Compound-archery-bow nocking-point locus in the vertical plane

Abstract

In general, the path taken by the nocking point of an archery bow in the vertical plane is not a straight line perpendicular to the rest position of the string; it may vary significantly from that. Any deviation from a straight line at a draw length around the full-draw position will potentially lead to significant inaccuracy if the draw length varies by small amounts from arrow to arrow. This can occur for a compound bow if the cams do not come to their full-draw position simultaneously. The path taken by the nocking point over the length of the bow's draw, and especially near the bow's brace height, can be expected to influence significantly the way that the arrow flexes and rotates about its centre of mass as it leaves the bow and enters the airstream. Hence it is of importance in relation to the arrow's position on the string relative to the bow's launcher (or arrow rest) at the brace height. This paper provides a model for the nocking-point locus in the vertical plane. While examples are provided for several configurations of compound bow, it is generally applicable to longbows and recurve bows as well. It is noted that asymmetric degrees of freedom in the cam configuration of a compound bow are required if the nocking-point locus is to be both straight and perpendicular to the rest position of the string, and that this cannot be achieved for some compound-bow configurations or for a longbow or recurve bow unless the arrow pass is in the geometric centre of the string.