importance of core lam material in all wood bows

[Echatham]

getting ready to take another stab at a trilam, bamboo, elm, and ipe.  im gonna use elm for the core cause its what i got but ive been pondering for a while what difference the core lam makes.  i know its not subject to a high percentage of the tension or compression forces like the back and belly are, so my own personal uneducated thinking is that the ideal would be just something light weight like elm or walnut, but i wonder if there are other factors to consider.  what say ye?

[BCWV]

Eric,

I'm wondering the same thing. My BBOs don't feel as snappy as the BBI I built but I love the look and feel of the osage. I used an Ipe core in my tri lam but also used hickory as a backing and it seems to be the quickest bow I've built yet. I'm going to shoot them through a chrono soon so I'll have facts instead of just going by feel and what I think I see.

I'm interested to see what the veterans say about core choices.

[Roy from Pa]

Why don't you girls just build glass bows and get it over with?  

Elm is a sweet core lam, light but strong, it is one of my favorites for a core lam. Bamboo is another really snappy core lam. And Osage is another great core lam. And walnut is a sweet contrast in color for a core lam.

So for speed, I'd say boo, then Osage, then elm.. But for looks I would say walnut..  

[Inuumarue]

My understanding was that the best cores have a high ratio of stiffness (MoE) to weight. The core doesn't take any of the extreme stresses, however it does get subjected to a number of shear stresses among other internal stresses.

I would be interested in knowing more as well. The only trilams I've experimented with had jatoba cores, more for looks than anything. I've got one such glue up waiting to be profiled. I debated an ERC core, and I'm kind of kicking myself in hindsight.

[Roy from Pa]

But what do I know?  

[Roy from Pa]

ERC is very pretty for a core lam. But it is not a high octane core material. But I don't build wood tri lams for speed. Just for fun and looks..

[Bowjunkie]

I like osage. With good glue lines and finish, it looks like one piece of wood and makes folks wonder how ya get all that deflex and reflex in there :^)

Seriously, I think Inuumarue covered it pretty well. I also think in low draw weights, low stress designs, the shear forces, and amount of actual work required of the core would be less than bows of heavier draw weight in higher stressed designs... so core wood choices may be more or less critical depending on such things.

[Black Mockingbird]

Non ones mentioned maple yet..its def a top 5 choice wood for cores...light yet strong in shear,great gluing properties,and its those reasons alone why it is preferred as the best choice in horn bows as well

[BCWV]

Originally posted by Roy from Pa:
But I don't build wood tri lams for speed. Just for fun and looks..

I'm not building em for speed either Roy but I'll take it if it comes along for the ride   :D

[Echatham]

Black mockingbird do ya mean hard or soft maple? Upon further planning my elm strip is only 66" long so that knocks an inch or two off my desired length.  So my other options on hand are osage with a couple pin knots or hard maple with some slight grain runoff and would have to be spliced in the handle.  This same osage board gave me problems as the belly of my first trilam due to a pin knot... But that shouldnt hurt it as a core lam right?  Oh i also have some good hickory... And more ipe.

[LESKEN2011]

I hope this is not off topic since it involves core woods for tri-lams. How important is straight grain for your core wood? I wouldn't have thought it would be much of an issue as long as you don't have any weak spots or knots in it. Also...where do you guys get your bamboo for lams? I have checked the local flooring places and all they are starting to carry is the horizontal and strand bamboo. Don't you need the solid vertical for bow laminations?

[LittleBen]

For a core those minor knots won't matter Eric.

My take on core materials is this
All other things equal
    a lighter core should be faster
    a core should glue well
    a core should look good
    a stronger core should make a higher weight     bow.


Lets say you three lams are roughly equal thickness, so your core lam is 1/3 of limb thickness.
if you were to use walnut at an SG of .50 vs. Ipe at an SG of 1.0 for the core, you would reduce the mass of the by 1/2*1/3= 1/6 or ~16%.

You would also reduce the stiffness and draw weight of the limbs by some margin also. Lets make some estimates. TBB v1 says that the outer 30% of either back of belly does 94% of the tension or compression work respectively. That leaves the remaining 6% of total work to be done by the core.

So if the Ipe is twice as stiff as the walnut as the SG would suggest, and the Ipe being very stiff does a bit more than 6% of the work, lets say 8%, then the walnut will do approximately 4% of the total work.

So total energy storage going from an Ipe core to a walnut core might be expected to decrease by <5%.


An even more conservative calculation, if we assume that the backing and belly slat do 75% of the work, and the core does 25% of the total work, and we again assume that the walnut does only half the work the Ipe would do, going from Ipe core to Walnut core woudl result in ~12.5% less energy storage.

So even when we make an extremely conservative calculation of two different core material and their performance, we can see that going from Ipe to walnut for a core, should reduce limb mass by 16%, but reduce total energy storage by less than 12.5%, likely less than 5-10%.

So basically what we find is that the lighter the core material, the faster the bow, assuming the core materials stiffness is roughly proportional to density.

Someone mentioned that a very light very stiff wood might be desirable for a core, and I would agree with this 100%. a high stiffness forces the core to store a greater portion of the total energy, and a low density keeps limb mass down.I wouldn't be suprised if sitka spruce actually fit this bill quite well.


FWIW maple is my favorite core material. I like the color and it glues/works well.

[BenBow]

I'll be watching this as I'm going to stick my toe into the tri-lam waters shortly. I'm going to try a wood I've used as the core in my lam bows that isn't normally used for bows but has been great in my glass lam bows. It will be a while before I can get started but I'll post picks and info when I do.

[traditional1970]

I just used oak for the core and am pleased with the feel.  
Hickory back, oak core, Ipe belly, oasage handle.  Worked fine.

[Zradix]

Ben..

I've had very similar thoughts.
I've thought sitka spruce would be a fun core to try if I can ever get myself into a pure experimental situation making a bow.
...seems like I'm always trying to make this years hunting bow...lol
..which is close to done..

...also thought about Poplars ( esp Balsam Poplar), and Aspens ( esp  Quaking Aspen)

coupled with only very good back and belly woods ie Hickory or boo backs...osage belly

[wood carver 2]

With regard to bamboo cores, which would you think might be better: laminated boo, eg. flooring, or natural boo planed both sides?
Dave.

[Echatham]

Think ill just go with osage on this one

[Bowjunkie]

Wise choice    

[Zradix]

Good luck Eric.

Roy....
Why don't ya think walnut would be as high octane as elm?

[Echatham]

For you guys that say maple do you mean hard or soft?