Construction of compound bow

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 the aircraft-grade 7075 aluminium alloy.

Limbs are made of composite materials and are capable of taking high tensile and compressive forces. The limbs store all the energy of the bow – no energy is stored in the pulleys and cables. A draw weight generally falls between 10 and 100 pounds enabling arrow speeds of 150 to 370 feet per second (46 to 113 m/s)

In the most common configuration, there is a cam or wheel at the end of each limb. The shape of the cam may vary somewhat between different bow designs. There are several different concepts of using the cams to store energy in the limbs, and these all fall under a category called bow eccentrics. The four most common types of bow eccentrics are Single Cam, Hybrid Cam, Dual Cam and Binary Cam. However, there are also other less common designs, like the Quad Cam and Hinged. The “let off” is a term that describes what happens as the cam rolls all the way over. This can be seen in the close-up picture. As the bow is drawn, the draw weight increases to a peak and then “lets off” a certain percentage of the peak draw weight before a stop (known as “the wall”) prevents the bow from being drawn further. The let-off is commonly between 65% and 80% of the peak weight for recently designed compound bows, although some older compound bows provided a let-off of only 50% and some of the most recent designs achieve let offs in excess of 90%.

The photo on the right shows the axle attaching the limb to cam is mounted at the edge of the cam as opposed to the center. As the string is drawn the cam turns and imparts force to compress the limb. Initially, the archer has the ‘short’ side of the cam, with the leverage being a mechanical disadvantage. High energy input is therefore required. When near full draw is reached, the cam has turned to its full extent, the archer has gained mechanical advantage, and the least amount of force needs to be applied to the string to keep the limbs bent. This is known as “let off”. The lower holding weight enables the archer to maintain the bow fully drawn and take more time to aim. This let-off enables the archer to accurately shoot a compound bow with a much higher peak draw weight than other bows (see below).

However, there are quite a few youth-oriented compound bows with low draw weights that feature no let-off and have a maximum draw length deliberately set farther than the majority of young shooters would reach. This effectively makes the bow function very similar to a recurve, with the draw length determined by the shooter’s preferred anchor point. This feature removes the necessity to adjust the bow draw length or use a different bow for different shooters (or to change bows as the shooter gets older). This type of bow is required for use in the U.S. National Archery in the Schools Program.

At the other extreme, one manufacturer, Concept Archery, is known for producing a compound bow with 99% let-off. Although it is quite unsafe to do so, such a bow can be drawn and pointed at the ground, and the mere weight of the bow will keep it drawn even if the grip is released and the bow is hung by the string (although extreme caution must be exercised when the bow is drawn to avoid accidentally disturbing the bow out of the let-off zone without first establishing a firm grip on the string and the foregrip).

Compound bow strings and cables are normally made of high-modulus polyethylene and are designed to have great tensile strength and minimal stretchability, so that the bow transfers its energy to the arrow as efficiently and durably as possible. In earlier models of compound bows, the cables were often made of plastic-coated steel.

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