Things that make you go HMMMM...!

[WESTBROOK]

Just getting my longbow figured out, started this spring, always been a recurve guy. Its a Robertson Mystical, marked #58 @ 28". The guy I got it from said It was 3-4 lbs lighter according to his scale, but anyway..I'm drawing dang near 30" and it is shooting 65-70 sitkas with 160 gr pts real nice, bare shafts real good too.
Today I tried some 65-70 lam birch with 125g tips, they were quite weak. Icould short draw an inch or so and they flew pretty good.

On a whim I tried some 75-80 lam birch with 160 g tips thinking they will be way stiff, suprise!, they fly great and a bare shaft flys right into the group. Did it several times 'cause I didnt believe it.

So what would make such a big difference in in spine requirements between sitka and lam birch shafts?
All shafts are 30" to BOP
Both have 160 g pts.
The only obvious difference is the shaft weight. The sitkas weigh 560g and the lam birch are around 760g. Will the shaft weight effect the spine that much.
What do ya think?

Eric

[vermonster13]

It's just like adding point weight to weaken spine. They are spined at 26, so you are putting a whole lot of weight up front.

[WESTBROOK]

Hi Dave,  Thanks

I know 200g difference on the tip will throw things out of whack, just never thought that weight over the length of the shaft would have that much impact.

Thanks again
Eric
HMMMM...

[ChuckC]

Are Sitka's full length tapered ?

ChuckC

[WESTBROOK]

Chuck, nope, just parrallel wood shafts.

Eric

[Artur]

Eric;

This is only a guess, and it might be considered as "reaching" by some... Perhaps the laminated shafts simply have more flex to them than solid, natural shafts. Then again, differing materials have differing characteristics, even when those materials are different types of wood.

[Cupcake]

Eric,

This is a subject that interests me a lot.  My theory is that it has to do with the natural frequency of the arrow.  The frequency is dependent on length, stiffness, moment of inertia,  modulus of elasticity and mass (weight).

The ratio of stiffness divided by the square root of mass directly affects the natural frequency.  If this is a correct theory, a heavier shaft must be stiffer to behave like a lighter shaft.  Your experience is in agreement with the math behind this idea.

I hope to do more investigation of this topic in my "spare" time.

Kevin

[DesertDude]

Kevin, Redant60/65 and Desertdude are lost??                 You forget we all don't work for NASA!!!