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A New Bow of Radical Design, Construction and Performance
Part 4 of 9

A comparison of the two types, when each is drawn to its most advantageous position, is still more convincing. Graph 3, shows the same kind of curves. In this case the reflex of type 2 is increased until the limbs are stressed the same amount when the string becomes radial, as are the limbs of type 6 when the string is at right angles. Type 2 has a limb stress starting at 55 pounds and rising to 83 pounds when the string becomes radial. Type 6 starts with a limb stress of 12.5 pounds and rises to 82 pounds when the string is at right angles to the limb. The area bounded by the curve 1 and lines AB and EB shows the amount of work that may be obtained from type 2. The area bounded by the curve 2 and lines CD and ED, shows the work that may be obtained from type 6. It is obvious that for the same limb stress, type 6 is far superior.

064_small (13K)       065_small (20K)
Graph 4
Effect of Bracing Height Type 6 Bow      
Graph 5
Effect of Bracing Height Type 2 Bow

Another interesting feature of the new bow is the manner in which the force-draw curve may be controlled by the bracing height. Curves 3, 4 and 6. Graph 4, show the drawing forces of the new bow as functions of the draw for bracing heights of 3, 4 and 6 inches respectively. The corresponding limb stresses are shown above by curves 3T, 4T and 6T.

Graph 3 shows a similar set of curves for the second type bow. It is interesting to note that for either type bow, the force at full draw is not changed very much by the bracing height. In the early part of the draw there is a marked change in the force for different bracing heights, the effect being more pronounced for the new type. This feature makes it possible to suit the bow to different weight arrows so that a more nearly constant acceleration may be obtained, (a lower bracing height being used for heavy arrows than for light ones.)

A unique feature of the new bow design is the angle at which the limbs are set forward. For maximum draw, for a given set of dimensions, it may be shown that the correct angle is about 49 degrees. However, it may be desirable to use angles as small as 30 degrees for other kinds of bows. It may also sometimes be desirable to pull the limbs beyond the position where they become straight. The amount that the limbs are reflexed, will depend on the amount of potential energy that is desired in the braced position. If desired, the reflex may be quite great so that in the unbraced position the tips are far behind the bow as in type 4. In general, it will be difficult to take full advantage of the short limb length in heavy bows. The limb must be thinner and will therefore be weaker. The strength may be increased by making the limb wider, but for practical reasons this cannot be carried far. If materials having higher elastic limits and higher coefficients of elasticity are used, full advantage of the short limb may be obtained. On the other hand there is a decided gain even if the limb length is not changed. In this case the angle that the limb may be set forward should be about 30 degrees and at full draw the limb will not reach its straight position. The draw will stop near the point where the force-draw curve begins to rise (Point A, Curve 2, Graph 2).