Post For Tim Baker ( Sinew)

[PatM]

Tim,   People are probably talking about energy storage  immediately on either side of the neutral plane when they say at or in  the neutral plane.

 The concern with the thin belly and moving  compression deeper is that the material deeper is no longer a good material to resist compression.

 No good if it suddenly wants to crease and fold.

 I'm still skeptical that belly material is less stressed at full draw after being reflexed.  Can't see how it can be shortened without being shortened.

[sleek]

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.

Yes. The back and belly are like batteries, storing energy. The energy is converted into the + and - at the NZ. The NZ gets its energy from the bow being bent, creating a potenial slip between the surface to be compressed and the surface to be stretched. The NZ takes the energy of slippage, and converts it into the other stresses. Technically, the NZ is under the cumulative stress of both the back and belly. I only broke it down like that to make it clear that there must be somethingin the NZ to make a bow work.


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.

This makes sense to me. However be careful with a thin belly, the thinner you get, the more exact you must be with its thickness, or stresses will concentrate in its thinnest part, and it will fail in compression.

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.

I disagree here. If the belly is very thin, you wont get much tension on the belly during reflexing. Thats why it is so easy to bend. The belly being so thin, the distanve of the surfaces to the bellys own NZ isnt great enought to leverage much sheer into the NZ. Whis is exactly what you are trying to do over all, use distance as leverage to force the NZ into more sheer. A thing belly being bent into reflex doesnt have extreme tension unless you bend it an extreme unrealistic amount.

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

Cant argue with results, and those are fantastic.

Arguments, suggestions and comments welcomed.

[Badger]

Normally it would take about a 50% increase in thickness for 400% increase. Tim, how much was your total increase in thickness, I am guessing more than double???

[PatM]

Normally it would take about a 50% increase in thickness for 400% increase. Tim, how much was your total increase in thickness, I am guessing more than double???

 Are you accounting for reflex?

[Tim Baker]

PatM:

" The concern with the thin belly and moving  compression deeper is that the material deeper is no longer a good material to resist compression."

If I understand, that would be true in the case of bamboo, not regular  wood.

" I'm still skeptical that belly material is less stressed at full draw after being reflexed.  Can't see how it can be shortened without being shortened. "

Less stressed per energy stored: Picturing a bow with equal wood and sinew thickness for now: When reflexed the belly surface is in tension, the belly back in compression, the NP about at center. As the limbs begin to be pulled toward straight, the compressed belly back works to come out of compression, stretching the sinew attached to it, storing energy. At the same time the belly surface begins to come out of tension. As the draw proceeds the belly back comes fully out of compression, stretching the sinew further, storing more energy, the belly fully out of tension but under no compression as yet. Note considerable energy has been stored before the belly surface feels any compression strain. As the draw progresses, since sinew is far easier to stretch than wood is to compress, the NP will rest toward the back of the belly, the compression load now shared by both inner and surface wood, surface wood less strained than otherwise.

Sleek:

" I disagree here. If the belly is very thin, you wont get much tension on the belly during reflexing..." 

The belly is about half the thickness of a same-weight all-wood bow, so can reflex far more, taking the belly to near-breaking tension.

Badger:

Thickness increase was 100%, double thickness, but sinew is much weaker in tension that weight went to 4 times instead of the 8 times if all wood.

PS: Thanks for starting this thread.

Tim

[PatM]

Tim, you still need to cut apart a few well used bows that were protected by sinew or perry reflexing and see that the belly still gets squashed.

Not sure why just the case with bamboo. In  your scenario the belly is a thin strip no matter what the material.

 I agree that the back is  storing more energy but  still not sure that the belly is protected.

 Again, sinew is "weaker"  in tension but it's the glue/sinew combined that you need to establish numbers for. 
  Does anyone have that?

 We would need to make a  sinew/glue fake tendon and test that to establish a number.

[willie]

I'm still skeptical that belly material is less stressed at full draw after being reflexed.

I have to agree with Pat. Suppose a straight belly lam is reflexed and put into tension. While the bow is being braced, that belly goes from being in tension to a "no strain" condition when it is straight again (half way to being braced). When fully braced or drawn it has the same strain as it would have had, if the bow had never been reflexed when the sinew dried. At brace the back in the reflexed bow, would of course be strained more than the belly, because it's "no strain" condition is the more reflexed position.

Assuming that sinew is capable of more strain than bamboo, it might be worthwhile to start the layup with a naturally deflexed bamboo belly. (can bamboo be trained to dry into deflex?) This might allow the bamboo belly to be drawn further before taking set, of course the strain in the back would be higher, but that could be the way to utilize the potential of sinew. 
Either way,  this means that back tension is the primary contributor to the much desired "exceptional early draw weight". Experimentation with tension side qualities may have some real potential for honing designs, as the compression qualities of bow woods are generally already understood.
I have not really come to any conclusions about what strain percentages constitute an optimum balance between the energy stored in the back vs. stored in the belly, but do believe exceptional bows have the balance we hope to duplicate. 


 

When reflexed the belly surface is in tension, the belly back in compression, the NP about at center.

NP about at the center of the belly lam? If so, I am confused. I would think that once the sinew is laminated, the NP of the limb might be above or outside the belly lam altogether, or at least so close to the belly back to mot make much difference. Perhaps I am misunderstanding the description of a NP inside the belly?

[sleek]

PatM:

" The concern with the thin belly and moving  compression deeper is that the material deeper is no longer a good material to resist compression."

If I understand, that would be true in the case of bamboo, not regular  wood.

" I'm still skeptical that belly material is less stressed at full draw after being reflexed.  Can't see how it can be shortened without being shortened. "

Less stressed per energy stored: Picturing a bow with equal wood and sinew thickness for now: When reflexed the belly surface is in tension, the belly back in compression, the NP about at center. As the limbs begin to be pulled toward straight, the compressed belly back works to come out of compression, stretching the sinew attached to it, storing energy. At the same time the belly surface begins to come out of tension. As the draw proceeds the belly back comes fully out of compression, stretching the sinew further, storing more energy, the belly fully out of tension but under no compression as yet. Note considerable energy has been stored before the belly surface feels any compression strain. As the draw progresses, since sinew is far easier to stretch than wood is to compress, the NP will rest toward the back of the belly, the compression load now shared by both inner and surface wood, surface wood less strained than otherwise.

Sleek:

" I disagree here. If the belly is very thin, you wont get much tension on the belly during reflexing..."

The belly is about half the thickness of a same-weight all-wood bow, so can reflex far more, taking the belly to near-breaking tension.

Badger:

Thickness increase was 100%, double thickness, but sinew is much weaker in tension that weight went to 4 times instead of the 8 times if all wood.

PS: Thanks for starting this thread.

Tim

Tim, I agree with your point on the belly on one condition. That being that a thin belly can bend farther in reflex than a thicker belly before it begins to fail in tension. You can bend a belly in reflex to the point if failure regardless of thickness, the only thing that changes in the bend radius. And perhaps I am daft for not taking what you said as having that meaning in the first place.

[sleek]

Tim, looking at the extreem reflexed belly you are making, its like a rubber band when unbraced. It is trying to spring back flat, but pulling the sinew into tension, until the forces equal, then it stops.

As you brace it, the belly is still trying to pull its self back flat, against the sinew, and still is under tension, not compression, until past its half way point ( when the belly is flat ) of being braced. All the work you put into it, has just been to stretch the back, at the expense of NO added belly compression.

Looking at it like this, when the belly lam starts off as flat, you force it into tension to add reflex, the belly doesnt go into compression until it has been bent back flat and into the brace profile. I dont believe you add compresion forces by reflexing a belly, but only add tension to the back. Even as you draw the bow, the sinew is now trying to pull the back, back into its flar state, the sinew doesnt add compressive forces either.  Only tje radius of the bows bend does.

[sleek]

^ this is a thought in prgress. I reserve the right to be nuts.

[Tim Baker]

I've been away from the scene for a time. Are there any critically witnessed fps numbers for 10gpp arrows from sinew-backed wood bows, with string type and mass noted? 

PatM:

" Tim, you still need to cut apart a few well used bows that were protected by sinew or perry reflexing and see that the belly still gets squashed."

It's sort of apples and oranges. The sinew on such bows was relatively thin, meaning the wood was about as thick as if not sinewed, so suffered much of the same degradation as a non sinewed bow of the same weight. Here the sinew doubles the thickness of a thinner limb, causing the NP to rest near the sinew, compression strain therefor supported not just by the belly surface, inner wood carrying much of the load.

" Not sure why just the case with bamboo."

Interior bamboo is weaker than surface bamboo

"In  your scenario the belly is a thin strip no matter what the material."

The belly is just about half as thick as a same-weight bare-wood would be. 

Good idea, and worth doing: Measuring the stiffness of a fake sinew-glue tendon. 

willie:

"  I would think that once the sinew is laminated, the NP of the limb might be above or outside the belly lam altogether, or at least so close to the belly back to mot make much difference. "

That's right; the NP is in close to the sinew, meaning that more of the belly thickness in made to do compression work, not just the belly surface as per usual.

The position of the NP would shift bellyward a bit if sinew/glue is stiffer than sinew alone as PatM thinks might be the case.

Tim

[Springbuck]

Ok, Tim, I've been thinking about this very thing for a while, never getting around to trying my prototypes.   So. A few ideas.

1.  Rough tiller a thin bow, maybe 1/4" thick, bamboo or wood.  Spiral wrap the entire thing in 1/8"  dia hemp craft cord.  Sinew back OVER the cord wraps, thus using using the cord to "lift" the sinew.   Secure sinew at tips. Cut off all the cordage on the sides and belly of the bow, leaving short parallel pieces running across the bow while the sinew runs lengthwise.

2.  Belly lam of wood or bamboo, center core of rattan splints, sinew backed.

3. Mollegabet style bow with wide shoulders and 60/40 bending limb/lever rstio.  Could be bamboo.  Sinew or silk cable is wound round from shoulder to shoulder, similar to the silk/bamboo bow from TBB.  Then flax or other "STIFF" cable is run UNDER the stretchy cable at the shoulders and back to the limb tips.  Dual cable system forces middle cable to stetch, but creates stiff levers.

Also. same as above, but lever is shortened and recurved.  A few small diameter bamboo sections laid crosswise act as string bridges and flax cable can be bound down to recurve. A cross-hatching wrap of cables can even be arrangel to improve stability of recurve.

4.  Small diameter, thicker walled bamboo is split and tiller roughed out.  This creates a "U" shape.  The bow limb is then wrapped candycane fashion witb twine, but in both directions (as the Meares Heath artifact was wrapped.)  Now, the bow can be cable-backed, the cable riding atop the twine wraps which span the open end of the "U".

 Alternatively, the "U" cross section bamboo bow limb is backed with a linen fabric strip, such that the fabric spans the open top of the "U", and the fabric laps over the sides however much needed.  This creates a hollow limb. 

Both these designs take advantage of the Poisson Effect.  The "U" cross section would attempt to flatten when bent but the wraps or the fabric would not allow it, increasing stiffness per mass.

Has anyone tried multiple cables?   Say three or four running abreast, following the front profile contours of the bow by riding in small grooves set into the string bridges. 

More to come. 

[willie]

Good idea, and worth doing: Measuring the stiffness of a fake sinew-glue tendon.

I agree, A sinew in glue matrix  might be different in both stiffness and elasticity, compared to sinew alone or wound into a cable. If you should happen to premake the sinew layup, I hope it could be successively applied on a bow with open bridges (attached only on the ends), one with a lightweight core and one with a conventional maple core.
Earlier I suggested that no work was being done in shear. I have reconsidered as there may well be shear deformation at the cellular level that contributes to energy storage in the limb. Hold a paperback book by the spine with one hand and moderately pinch the opposite edge with the other hand, then bow the book. There is a little bit of shear between each page.  Imagine if the pages were glued with a flexible glue (the lightweight core condition), and finally if the pages were glued with a hard glue.  All three limbs would differ in their stiffness or stored energy per thickness, and each have their own degree of hysteresis.

[gfugal]

Tim, I hope you don't think we're jumping on you like a pack of rabid dogs  . I think it sounds super interesting, just trying to understand it. I appreciate you joining and look forward to your future posts. You're practically a legend to me for your pioneering work in bow-making, using your scientific approach to dispel dogma and all. Now you can see we're skeptical of everything including your new ideas haha. I personally would love to see this bow you made. Do you have any pictures? Maybe you've tested it in a chronograph? 

[Tim Baker]

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. Pondering the problems and solutions now. Meanwhile, the present configuration of this experimental bow eliminates the problems.

Using a maple core would effectively be the same as starting with thicker wood.

If the glue is rubber-like, allowing some sliding between back and belly slats, for example, there would be a small amount of energy stored in the rubbery glue, but far less total energy stored due to reduced stretching of the back and compression of the belly. If the glue is rigid, not allowing the back and belly slats to slide, there is no energy stored in shear, all energy again stored chiefly at and near back and belly surfaces.

gfugal:

Thanks for the kind words, but don't stress; skepticism is valuable and appreciated, and it's been positive and civil. It's following pure Comstock protocols: "We're of no value to each other if we can't disagree."  I'm brand new here and don't know the photo posting ropes yet. I think I've sent Badger--Steve--a photo of the bow, or if not I will, and maybe he can post it. Steve is going to the MoMeet shootoff in couple of months so I'm making an updated version for him to shoot there. Steve lives not far away so when I finish the tillering touchup with sinew we'll chrono the present version and post the results.