Back in the days when wood was the principle material for making arrow shafts, there were a couple of technically oriented archers who pondered over the process of selecting just which wood was most ideally suited to this task. An archer/engineer by the name of W. J. Rheingans of Milwaukee, Wisconsin published an article in 1933 entitled “Debunking Spine”. He described the arrow testing machines that were just beginning to appear in private use to measure those mechanical characteristics of various varieties of wood that would make them desirable for use as arrow shafting. In his calculations he introduced the weight or density of the material as one of the factors affecting arrow flight along with the spine or stiffness or the material. He devised a spine rating number (N) which, when simplified by standardized test methods, gave a true indication of the spine relative to use for arrow shafting. All of this considered that, in keeping with the technology of the time, it was necessary to have an arrow that was stiff enough to keep from breaking due to the applied force of the bow, and yet supple enough to bend around the bow handle when it was shot. Remember that cut-past-center was a technology that was still in the future.

Rheingans concluded that measuring stiffness over a fixed distance (26-inches) with a 2 pound weight suspended in the center of the span using a 28-inch billet with a predetermined square cross-section, gave adequate correlation with the dynamic spine so that it alone could be used to determine appropriate spine for matching arrows to a bow.

Four years later, in 1937, Rheingans and Forrest Nagler combined their studies in an article entitled “Spine and Arrow Design” in which they discussed many of the spine measuring machines in use as well as comparative results. They recommended that the deflection of the billet (arrow shaft) in inches as measured on the 26-inch span with a 28-inch billet length under a center load of 2-pounds be adopted as a universal method of providing arrow interchangeability. Thus a standard of measuring spine for arrows came into being. However, this method was only used for wooden arrows.

After World War II with the release of aluminum alloys back to the civilian market, Doug Easton’s aluminum alloy arrows slowly took over the market. For some reason, probably having to do with the length of arrow shafting, the standard span used became 28-inches with a shaft length of plus 1-inch to allow for axial traveling during deflection. Shorter arrows were accommodated by a span length of 23-inches. The deflecting weight was set at 1.94-pounds (880 g) at the center of the span. This method was the most commonly used technique in the late 1990’s when work began on a new ASTM standard to measure arrow shaft static spine.

The work of the AMO Standards committee was completed in the year 2000, and a new ASTM standard was issued. This standard, identified as ASTM F 2031-00, has recently been revised to correct a minor error and is now approved and will be reissued as ASTM F 2031-05. It does not effect a change to the method of measuring wooden arrows because of long history of usage and data in that area.

It is important to note that the spine measurement determined by Standard F 2031-05 is identified as static spine. This is to distinguish it from dynamic spine which is decidedly different. Static spine is a measurement of the stiffness of the arrow shaft taken statically. Other elements of the arrow assembly are not considered in this measurement – just the shaft alone.

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However the individual weight and placement of the point, the fletching, the nock, the cap dip, the
cresting and any other elements of the arrow affect the way it functions dynamically. They affect its natural frequency of vibration and hence its recovery rate, the placement of its nodes of vibration, the balance point, and also its stability. Adding weight to the point reduces the stiffness while adding weight to the rear of the arrow increases its stiffness. Adding point weight will move the center of gravity forward usually increasing stability, while adding weight to the rear of the arrow will move the center of gravity to the rear and reduce stability. These are all the effects of dynamic spine which can override a carefully chosen shaft selected by static spine alone.

Granted, an arrow shaft chosen on the basis of static spine will have some tolerance based on normal choices of point weight and fletching material and therefore launch and fly quite well.
Today’s compounds are cut well past center and quite often are shot with release aids that drastically reduce lateral bias from finger release. They offer considerably more tolerance for spine than do recurves and long bows that require the arrow to bend around a projecting pressure pad. Added tolerance for spine variation is a decided plus in a bow. However, it is good to know that dynamic spine is the final contributing factor and that static spine is primarily a place to start tuning the arrows to the bow.

One interesting phenomenon that Rheingans and Nagler observed in their work was also corroborated by the work of Hickman, Klopsteg and English. Values of static spine suitable for shafting for a given bow are normally constant but actually are affected by the weight of the bow.
Bows of heavier weight actually require decreased values of deflection while bows of lighter weight require increased deflection values. They attributed this to the fact that as bows became heavier and used heavier arrows, the dynamic efficiency of the bow and arrow combination was increased, and a stiffer arrow was required. Lighter weight bows using lighter weight arrows caused dynamic efficiency to be decreased, thus allowing the use of arrows of reduced stiffness.

When we read values of spine from manufacturer’s arrow spine charts what we are seeing is the deflection of the arrow shaft in thousandths of an inch, from a load of 1.94-pounds suspended in the center of a span of 28-inches in length. This is true for all except wooden shafts. Here it is the deflection of a 26-inch span in thousandths of an inch, from a load of 2-pounds suspended from the center of the span. The smaller is the deflection the stiffer is the shaft.

Today there are computer programs available that simplify shaft selection at least as far as static spine is concerned. I am not aware of any that will accurately predict the variations possible with the options that dynamic spine can offer.