January 14, 1943
I was delighted with Forrest Nagler's article "Bow Strings" which appeared in the January number of the American Bowman-Review. The tabulated data throws considerable light on the differences in bow string material and construction. The variation in the "K" and "KT" values for the string listed is 62.5% and 79% respectively. Since "KT" is a function of "K" it is evident that many string makers are handicapping themselves by the use of inferior materials. It would be interesting to see what the variation in "K" would be with all the makers using the same thread.
My main interest in Mr. Nagler's article centered on the data tabulated for bow string No. 13. Why did this string which had the greatest potential strength have such a low "K" value? The weight of the string seemed high, so I compared it with the unit weight of the thread and the minimum length of thread required to make a string 21½" long with a double loop. The resulting deductions follow:
- By cutting the single threads 28 inches long the weight of the 21½" double loop string No. 13 could be reduced in weight from 48.5 grains to 30 grains. (Wt. = 28 x 18 x 6.23 / 119= 26.4 grains, say 30 grains after wax is added.
- The making efficiency (actual breaking load divided by the potential breaking load) is only 65.7%. This is less than average. It is believed that a well made string should go at least 80% to 85%. Some went higher.
- The "K" value that may be reasonably expected for a string made from the same material as string No. 13 is arrived at by taking the weight (30 grains) from "a" above, the making efficiency (85%,) from "b" and keeping all other data the same as tabulated. Then K = 18 x 17 x .85 / 30 = 8.66, which is better than double the "K" shown in the table for the same thread.
From the foregoing it is apparent that "K" is dependent upon strength of materials, weight of materials, economy of materials, moisture content, equal tension on threads, arrangement of threads, amount of twist, and type of construction. Is there any wonder that bow strings vary so much in strength and efficiency?
Yours very truly,|
Robert J. Dunbar
U. S. Engineer Area Office
P. O. Box #1111
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