Bow Design Figure 9 – Illustration of draw force patterns for different types of bow
of the drawn arrow. A simple visual check as the arrow is drawn reveals when the tiller is correct, or in which direction it is out of adjustment. Screw adjustments at the limb/ riser joints allow corrections to be made.
UPPER PART OF STRING
DRAW FORCE LINE
arrow clips onto the string, would move directly towards the bow hand. Since the arrow rests above the bow hand, this would give a relative downward motion to the rear of the arrow producing a vertical oscillation in the arrow (known as porpoising). To avoid this, the handle is below the half-way point and the upper limb is not as stiff as the lower one. Thus when the bow is drawn, the riser tips forward by about 5°. The upper and lower segments of the string are at different angles to the draw force line and so have different tensions. However, the arrow now bisects the string angle and when it is loosed; the string tensions equalise and the force is directed along the line of the arrow. [Fig 10]
LOWER PART OF STRING
Figure 10 – Geometry of forces (Alison)
Figure 11 – Total limb deflection, unstrung to full draw
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Laminated bow limbs usually have a tapered wooden or foam core with fibreglass or carbon-fibre outer layers. A bow limb is thought to have the highest deflection of any commercially available laminated structure. [Fig 11] The difference in stiffness between the limbs is known as the ‘tiller’ of the bow. Actually measuring the tiller of a bow is not easy and various ways of inferring it by measuring the string position are often used. During the research, we experimented with a device made to fit behind the button. This was a simple pointer which could be adjusted to indicate where the nocking point on the string lay, relative to the riser, before the bow was drawn. (Some sight systems can be reversed and adjusted to do the same thing.) [Fig 12] When the arrow is shot, this is where the nocking point would return to. As the bow is drawn and the riser tips forward, the original nocking point rises. If the force propelling the arrow is to act directly along the shaft, then this point must lie on the line
A bow, shot by hand, doesn’t shoot in a nice straight line. The string rolls off the fingers of the draw hand and moves the tail of the arrow sideways along with the string. This bends the arrow and gives a sideways component to the propelling force, pushing the front of the arrow against a sprung button mounted on the riser. The bent arrow begins to straighten as it is accelerated forward and moves away from the button. It also begins to lift and loses contact with the arrow rest (a light support, which may be brushed aside if touched by a fletching). The tail of the vibrating arrow moves laterally through the central plane of the bow and then back again before clearing the riser. All this takes some 3-4 milliseconds. It is important that the stiffness of the arrow shaft is matched to the bow characteristics to avoid the arrow colliding with the riser. The arrow continues on its way with a lateral vibration of some 250-350 Hz, which does not die away for some distance. This lateral motion of the string and the subsequent arrow vibration is difficult to reproduce with any bow shooting device for laboratory tests or on an arrow in a wind tunnel. (Currently go to
bbc.co.uk and follow links to ‘Sport’ and ‘Archery’ for research video.)
The sprung button can be adjusted for lateral position and pre-load. Alternative springs are available to change the button stiffness. The arrow rubbing on the button for the first few inches of movement sets up its initial line of flight. Whenever anything changes on the bow (including with time), archers have to adjust the button to ‘tune’ the bow to give a consistent flight in a simple vertical plane. This allows the sights to be adjusted in the same vertical plane to allow for different target ranges without having to change the lateral adjustment.
The bow window moves the structure of the riser to one side, which inevitably leads to a slight twist in the top limb when the bow is drawn. As a designer of structure, I have suggested to several bowyers that if the cross-section around the window were changed to a channel, the flexural centre could be kept in line with the bending plane, eliminating the twist in the riser, but, as yet,
LINE OF ARROW
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