Tiller
The tiller is the difference between the normal distance between the bowstring and the upper or lower limb, which can be recognized by the limbs of a sport bow , measured at the point at which the limb enters the limb holder (limb pockets) of the middle section of a sport bow. The tiller is a quantity that indicates the ratio of the tensile stress of the lower to the upper limb. The manufacturing or setting process is called Tillern .
Force distribution
Because the recurve bow is usually supported (gripped) in the handle shell and its geometric center, it is not possible to bring the energy / force center of the limbs and the pivot point or force application of the bow hand to one point. The center of force of the arch, depending on the design of the arch system or the middle section (middle section), is approx. 35 mm to 45 mm above the pivot point or the symmetrical central axis. The shooter-specific pressure point position shifts the theoretical pivot point up or down depending on the hand position or the selected grip geometry, which influences the coordination of the limb synchronization. Furthermore, the grip position of the bow hand means that the arrow comes to rest above the axis of symmetry. However, so that the limbs enable a force-synchronous ejection of the arrow, ie to transmit a uniform force and an optimal acceleration behavior to the arrow, it is necessary to adapt the force geometry of the limbs to this fact. One possibility is to select the limbs of different strengths accordingly, or to adjust their angle of attack in the limb pocket of the middle piece, which is achieved with the change in the upper tiller. The changes in force caused by the gunner, which influence the overall system and thus fix the force curve individually, must be considered as person-related. Often the bow manufacturers compensate for this in advance by setting the limbs and / or by setting the limbs at different angles. The adjustment options of today's devices take these individual optimization requirements into account. Tiller adjustment options, adjustable arrow rests and the like belong as standard to a modern, easily optimized construction, which should also be taken into account when choosing.
In the case of a crossbow , the bow is held exactly in the middle by the rifle- like structure and the string is tensioned exactly in the middle by a holding device. When the trigger is released, the two limbs (right and left) can move forward with the same force and accelerate the arrow / bolt .
Longbow
With the usual longbow , which is grasped with the bow hand exactly in the geometric center, the problem described above also arises: The bow hand presses / holds the bow in the middle, but the arrow lies slightly above the bow arm on the index finger , more precisely on the metacarpal joint of the index finger, the bow hand. Seen from the side, the arrow is therefore slightly above the central axis of the arc. If the arrow is not only between the index and middle finger of the draw hand, but the bowstring is also stretched with the ring finger (Mediterranean grip), the power transmission axis of the limbs shifts upwards accordingly. The archer compensates for this by nocking the arrow with its end (nock) about one arrow diameter (rule of thumb is approx. 8 mm) above the normal of the arrow support point on the bow or the bow hand and the string. This initially compensates for the asymmetrical arrow rest. Experience and physical tests show, however, that it is not enough for a modern archer to simply change the position of the nocking point on the string.
The high accuracy required in competition also requires a reaction to the fact that the two limbs are no longer the same length due to the relocation of the handle and the arrow rest. Because this has the consequence that the two limbs would not move with the same strength in such a way that they absolutely simultaneously exert the same force on the arrow over the same period of time. The shorter lower limb must therefore be slightly stronger so that the limb work = tensile force x extension travel is the same on both sides of the arrow axis on the extended bow.
When making a longbow, the different throwing strengths of the upper and lower limbs are achieved by hanging the bow horizontally on the handle on a clamping device (tiller board) and using weights or a pulling device to load the tendon downwards in certain steps. The side view shows whether the part of the bow involved in the ejection of the arrow first bends evenly and secondly whether the tiller geometry leads to the desired result. If this is not the case, the too strong limb is weakened accordingly by suitable material removal on the limb side (= pressure-loaded limb side facing the shooter). This is known as tillern .
Yumi
A specialty is the Japanese longbow yumi , which is grasped in its lower third. A practice-oriented theory says that the yumi must also usually be capable of being shot from a horse or knee, especially in battles. However, since the excess length of approx. 2.2 m does not allow a middle grip in these situations, the move has been made to adapt the bow geometry to this circumstance without sacrificing shooting technique.
Another theory is that the technology is probably due to the fact that several thousand years ago for making a Kyūdō One sheet has been driven not so much effort. They took a small, roughly fitting tree branch, stripped it and attached a string to it. In order to keep it in balance and tighten it, you had to reach a little deeper so that the upper, longer and thinner part of the bow remained roughly in harmony with the lower, shorter and thicker part.
After the introduction of composite construction, however, the arches were roughly the same thickness at the top and bottom. This inequality is compensated for by a movement of the bow hand, which pushes the bow forward with its upper tip when it is fired. As a result, the long side of the arch - long way - is slightly accelerated, the lower side of the arch - short way - is slowed down slightly. Only then is it possible to coordinate the different strengths of the two limbs so that a clean, horizontal shot is possible.
A recognizable on the right shot window of the Japanese longbow, which results from the arc geometry, enables the arrow shortly after the release the string without further sheet Contact - similar to a firing Folding Arrow Rest in compound bow - to the plate to shoot. The length of the arrows, around 100 cm for the Japanese bow, ensures good directional stability at the usual shooting distance of 28 meters.
literature
- Ekkehard Höhn, Karl-Heinz Hörnig: Traditional tuning, fine-tuning of longbows and recurve. Hörnig, Ludwigshafen 2000, ISBN 3-9805877-1-1 .