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Getting The Most Out of The Bowstave
Part 5 of 5

In my earlier days of bow making, I followed tradition, for want of a better guide, in deciding on location of handle, relative to limbs, and on the length of the rigid middle section. Usually I made the lower limb shorter than the upper, and made it considerably stiffer. I abandoned that practice some years since, and invaribly make the limbs of equal length. My trend has lately been to locate the handle in its entirety below the middle of the bow so that both halves of the drawn bow are perfectly symmetrical with respect to the arrow. A properly designed and constructed bow, embodying this idea, is the sweetest acting weapon one could desire. Necessarily it requires a considerably longer rigid middle section than does the traditional bow.

This led me to investigate by experiment the effect of different lengths of rigid section upon the force-drawn curves, and especially the force at full draw, of a straight bow with equal limbs. The problem was kept as simple as possible by using the same limbs in every experiment. Limbs of the same length and stiffness were securely attached to rigid sections of different lengths. Thus six different model bows were made and tested, having the same limbs, but different rigid sections. The latter were on a relative scale of 0, 6, 9, 12, 18 and 24 inches, respectively, and the limbs on the same scale were 26 inches long. Although the problem has not been exhaustively investigated, several interesting facts have been found.

  1. For the same length of draw in each case, the radius of curvature of the drawn limbs increases with increasing length of rigid section. When the length of this section was changed from 0 to 24 inches, the radius increased by 17%. The limbs were therefore under 17% less stress with the 24-inch rigid section.
  2. Corresponding to the smaller stress with increasing length of rigid section, we find decreasing force at full draw. For example, at 28 inch draw, we have the following values:
Length in Inches of Rigid Section Force in Pounds at 28 Inches
0 46.5
6 39
9 36.7
12 35
15 34
18 32.8
21 31.8
24 31

This result, together with the fact that the radius of the drawn limb is greater as the rigid section becomes longer, obviously means that a stiffer limb is needed to maintain the drawing force constant as the rigid section is lengthened. This increased stiffness may, as we have seen, be obtained either by shortening the limbs, or by making them thicker, or by making them wider. Since their radius is lessened, they may be made either shorter or thicker without exceeding the strength limit.

This may be put in another way. Starting with a bow of given length, our experimental results suggest that an increase in length of the rigid section, with a corresponding decrease in the length of the limbs, is permissible from the standpoint of strength of the wood, and desirable from the standpoint of cast. There is a limit, of course; this is reached when the ratio of the radius of the fully drawn limb to its thickness reaches the limiting safe value for the wood, as previously mentioned. Having found that the long rigid section is a feature with-out serious disadvantages, and with some definite advantages, we are reassured about our departure from traditional design in putting the arrow plate at the middle of the bow. For, in so doing, we have a completely symmetrical bow with equal limbs, efficient in transforming the energy of elasticity at full draw into the energy of motion of the speeding arrow, and shooting without jar. If, in addition, we adopt the design feature, mentioned in earlier articles by the present writer and others, of offsetting the handle or of thickening the limbs slightly on the arrow side of the bow, it will result in a bow that will minimize arrow slap against the arrow plate. Such a bow is "insensitive" to large variations in spine or weight in the arrows, and requires far less critical matching of arrows to the bow than does the conventional type. Thus we may at least closely approach, if not completely realize, our ideal of getting the most out of the bowstave.