A compound bow
is a modern bow
that uses a levering system, usually of cables and pulleys
, to bend the limbs.
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 1969 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
The central riser of a compound bow is usually made of aluminium or magnesium. Many risers are made of the aircraft-grade 6061 aluminum alloy. 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.
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.
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
- 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 one manufacturer (Concept Archery) produces 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 keep the limbs bent. 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 longbow or recurve.
- The bow is resilient to temperature and humidity changes giving the bow superior accuracy, velocity, and distance in comparison to bows made of natural materials.
- The pulley system usually will 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. 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.
- The design of the pulleys (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. Single cam bows are generally faster than other compounds, reaching speeds close to 100 m/sec. / 315 fps. Single cam bows are less adjustable than twin cam bows, so to change the draw length of a single cam bow, the cam and string must usually be replaced.
- Compound archers usually use a mechanical release aid to hold and release the string. This attaches to the bowstring near the point where the arrow attaches, and permits the archer to release the string with a squeeze of a trigger or a slight increase of tension. The use of a release aid gives a more consistent release than the use of fingers on the string.
- In tournaments, compound archers are usually allowed the use of a magnifying sight. This is not allowed for other bow classes.
- 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.
- 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
AMO standard 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
/ 3 joules per kilogram but can reach 1.4 ft·lbf/lb / 4.2 J/kg.
Efficiency of the bows also varies. Normally between 70-85% 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 a 35 g / 540 grain arrow when shot from a bow with a peak draw weight of 270 N / 60 lbf and draw length 76 cm / 30 inches IBO speed is defined as the initial velocity of a 22.7 g / 350 grain arrow shot from a bow with a peak draw weight of 300 N / 70 lbf and a draw length of 76 cm / 30 inches.
Brace height is the distance from the deepest point of the grip to the string at rest. Typically a shorter brace height will result in an increased power stroke.
Arrows used with compound bows differ very little from arrows shot by recurve archers. 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 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 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
Bow construction techniques
- (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