The authors suggest that there would be a great simplification in selection of arrow material if archers would specifically and definitely adopt as a measure of spine, the deflection, in inches, of a square test bar when supported on 26" centers and loaded with a 2 lb. weight in the middle. The specimen under test to be of a uniformly square cross section and 28" long. The actual size of the square is immaterial, since the weight takes care of it, but it is recommended that it be about 5/16" as for this size, the two pound weight will give easily measurable deflection or about the same as a 11/32" dowel.
The spine rating number, as defined above, would then be:
Where D equals the deflection in inches caused by a two pound weight suspended one-half way between two supports 26" apart.
W equals the weight in grains of the square test bar, 28" long.
Note especially that dimensions of the square do not effect the determination of spine of wood. The test bar need be of no definite size as long as it is square, of uniform cross section and 28" long. Incidentally, if it happens to be more convenient to test a round dowel, the spine rating Number "N" can be obtained in the same manner and when multiplied by the factor .95 will give a direct comparison with spine rating number obtained from a square bar.
The lower the value of "N", the better the material for arrows. However, the authors believe that this latter statement should be slightly modified for heavy bows, as will be shown towards the end of this article.
The following spine rating numbers, using the above standards, are submitted of the best, the average, and the poor grades in all four of the best arrow woods. Birch is also included to indicate how far it is from a good wood.
|SPINE RATING N|
|P. O. Cedar||39.0||43.0||47.0||70|
The selection of good arrow material can be made still simpler by standardizing on a 5/16" square test bar, 28" long. If laid on supports 26" apart, the deflection due to a two-pound weight hung midway between the supports will give a good indication of the spine of the material, provided the weight is within certain limits. The following figures are submitted of some of the best arrow woods:
Length of test bar—28".
Size of test bar—5/16" square.
Distance between supports—26."
Two pound weight applied midway between supports.
| Upper Weight |
Limits of Bar,
| Maximum Deflection|
Limit of Spine
Rating No. N
|Sitka Spruce . . . . . .||290||.490||41.0|
|P. O. Cedar . . . . .||340||.370||42.0|
|Douglas Fir . . . . .||400||.270||43.0|
|Norway Pine . . . . .||350||.420||50.0|
If the bar under test does not weigh more than shown for each particular wood and if the deflection is no greater than shown, it is an indication that the material will make good arrows: