In selecting a material suitable for the making of either flight or target arrows, one of the most important qualities usually considered is the so-called spine of the material. Spine has frequently been looked upon as some mysterious property, incapable of being measured or determined by the ordinary material tests. In the past the archers have been led to believe that the selection of suitable wood or other material for arrows is an art which only a gifted few possess and that in any case it can only be acquired after years of experience at the feet of the masters.
The purpose of this discussion is to show that there is nothing mysterious about any of the mechanicalor physical properties of a material that can directly affect the flight of an arrow. It will also be shown that we can use an exact science instead of art in selecting proper arrow material since all the properties affecting the flight can be determined by the use of a balance scale, a rule and a static bending machine. Thus can the beginner in archery select suitable material for his arrows as readily and truly as those with years of experience.
The writer, however, does not want to create the impression that this applies in any way to the actual making of arrows. That is still truly an art in spite of modern machine methods.
A careful study of the behavior and ballistics of arrows will indicate that the only mechanical properties of a material that can affect their flight are stiffness and the speed with which it returns to its original shape and stays there after being deflected and that the only physical property directly affecting the flight is the density of the material. Since it will be shown in the following discussion that the speed of return of an arrow after being deflected depends upon the stiffness of the material, the properties enumerated above can be reduced to stiffness and density.
The speed of return of an arrow to its original shape has frequently been attributed to dynamic properties of the material. Recently several machines have been designed and built to measure such dynamic qualities, hoping thereby to arrive at some property of the material not shown by static bending tests. In studying the behavior of an arrow after being deflected either by passing around the bow or due to the column action of the propelling force or a combination of both, it will be found that the arrow actually vibrates back and forth and will come to its original shape and stay there only after a certain number of cycles (wiggles). The number of these cycles and the time it takes an arrow to stop vibrating depend upon the damping action of the material and the resistance of the air, which in turn depend upon the velocity and the amount of deflection.