Primitive Archer
Main Discussion Area => Bows => Topic started by: fiddler49 on May 10, 2013, 01:10:52 am
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I've been working with wood backings lately and they have given me some insight into why cloth backings glued up with tite bond type wood glues really don't work!
Yes thats a bold statement but hear me out. To hold down and keep a splinter from lifting on the back of a bow the backing whether cloth, glass, sinew or wood has to span across
the splinter like a bridge and not flex in the 90 degree axis of the bending bow. I've come to this conclusion because of seeing how a splinter lifted the linen backing of a 40 lb hickory bow I made. I could visually see it's outline through the linen. Luckily it didn't brake! Now just recently I was trying to repair a lift splinter on my tri lam take down by glueing on a very thin 1/4 sawn purple heart patch over the offending splinter. I sanded the purple heart down to less than a 16 nth of an inch thick. Shot the bow a few dozen times and heard the distinct tic of popping wood. I let down my draw and checked over bow. Found the exact same splinter had lifted right through the purple heart. It was in the same place and had the same shape as before. Now the light bulb went on!!! The backing needs to be strong enough to bridge across the offending bad grain to mechanically hold it down!!! How could I make the backing stronger? First make it thicker say 1/8 inch. So how does this apply to linen and other similar backings? Well I then remembered the linen backing on my hickory bow and how the splinter just pushed up the linen like a small springy finger. Maybe I'm using the wrong glue for the job? Wood glues like tite bond
can't fill gaps! Cloth is full of gaps!! Fiber glass is like cloth! but you use epoxy type glue that is a gap filler. When you cover linen cloth with wood glue it becomes brittle like a corn chip!!! When you cover fiber glass with epoxy it becomes very strong and can bridge across splinters without lifting. The same for sinew and hide glue if enough layers are applied or to raw hide if it's thick enough to bridge those pesky splinters with out deforming! A good backing on a bow could even be tied on with out glue and it would still do it's job of holding down violated grain and splinters but the cord or thread would add weight to the limbs so glue is the mechanical method of choice! If your building a backing in place by layering fibers then a gap filling glue would be the only choice!!! All two dozen of my linen and tite bond backed bows have eventually broken! except two.
Those two wouldn't have likely broken to begin with. My conclusion is wood glue and linen or silk backings are a bower's myth! Yes there will be lads that will stand by the method as tried and true but looking more closely with a macro and even micro point of view of whats really happening with the wood and backings will make a lot of sense!
cheers fiddler49
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Can you please describe in detail how YOU apply clothe backings.
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All two dozen of my linen and tite bond backed bows have eventually broken! except two.
Those two wouldn't have likely broken to begin with.
My experience with grain runouts on hickory and linen/TBII backing are quite contrary to yours. I had ONE out of about 35 fail.
The failure involved heating the belly and trying to work some recurve into the tips of the bow. In doing so, I managed to create a crease in the back of the bow (probably both wood and fabric). When stressed, the crack followed the crease across the limb. That, in itself, would have been the seeds of failure of a flawlessly chased growthring on a flawlessly tillered self bow.
Other than that, I have not ever had a splinter lift under a linen/cloth backing.
So now we have a conundrum. Two anecdotal bodies of evidence with two major variabilities...the bowyer and his techniques. Any chance you live close to Rapid City SD? We could go together on this and run this out to a more scientific conclusion.
Keep this in mind, just because you can't run a 4 minute mile doesn't mean it ain't possible.
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gonna have to chew on that one awhile... it is an interesting theory and worth doing some experimenting and testing on, personally i have only had 1 (one my first bows) out of seven or eight cloth backed (all glued with titebond 2 and 3) board bows break and i believe it was becuase a combination of beginers awfull tillering skills and bad wood choice, but after gaining quite a few years of experiance and decovering the tillering gizmo i have not had a cloth backed board bow break since.
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The question is was the wood good enough to be left un-backed in the first place.
Do the maths on how much strain is felt by a bows back......you will get some pretty high numbers. ;)
Do some tests on linen/silk etc etc and see if the numbers match up.....they won't!
These backings may add a little insurance on a bow that was good enough to be left unbacked but they won't hold together wood that isn't close to being good enough for a selfbow.
The problem with this debate Fiddler is that it's open to subjectivity, I suspect you will have lots of people saying 'yeah but I made a linen backed bow and it hasn't broken'...fine but I bet you could take the backing off virtually all these bows and they would still pull to full draw. Do some 'scientific' testing and the answers will be clear.
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just because you put a cloth backer on a bow doesn't guarantee success, there is a difference between "iffy" grain and " I need to where a helmet when I draw this" grain, it's not a cure all for lousy grain, Bub
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2 thoughts:
1. what has already been said... your experience isn't necessarily everyone else's. I would wonder what you did differently on the two that didn't splinter.
2. The value of a cloth backing is NOT guaranteed to prevent splinters from lifting, but more to keep the bow from breaking. I have my second red oak bow still serviceable but retired because it lifted a bunch of splinters. I can guarantee you those are due to my novice skills at the time (i'm a little better now ;)). I kept hearing the dreaded TIC but couldn't find it till one day i flexed the bow and sighted down the limb and my jaw hit the floor when i saw a whole bunch of splinter outlines. That bow would have broken 50 times over without that backing, which held nicely. I suspect that i could still put another 500 - 1000 arrows through without problems, other than loosing a little more # with each splinter that lifts.
Did you CA glue the splinter down before you tried that patch???? That may be the missing ingredient ;D
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"Do the maths on how much strain is felt by a bows back......you will get some pretty high numbers.
Do some tests on linen/silk etc etc and see if the numbers match up.....they won't!"
So... I'm not saying you're wrong but help me with my math a bit...
The modulus of Rupture for tension strong woods like Yew or Osage is roughly 150 Mpa (see my comments on this below -- could be quite a bit higher).
The tensile strength of common silk for example is 500-1000 Mpa (lots of variability depending on where its gathered from).
Those numbers suggest to me that silk fibers are stronger than wood fibers. But I'm sure there is more to this equation.
I found some strain numbers from a thread on PP long ago and these may be inaccurate but they stated that roughly:
For a 28" long bow limb (single limb), with a thickness of 0.44 inches and a width of 1.25 inches, drawing 35 lbs of force, the amount of tension stress along the back of the bow (assuming evenly distributed stress) was 28,000 PSI. 28000 PSI is equal to about 193 Mpa. This is greater than the reported rupture strength for wood (which confused me at first, but I take the posted rupture strengths with a grain of salt because I doubt the testers consider grain orientation the way a bowyer would).
However its quite a bit less than even the conservative estimates for the tensile strength of Silk fibers.
My conclusion (perhaps wrong) would be that a silk backing.. if applied to wood with enough force... could protect the back of a bow that may otherwise break (depending on the severity of the break).
What say you?
Gabe
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This kinda spooks a new guy just finishing his 1st board bow with a silk backing on it.
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Take a breath and relax Ed. You'll be fine.
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LOL, Thanks Pearl Drums.
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Keep in mind a few things.
IMO, bleaching cloth- silk, linen, cotton, WHATEVER-
WEAKENS IT.
Also in TBB it warns NOT to use cloth backings on a board stave unless its nearly flawless because they really aren't that strong. I have had good luck with burlap on a board stave, but it was overbuilt, a light draw, and I put 2 layers on with TB2...
Plus the thing was not in danger.
Cable backs are nice.
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Ok, would this be a fair test of the fabric backing?
1) Build a bow with a decent amount of strain to the limbs. For example, my 26" draw, 56" nock to nock with stiff handle of 4" and 1.5" fadeouts at approximately 45-50 lbs of draw weight. This comes out to less than the old "double your draw length plus 10%, therefore setting the limbs up to be stressed a bit more than normal.
2) Make the bow from hickory and make the limbs narrower than the accepted norm, again adding a slightly higher degree of strain to the back fibers, i.e. 1.5 inches at the fades vs the usual 1.75"
3) Chose a board that has a few grain runouts in mid limb, thereby encouraging limb failure to happen in a more predictable point.
4) Back with a light linen/canvas material and TBII
5) Given that a really crappy piece of wood is not a fair test On the other end of the spectrum, a flawless piece is not fair either. The idea is to figure on making the bow slightly less than the low end of acceptable. Ex: On a scale of 1-10, with 10's being guaranteed to shoot great and survive, a 1 failing at floor tiller, this bow would be a 4.
The plan would be to photo-document the process and then suit up in some borrowed armor from one of the local Society For Creative Anacronism nutjobs and overdrawing this thing back to my ear while asking forgiveness from the Bow Gods.
I think I have the 1x2, I know I have the 6 oz linen (gorgeous stuff imported from Russia, leftovers from making a nice Colonial shirt), TBII, and the tools. Just gonna have to see if I can make the time in my schedule.
Suggestions on how to modify this to be a fair test of the theory? After all, this is a good and proper smoke test, the idea is that the bow will be built to fail. Am I missing something, is there something to add, are there better criteria?
I'm not interested particularly in proving you wrong, Fiddler, any more than I wanna prove myself right. I want an answer that can be considered reliable.
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Keep in mind a few things.
IMO, bleaching cloth- silk, linen, cotton, WHATEVER-
WEAKENS IT.
Also in TBB it warns NOT to use cloth backings on a board stave unless its nearly flawless because they really aren't that strong. I have had good luck with burlap on a board stave, but it was overbuilt, a light draw, and I put 2 layers on with TB2...
Plus the thing was not in danger.
Cable backs are nice.
If you think bleached muslin, cotton, or linen is "weakened", I suggest tease one of those burlap cords loose and test it with a scale. I cannot for the life of me figure out why people persist in using burlap?
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if you have a neer flawless board you don't need to back it with anything BUT air, it's ludicrous to back something that don't need it and add mass to it ::)
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when I read about cloth backings in the TBB, I thought it sounded like a good idea......so I thought I test it out on an old rubbish bow first....I got some good quality strong linen cloth and glued it with TB2....then tested it to destruction.....it broke clean through the limb as if there was no cloth there....I tried the same with silk..the same thing....IMHO....cloth backings do nothing at all and are a waste of time except for looks.
Except using epoxy maybe? tried gluing black linen on a back with epoxy, everyone thought it was fibreglass...looked just like it...this time it did maybe up the performance a little bit....but when I tried breaking it...it broke clean through as wood normaly does....TBB says it protects the back...but not enough to make it worth bothering with I'd say
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when I read about cloth backings in the TBB, I thought it sounded like a good idea......so I thought I test it out on an old rubbish bow first....I got some good quality strong linen cloth and glued it with TB2....then tested it to destruction.....it broke clean through the limb as if there was no cloth there....I tried the same with silk..the same thing....IMHO....cloth backings do nothing at all and are a waste of time except for looks.
Except using epoxy maybe? tried gluing black linen on a back with epoxy, everyone thought it was fibreglass...looked just like it...this time it did maybe up the performance a little bit....but when I tried breaking it...it broke clean through as wood normaly does....TBB says it protects the back...but not enough to make it worth bothering with I'd say
Was the linen bleached? How thick was it?
Try rawhide...
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it was top quality, expensive Irish linen (Irish is traditionally the best...so they say) and pretty thick...not a thin fine cloth....dunno if it was bleached though...cant see why it would be....
yeh, I agree...rawhide works great and even if the bow breaks the rawhide holds it together....better than cloth in all respects
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Dragonman I think the error in your test is using a "rubbish bow". I assume that means destined for failure. And your assessment of "broke as if the cloth wasn't there" seems like a fair amount of assumption. Can you prove it didn't hold on for an inch or two longer than otherwise?
Just for the sake of science, i may take my old retired bow out and see how far i can yank'r back before she goes to pieces...
JW, could i borry some o' that armor? ;) BTW, can't wait to see the video of you performing this test >:D
go figure, the thousands of bows that have been made with that useless cloth backing just weighing them down ::)
Actually i look forward to seeing if we can really test this. After all, don't wanna use the stuff if'n it don't do nutn. ;)
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The only real test would one series of limbs that was backed and another set unbacked as controls. Both sets of limbs would have to be engineered to fail at exactly the same set of conditions.
Now I am wondering if I can manufacture a set of limbs, artificially induce an incipient splinter, THEN back it and test to death.
Either way, I gotta have my friend Mikey, the tattoo artist, do a very old school inking on the limb that says "Born To Lose"!
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Keep in mind a few things.
IMO, bleaching cloth- silk, linen, cotton, WHATEVER-
WEAKENS IT.
Also in TBB it warns NOT to use cloth backings on a board stave unless its nearly flawless because they really aren't that strong. I have had good luck with burlap on a board stave, but it was overbuilt, a light draw, and I put 2 layers on with TB2...
Plus the thing was not in danger.
Cable backs are nice.
If you think bleached muslin, cotton, or linen is "weakened", I suggest tease one of those burlap cords loose and test it with a scale. I cannot for the life of me figure out why people persist in using burlap?
Well, Burlap is free for me. I do this on a zero budget. I cut my own wood for the most part.
Plus burlap is jute fibers...
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Another thought to ponder:
A tension break in a bow I think can be different from lifting a splinter. The whole conversation got started cuz someone said that the fabric won't hold down splinters. I may concede on that, given that it doesn't mean that it won't hold the bow together once the splinter lifts. This has been my experience and countless others-- a splinter lifts and the bow holds due to the backing.
I know, a splinter lifts due to tension stresses and can break a bow. But A BOW CAN BREAK IN TENSION without ever lifting a splinter. That's a different beast all together. I had a silk backed bow snap once that was doomed to failure (wood was too brittle/dry rot). It broke clean across the limb (the backing "broke" flush with the wood)... no splinters. A break creates sharp edges... is it not possible that the fabric backing is more or less "cut" by the edges projecting toward the underside of the backing of the broken limb (a la my bow break and dragonman's experience)??
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I'm just telling it as I saw it Hat...I realise it wasnt the most scientific of tests... and it doent prove much in the eyes of others,.but it convinced me not to bother with cloth backings....the amount of protection is very minimal.....so I thought I'd mention it....yeh, your right probably it held on a second longer... all I am saying is that in my humble opinion...it dont do a lot
TBB led me to believe it would prevent that type of breakage...where the limbs blow apart, but it didnt...didnt even come close..
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It just came to me...as the original post says..its the glue that makes the threads brittle..they may be as strong as wood fibres ( linen fibres) , but not once they are soaked in glue and dried hard...then they lose their tensile strength.....
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Anyone else run any tests? Seems if I recall right back in the 40's they use compressed paper for backing? I have used paper drywall tape many times in the past, also have had good luck with cloth as well.
VMB
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Fibergl@$$ wallboard tape, applied in 3 layers, over a board bow, makes a very good backing...
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Squirrel, FB is a very bad word.
I'd repeat what I said earlier. Silk and Linen fibers are both MUCH stronger than wood fibers.
The force per area on a bows back is much weaker than the strength per area of silk or linen fibers.
The real question probably lies in how a bow breaks under tension.
Gabe
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just because you put a cloth backer on a bow doesn't guarantee success, there is a difference between "iffy" grain and " I need to where a helmet when I draw this" grain, it's not a cure all for lousy grain, Bub
Yep! X2 what bubby said.
I backed two bows with cloth and tightbond, one a pecan and the other osage, that raised splinters on their backs. Both bows were still shooting and may have lasted a long while unbacked, but I glued some 7 oz. linen canvas on them with tightbond II and they are still shooting a couple years later. You can see the outline of the splinters through the cloth, but they have never raised any further. I think it saved those two bows. If a bows back is going to break, cloth ain't going to save it. I have only backed a few of the bows that I have made so take my post with a grain of salt. I don't like fooling around with backings anyway and prefer a simple selfbow these days. I have made enough bows now that I don't waste time trying to administer CPR or patch up a piece of wood that doesn't want to be a bow.
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I've had 2 bows break with linen backings - One I posted about recently - a 45# 68" ntn bow - top limb snapped clean in half - no splinters, just a sudden snap. Trying to get to 30" with a "square" tiller and a 10" stiff handle - a combination of too dry wood and bad tillering and long draw length. Another was with an extremely light linen backing that I heat treated to death to the point that the glue became brittle. Getting over zealous with the heat gun I guess. But I've had many more bows hold together than have broken with linen backings.
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Wow, great discussion! Of course this appeals to the engineer/ physics teacher in me...I'm mostly loosely replying to Rossfactor since I have 4 working bows and about 15 that exploded in my hands--none of which were backed.
One measure used to evaluate a bow's breaking point is the the rupture tension of wood. A different measure--which may or may not be more useful--is Young's Modulus which is basically tension divided by compression...loosely.
Lifting a splinter, to me, is not just about tension--it's the relationship between tension and compression. While we think that linen/ silk/ cloth works only in tension (and I guess technically it does), I am throwing this out there (a bit on the Devil's Advocate side) that cloth's tension strength is what "pushes back" the compression forces felt by the belly of the bow.
I might even argue that the reason a splinter lifts is soley due to the compressive forces. Think of it this way--push your finger into a balloon--the balloon stretches around your finger. Yes, it's the tension you feel on your finger, but you compressed the balloon. The balloon is the tension capability of a bow's back, and the compression is your finger. By adding cloth, you are adding to the force that the bow's back pushes on the compression forces, like the balloon.
Where the cloth helps is where wood is not uniform in density, like at growth rings/ hidden knots/ grain run off. The compression forces that push on the back of the bow are higher in less dense places, putting more force on the ring separations and hence the splinter lifting. There's little doubt in my mind that cloth will help add the tension strength needed to "push back" the compression forces. Newton's 3rd law--forces in pairs--hence the "push back".
So I am in agreement with Rossfactor that, silk, properly applied, would prevent the wood from breaking. However, force applied that exceeds, fundamentally, the capability of a given wood species Young's Modulus will break, regardless of the backing.
Backing does two things, at least in my head scientifically: 1) it adds tension in the places the wood cannot due to nonuniform density and 2) helps compensate for Bowyer mistakes--cut too deep with a bandsaw, poor tillering, poor grain runoff, etc.
A good piece of hickory, 2.5 inches wide, and 9/16th width and 68 inches NTN should be able to draw 60# without breaking or lifting a splinter because the wood itself is capable. Now, if I were trying for 60#, I'd back it because a) my choice in wood is not the best yet 2) I make lots of mistakes.
So no, I don't think adding silk or anything else make the wood "stronger" per se, but it definitely helps the wood live up to its capability independent of Bowyer skill. If ya got suspect wood and some grain runoff, I'd absolutely back it.
Rossfactor: those weird numbers about the wood failing the silk making have to do with the way it was tested, I suspect. Tension testing, when testing for just tension, is quite different than bending something till it breaks. That's why I prefer Young's modulus versus tension rupture...not that there's necessarily a whole ton of difference, it opens the floor to multiple different analyses, versus just the rupture tension.
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Hi Hurlbri...thats all basically common sense put into fancy language, in my opinion...I think most bowyers here.. probably realise this..you cant argue that a backing will offer some degree of added tension and also some added protection....the conclusion that I came to ( and Fiddler came to , too, I think) is that; threads of cloth ,once soaked in glue and hardened lose their tensile strength and become brittle...hence , they dont really do much at all?
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Look, I'm no good with the mathematics or physics side of things, but these have been my experiences with unbleached linen backings so far:
-Out of the five or so bows I've made in the past couple months, each one has had a linen backing secured with either titebond II or III.
-One, that I made for myself as a test, had some pretty decent grain run-off halfway up the top limb, and towards the tip on the lower limb. Unbacked, with the poundage I was shooting for, I can almost guarantee you they would have broken. All of them are from store-bought, 1x2 red oak boards.
-I have not heard, seen, or been told of any cracks or splinters from the people who got my bows. I'll check them over once in a while to make sure, haven't had any problems yet.
-I took the linen off the bow I mentioned above, with the run-off, to see how it had held up after a year of shooting. No signs of undue stress, no cracks, and with average set for a 72" board bow and my mediocre tillering skills.
I'm not saying you're wrong, just that it's a pretty bold claim to make when most people have seen so much success.
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To paraphrase what Hurlbri said...
When you back a bow, you help the back of the bow overcome tension/streching forces created by the compression in the belly.
Every action has an equel and opposite reaction. When you have compression, you have tension in SOMETHING.
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... ::)
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I backed a bow with parrallel strands of linen, secured with TBIII - I had to be careful filing off the excess at the edges because the file would pull the strands of linen off the back, indicating to me that the tension strength was still in the linen, despite the glue.
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Read somewhere that it's the "linen-glue matrix" that gives it strength. Course that could be one of those unverified passed-on things, but I don't think so. Just because it's not as "springy" doesn't mean it's lost strength.
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Thats true about a fiber and glue matrix being strong but that only applies to epoxy. Wood glue/ tb 2 and 3
looses strength if you try to fill gaps and cloth is full of gaps. cheers fiddler49
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If you size the back proper (like applying sinew) let set till tacky then apply a wet backing (cloth prewetted with glue) WHERE ARE YOUR GAPS? I have never seen any. I don't agree.
VMB
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Lad, you need to read up on glues. Tb2 and tb3 won't bridge gaps with any strength. There are lots of gaps in the cloth. People don't use tb2/t'b3 with fiber glass but do use epoxy. The reason is epoxy will bridge gaps in the fiber matrix.
cheers fiddler49
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Very interesting thread. The only time I've had a splinter lift and cause failure was with a sinew backed ipe bow. The splinter popped up right through 1/8" of sinew. It didn't break but it made a nice "crack" sound. I retired the bow and started wrapping the sinew tightly when backing all the other bows I've made. No problems since.
I haven't used cloth, though. Seems a little weak to me so I tend to agree with the original idea of the post.
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If you size the back proper (like applying sinew) let set till tacky then apply a wet backing (cloth prewetted with glue) WHERE ARE YOUR GAPS? I have never seen any. I don't agree.
VMB
Tb needs flawless mating surfaces for a good bond.
Every glue has a tech specification sheet - it's a good idea to get hold of one from the glue manufacturer. It can also be extremely useful to contact the makers of any glue you use and just speak to them about your intended use. Glues aren't quite as simple as people may think. :)
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The problem with cloth backings is that people don't add enough depth of backing. Every other backing we use whether thin hickory, bamboo or sinew is built up to even a modest depth of fibers. Expecting a strands worth of cloth with thin gaps between each run of fiber to hold up the same way isn't really a fair comparison.
Linen phenolic has incredible tensile and compressive strength and oddly paper phenolic tests out with higher tensile strength despite not having linear fibers.
The "fiber" backing used in the pre fg era with great success were paper or linen but they were all much thicker than what people consider adequate now.
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That's a good point Pat.
What would be the best glue brand/type for applying a cloth backing then? In terms of ease of application and gap bridging? TB is easy to just squeeze out a crap-ton to cover a whole bow back and saturate the cloth. Epoxy to me is those little mixing syringes. I'd need a a dozen of those to do a whole backing job.
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Another problem with any woven backing (cloth, burlap, that fiberglass drywall tape garbage) is that exactly half of the fibers in the material are running perpendicular to the length of the limb, and serve absolutely no function other than to add dead weight and rob your bow of performance. I would suspect that it would also create more slack in the fibers that are actually under tension, just because of the fact that the fibers are woven.
I've never tried a fabric backing just because it doesn't seem to make sense to me when there are so many other better options available.
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I think Pat M is on to something with backing thickness. Even a wood backing needs to be thick enough. Somewhere between 1/8" and 1/6" or a splinter can lift right up threw it! So what kind of glue was used on phenolic type backings? cheers fiddler49
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more fodder for the fire:
http://www.primitivearcher.com/smf/index.php/topic,39894.0.html (http://www.primitivearcher.com/smf/index.php/topic,39894.0.html)
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I've been following this post for the past couple days and decided to contribute my 2 cents worth--like my daughter says, "opinions are like ....", anyway. It seems that any material that can be applied to the back of a bow and prevent or lessen the likelihood of the bow's exploding is good. I agree with PatM that it would take 2 or 3 layers of linen to equal the depth/mass of another material such as rawhide, goat skin or sinew. That's one reason I can't buy into the argument that multiple layers of linen serve no purpose. While doing so will add weight, it still won't be any heavier than a leather backing.
And to say that with time Titebond ( I've never tried TBIII) will dry and get brittle just isn't true. I have a partially full quart bottle of TBII with dried glue in the nozzle and around the cap. Every now and then I have to remove it to keep the glue flowing, yet its still flexible and pliable.
I don't have a degree in engineering, but it seems that part of the strength of a material is its cross pattern or perpendicular weave. While sinew will stretch and retract linearly like a rubber band, the cross design in woven material would lend additional support to a linear grain, especially in woods that aren't interlocking grain like elm or hackberry are. For example the heavy leather wrapping found on the Meare Heath bow which also gave evidence of having been drastically, if not purposely, grain violated on its back.
Regardless, I've heard that cracking sound before, and in one instance wanted to believe that it had somehow come from the tillering tree, so I continued to shoot this cracked linen backed hickory short bow for several days until the limb was bending so far and so easily that the break could no longer be ignored. The bow never exploded, maybe because of the cloth backing...who knows. Just my 2 cents.
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I dont have experience with cloth backings, but if I were to use linen or silk cloth backing I would use hide glue. I find it peculiar that on this forum, hide glue hasnt been mentioned once in this thread for applying cloth backing. I recall reading an article from the Society of Primitive Technology publication that determined through testing that the tensile strength of a sinew backing was more the result of the hide glue in the sinew/glue matrix than was the sinew. If I am going to apply a protein based backing (whether it be plant based linen, or animal based silk), I am going to opt for a protein based glue...hide or fish glue.
Look I even found the article I mentioned : http://www.primitiveways.com/secrets_of_sinew.html
After reading this article again, I am wondering if shrinkage of the cloth has a lot to do with it.
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Yeah, Carson. I started to mention it, but felt like I didn't have enough experience with it to chime in. I only used hide glue once for snake skin, but loved how it turned out. I used titebond on the only linen backed bow I made with success, but wondered how it would have turned out with the hide glue.
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That's worth giving a try Carson, i hadn't considered it. Guess in my mind hide glue is for "primitive stuff", but I bet you're right. If i remember correctly from TBB, hide glue is stronger than TB2. The only downfall is moisture weakens it.
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fiidler, but your backing did work.
You said, " I could visually see it's outline through the linen. Luckily it didn't brake!"
A poorly chosen board with many run offs will break no matter which backing is used as will a poorly tillered bow.
I always advise beginners to back early efforts for ADDED insurance.
Jawge
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I've been following this post for the past couple days and decided to contribute my 2 cents worth--like my daughter says, "opinions are like ....", anyway. It seems that any material that can be applied to the back of a bow and prevent or lessen the likelihood of the bow's exploding is good. I agree with PatM that it would take 2 or 3 layers of linen to equal the depth/mass of another material such as rawhide, goat skin or sinew. That's one reason I can't buy into the argument that multiple layers of linen serve no purpose. While doing so will add weight, it still won't be any heavier than a leather backing.
And to say that with time Titebond ( I've never tried TBIII) will dry and get brittle just isn't true. I have a partially full quart bottle of TBII with dried glue in the nozzle and around the cap. Every now and then I have to remove it to keep the glue flowing, yet its still flexible and pliable.
I don't have a degree in engineering, but it seems that part of the strength of a material is its cross pattern or perpendicular weave. While sinew will stretch and retract linearly like a rubber band, the cross design in woven material would lend additional support to a linear grain, especially in woods that aren't interlocking grain like elm or hackberry are. For example the heavy leather wrapping found on the Meare Heath bow which also gave evidence of having been drastically, if not purposely, grain violated on its back.
Regardless, I've heard that cracking sound before, and in one instance wanted to believe that it had somehow come from the tillering tree, so I continued to shoot this cracked linen backed hickory short bow for several days until the limb was bending so far and so easily that the break could no longer be ignored. The bow never exploded, maybe because of the cloth backing...who knows. Just my 2 cents.
This has been my experience with the TB products as well.
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i think the bowyer has too large of an impact on the final product to approach this from a finished bow standpoint. This engineer sees a set of near identical strips of wood (3/4" wide and 3/16" thick) ripped from the same board. make sure to randomize the strips then select half to be bent till broken as is and half to be glued up with linen and TBII. Since wood is wood, we need a larger sample size to make the statistics good, i'd say 10 each which still probably isn't enough but it is more practical than doing 100 each.
I would then clamp the end of a strip in a vise and pull on the other end with a scale until it breaks, noting the distance it was pulled when it broke and the poundage it broke at.
Now someone is going to say, "yeah but this isn't a bow". Well you could make it a simple bow by doing a quick pyramid taper on the strips, but you are already starting to creep into the "bowyer influence" area when you do that. The point is to see if adding a cloth backing has an impact on how wood breaks and i think the above outline would do just fine and could be done in a day for some ambitious person.