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String Weights
Part 4 of 4

With a view towards relieving the handicaps suffered by many good bows in having to accelerate a "rope" of a string, the following notes are submitted.

A bow has to do three jobs:

It has to accelerate itself;
It has to accelerate the string;
It has to accelerate the arrow.

The first two jobs are useless or parasite loads which should be reduced as much as possible. Hence, the efforts to improve designs of bows and of strings. In passing, it may be noted that Curt Hill has recently sent in some figures indicating that bows may be so good that they will have a draw pull of more than 5 pounds per ounce of weight of the bow itself (for example, a 20-ounce bow that will pull 100 pounds). More of that subject later.

And to continue on the string: Hickman and Klopsteg have figured that a third of the string weight may be considered as equivalent arrow weight. The penalty of excess arrow weight in reducing its velocity is definitely known; and velocity is the only thing that will increase the range of an arrow or flatten its trajectory. For a concrete example, assume a 75 pound bow and a feasible 40 grain string in contrast to, say, a 70 grain string. The equivalent to the 30 grain saving is 10 grains at the arrow. On a 200 grain flite arrow good for 500 yards with the 70 grain, the velocity would be increased from about 300 feet per sec. to 306 feet per sec., and the range from 500 yards to 512 yards. If the arrow weight were increased by the 10 grains equivalent saving in string weight and used with the 40 grain string, it would start off at the original 70 grain string velocity, but would slightly exceed the original 500 yards, but not by as much as with the higher velocity.

E. L. Keasling of Downey, Cal., recently sent in some reports of string tests and some samples for checking, which prove that assembled strings can be made up with a strength greater than that of their composite threads. This is accomplished by reverse twist 3-strand assemblies, as is done in a manila rope. Single strand twists have difficulty in reaching 100 percent.

Fortisan is at present the strongest material for its weight, dependably available, and with low enough stretch to permit it to be used for bow strings. It is equalled in its weight ratio only by very rare samples of linen and stretches only 30 or 40 percent more than linen. Probably no equal samples of linen are available commercially at the moment.

Starting with a factor of 85,000 (strength divided by specific gravity), we find that our best threads, including numerous samples of Fortisan, pull about 2.8 pounds per grain of weight of 10 ft. lengths. Textile experts will, of course, translate this into "deniers."

Starting with this basis, and using the rule that each pound of bow-pull requires about 4 pounds of string strength to give a reasonable life in target and hunting strings, we can quickly come to these conclusions. A 5 ft. string may use 6 ft. threads and 10 strands of 333 size which has 28 pounds strength, will require 60 feet weighing 60 grains for a total of 280 pounds strength, good for a 70 pound hunting bow. If we add a little for serving, this works out even more simply to the conclusion that "the weight of a high-grade string in grains is about equal to the draw pull of the bow in pounds."

Flite bows—because of their shorter length and because the question of durability is so reduced that the string may need only 2 pounds of strength per pound of bow pull, permit of half the above ratio. In other words, whereas the best string for hunting and target bows has a weight in grains about equal to the pounds pull of the bow, a flight string has a weight in grains equal to about half the pounds pull of the bow. (Now, take out your pet strings and see how far they are off from this minimum.)

Incidentally, our new materials and best technique in building the strings which result in these low weights, also result in strings of such small diameter that they are surprising to the eye; and for ordinary arrow nocks require padding by double serving at the nocking point. Caution is also necessary in connection with Fortisan to make sure that all abrasive points are served, as it is inferior to linen in withstanding rubbing.

editor's note:

As old as the paradox is the idea of a center shot bow. The accompanying article is included, as it is the most nearly factual of any article located that deals with the subject. Since there is nothing symmetrical about the release or motion of the rear end of the arrow, there is no reason why the front end should be so. A "pinch grip release" approximated by "flippers", "hooks" or "Turkish thumb rings", might require an approximation of the center shot bow, so far as lateral displacement is concerned, but the Mediterranean release, which jumps the string sideways frequently as much as five-eighths of an inch, does not.

In a vertical direction, the much greater offset or unsymmetrical location of the bow hand and the drawing fingers, inequalities of limbs, and the fact that the arrow is out of line with the forces that drive it, introduce features of unbalance even more significant than those that might be corrected by the center shot principle. Even tho they may disagree with opinions or conclusions, the editors feel, however, that the facts presented are worthy of study.