Spine Testers
Spine testers are in reality nothing more than deflection measuring instruments. The spine is obtained by using this measured deflection in combination with the weight of the test bar or dowel as has been described above.
Four photographs are attached showing four simple variations of spine testers. All of these measure the deflection of an arrow or a test bar in thousandths of an inch, the arrow being supported on some standard length which the authors suggest to be 26", and loaded with a two-pound weight.
The portable spine tester, Figure No. 1, was built by C. D. Magee, shown to Dr. Elmer in 1931, and we believe was later described by him in "archery", revised edition. Figure No. 2 (Nagler) is an easily made adaption of a simple measuring device, demountably attached to anyone's bench. Figure No. 3 (Green) is a permanent wall mount for a shop basement. Figure No, 4 is a completely portable one, designed and built by Walter Rohde. All comprise fixed supports allowing the arrow or test bar to slide or swing as it deflects. Magee's machine comprises a vertical screw for adjusting the pointer to the zero point on the scale. No. 2 has an eccentric grooved pulley which does the same job when rotated. No. 3 and No. 4 adjust the scale. All are satisfactory. Anyone capable of building bows and arrows, can easily improvise a machine along this line. Also, all of them measure the crookedness of a shaft, when it is rotated. (Barrelled arrows ground between centers in the Canadian brand of two minute arrow machines, regularly show less than .02" throw.)
