Home > Books > Archery: The Technical Side >
Debunking Spine
Part 5 of 7

Another relationship can be expressed as follows

 (4)

Where Wt is the weight of the dowel as tested.
Dt is the deflection as tested of the dowel weighing Wt.
Dn is an arbitrarily selected deflection for which comparisons of weight are to be made
Wn is the weight of the dowel necessary to give deflection Dn.

The above relationships (3) & (4) hold true no matter what the diameter of the test dowel as long as it is constant over the entire length. This means that it is possible to take a group of dowels of assorted weights, diameter and material, weigh them, measure their deflection in a static bending machine and by using the above equations either obtain the spine rating number or the relative weights required for a fixed selection deflection.

Equation No. 3 was checked as follows. Roy Case made a number of dowels, determined their weight and deflection and computed "N" the spine rating number. He then reduced the diameters of all the dowels by a considerable amount. The new weights and deflection were then determined and a new rating number computed. The new rating numbers checked the original numbers exactly in all cases.

In order to illustrate the practical use of equations No. 3 and 4, Table No. 1 was prepared showing the properties of various materials. The dowels were selected from a bunch made up by Roy Case and were tested in his static bending machine.

The table shows the kind of material, its source, weight of the test dowel in grains, test deflection in inches with a distance of 28 inches between supports and a two pound weight suspended from the center, the diameter of the dowel and the spine rating number. Nearly all the diameters were alike. This is just a happenstance and is not necessary to obtain the rating number. The rating number was divided by 1000 so as to give smaller figures. Note that the dowels are all listed in the order of their rating number. The last two columns of the table show the computed weights and diameters of the dowels for a deflection of 1.2" with a two pound load.

No Kind of Wood Source of Wood Wt. inGrains TestDefl.inInches Dia. of Dowel inInches Rating=N/1000 Computed For a Defl. of 1.2" Wt., Grains TABLE No. 1 Length of dowel, 29¼ inches; Distance between supports, 28 inches;Weight suspended from dowel, 2 pounds. 1 Spruce Local Dealer 297 .595 .342 52.4 210 .288 2 Sitka Sp. Kore Duryee 332 .488 .380 53.9 212 .303 3 Sitka Sp. Kore Duryee 341 .475 .380 55.2 215 .301 4 P. O. Ced. J. E. Stone 392 .375 .380 57.8 219 .284 5 P. O. Ced. Ullrich 385 .405 .380 60.0 224 .290 6 P. O. Ced. J. S. Stone 434 .330 .380 62.2 228 .276 7 P. O. Ced. Ullrich 413 .362 .380 62.3 228 .282 8 Sitka Sp. Kore Duryee 367 .464 .380 62.5 228 .300 9 P.O. Ced. J. E. Stone 451 .319 .380 64.8 232 .272 10 Imp. Norway Pine Bean 399 .441 .380 70.0 242 .296 11 Alaska Cedar Kore Duryee 378 .500 .380 71.1 244 .305 12 Imp. Norway Pine Bean 436 .404 .380 76.5 253 .289 13 Alaska Cedar Kore Duryee 362 .585 .380 76.8 253 .317 14 Yellow Cedar Alaska 340 .672 .342 77.8 254 .296 15 Fir Local Dealer 509 .330 .380 85.8 267 .275 16 Bamboo, Solid So. Bend Bait Co. 301 .950 .250 86.0 268 .236 17 Comm. Birch Local Dealer 495 .362 .380 88.9 272 .281 18 Tubular Alum. Nagler 340 .800 .250 91.0 . . . . . . 19 Poplar, Sapwood R. C. Berrey 428 .500 .380 91.9 277 .306 20 Wis. Wh. Birch Vilas Co., Wis. 455 .450 .380 92.9 279 .298 21 Fla. Jun. Roy Case 310 .994 .380 95.8 282 .362 22 Longleaf Yel. Pine R. C. Berrey 371 .330 .380 107.8 300 .275 23 Yew Ullrich 442 .875 .355 171.0 378 .328 24 Lemon-wood . . . . . . . . . . . . 520 .850 .319 230.0 437 .292 25 Osage Nagler 590 1.290 .319 450.0 611 .325

The table will show that there is not a great deal of difference between the first nine specimens listed. Also note the great variation between different specimens of the same kind of wood. Due to this variation the table of course does not indicate definitely which is the best kind of material to use in arrows. For instance the specimen of Norway pine tested may have just happened to have a low rating whereas another sample might be as good as the best Port Orford Cedar or Spruce. However in a general way the table does show that the three woods Port Orford Cedar, Norway Pine and Spruce make the best arrows. The fact that experience has always shown these woods to be the best arrow material would tend to prove that the method given above for rating materials for arrows is correct.