Differences between two spine declarations?

Question

I shoot Easton arrows. These arrows are normally labeled with "500" or "300" etc. to tell you the spine of them.

However, I've recognized a few brands which label their arrows like "3545" (or something). Note: I can't really tell if this is an accurate number but the formatting is always like this (4 digits which seem to be completely random in my eyes :) ).

So, what does the second annotation mean? What's the difference to the "common" declaration of the spine value? (And why on earth do we need both of them?)

Answer 1

That's a different measurement.

In the 4 digit number the first 2 digits specify the diameter of the shaft in 64ths inch. And the second number specifies the wall thickness of the shaft in 1000ths of an inch.

So 3545 would be an arrow that is 35/64ths inches thick and the wall of the arrow is .045 inches thick.

This is described at the bottom of this article:

http://www.huntersfriend.com/carbon_arrows/hunting_arrows_selection_guide_chapter_3.htm