Cloth backing and tite bond/wood glues, the myth!!

[fiddler49]

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

[vinemaplebows]

Can you please describe in detail how YOU apply clothe backings.

[JW_Halverson]

  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.

[killir duck]

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.

[mikekeswick]

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.

[bubby]

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

[k-hat]

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

[rossfactor]

"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

[Ed Brooks]

This kinda spooks a new guy just finishing his 1st board bow with a silk backing on it.

[PEARL DRUMS]

Take a breath and relax Ed. You'll be fine.

[Ed Brooks]

LOL, Thanks Pearl Drums.

[Thesquirrelslinger]

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.

[JW_Halverson]

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.

[JW_Halverson]

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?

[bubby]

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