Post For Tim Baker ( Sinew)

[willie]

Belly compresses, but it WANTS back to stretch.  Back stretches, but it would rather force belly to compress.  I don't think the shear forces actually STORE much energy  (because shearing is all or nothing, catastrophic) but the whole rest of the bow must EITHER store energy by bending, or shear and collapse.

this thread has me thinking a little bit different about shear and energy storage. the cells in wood are a bit longer than those in the pic, but they are not free to slip along side each other. this resistance to slipping causes distortions in the cell walls, and the cells want to straighten when the load is released just like the tension or compression distorted cell want to return to it's original shape. In fact this resistance to shear causes the compression and tensile stretching. If the cells are free to slip, the limb has to be much thicker to resist the same bending load.

id wager that is what we call hysteresis. Its not actually stored energy, but energy lost, as Tim ( i think but maybe someone else ) puts it, due to internal friction.

hysteresis is when the cell does not want to return to shape rapidly.

[DC]

Could you use a, say, 1" dia. whole bamboo. That gives you a thin belly, light center(air) and a thin back to sinew to. Just spitballin"

[Tim Baker]

BowEd:

" Are you suggesting a purposeful and localized amount of slippage in parts of the limb might be helpful?
Did you mean " rigid flexible enough to prevent the narrow outer-limb sinew from stretching dangerously"

The provisional thinking is that a spacer with some give in it would allow the sinew to be elevated without the failings of in-air sinew that narrows from grip to tip. Such sinew would be over-strained in its narrowed portions. A some-give spacer would be set in place with the belly in large reflex and firmly glued to belly and sinew. As it's braced and drawn the narrower portions of the sinew would struggle to over stretch but be prevent, only slight give in the spacer, all of the sinew made to work just as if directly glued to the belly, minus a small % of stored energy lost to the give.


BowEd: 

I"ll do it in a slightly different way and see if we get similar figures: I'll weight a couple of ounces of sinew, apply glue as if applying it to a bow, but let each bundle separately air dry, suspended by a thread. They should dry quickly. 10 days should do it, but will wait till no weight loss whatever.

Sleek:

This dense-foam glue is really interesting. It's harder to thumb-nail dent than pine--just tested it--but is half the mass, per float test. The first batch was made accidentally by whipping powdered glue into hot water. Knox gelatin in this case. Being self-insulated, drying/shrinking time is long, It's probably not worth the trouble to fashion a large enough batch for the sinew spacer here, but might have other uses. It might be a new paleo technology.

[Springbuck]

 PatM:   You misunderstood the perry reflex comment.   I meant that the core material and belly would have perry reflex between them and the sinew and core would also have it between them because the sinew would be applied with the core material in feflex as well."

 Ah, yes.  I did misunderstand it, then.

[Springbuck]

   How many layers?
[/quote]

  Good question.  How thick is a layer.  I was thinking of a pretty short little West Coast kind of bow I tried once, and I put on two layers of bundles smaller than a pencil.

[Springbuck]

Could you use a, say, 1" dia. whole bamboo. That gives you a thin belly, light center(air) and a thin back to sinew to. Just spitballin"

You could, and I had a similar idea about what I've heard call a "loose" laminate bow, where the bow is made by stacking thin lams that are not glued.  (Essentially a bundle bow, but stacked slats, not sticks.)   

But, the bamboo immediately UNDER the sinew then is still resisting the stretch.  You really wouldn't get the most out of your sinew. 

[Springbuck]

 "this resistance to slipping causes distortions in the cell walls, and the cells want to straighten when the load is released just like the tension or compression distorted cell want to return to it's original shape."

Oh, yeah!  Their position relative to each other is probably more "fixed" than their shape. 

  "In fact this resistance to shear causes the compression and tensile stretching. If the cells are free to slip, the limb has to be much thicker to resist the same bending load."

  Yup, kind of like a "loose laminate" bow.   It takes a stack of lams to equal the stiffness of two lams glued together.

 

id wager that is what we call hysteresis. Its not actually stored energy, but energy lost, as Tim ( i think but maybe someone else ) puts it, due to internal friction.

hysteresis is when the cell does not want to return to shape rapidly.
[/quote]

[sleek]

Could you use a, say, 1" dia. whole bamboo. That gives you a thin belly, light center(air) and a thin back to sinew to. Just spitballin"

You could, and I had a similar idea about what I've heard call a "loose" laminate bow, where the bow is made by stacking thin lams that are not glued.  (Essentially a bundle bow, but stacked slats, not sticks.)   

But, the bamboo immediately UNDER the sinew then is still resisting the stretch.  You really wouldn't get the most out of your sinew.

Cut slits in the back so it opens up when bent rather than stretch, then lay sinew over that.

[sleek]

hysteresis is when the cell does not want to return to shape rapidly.

Id say thats true, and i guess my statement was too absolute. Sheer must deform cells, and that deformation is wasted energy, as is it when the cells relax. A perfect core would be impossible to deform at all.

Modifing to say,  the core should also offer no tension or compression abilities.

[Springbuck]

  Here is just another idea to throw out there.

  Diagram shows half a pyramid limb.  Yellow lines are 3/8" dia rattan cross-pieces bound in place with twine.  Orange rounded rectangles are sinew cables, each varying in thickness or number of strands to correspond with its position on the limb: more toward the handle, fewer toward the tips.   The green lines are just lashings of twine to keep the sinew riding down the middle of the bow.

 Side view shows how the cables ride "in the air", but the fixed position of each cross piece forces the cables that correspond to THAT segment to stretch as much as that cable wants to bend.

My intuition tells me that the number of strands toward the tips will be lower than toward the handle, but that the amount of WOOD will diminish faster than the amount of sinew, which risks a tip-heavy bow, again.

[Springbuck]

  Here is one I HAVE done, experimentally.

  Mollie-type, 1.75" wide bamboo slat, bent into reflex by tying down handle and placing limbs on blocks.  Shoulders are cut in at nodes.  (Black lines)   The whole middle, from shoulder to shoulder, got a nylon cable (artificial sinew) that was run up the back, around the top shoulder, down the back, and around the bottom shoulder.  Repeated a bunch of times. (orange lines in diagram) I THINK I followed the TBB; total diameter=1/5 area of the cross section of the base of the limb, about.  Several rattan dowels were placed to bridge the cable.   

The separated cable(s) were laced together with hemp craft cord in a zig-zag to center it/them. (light green lines)  Then, a HEMP cable (darker green) was run UNDER the nylon at the shoulders, up the lever, and around a rattancross-peg lashed toward the tip, again with the lever in slight reflex.  It cinches the nylon cable a lot more than the diagram shows.    This cable was "set" by forcing the levers back straight, and bound in place.   Then the whole bow was forced back to straight to "set" the nylon cable. 

This bow bent fine, and I could adjust tiller by scraping the sides and messing with the string bridges.  I don't remember ever shooting it, though, because I have a mental block about bows that wide and flat without narrowed handles, and couldn't see a way to build up and narrow it and still have it act like it had fades. 

 

[BowEd]

Tim...I suppose that should work too.Although many times while sinewing while smoothing the sinew down laying it in place extra glue from my fingers goes on too.I should think doing it the way you propose will come in with a lighter weight of glue percentage yet is what I mean.

[willie]

Springbuck, in your first example, varying the thickness of the cables as you go down the limb may be a good way to "taper" the tension side. I assume that each bridge is well anchored to the limb so that the tension can step down as you move out the limb? Perhaps the bridges float and even though the entire cable assembly is under the same tension,  the amount each cable stretches will change on account of each being successively smaller diameter, as you move out the limb?

Am building a bow that Springbuck and this discussion inspired. It is a bamboo HLD. About 5/8" wide and 42" overall. Hopefully it will survive to be the new shooter for an 8 year old.

Tim, do you mind that I post it here in the spirit of experimentation? I don't want it to take away from your work. If so, I can put it in another thread.

Without  the bridges, the belly pulls 7.5lbs @ 10" draw.  I hesitate to draw it further as the edges of the sectioned bamboo seem prone to raise a splinter, but hope to build a cable that will double that draw weight (is that reasonable?), placing the lips of the U section at the NP of the limb, and not have to be so concerned about raising splinters. When adding a cable to a previous bow the tiller changed, and  I ran out of wood as I re-tillered the belly,  but the bridges were of uniform height.  This time, I think I can "mess with the bridge height" and keep the same bend. ie." tiller"  the rest of the way with cable tension, bridge heights and placements, while leaving the belly as is.

[Tim Baker]

BowEd:

" ... many times while sinewing while smoothing the sinew down laying it in place extra glue from my fingers goes on too."

Might as well be totally accurate, so your above should be accounted for. So the new plan is to "sinew back" a piece of paper, applying the sinew just as if on a bow except separating the bundles a bit to speed drying. Part of the reason sinew on a bow has a long drying time is that much of the backing's water penetrates into the bow, the paper in this case preventing such, and allowing the bundles to dry on all sides. I'll weight the dry sinew and paper before, and once dry. 

willie:

Elevating sinew makes it do more work, allowing a lower mass of it than otherwise to carry the entire tensions load, in turn allowing thinner wood, in turn allowing exceptionally large reflex. That's the gist of this thread, so anything related at all is valuable, everyone benefiting.

[BowEd]

BowEd:

" ... many times while sinewing while smoothing the sinew down laying it in place extra glue from my fingers goes on too."

Might as well be totally accurate, so your above should be accounted for. So the new plan is to "sinew back" a piece of paper, applying the sinew just as if on a bow except separating the bundles a bit to speed drying. Part of the reason sinew on a bow has a long drying time is that much of the backing's water penetrates into the bow, the paper in this case preventing such, and allowing the bundles to dry on all sides. I'll weight the dry sinew and paper before, and once dry.

willie:

Elevating sinew makes it do more work, allowing a lower mass of it than otherwise to carry the entire tensions load, in turn allowing thinner wood, in turn allowing exceptionally large reflex. That's the gist of this thread, so anything related at all is valuable, everyone benefiting.

I agree Tim.The wood itself absorbs that moisture a lot and has to leave too.Sounds like a good plan you stated.Quite a bit faster to find out doing it on paper.Don't think anyone has actually brought this to a complete experiment finding out exactly how much weight the glue is on line like this yet.Your bow too as an example of an experiment.It'll be interesting.
It might be some people squeeze out more before applying too,but should'nt be much.Consistency of glue[thin or thick] might be a factor. Somewhere in between should be good.I know it took me a while sinewing a few bows to get into a satisfying procedure.I really can't prove the 30 to 40 % mass glue weight final figure I come up with is ideal for strength,but just from sinewing many bows now that hav'nt failed.Some with reflex over 12".Pat M seems to think so.Think Adam Karpowiz has stated so too.Marc St. Louis has done a number of them himself besides others that I know not of.Loefflerchuck makes some authentic extreme all horn bows too.Actually never had sinew fail on a bow besides putting too much reflex on a red cedar....lol,and the glue and sinew did'nt really fail.It delaminated the red cedar with a resounding big big bang.I got away with not wearing a helmet on that one....lol.I think I might have had the red cedar itself just plain too thick to take the bend myself.Something to think about with the cotton being used.Delamination should'nt occur though if the reflex is'nt too extreme I would think.
You should pack a bag and drive up with Steve when he goes to Mo Jam this year with the bow.I and I'm sure many others I know who are  bow makers were'nt fortunate enough to make the first get together when all of you guys got together to settle opinions.A lot of info was found out that's for sure.