Compound bow

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

The pulley/cam system grants the wielder a mechanical advantage, and so the limbs of a compound bow are much stiffer than those of a recurve bow or longbow. This rigidity makes the compound bow more energy-efficient than other bows, as less energy is dissipated in limb movement. The higher-rigidity, higher-technology construction also improves accuracy by reducing the bow's sensitivity to changes in temperature and humidity.

The pulley/cam system also confers a benefit called "let-off". As the string is drawn back, the pulleys rotate. The pulleys are eccentric rather than round, and so their effective radius changes as they rotate. The pulleys feature two cam tracks. An inner cam track which connects mechanically to the limbs or opposite cam and an outer cam track which the bowstring runs through. As the bow is drawn the ratio of bowstring pay-out and bowstring take-up relative to limb-weight and leverage of the cams changes. By manipulation of the shapes of these cam tracks, different draw-stroke profiles can be created. A compound bow can be soft-drawing with a slow build-up to peak weight and a gradual let-off with a long "valley" at the end. It can also be hard-drawing with a very fast build-up to peak draw-weight, a long plateau where weight is maintained, and a quick let-off with a short valley. The let-off itself is the result of the cam profiles having passed center and approaching a condition very similar to a cam-lock. In fact some compound bows, if the draw-stops or draw-length modules are removed, will self-lock at full draw and require professional equipment to unlock safely.

The compound bow was first developed in 1966 by Holless Wilbur Allen in Billings, 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, before the invention of compound bows, composite bows were described as "compound". This usage is now outdated.

A bow's central mount for other components such as the limbs, sights, stabilizers and quivers is called the riser. Risers are designed to be as rigid as possible. The central riser of a compound bow is usually made of aluminium, magnesium alloy, or carbon fiber and many are made of 7075 aluminium alloy.

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