Sometime ago I showed an expert archer friend of mine a bow I had just finished. He said, "Well it's a good bow, but why spoil it with a heavy string? " His criticism was well taken. Up to then my strings were made large so that they would last a long time and they did. They were to the arrow what the anchor rope is to the ship. Well I started to figure how to make a string that would allow the bow to shoot well.
But how important is this matter of bow string in determining the cast of the bow? We may refer to an article by Dr. C. N. Hickman where he records experiments with different weight bows and arrows in an apparatus capable of accurately determining the effect of weight of string on the speed of the arrow. The speed of the arrow is decreased approximately as much as though two-fifths of the increase in string weight were added directly to the arrow. If one uses a 200 grain string when a 100 grain string would do, the loss in cast would be equivalent to that obtained by adding 40 grains to the weight of the arrow. This may amount to 5 to 8 percent loss, which is rather small. Probably string weight is over-emphasized by some archers. However it is one of those details which every good archer is interested in having correct.
In designing a bow string the first question is—how strong must the string be? The fundamentals necessary to answer this question have been printed in articles by Dr. Hickman. In applying the principles of physics to this problem Dr. Hickman has had to make a number of assumptions about bow design to arrive at his general relations. However one may calculate from the Hickman charts what the static stresses on the string would be for various types of bows. I believe the results given in Fig. 1, furnish a very good guide to follow. The effect of shortening the bow is to decrease the stress on the string—a fact which may well be taken into account by flight shooters. The maximum stresses only are given. With short bows this maximum comes at full draw; with long bows the string is stressed most at the braced position. The pull on the bow string usually goes through a minimum when the bow is partly drawn.
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Fig. 1: Maximum Static Stresses on Bowstrings