Compound Bow Basics

The invention and successful marketing of the compound bow changed the face of bowhunting forever. There is little doubt that without the compound bow, which made it easier than ever for the novice to learn quickly how to shoot a bow accurately, bowhunting would not be nearly as popular as it is today.

(Editor's Note: Bow manufacturers come out with new innovations each year this is more of a historical look at the evolution of the compound)

While both traditional and compound bows use limbs attached to a handle or riser and a bowstring to propel an arrow shaft forward, the differences between the two are dramatic. The sleek, uncluttered lines of traditional equipment speak volumes on old-school simplicity and tradition. On the other hand, a compound bow with its system of cables and wheels and adorned with accessories like a stabilizer, wrist strap, multiposition arrow rest and fiber-optic bow sight screams modern-day technology.

Yet this fact connects the two types: Both are short-range tools that force the hunter to get extremely close to game without being detected, to make a killing shot possible.

Advantage of a Compound

The advantage of the compound bow over a recurve or longbow lies in the compound's system of round or eccentric wheels and cables that work together as the bowstring is pulled back to reduce a given bow's holding weight well below its listed draw weight.

Here's how it works. When the traditional archer draws the bowstring back, the bow reaches its peak draw weight the heaviest amount of pressure needed to draw the string back at full draw. There the archer must hold that pressure on his or her fingers until he or she is ready to release the arrow. Let's say that's 70 pounds. It takes a lot of strength to hold that much weight for any length of time, which is one reason traditional archers who shoot instinctively draw and release the arrow in one fluid motion that doesn't require them to hold the bow at full draw for more than a few seconds.

With a compound bow, the archer has a distinct mechanical advantage provided by the bow's wheels and cables. A compound bow with a peak draw weight of the same 70 pounds and a let-off of 65 percent a common let-off for today's bows also forces the shooter to put 70 pounds of pressure on his fingers as the bow is drawn back to about the halfway point.

But thanks to the mechanical advantage provided by the bow's pulley system, that 70-pound peak weight is reduced as the shooter continues to pull the bowstring all the way back to full draw, where he has to hold only 35 percent of the bow's peak draw weight on his fingers until the time of release: that's just 24.5 pounds of pressure! This allows the archer to aim longer at the target without muscle fatigue. Longer aim time facilitates the use of a bow sight, which means you'll shoot a more accurate arrow.

In the early days of compound bows, the common let-off was about 50 percent. Today, let-offs commonly fall between 65 percent and 85 percent. Bows with high let-off are increasingly popular despite the fact that, for entry into their record book, the Pope and Young Club will not recognize animals taken with bows with a let-off of more than 65 percent.

Compound Bow Power

One common misconception among compound bow shooters is that their bows have an inordinate amount of power to drive their arrow shafts deeply into the target. For many years, a bow's draw weight was the common way many archers determined power potential. Simply put, if your bow had a peak draw weight of 70 pounds, and mine had a peak draw weight of only 60 pounds, your bow had more power.

Maybe. And maybe not. Many more factors go to work here than just draw weight: Differences in draw length, bow style and the power stroke of different bow designs also contribute.

Today, thanks to variations in basic bow design, it is possible for a particular bow with a draw weight of 60 pounds to shoot arrows of equal weight and length with more kinetic energy (KE) than a bow with an 80-pound draw weight.

To understand how this can be, you must first understand the basics of how a bow works.

When you draw the bowstring back, you exert effort. This energy is transferred to and stored in the bow's bent limbs. When you release the string, most of this stored energy but not all transfers to the arrow shaft, which uses the energy to fly through the air and penetrate the target. Leaving the bow, the arrow converts the stored energy into Kinetic Energy (KE), which is defined as the energy of mass in motion. Bow power is based on the fact that KE has the ability to do work; therefore, the more KE the arrow possesses, the harder it will hit and the deeper it will penetrate.

The key to bow power is having a bow that stores as much energy as possible, then delivers most of that energy to the arrow. The amount of energy transferred to the arrow is known as bow efficiency. Most of today's compound bows deliver 70 percent to 80 percent of stored energy to the arrow shaft.

With compound bows of the same draw length and draw weight, the key factor in a compound bow's ability to store energy is the shape or design of the cam. For example, a round-wheel bow with a 60-pound draw weight might store 1 foot-pound (fp) of energy for every pound of draw weight, for a total of roughly 60 fp of stored energy. However, this same bow with an energy wheel might store almost 1.5 fp of energy for every pound of draw weight. That would translate into nearly 90 fp of stored energy, or 33 percent more!

The bow's power stroke the distance over which the bow delivers power to the arrow upon release of the string is the other factor that affects the amount of energy a bow can store. The shooter's draw length and the bow's brace height the distance measured between the bowstring and bow handle when the bow is in the undrawn position are the two factors that most affect power stroke.

Brace Height can Lengthen Power Stroke

Brace height is a function of bow design; in general, a bow with a shorter brace height lengthens the power stroke, which in turn produces more stored energy. And a longer draw length will also produce more stored energy, simply because drawing the bowstring is what causes energy to be stored in a bow's limbs.

To determine your bow's kinetic energy, all you need are two numbers: the weight of your arrow shaft and its initial velocity.

Weigh your shaft complete with arrow point attached on a grain scale, then shoot it through a chronograph, and you'll have what you need.

Next, plug the numbers into this formula: KE = arrow weight in grains X velocity2 / 450,240

For example, a bow shooting an arrow shaft weighing 525 grains and leaving the bow at 250 feet per second (fps) has a kinetic energy of 72.88 fp of energy.

How much KE do you need for bowhunting? That depends, of course, on many things. First, no generally recognized KE minimum has been established by the various national archery and bowhunting organizations for bowhunting game animals. For deer-sized game, 35 fp the KE generated by a bow shooting a 400-grain arrow at 200 fps is probably enough if you place your arrow, tipped with a razor-sharp broadhead, in the animal's vitals at short range. Larger game, like elk and bears, require more KE to get the job done efficiently.

The secret is to employ an efficient bow that you can draw comfortably and shoot accurately, then place the shaft into the animal's vitals. Even the most powerful bow in the world won't give you an edge if you can't shoot it with consistent accuracy.

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