A Little Bow Geometry

Everything about archery consists of tradeoffs, starting with the design of bows.

Recurve Bow Geometry Diagram
Consider the recurve bow at brace in the figure. The horizontal centerline of the bow is midway between the arrow rest hole and the pivot point of the grip. The nocking point is about one half inch above the level of the arrow rest, so that arrow (depending on size) is attached to the string about 3/8˝ above level and an additional inch or so above the previously mentioned centerline. All of these are examples of many of the various tradeoffs necessary to design a bow.

Basically this results in the archer’s fingers (plus tab), being 2-2.5˝ high, being practically centered on the bowstring (see fingers in diagram in relation to bows centerline (CL) which is also the string’s centerline). The bow hand is on the bottom half of the bow, creating what is called a “tiller” problem. (The word tiller means the same as the word tiller associated with sail boats; it means a thing “to steer.” By holding the bow asymetrically, that is on the bottom half, we in effect have made the top limb longer. Many people adjust for this by turning the limb screws to create a slightly weaker limb on top than bottom. (In the old days, they actually sanded one limb more than the other to make it weaker.) Others address this issue by adjusting the nocking point location, leaving the limb bolts alone. If you move the nocking point up, you are decreasing the leverage you have on the top limb, making it effectively stronger, etc. and apparently only small adjustments in nocking point location are necessary to adjust for the problem that comes from holding the bow on its bottom half.

The same issues come up whether you shoot a longbow, recurve bow, or compound bow. The simplest approach is to set the tiller at “even” and then adjust the nocking point location while tuning (bare shaft test). Tiller is determined by measuring from the top or bottom of the riser to the bowstring (at a right angle), then “tiller = top tiller measurement – bottom tiller measurement.” Typical recurve settings are +1/8˝ to +1/4˝ (string closer to bottom limb than top).

This situation has benefits, though, in that as the bow is drawn, a torque is created hinging on the bow shoulder that helps to raise the bow. Basically the draw is part “back” and part “up” (using the bow arm as a boom.

Full Draw Vectors

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