The majority of today's bowhunters utilize compound bows, but how many really know the science behind how they work?  Here is a basic overview of how a compound bow hurtles an arrow towards our quarry with more speed, accuracy, and kinetic energy than ever before.

As a bow is drawn it's eccentrics turn, wrapping up cable and pulling the limb tips toward each other. Energy is stored in the flexing limbs, cables, and cams. The more energy your bow stores as you draw it, the faster your arrow will fly when you release the string and the more kinetic energy, or "punching power", it will carry when impacting game. Energy storage depends upon three things: the distance the string is drawn, the shape of the eccentrics on the bow (which affects how the bow "feels" when you draw it) and your bow's maximum draw weight.
 

A bow's let-off is created by lever arms within the eccentrics. When the bow is in the undrawn position the lever arm between the axle and string is much shorter than the lever arm between the axle and cable (or harness). Since the cable has the mechanical advantage, the string is under considerably more tension.

As you draw the bow this system of levers changes. At full draw the cable is very near the axle, while the string has moved a considerable distance from it. Now the lever arms have actually reversed, with the string gaining the advantage. That is why the holding weight drops off. With a high let-off bow these lever arm differences are simply more exaggerated.

The Archery Manufacturer's Organization (AMO) is an industry standards committee that has put forth a method for measuring arrow speed. AMO speed is found by shooting a 540-grain arrow from a 60-pound bow with a 30-inch draw length. You can request the "AMO speed" for any bow on the market. The International Bow hunter's Organization also promotes an arrow speed standard that is becoming more accepted than the AMO standard. The IBO speed is found by shooting a 350-grain arrow from a 70-pound bow with a 30-inch draw length. IBO speed will always be considerably higher than AMO speed, but both serve as good apples-to-apples comparisons.

Bows with AMO speeds in the range of 200 to 215 fps (275 to 290 fps IBO) are low performers with designs that are highly forgiving of small lapses in form, and are best used for target archery. Bows possessing AMO speeds in the 215 to 235 fps range (290 to 315 fps IBO) are the mid-performers - combining decent speed with shoot ability - and are a fine choice for most bowhunting situations. Once the AMO speed tops 235 fps (315 fps IBO) you are dealing with a different animal. This hot speed bow is best reserved for experienced archers under velocity-critical hunting conditions.

One of the biggest factors affecting both speed and accuracy is the bow's brace height: the distance from the string to back of the grip when the bow is in its undrawn state. Seven inches is kind of a break point for brace height. As this distance becomes longer the bow becomes more forgiving and more accurate. As the brace heights become shorter than 7 inches the bow becomes more critical and harder to shoot well with less than perfect shooting form.  The reason for this is that the shorter the brace height, the longer your arrow stays on the string before taking flight.  This means any flaws in shooting form both before and during the shot will be exemplified and adversely effect accuracy.

Unless you really need a fast bow to compensate for range estimation errors (typically these become a factor on shots past 25 yards) stay away from the hottest bows and stick with the mid-performers and arrows weighing roughly 6 ½ to 7 ½ grains per pound of draw force. For example: 60 pound bow - finished arrow weight target should be 390 to 450 grains.  This combination will provide you with enough speed, trajectory, and kinetic energy to successfully harvest the majority of North American game animals.