being sliced in two, but somehow now it's two 10lb bows instead of two 40lb bows, Why? And where was all of that extra energy hiding?

When whole, at full draw, the back was being stretched longer, the bell compressed shorter,  high shear forces in play between the two, much energy stored as a result. At release the shear forces are free to return to zero, doing work in the process 

If sinew is not rigidly attached to the belly then shear force between the two can't exit, and otherwise resulting stored energy won't exist.

The dynamics become way more interesting and complicated when seeking optimum designs where large-volume sinew is both elevated and rigidly attached. Not fully thought out yet. 

Not enlarged on yet, there are problems when straining  a uniform-diameter cable due to non-uniform stresses along it entire length. More later.

Early on I was infatuated with Haida and related designs but couldn't get them to perform as well as well-designed same-weight self wood bow. I love the idea of them, and would love to see one perform as hoped. Somehow it seems they deserve to. 

Thanks for the targeted questions. They help sort things out.

Tim

[Tim Baker]

Willie:


Gee I'm dumb. That shear argument doesn't hold up. When at 80lbs most of the energy is stored at and near back and belly surfaces, shear not involved. Gotta think more before typing.

Tim

[willie]

If sinew is not rigidly attached to the belly then shear force between the two can't exit, and otherwise resulting stored energy won't exist.

Tim,  I think your explanation of shear forces is right on. And without force you cannot have energy, right again.
But you can have force without energy. You can push against a locomotive, but if it does not move, you have done no work. If you push harder and it moves, then energy is 1/2 the mass of the locomotive times the velocity squared.

A bow limb is 3 foot long piece of wood and you are trying to shear it along the length. Lots of force required, but if it doesn't deform to any significant degree, no appreciable amount of energy is stored. Your design still needs to be able to resist this force Of course all most all of the energy in a bow is stored in the back and belly because they actually stretch and compress.

I didn't mention earlier how much trouble it was making "cable nocks" for the bow pictured earlier. It started life about a foot and a half longer, but by the time I rebuilt the attachments three times, the bow was only 40" long.  I was determined to build up the "cable nocks"  until something else  broke. The birch "back" under the bridges sheared at 70# pull. To keep the cables attached to the ends of the limbs, I added the birch blocks shown below. They are approximately 3/4" wide by 2.5" long, glued with a good epoxy. I suppose that a similar sized short lam of decent wood, at the ends of your limbs and on top of the bamboo, could anchor the ends of your sinew layer. Couldn't the remainder of your core be the whipped hide glue, so long as it was firm enough to not collapse and maintain the needed elevation?

[PatM]

I picture a strip of this Maple moulding  as a intermediate strip to raise the sinew.
 
  Repair the gap in the non sponsor link and paste it.

https://w ww.royalwoodshop.com/mouldings/product/burlap-decorative-bur101/?

[sleek]

This is cholla cactus. It is mostly air, very resisnt to being crushed, and woody. This would make a perfect core by cutting it down to strips, and laying sinew over it. I vant think if a more perfect way to do this.

[sleek]

Willie:


Gee I'm dumb. That shear argument doesn't hold up. When at 80lbs most of the energy is stored at and near back and belly surfaces, shear not involved. Gotta think more before typing.

Tim

Tim, i can assure you that sheer forces there and are strong in the neutral zone. All of the compression forves at the belly are mirrored by the twnsion forces at the back, but it is sheer force that is the conversion factor between tension and compression. Energy is stored at the back and belly, but only because those at the surfaces where the energy accumulates. Its coming from the neutral plane. You could say the neutral plane is the compression and tension force generator, sending energy out to the flat surfaces above and below it.
 

[DC]

Is there a reason engineers named it the "Neutral Zone"? If all the stresses and strains were in the middle "I" beams would be "O" beams wouldn't they?

[sleek]

Is there a reason engineers named it the "Neutral Zone"? If all the stresses and strains were in the middle "I" beams would be "O" beams wouldn't they?

All i know is that because its under 0 compression, and 0 tension, its the middle, so its neutral in those forces. But it is where all the sheer forces are located.

[gfugal]

This is cholla cactus. It is mostly air, very resisnt to being crushed, and woody. This would make a perfect core by cutting it down to strips, and laying sinew over it. I vant think if a more perfect way to do this.

I used to live in Santa Fe New Mexico when I was growing up and chollas were everywhere. When they died they left that woody Skelton. They are surprisingly resilient strong and somewhat flexible if I remember correctly. I remember trying to break them thick sticks as a bat with no avail. After I moved from there and got into bow making I have often thought of whether a bow could be made from them, maybe as a laminate. The problem i see is that they have all those whole (which would reduce mass) but if you used glue with them they would fill up with glue as dead weight which weighs more than the wood that might have been in it's place.
 

[Badger]

  I was playing with corn cobs yesterday, the hollow core is extremely light and extremely strong, glued side to side it would make an excellent cable bridge.

[sleek]

This is cholla cactus. It is mostly air, very resisnt to being crushed, and woody. This would make a perfect core by cutting it down to strips, and laying sinew over it. I vant think if a more perfect way to do this.

I used to live in Santa Fe New Mexico when I was growing up and chollas were everywhere. When they died they left that woody Skelton. They are surprisingly resilient strong and somewhat flexible if I remember correctly. I remember trying to break them thick sticks as a bat with no avail. After I moved from there and got into bow making I have often thought of whether a bow could be made from them, maybe as a laminate. The problem i see is that they have all those whole (which would reduce mass) but if you used glue with them they would fill up with glue as dead weight which weighs more than the wood that might have been in it's place.

The trick is, dont use so much glue it fills up. Apply a layer of glue over it, lay your sinew down, probably best to use the long solid back strap piece, let it dry, then lay your shredded sinew over it.

The back strap sinew would act like a canvas covering the holes, and give a solid bed to attatch your sinew layers to.

[sleek]

  I was playing with corn cobs yesterday, the hollow core is extremely light and extremely strong, glued side to side it would make an excellent cable bridge.

I bet corn cobs would work great as well.

[Tim Baker]

Agreed,  there is shear force at the neutral plane of a bending beam/bow, but since there is no movement at the NP no energy is stored. Energy is stored only as the back is stretched and the belly compressed. 

Some thoughts on advantages the experiment-bow here should have over conventional self or sinew-backed bows:

Since draw weigh rose 400%, to normal draw weight, means the belly alone was initially of unusually low thickness/draw weight. Being so thin allows extreme reflexing, for exceptional early draw weight, for greater total energy storage.

Extreme reflexing draws the belly surface into extreme tension, reducing its surface compression load when drawn, and allowing deeper non-surface wood to carry a larger portion of the load. 

Draw weigh rose 4 times but mass less than doubled, raising efficiency by reducing energy lost to limb vibration. 

Arguments, suggestions and comments welcomed.