Sinew question

[DuBois]

I agree with Joachim. Thanks for all the interesting conversation folks.
Check out pg 106 TBB 1 for another good explanation.

[joachimM]

Check out pg 106 TBB 1 for another good explanation.

When in doubt, read the TBB series (there's a reason it's called the traditional bowyer's BIBLE): the answer is probably written out loud and clear somewhere. Tim Baker clearly explains in a few lines what took us 4 pages...
For those who don't have the TBB series yet: it's the best investment in bow-making material you can think of.

[DC]

I've read the TBB at least a half dozen times. Sometimes you just miss things and sometimes you just don't understand. I've never been able to relate to the seesaw analogy. I'll have to sit back and think on it for a while. Thanks for the page number.

[PatM]

Keep in mind that early TBB contain errors, some of them in the musings of Tim Baker.

[bradsmith2010]

yes I think I remember even Tim saying in later volumes that there were some things that had been re evaluated,,but I will say for the time they came out,,,they had great info,, and still do,, back then,,,, there was not the collective knowledge that there is now,,,, its just mind blowing what we have at our finger tips today,, lots of brilliant bow makers that will share knowledge ,,,its amazing,,

[DuBois]

Yeah, ya know the more I look at the info in tbb I referenced, the more I think I see a new twist.

tbb has column 1 as % of limb depth at back or belly and column 2 as tension or compression work done at these depths.

1%                        6%
5%                       27%
10%                     49%
20%                     78%
30%                     94%

To me it seems it should read as:
% of distance from neutral plain in column 1 = % of work done.

Farther from neutral plain-more work done. The sinew keeps the belly from crushing by moving the neutral plain closer to the belly surface.

[willie]

re-evaluated, yes, sometimes a good thing.

I made a comment earlier, that I need to re-evaulate.

sinew is often used to its best advantage with low compression strong wood

is there room left in this thread discuss the merits of using sinew on hardwoods, hickory etc, vs softer woods juniper etc?
looking back over old threads, it seems some folks definitely have some preferences as to what kind of wood sinew works best on.
 what are your thoughts on using sinew on a dense wood?  is it another way of trapping the back?


DuBois

i had to reread that twice also.....

[loon]

eh I'm pretty sure sinew moves the neutral plane farther from the belly (closer to the back)

[BowEd]

Sinew is some crazy wonderful stuff.Maybe someone else can verify this as to happening to them too.While tillering a sinewed bow on the tillering tree removing wood and working my way to my draw weight pulling to my target weight.The amount of set it took pulling was returned after removing wood from the belly.So I quess the sinew was over powering the limb to a certain degree.These things don't happen while tillering till a person puts enough sinew on.Kind of off the subject I quess but the neutral plane movement might have something to do with it.

[willie]

Ed, have you ever seen the experiment where a bow that had taken set was sawed down its length separating back from belly?
the back returned to shape because it was being held into deflex by a permanently deformed belly that did not want to relax.
the belly stays the same because the set is....well, permanent. I believe that a bow that recovers from set after a long period of time has had its back pulling constantly, albeit gently, all straightening the bow out.
the sinew in your example started pulling when it began to dry, and that tension is always in the bow. you were removing the most highly damaged/ deformed part of the belly, the surface.


Dubois

He is not saying that 78%  of the work resides at a depth that is  20% of the way to the neutral axis.
He is saying that 78% of the work is done between a 20% depth and the surface

the middle third of thickness is pretty close to doing nothing.

[gfugal]

Keep in mind that early TBB contain errors, some of them in the musings of Tim Baker.

Do we know what specific errors there were? just curious so I have a heads up when I read them.

What you have to understand is the only factors that affect the total tension or compression stress the specific fibers undergo, is their distance from the core line from a side profile or how far they bend. If two bows were the same thickness but one had a much stiffer material on the back, like sinew, there still wouldn't be any difference from the compression forces on the belly. The latter bow would have a higher poundage, due simply to the stiffer sinew, not that there was more compression on the belly.

I realize this isn't quite correct, and  It is true the stresses on the back or belly depend on the thickness of the limb and how far the limb bends, but those aren't the only factors. The stiffness of the materials play a much larger role than I previously thought. I would like to make a new stab at it. correct me if I'm wrong

So joachimM and Tim Baker in TBB1 were saying that the neutral plane is 2/3 from the belly. If my calculations are correct this basically translates to a compression stiffness of about 1/2 that of the tension (or 50%). In the research paper that Willie posted they calculated the compression stiffness to be about 70% of tension stiffness. Likewise, they found that the neutral plane didn't shift as much as 2/3. It should be kept in mind that the difference in wood species between their compression and stiffness is going to very quite a bit. Would It be a good generalization that the compression stiffness of wood is about 60% that of tension? This is the average between the 50% and the 70% from the two sources.

As far as neutral plane shifting is concerned, the side of the plane which is bigger will undergo more tension or compression. The neutral plane itself will shift towards the side that is stiffer. Therefore the other side will be bigger and stressed more. Wood is stiffer in tension so it shifts closer to the back. If a laminate bow had a stiff wood on the belly, the plane will shift towards the belly thus stressing the back more. Additionally, if you have a less stiff material like sinew on the back the plane will shift towards the belly. this is because the belly wood is stiffer than the sinew. Although it may not be stiffer since the compression MOE of wood is about 60% that of tension. So a 7 GPa MOE for sinew will be stiffer than the belly stiffness of yew (9.6x60% = 5.76 GPa). Then again not all sinew will be 7 GPa in stiffness, the low end of sinew is 2.7 GPa which is much less stiff. So really it's just a big "depends", but I think it's safe to say it will most likely be less stiff, thus shifting the neutral plan towards the belly. However, the neutral plane will also shift towards whichever side you add material too. Therefore, like Joachim said, the shift towards the belly from sinew's less stiff properties is often offset by a shift towards the back by simply adding the material.

So I will try to translate that to composite application. Any add-ons will aid the side it's added on to for two main reasons. First is because the outer fibers do most of the work and the add-on's are more likely able to handle that stress. Second is because simply adding on any material will shift the plane towards that side because thats where you made the limb thicker. There is however, a third factor that affects the bow. Depending on the stiffness of the add-on material, it will stress or relieve stress to the opposite side. for example, a less stiff material may unstress the belly, while a stiffer one will stress it. The same is true with add-on belly material like horn.

[willie]

eh I'm pretty sure sinew moves the neutral plane farther from the belly (closer to the back)

Loon

imagine gluing a 1' x 1' x 60 strip of styrofoam to a steel band of the same width and length, and bending like a bow.

if the styrofoam is on the back, its gonna stretch on the outside before the steel compresses. In actuality the steel will shorten, but very very little.

flip it over and put the steel on the back and the styrofoam on the belly. Again, the styro will make the adjustments, while the steel stretches very little. the neutral plane (the place in the cross section where the length does not change), is very close to the glue line in either orientation.  If the strip was just the styro alone, the NP would be in the center, but the addition of a weaker materiel on the outside of the bend, moves the NP towards the stiffer materiel, on the belly.

[willie]

greg

the discrepancies you note between data are valid. It is generalized by many that wood is twice as strong in tension as compression. There has been very little comprehensive testing of wood in pure tension, because there are no real applications for using wood in pure tension. An example would be to design a wood member for lifting.  The extra reinforcement needed for fasteners and glue to attach the member to something else, in order to realize the full tension qualities of the "wood cable" are impractical. As wood is most often used in compression or bending, and since compression limits almost all bending applications , the research is just not there. Some theorize that the difference might be as high as three to one. Comprehensive tests on high quality wood is of interest around here sometimes, but wood like that was in the paper,  was typical lumberyard stuff that was kiln dried and might have been harvested half rotten, bug eaten or run over by log skidders, all the defects that have to be considered when rating for construction grade purposes.

[BowEd]

willie...I've seen sinew literally delaminate healthy good red cedar before while bending.It also chrysalled a black cherry bow that was into too much reflex also.I've seen bamboo crush pecan but not breaking the bow.Just leaving a useless ripple on the belly with a hinge in the limb.You could very well be right saying the neutral plane moves to the stiffer material.I'm sure many others have memories of different experimental bows not working out they could mention.
gfugal....Your analogy in your last paragraph rings home true to me in reasoning from making bows.I've seen bamboo probably too thick crush the belly of elm because it was too stiff of material.What is said it over powers it.Sinew being less stiff would'nt near as easily.Density difference cannot be the determining factor either between the two.It's the elasticity of sinew that is the difference.
The last few years I've had good success sinewing hickory.To me more of an imbalanced type wood naturally tension versus compression.It may very well be why sinew is such amazing stuff.It does'nt seem to overpower any wood that I know of in my experience.Unless it is used to put certain woods into designs they can't handle.As far as where & which way that neutral plane shifts I don't have a comfortable hold onto that yet really.I go with what works.I don't worry why.I probably should.I just let time tell me through the school of hard knocks breaking and making bows from different woods.

[Stick Bender]

I will take the School of hard knocks knowledge every time over theory , only because its realistic I have read every bow building book I can get my hands on & it seems you always find 1 guy says this as gospel & then another guy says somthing that disputes that but when some body that I trust says I tried this & it didnt work or it broke ,Im all ears I think practical application knowledge is far superior then theory ,Im not saying theory isnt practical Im just saying one way is superior in my mind but thats just me.