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A compound bow is a modern bow that uses a levering system of cables and usually cams and pulleys to draw the limbs back.
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 and patented by Holless Wilbur Allen in Missouri in 1967 and has become increasingly popular. In the United States, the compound is the dominant form of bow.
The central riser of compound bows is usually made of aluminium or magnesium. 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. Many risers are made of the aircraft-grade 6061 aluminum alloy.
Limbs are made of composite materials and are capable of taking high tensile and compression forces. The limbs store all the energy of the bow - no energy is stored in the cams 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.
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). 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).
Efficiency of the bows also varies. Normally between 70-80% of the stored energy is transferred to 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 (35 g) arrow when shot from a bow whose peak draw weight is 60 lbf (270 N) and draw length 30 inches (76 cm). IBO speed is referred to the initial velocity of a 350 grain (22.7 g) arrow shot from a bow with a peak draw weight set to 70 lbf (300 N) and a draw length of 30 inches (76 cm).
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.
Arrows used with compound bows differ very little from conventional arrows shot by recurve archers, although shafts of arrows used with compound bows are usually made of an aluminum alloy, or carbon fiber, or a combination of these. Due to the greater forces that a compound bow places on the arrow, wooden arrows intended for a recurve bow may break when shot from a compound bow, possibly driving the broken shaft into the archer's arm. Manufacturers also produce shafts with different stiffnesses and lengths in the same model of shaft to accommodate to different draw weights and draw lengths.