The Compound Bow

A compound bow is a modern bow that uses pulleys or cams, usually at the end of each limb to amplify the amount of force delivered to the arrow greatly increasing its speed of flight. 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. Upon release of the bowstring the turning of the pulleys or cams cause the bowstring to accelerate at a much higher rate imparting much more of the stored energy to the forward motion of the arrow. 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.
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. In most cases 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.

Advantages over traditional bows

Mechanical advantages of pulleys

• As the bow is drawn, the draw wait 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 (670–900 N).


• 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.

• 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.

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 shot from a compound bow, possibly driving the broken shaft into the archer's arm.