Compound bow
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A compound bow is a modern bow that has pulleys or cams at the end of each limb through which the bow string passes. As the bow is pulled back (drawn) the pulleys or cams turn which, in turn, reduce the amount of force needed to completely draw the bow. They are little affected by changes of temperature and humidity and give superior accuracy, velocity, and distance in comparison to the traditional longbow. They were first developed and patented by Holless Wilbur Allen in the USA in the 1960s and have become increasingly popular.
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[edit] Construction of compound bow
The riser (body) of compound bows is usually made of aluminium and magnesium. Risers are rigid to mount sight, stabilizer, and arrow quiver accessories. They also aid in dampening the vibration occurring from aftershock in firing a compound bow.
Limbs are made of composite materials. The limbs serve as energy storage when a bow is drawn.
A composite bow is made from different materials laminated together, usually applied under tension. Modern composite bows use laminated wood, plastic, fiberglass and carbon foam.
In the most common construction there is the riser (body) into which limbs are attached. At the end of each limb there is a cam, whose shape may vary. The cams normally have two concentric oval or more irregular shaped perimeters, around which the string and cables go. Cables travel between the cams, and at one end are attached to the string and at the other end to the cam axles.
Bowstring is normally made of high-modulus polyethylene.
[edit] Advantages over traditional bows
[edit] Mechanical advantages to pulleys
- As the bow is drawn, the draw weight increases to a peak and then "lets off". The let-off is usually between 40% and 60% of the peak weight, but some concept bows have a let-off of 99%. This enables the archer to hold the bow fully drawn and take more time to aim.
- The pulleys enable the archer to draw a bow with a much higher draw weight than they could manage with a conventional single stringed bow (there are very few people alive today who could shoot accurately with a single string using the draw weights of the longbows found on the Mary Rose). (note: Replicas of the Mary Rose longbows have draw forces ranging from 150 to 200 lbs[1].)
- The string continues to accelerate from the release to rest so imparting more power (and hence speed) to the arrow.
- The "break" of the draw weight helps the archer achieve a consistent anchor point (the amount of force required to hold at full draw), further increasing accuracy.
[edit] Other advantages
- The bow is resilient to temperature and humidity changes giving the bow superior accuracy, velocity, and distance in comparison to bows made out of natural materials such as the classic longbow.
- The pulley system usually will include some rubber-covered blocks that act as draw-stops. This provides a solid "wall" that the archer can draw against. These draw stops can be adjusted to suit the archer's natural draw-length.
- Archers in modern archery competitions usually use a release aid to hold the string steady. This attaches to the bowstring at a point and permits the archer to release the string with a pull of a trigger.
- Compound archery (unlike recurve and traditional archery) usually permits the use of a magnifying sight.
[edit] Quantities describing compound bows
Draw length is the distance from string at full draw to the lowest point on the grip plus 1.75 inches.
Peak draw weight is the maximum force needed to draw the bow to full draw. It is usually given in pounds, although SI-units should be used.
Because the draw force may increase more or less rapidly, and again drop off more or less rapidly when approaching peak draw, bows of the same peak draw force can store different amounts of energy. Norbert Mullaney has defined the ratio of stored energy to peak draw force (S.E./P.D.F.). This is usually around one foot-pound per pound but can reach 1.4 ft-lbs/lb.
Efficiency of the bows also varies. Normally between 70-80 % of the stored energy is transferred on the arrow. This stored energy is referred to as potential energy. When transferred to the arrow it is referred to as kinetic energy. The product of S.E./P.D.F. and efficiency can be called power factor. There are two measurement standards of this quantity, AMO and IBO speed. AMO is defined as the initial velocity of 540 grain arrow when shot from a bow whose peak draw weight is 60 lbs and draw length 30 inches. IBO speed is referred to the initial velocity of a 350 grain arrow shot from a bow with a peak draw weight set to 70 lbs and a draw length of 30 inches.
Brace height is the distance from the center of the bow’s riser (handle) to the string at rest. Typically a shorter Brace height will result in an increased power stroke.
[edit] Arrows used with compound bows
Arrows are normally made of aluminium alloy or carbon fiber or combination of these. Stiffness of the arrow shaft should match the bow draw weight and tip weight. Arrow length is always chosen to accommodate the draw length.
Fletching adds a small drag to the arrow in flight to keep the arrow accurate and precise.
Due to the greater forces that a compound bow places on the arrow, wooden arrows intended for a recurve bow may break when fired from a compound bow, possibly driving the broken shaft into the archer's arm.
[edit] Notes
- ^ "Longbow", by Robert Hardy (on line summary)
