Laminations

[bow101]

Compression and tension are focused on the outsides. The middle is less critical so the lighter the better.  It really doesn't matter that much though 

I meant this post be about wood bows, it seemed to spark controversy about all bows included FG as well.  I think adb make good sense why use a lighter wood in the core.  what benefit wood there be..?

My theories about tri-lams revolve mostly around glue joints... much like plywood is stronger than a single piece of wood. Also the neutral plane of a glue joint is a factor I believe.

Flexibility seems to ring a bell here to.  I noticed that over an over stressed bow of a sinlge material that has taken a lot set is rather ruined where a lam bow still has some zing to it.

[JG]

Nice discussion combining therory and practical experience

to adb:
     "Also the neutral plane of a glue joint is a factor I believe."

could you explain a little bit, what you mean. Because I do not see the neutral plane in the glue joint in a TriLam in general. It is possible to design a bow like that, but only for one glue joint. physics won't let you.

JG

[Del the cat]

Surely a key point in this discussion is that the neutral plane (on a wooden laminated bow) isn't in the middle, and that's probably why the core wood is under compression.
E.G A bamboo backing will pull the neutral plane towards the back, whereas a backing of wet spaghettii (wet lasagna sheets are better  ) would move the neutral plane towards the belly and the core would then be working in tension.
So my guess is it's more about the combination of woods rather than the individuals. You could choose a backing to privide power or just protection, the best core wood may well be different for each.
Anyhow, what the heck do I know? I don't make laminates
Del

[toomanyknots]

I meant this post be about wood bows, it seemed to spark controversy about all bows included FG as well.  I think adb make good sense why use a lighter wood in the core.  what benefit wood there be..?

My personal opinion differs on this. I know from experience as well that using a light core, like a light piece of yellow heart (my yellow heart is pretty light) or maple verses a dense core like ipe does in fact make a difference in cast, at least with the english longbows I make. I like maple because it is not overly dense, but it works as a core. Same with yellow heart. Purple heart works, and looks great, but is dense. Ipe and osage, the same. I do notice it is important to taper the core at the tips if it is weaker in compression than the belly, you don't wanna have more core than belly at the tips if you use say a maple core and an ipe belly, as it will cause it to take more set at the tips. You want the belly wood you selected specifically because it is good at compression, actually being the one taking most of the compression. But other than that, I have never noticed any difference in set or any other issue.

[Inuumarue]

If we get technical and start over analyzing everything it is all very dependent on the Modulus of Elasticity (MOE) and the thickness of each lam.  Like Del points out, a bamboo backing can pull your neutral axis (NA) towards the back of the bow. BUT, this requires that the bamboo be "stronger" than the belly, belly/core materials.   For quick and clean examples lets take a bow limb and make each lam an equal thickness and of rectangular cross section.  and we will use the following MOE information for giggles: Bamboo 20GPa, Hard Maple 12.6 GPa, Ipe 22 GPa  (and for fun Glass at 80GPa which is a rough approximate)

Bamboo backed Maple:  Significant shift of NA to the back of the bow, glue line is under compressive force.  This puts some compression force on the belly side of the bamboo

Bamboo backed Ipe:  Slight shift of NA to the belly of the bow, glue line under tensile force. Some tensile force on the back of the Ipe lam.

Now lets look at some Trilams with three lams of equal thickness and rectangular cross section.

Bamboo backed Ipe, Maple Core: Slight shift of NA to the belly,  majority of the core is in tension.  one glue line is in tension, the other is in compression.

Bamboo Backed Ipe, Ipe core: Large shift of the NA to the belly, probabably right next to the ipe-ipe glue line in the core. Core is mostly under tension.  One glue line firmly in tension, second glue line near neutral.

Bamboo backed Maple, Ipe Core: Large shift of the NA to the back of the bow, probably near the glue line but in the ipe core. Core is mostly under compression.

Glass backed Glass, Maple core: NA at center of the limb, however due to the discrepancy in MOE the maple carries little force, that said there are a large number of shear forces due to the distortion that the maple must undergo in movement. 


That is a horrifically simplified version of the relationships and assumes that all of the material properties are equal throughout the thickness of the material (bamboo in particular).  Rounding the belly of the bow will shift the NA to the back, using different thicknesses in relation to one another changes it as well.

Adam

[DavidV]

I don't know much about bows or physics but I always thought of them like a steel I-beam in that the core is only used to separate the back and belly while also being able to stand up to the shear forces.

However I'm beginning to doubt that after having seen a trilam on this site with a fretted bloodwood core. If that fretting was caused by the shear force or the compression I don't know...

[toomanyknots]

If we get technical and start over analyzing everything it is all very dependent on the Modulus of Elasticity (MOE) and the thickness of each lam.  Like Del points out, a bamboo backing can pull your neutral axis (NA) towards the back of the bow. BUT, this requires that the bamboo be "stronger" than the belly, belly/core materials.   For quick and clean examples lets take a bow limb and make each lam an equal thickness and of rectangular cross section.  and we will use the following MOE information for giggles: Bamboo 20GPa, Hard Maple 12.6 GPa, Ipe 22 GPa  (and for fun Glass at 80GPa which is a rough approximate)

Bamboo backed Maple:  Significant shift of NA to the back of the bow, glue line is under compressive force.  This puts some compression force on the belly side of the bamboo

Bamboo backed Ipe:  Slight shift of NA to the belly of the bow, glue line under tensile force. Some tensile force on the back of the Ipe lam.

Now lets look at some Trilams with three lams of equal thickness and rectangular cross section.

Bamboo backed Ipe, Maple Core: Slight shift of NA to the belly,  majority of the core is in tension.  one glue line is in tension, the other is in compression.

Bamboo Backed Ipe, Ipe core: Large shift of the NA to the belly, probabably right next to the ipe-ipe glue line in the core. Core is mostly under tension.  One glue line firmly in tension, second glue line near neutral.

Bamboo backed Maple, Ipe Core: Large shift of the NA to the back of the bow, probably near the glue line but in the ipe core. Core is mostly under compression.

Glass backed Glass, Maple core: NA at center of the limb, however due to the discrepancy in MOE the maple carries little force, that said there are a large number of shear forces due to the distortion that the maple must undergo in movement. 


That is a horrifically simplified version of the relationships and assumes that all of the material properties are equal throughout the thickness of the material (bamboo in particular).  Rounding the belly of the bow will shift the NA to the back, using different thicknesses in relation to one another changes it as well.

Adam

Very thought provoking, thanks for posting! Am I the only one loving this thread...

[zenart]

[/quote]

Very thought provoking, thanks for posting! Am I the only one loving this thread...
[/quote]

You are not alone. Neutral plane topic is always fascinating, particularly when it involves glue joints themselves. Theoretical stuff aside, I'm finding everyone's hands-on experience especially helpful. .Ron