In the beginning, prehistoric man hunted with stones and wooden spears. Archeologists are not sure when man first used the bow and arrow for hunting, but likely it occurred in Africa. Cave paintings by prehistoric man artistically demonstrated use of the bow. Since the invention of the bow produced an efficient hunting technique, the invention is considered as significant as the use of fire, the wheel and speech in the civilization of man.

The technology of the bow and arrow has evolved to make bows more powerful and accurate. Initially the standard bow throughout history was the long bow which was built from a straight piece of wood held bent with a bow string. Later the recurved bow developed when it was discovered that the tips of the bow limbs could be permanently curved away from the bow string to add extra spring or power to the limbs to launch the arrow faster. Today, thanks to the invention of the compound bow over 70 years ago, this bow has become the preferred bow for recreational target shooting and hunting due to its amazing engineering.

The long and recurve bows relied on the sheer power of the archer to pull the bow string which bent the limbs. After releasing the bow string the limbs straightened launching the arrow. The feature of the compound bow which led to almost universal popularity is the ease of holding the fully drawn bow string while aiming at a target. By the addition of cams, which look like lopsided five to six inch diameter wheels at the tips of the bow limbs, the initial strong effort needed to pull or draw the bow string drops off by an average of 50%. This means the archer may pull the bow string with a 50 lb effort but this drops off to only 25 lb while aiming the arrow. This revolutionary feature is accomplished using a large egg shaped cam wheel pivoting at the tip of the bow limb. The dictionary describes a cam as “an irregular shaped disc or cylinder that imparts a rocking motion to any object it contacts”. In my opinion the compound bow which has large wheel-like cams at the end of the bow limbs, resembles a child’s first push bicycle.

The phenomenon of a drastic drop off in effort occurring during use of the compound bow intrigued me. To analyze how the cams accomplish this drop off, I observed the cam rotation and cable action having my 20 year old compound bow secured in a bench vice. Cams work based on a simple principle of physics-when a handyman or woman tries to loosen a bolt by holding the wrench at the end farthest away from the bolt head, a mechanical advantage is gained making it easier to loosen the bolt than if the wrench is held close to the bolt head.

When the compound bow string, which sits in a shallow groove on the cam wheel, is pulled, the cam pivots around a rod holding it at the tip of each limb. At the same time and most importantly, a second but much smaller cam lays flat and solidly fixed to the larger cam. A thin cable attached to the small cam runs to the opposite limb where it is firmly attached. As the large cam rotates, turned by the bowstring, the small cam also rotates winding the cable around itself. This shortens the cable attached to the opposite limb thereby pulling the limb down. Drop off pulling the bow string occurs because both cams are positioned so that at full draw, the bow string and cable attachment on the cam are at maximum distance from the cam rotation point. This position provides maximum mechanical advantage like with the wrench which translates into less effort to hold the bow string. When the bow string is released, each limb straightens instantaneously with a force equal to the initial draw effort launching the arrow.

The compound bow, a creative tool, is powerful, lightweight, durable and expensive but, should provide a lifetime of recreational use.