The term arrow spine is misunderstood (I know it was . . . by me for many years) and I wonder when it was first used? In England I know there was a term “whippiness” used to indicate arrow spine and an arrow of weak spine was often referred to as being “whippy” or “too whippy.”
If anyone out there knows who first used this term and why it was chosen (it suggests a relationship with human spines, but that is just a guess) I will very much appreciate having that information.
Misconception: Arrow Spine is Not Arrow Stiffness
The most common misconception is that spine is a measure of stiffness, but it is not. If you were to put a rather thick copper wire in a spine tester and hang the weight from the middle, you would get a “reading” indicating the “spine” of that “shaft.” But when you take the weight off of that “shaft” you would find the shaft stays quite bent. It will “spring back” slightly but not all of the way back to straight. So, this “shaft” possess some “stiffness” but not the attribute we require of arrow shafts.
That desired attribute is resilience. We need the arrows to spring back to their former shape completely. According to Merriam-Webster, resilience is:
the capability of a strained body to recover its size and shape after deformation caused especially by compressive stress.
An Aside When an arrow is bent, the outside of the “bow” expands the material of the shaft and the inside of the bend compresses it. So, there are “compressive forces” involved in arrow flexing.
And we certainly do not want arrow shafts that are stiff and brittle, for example. They need a certain springiness.
Arrow spine as measured on spine testers is called “static spine” to differentiate it from “dynamic spine” which is the arrow’s resilience based upon its behavior while being shot. In a static spine tester, you can swap out arrow points to your heart’s content and you will get the same reading from the tester. Same is true for nocks and fletchings. But each such change affects the arrows dynamic spine, that is its behavior.
The masses of the point and fletchings change how easily the point and rear of the arrow move. For example, heavier arrow points make the arrow point harder to move (due to having more inertia) which means the forces acting making the arrow shaft flex are going to act more on the other parts of the arrow. Heavier arrow points make the arrows behave “whippier,” that is as if the spine of the shaft were weaker. Lighter points have the opposite effect.
We do use, and I have used, “stiffness” as a stand-in for arrow spine and will probably continue to do so, but that is only a rough approximation.
Misconception: Higher Spine Numbers Do Not Mean More Spine
Static spines have come to be standardized. With a shaft of a specified length is supported on two uprights spread a certain distance apart, a weight is hung from the center of the shaft and the amount that the shafts sags is measure in thousandths of an inch. If the arrow’s center sags one half of an inch, or 500/1000 inch, we say that the shaft has a spine value of 500. The more resistant to bending the shaft is, that is the more resilient, the less it sags and the lower the spine number is. If the arrow’s center sags one quarter of an inch, or 250/1000 of an inch, we say that the shaft has a spine value of 250. That is a very “stiff” arrow for target purposes. Common arrow spines reach from that low value upward to even 2.000 inches or a spine value of 2000. Those arrows are for children drawing very lightweight bows.
I will come up with a suitable prize for anyone who comes up with the name of the person who first used the term and what the derivation of that term was (an allusion to having a stiff spine as a person is a possibility). Let me know. You know where I am.
Oh, why? Because enquiring minds want to know, that’s why.