Compound bow

A compound bow is a modern bow that uses a levering system of cables and usually cams and pulleys to draw the limbs back. The limbs of a compound bow are usually much stiffer than those of a recurve bow or longbow. This limb stiffness makes the compound bow more energy efficient than other bows, but the limbs are too stiff to be drawn comfortably with a string attached directly to them. The compound bow has the string attached to the pulleys, one or both of which has one or more cables attached to the opposite limb. When the string is drawn back, the string causes the pulleys to turn. This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy. The use of this levering system gives the compound bow a characteristic draw-force curve which rises to a peak weight and then "lets off" to a lower holding weight. The compound bow is little-affected by changes of temperature and humidity and gives superior accuracy, velocity, and distance in comparison to other bows. The compound bow was first developed and patented by Holless Wilbur Allen in Missouri in 1967 and has become increasingly popular. In the United States, the compound is the dominant form of bow. In literature of the early 20th century, composite bows have been described as "compound".

Construction of compound bow

Browning Compound Bow Pulley System Closeup

The central riser of compound bows is usually made of aluminium or magnesium. Risers are designed to be as rigid as possible. The riser is the central mount for other components such as the limbs, sights, stabilizers and quivers. Many risers are made of the aircraft-grade 6061 aluminum alloy. Limbs are made of composite materials and are capable of taking high tensile and compression forces. The limbs store all the energy of the bow - no energy is stored in the cams and cables. 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 utilizing 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. Compound bow strings and cables are normally made of high-modulus polyethylene and are designed to have great tensile strength and minimal stretchability, in order 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.

Advantages over traditional bows

Technical advantages

• As the bow is drawn, the draw weight increases to a peak and then "lets off". The let-off is usually between 65% and 80% of the peak weight, and at least one manufacturer (Concept Archery) produced a compound bow with 99% let-off. The "let off" is a term that describes what happens as the cam rolls all the way over. If you look at the photo to the right you can see that 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. Once the cam turns all the way around the least amount of force needs to be applied to the string to compress the limbs. This is known as "let off". This enables the archer to hold the bow fully drawn and take more time to aim.
• This let-off enables the archer to accurately shoot a bow with a much higher peak draw weight than they could manage with a conventional longbow. 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 to 900 N).^[1])
• 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. This helps the archer achieve a consistent anchor point and a consistent amount of force imparted to the arrow on every shot, further increasing accuracy. Also the design of the pulleys (cams) directly controls the acceleration of the arrow. What is termed a "mild cam" will accelerate the arrow more gently than a more aggressive cam. Novice archers will typically shoot a mild cam where as a more advanced archer may choose to use a more aggressive cam to gain speed. Bows can be had with a variety of cams, in a full spectrum from mild to aggressive. Modern 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 and are often called "solocam" bows. This design eliminates the need for two separate strings 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. Solocam bows are generally faster than older compounds; reaching speeds close to 315 fps.^[2] In order to change the draw length of a solocam bow, a different cam and string are usually required as they are less adjustable.

Circumstantial advantages

• 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 squeeze of a trigger. The use of a release is also a big advantage because compared to the traditional finger on string release the mechanical release leaves the string in a consistent manner every time you touch the trigger.
• Compound archery competitions (unlike those for recurve and traditional archery) usually permit the use of a magnifying sight.
• Stabilizers and dampeners are particularly well-developed for the compound bow. They allow the archer to shoot even more accurately, by reducing the movement of the bow when the string is released.

Circumstantial disadvantages

• The relatively low holding weight of a compound bow compared to a recurve bow makes the compound more sensitive to certain form faults when the archer is at full draw. In particular, it's easier for the archer to torque the bow around the vertical axis, leading to left-right errors.

Quantities describing compound bows

Draw length is the distance from the string at full draw to the lowest point on the grip plus 1.75 inches (4.45 cm). Because the draw force may increase more or less rapidly, and again drop off more or less rapidly when approaching peak draw, bows of the same peak draw force can store different amounts of energy. Norbert Mullaney has defined the ratio of stored energy to peak draw force (S.E./P.D.F.). This is usually around one foot-pound-force per pound (3 joules per kilogram) but can reach 1.4 ft·lbf/lb (4.2 J/kg). Efficiency of the bows also varies. Normally between 70-80% of the stored energy is transferred to the arrow. This stored energy is referred to as potential energy. When transferred to the arrow it is referred to as kinetic energy. The product of S.E./P.D.F. and efficiency can be called power factor. There are two measurement standards of this quantity, AMO and IBO speed. AMO is defined as the initial velocity of 540 grain (35 g) arrow when shot from a bow whose peak draw weight is 60 lbf (270 N) and draw length 30 inches (76 cm). IBO speed is referred to the initial velocity of a 350 grain (22.7 g) arrow shot from a bow with a peak draw weight set to 70 lbf (300 N) and a draw length of 30 inches (76 cm). Brace height is the distance from the center of the bow’s riser (handle) to the string at rest. Typically a shorter brace height will result in an increased power stroke.

Arrows used

See also: Arrow

Arrows used with compound bows differ very little from conventional arrows shot by recurve archers, although shafts of arrows used with compound bows are usually made of an aluminum alloy, or carbon fiber, or a combination of these. 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 or the arrow may shatter because of the changes in force applied to it during release. Manufacturers also produce shafts with different stiffnesses and lengths in the same model of shaft to accommodate to different draw weights and draw lengths. Arrow stiffness (spine) is an important parameter in finding arrows that shoot accurately out of a particular bow (see Archer's paradox).

External links

• Extensive article on the compound bow Twenty-five years after Allen's patent of December 1969.
• photo history detailing the development of the compound bow
• Photos of compound bows from the 1970s

See also

• Bow (weapon)
• Crossbow
• English longbow
• Longbow
• Flatbow
• Archery
• Horse archer
• Binary Cam

Bow construction techniques

• Self bow
• Cable-backed bow
• Compound bow
• Laminated bow
• Composite bow

Bow forms

• Recurve bow
• Decurve bow
• Deflex bow
• Reflex bow

References

• (1992) The Traditional Bowyers Bible Volume 1. The Lyons Press. ISBN 1-58574-085-3
• (1992) The Traditional Bowyers Bible Volume 2. The Lyons Press. ISBN 1-58574-086-1
• (1994) The Traditional Bowyers Bible Volume 3. The Lyons Press. ISBN 1-58574-087-X