So many inquiries have been made as a result of articles and as a result of inspection of some of the spine testing machines by visitors at the Canadian National Tournament that the following notes and descriptions are thought to be of general interest.
Spine has partly emerged from the mystery that has always surrounded it to the extent that there have been recent attempts to define it. The definition in Canfield's Handbook Of Archery Terms reads:
The authors take exception to that definition as it cheerfully scrambles arrow design with quality of wood.
English, in his "arrow factor", took into account weight and stiffness. However, his formula as applied to arrow design seems questionable as will be shown later since it contains the weight factor. On the other hand, if his formula is used for the selection of arrow material, it will lead to erroneous conclusions, since the exponent of the weight factor is not correct.
The authors believe that the word "spine" should be used only in connection with the selection of a material suitable for arrows. Spine can, therefore, be defined as "a measure of the mechanical and physical properties of a material, which make it suitable for use in arrows". What are these mechanical and physical properties which determine good arrow material? One of the authors discussed this quite thoroughly in the May 1933 " archery review" ("Debunking Spine", by W. J. Rheingans) where it was shown that the only properties of a material which can affect the flight of an arrow are stiffness and weight.
Spine was then more definitely defined as the relationship between the stiffness (as determined by deflection) and weight of the material. The relationship was expressed in terms of a spine rating number, as follows:
Where W equals weight of the test bar or dowel for a given length.
D equals tested deflection for a given weight suspended on the test bar between supports a given distance apart.
The factor 1000 is used to reduce the rating number N to smaller units.
Assuming that the smaller the deflection of a material for a given weight of the test bar, the more suitable it is for use in arrows, we find that best material is indicated by the lowest Spine Rating Number N.
From practical experiences, we know that the best arrow woods are undoubtedly the group comprising Sitka Spruce, Douglas Fir, Norway Pine (imported), and Port Orford Cedar. It is more than a coincidence that these woods that have stood the test of ages, experts, and analysis also have the lowest Spine Numbers. This fact alone convinces the authors that the only measure of spine is the ratio between weight and deflection.
Adhering to the above formula, it is very easy to standardize on a method of determining whether a material is suitable for use as arrows.