The compound bow is a bow that uses a levering system, usually of pulleys and cables, to bend the limbs and store energy. Compound bows are widely used in target practice and hunting.
The pulley/cam system gives the user a mechanical advantage, so the limbs of a compound bow are much stiffer than those of a recurve bow or longbow. This makes the compound bow more energy-efficient than other bows, as less energy is dissipated in limb movement.
The pulley/cam system also gives a benefit called "let-off." As the string is drawn back, the cams rotate. Most cams are eccentric rather than round, and so their effective radius changes as they rotate. Each of a compound bow's two cams features two tracks: an inner track which connects to the opposite limb or opposite cam through cables, and an outer track through which the bowstring runs. As the bow is drawn, the ratio of bowstring pay-out and cable take-up relative to limb-weight and leverage of the cams changes. By changing 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. Many compound bows offer 65–85% let off once they are pulled to full draw. This allows the shooter to relax and concentrate on the intended target at which they are shooting. This in turn gives the archer a further advantage of more precise shooting.
Courtesy and with permission of World Archery
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