More discussion about backings.......

[bushboy]

I understand know .I was posting in regards to what I have witnessed in lams that I've tried though the years and not science type stuff!I should have taking more time to read the complete thread!my bad!

[bushboy]

I don't mean to be a smart a$$,all I know is if the belly gets to thin it's not good.

[gfugal]

I don't mean to be a smart a$$,all I know is if the belly gets to thin it's not good.

Ther might be something to that but I think.

willie; "If the thickness of backing doesn't matter, why not a backing only a couple of thousandths thick?"

 I wonder.  Baker talked about backing some bows with hickory veneer.  Thickest veneer I have seen is 1/40".  MY real thinking here is that, while following longitudinal grain exactly is so vital to the process, it's essentially impossible to do.  A backing that thin is just too prone to flaws.  But, I've seen PearlDrums suggest someone slap on two 1/16" hickory backings.  Does that equal one 1/8" backing?

  I think the same about bellies, too.  I once glued on a belly lam of ipe, less than 1/8" thick, to a black locust bow that I had screwed up.   I ground the belly flat on the belt sander, clamped some of the set out of it on a form, and applied the ipe to the belly.  That ipe basically crumbled at half draw.

I wonder if there is like some critical thickness for the wood fibers where if you go too thin then you hinder its ability to compress?

[willie]

I wonder if there is like some critical thickness for the wood fibers where if you go too thin then you hinder its ability to compress?

I think that the often repeated adage that the top 10% of thickness does half of the work, is only part of the story. Proportionately less work is done as the depth approaches the neutral plane, but the other half of the work is still done below the top 10. If you have a thin but stiff layer taking strain, over a weaker underlayment, once the top half chrysals, things go down hill fast.     

[Marc St Louis]

I'm not quite sure what properties heat treating changes. It's possible that it doesn't make the belly that much stiffer, but maybe rather improves the wood cell's integrity so they are less likely to collapse and damage. So the benefit it gives of keeping the belly sound may be worth the risk of it potentially causing the back to break, I would still not heat treat a wood that is already weak in tension or has some defects if it was going to be a self-bow for the very reason you mention.

I've had Oak, Osage, HHB and Maple bows explode violently after heat-treating.  It's pretty hard to say what type of failures they were but I would be willing to bet they were tension failures.  The collapse and damage would be related to the elasticity of the wood and not it's strength/stiffness, I've always maintained that heat-treating does nothing for the elasticity of the wood.