Compound bow

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A compound bow is a modern bow that uses a levering system, usually of cables and pulleys, to bend the limbs.

The limbs of a compound bow are usually much stiffer than those of a recurve bow or longbow. This limb stiffness makes the compound bow more energy efficient than other bows, but the limbs are too stiff to be drawn comfortably with a string attached directly to them. The compound bow has the string attached to the pulleys, one or both of which has one or more cables attached to the opposite limb. When the string is drawn back, the string causes the pulleys to turn. This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy.

The use of this levering system gives the compound bow a characteristic draw-force curve which rises to a peak weight and then “lets off” to a lower holding weight.

The compound bow is little-affected by changes of temperature and humidity and gives superior accuracy, velocity, and distance in comparison to other bows. The compound bow was first developed in 1966 by Holless Wilbur Allen in Missouri, and a US patent was granted in 1969. The compound bow has become increasingly popular. In the United States, the compound is the dominant form of bow.

In literature of the early 20th century, composite bows have been described as “compound”.

The central riser of a compound bow is usually made of aluminium or magnesium. Many risers are made of the aircraft-grade 6061 aluminum alloy. Risers are designed to be as rigid as possible. The riser is the central mount for other components such as the limbs, sights, stabilizers and quivers.

Limbs are made of composite materials and are capable of taking high tensile and compressive forces. The limbs store all the energy of the bow – no energy is stored in the pulleys and cables.

In the most common configuration, there is a cam or wheel at the end of each limb. The shape of the cam may vary somewhat between different bow designs. There are several different concepts of utilizing the cams to store energy in the limbs, and these all fall under a category called bow eccentrics. The four most common types of bow eccentrics are Single Cam, Hybrid Cam, Dual Cam and Binary Cam. However, there are also other less common designs, like the Quad Cam and Hinged.

Compound bow strings and cables are normally made of high-modulus polyethylene and are designed to have great tensile strength and minimal stretchability, in order that the bow transfers its energy to the arrow as efficiently and durably as possible. In earlier models of compound bows, the cables were often made of plastic-coated steel.

AMO standard draw length is the distance from the string at full draw to the lowest point on the grip plus 1.75 inches / 4.45 cm.[1] 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-force per pound / 3 joules per kilogram but can reach 1.4 ft·lbf/lb / 4.2 J/kg.