More discussion about backings.......

[Springbuck]

Badger: "I have also used bamboo backings on woods like maple that ended up being 50% of the thickness."

Some people would call you a maniac. 

I left bamboo out of the discussion, because the fibers inherently MUST be unviolated, unless I was dumb enough to flatten the nodes.

And I agree, too, that gluing up very thin lams then requires both perfect selection and perfect handiwork, esp on the back and belly.

[Springbuck]

 DC: "Does this mean that we could tiller back and belly as long as we keep the back flattish?"

  This is partly why I brought it up.  YES, this is exactly what decrowning is, and it's exactly what working with a board stave means, too.  Again, TBB basics.  "it won't break if it isn't under a breaking strain, and you can reduce strain a lot by making things wider.

 The key, of course, is to saw out a backing from a straight tree, and follow the grain perfectly.  I, too have done just a bit of sanding to my backing when the limb ended up thinner than expected and I was worried about belly thickness.

gfugal: "I disagree that "too thick of a backing will overpower the belly". 

   Yeah, I'm saying that if your backing is going to overpower your belly, maybe your limb needs to be wider and thinner.  Maybe THAT could require you to use a thinner backing, but that comes at it from the wrong end.

 Badger:  When I first started making bows, I must admit that I was surprised when reports (and my own experience) began to come back that vert-lam bamboo flooring didn't make a good backing.  Looking at it, I was sure it would have acted like quartersawn wood, which it doesn't.  Likewise, I tried a few "board" bows from flooring, and very soon discovered they needed linen cloth backs to survive much draw weight at all.

  I suspect, Steve that since you use bamboo, the crown takes on the function of the trap. 

[Springbuck]

Willie: "isn't strain (% stretch) proportional to stiffness?  ie.  a backing that is twice as stiff as a belly will stretch half as much as the belly will compress?"

   I think this is the main crux of both my confusion, and my argument.  That's what I understood early on, that the backing should stretch a bit to balance the belly compression, move the neutral plane, and not overwork the belly.   Then TBB says, nope, we were wrong.

 Somebody had a great thread on here maybe a year ago with a graphic about amounts and percentages of wood stretching, and that really seemed to show that all woods stretch about the same.  That convinced me that the differences in wood species, width, and stiffness really came down to "will it break?" rather than "how far will it stretch?"  Because the answer for all woods was basically "somewhere between one and two percent".

  But, here you and other guys I know, good and successful bowyers I have been learning from for 20 years, do it and think it this other way, and it works.

[DC]

I think that when two craftsmen do things different ways for different reasons and they are both successful that their reasoning is probably wrong. At least one of them, anyway. So many things are done because,"That's the way I was taught to do it for this reason." Question everything

[willie]

Then TBB says, nope, we were wrong.

having too strong a backing did not work out well in some experiments I made last winter, in fact I came to the conclusion that stiffer backing is helpful only in the amount required to keep an iffy back from breaking, but beyond that, more stiffer materiel on the back only serves to crush the belly sooner.

I guess what we need to define is what constitutes  the "balance" we look for in a good design?  Should we build to make the back and belly have equal strain?  I have always wondered if lesser performing designs often suffer from more hysteresis because one side stretched more than the other.

[Morgan]

I understand that the back is under tension and belly under compression. I cannot understand why back material would increase or decrease the amount of compression the belly is under?? Isn’t bend radius and limb thickness as a whole responsible for the amount of compression?

[willie]

Morgan,
if the back cannot stretch, then the belly has to compress more

a different but extreme example of the effect is

       http://www.primitivearcher.com/smf/index.php/topic,62668.msg878994.html#msg878994 

[gfugal]

I understand that the back is under tension and belly under compression. I cannot understand why back material would increase or decrease the amount of compression the belly is under?? Isn’t bend radius and limb thickness as a whole responsible for the amount of compression?

It's called the neutral plane. In a homogenous material of similar compressive stiffness and elastic stiffness, the neutral plane is in the middle of the limb's thickness. However, once you start combining materials of different stiffness, the plane shifts to the side that has the stiffer material. If it shifts to the back the belly is under more compression if it shifts to the belly the back is under more tension. Generally, bows break in tension before compression so we tend to either place a less defect riddled backing or a stiffer backing to shift the plane towards the back. The latter reduces the tension the wood fibers under the backing experience. However, if you do very stiff material, or way too much of just a little bit stiffer material, you'll end up crushing the belly.

[JNystrom]

It's called the neutral plane. In a homogenous material of similar compressive stiffness and elastic stiffness, the neutral plane is in the middle of the limb's thickness. However, once you start combing materials of different stiffness the plane shifts to the side that has the stiffer material. If it shifts to the back the belly is under more compression if it shifts to the belly the back is under more tension. Generally, bows break in tension before compression so we tend to either place a less defect riddled backing or a stiffer backing to shift the plane towards the back. The latter reduces the tension the wood fibers under the backing experience. However, if you do very stiff of a material, or way too much of just a little bit stiffer material, you'll end up crushing the belly.

But, but... heat treating! Why would you do heat treating if bows gave up in tension? Anyway, nice explanation for neutral plane, that's what i tell to myself also.

[bushboy]

If generally,wood is 8x stronger in thickness,wouldn't this put more strain on the belly with a thicker lam?

[gfugal]

But, but... heat treating! Why would you do heat treating if bows gave up in tension? Anyway, nice explanation for neutral plane, that's what i tell to myself also.

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.

[gfugal]

If generally,wood is 8x stronger in thickness,wouldn't this put more strain on the belly with a thicker lam?

So we're not talking about width vs thickness in this discussion I don't believe, so I'm not sure how this is relevant. Were only concerning ourselves with the differences in stiffness between materials.

[bushboy]

I'm picking up what your putting down gfugal!makes sense!

[bushboy]

And stiffness isn't affected  by thickness right?not a engineer here.

[gfugal]

And stiffness isn't affected  by thickness right?not a engineer here.

I'm not one either. I actually learned most of this from this site haha. Either that or my brother or father who are engineers.

You bring up a barrier in language. I'm using the word stiffness in replace of Modulus of Elasticity (MOE) also known as Young's Modulus. It's true that a thicker piece of wood is going to be stiffer when you try and bend it than a thinner piece. However, if you have two sperate pieces of the same thickness but different material that's where MOE comes into play. If you try to bend a quarter inch thick by 2 inch wide piece of steel, it's going to take a much larger force than quarter inch thick by 2 inch wide piece of plastic.