Comparative compressive strengths

[Wooden Spring]

Is there a way to determine the comparative compressive strengths of woods? In multi-laminate bows, say with 3 laminates, I can imagine that one would want the material with the highest compressive strength on the belly... How is relative compressive strength determined? Specific Gravity? Bend / Break test?

Or, does it matter at all if both the core lamination and the belly lamination are high in resistance to compression, which one goes in the core, and which one goes on the belly?

[bushboy]

i use the 3 you listed in a less than scientific fashion.first by gauging density of a piece of stock by the weight of it by feel as compared to the norm.bend tests and break tests.not sure about core wood.

[Badger]

   The middle lam is under so little compression I don't think it matters all that much, mass seems to be a bigger consideration for the middle lam.

[Wooden Spring]

   The middle lam is under so little compression I don't think it matters all that much, mass seems to be a bigger consideration for the middle lam.

That's what I was thinking, but I was curious because of the results that I obtained from two glue-ups...
Both glue ups were on the same form, inducing 2" of reflex.
Glue-up #1 was 1/8" Hickory back over 5/8" Jatoba, and retained 1" of reflex coming off the form.
Glue-up #2 was 1/8" Hickory back, 1/8" ipe core, and 5/8" jatoba belly. It retained almost the full 2" of reflex.

So, did merely the extra glue joint account for the additional reflex? Or did the rigidity of the ipe help to maintain the reflex?

I'm making a mollegabet out of the tri-lam (Glue-up #2) that I expect to be a heavy draw weight, (I'm good with that) but I'm curious to see its performance next to a mollegabet of the exact same dimensions made from Glue-up #1.

That's just how I learn... Throw a questionable variable into two identical bows, and see how they perform...  My take on the scientific method I suppose... I just wish that ipe dust didn't make me see pink elephants...

[Don Case]

#2 was 1/8" thicker, would that account for it?

[DavidV]

There happens to be a measurable term for it called Compression Parallel to The Grain. Or sometimes Crushing Strength. For most species the data is on the wood database or the forest product laboratories.

Disclaimer: there is a lot of variety between individual trees of the same species.

[Wooden Spring]

#2 was 1/8" thicker, would that account for it?

I originally thought the same until I roughed out the bow and it held the same reflex. So it'll be interesting to see them side-by-side...

[willie]

ws

How is relative compressive strength determined? Specific Gravity? Bend / Break test?

Tim Baker developed a simple test that was reported in TBB.  Some more recent work with this can be found
http://paleoplanet69529.yuku.com/topic/58080/Standard-Bend-Test

Badgers findings confirm the effects predicted by the basic principles of mechanics and strength of materials.

Other bowyers have prestressed laminates in the glue up to improve performance
http://www.primitivearcher.com/smf/index.php/topic,23698.0.html

And still other bowyers have found less well described reasons to prefer heaver woods in the cores of laminate bows. If this preference can be quantified better, it may well be that the performance gains in these designs may outweigh the losses normally expected from using more massive cores.

willie