The function of the cam systems (known as the ‘eccentrics’) is to maximize the energy storage throughout the draw cycle and provide let-off at the end of the cycle (less holding weight at full draw). A traditional recurve bow has a very linear draw weight curve – meaning that as the bow is drawn back, the draw force becomes heavier with each inch of draw (and most difficult at full draw). Therefore, little energy is stored in the first half of the draw, and much more energy at the end where the draw weight is heaviest. The compound bow operates with a different weight profile, reaching its peak weight within the first few inches of the draw, and remaining more flat and constant until the end of the cycle where the cams “let-off” and allow a reduced holding weight. This manipulation of the peak weight throughout the draw (accomplished by the elliptical shape of the cams that change leverage and mechanical advantage) is why compound bows store more energy and shoot faster than an equivalent peak weight recurve bow or longbow.
The design of the cams directly controls the acceleration of the arrow. What is termed a “soft cam” will accelerate the arrow more gently than a “harder” cam. Novice archers will typically shoot a soft cam whereas a more advanced archer may choose to use a harder cam to gain speed. Bows can be had with a variety of cams, in a full spectrum from soft to hard. Some pulley systems use a single cam at the bottom of the bow and a balanced wheel at the top of the bow instead of two identical cams. This design eliminates the need for buss cables and instead uses a single string that begins at the cam on the bottom of the bow, travels over the wheel on top, around the bottom cam again, and ends attaching to the top limb.
When a compound bow is drawn, the limbs are pulled in toward each other, by the buss cables, unlike a longbow or recurve where the limbs flex in the direction of the bow string. This difference allows modern compounds to have limbs that are horizontal instead of angled. The horizontal limb configuration minimizes the recoil and vibration felt by the shooter when the arrow is released. The compound bow is resistant to temperature and humidity changes, giving the bow superior accuracy, velocity, and distance in comparison to bows made of natural materials.
The pulley system will usually include some rubber-covered blocks that act as draw-stops. These provide a solid “wall” that the archer can draw against. These draw stops can be adjusted to suit the archer’s optimum draw-length, which helps the archer achieve a consistent anchor point and a consistent amount of force imparted to the arrow on every shot, further increasing accuracy.