Almost all archers and bowhunter of the modern world use the compound bow. Very few bowhunters know how this bow works. The physics behind the compound bow is what makes the arrow fly so straight and accurate.
Energy in the Compound bow
As the bow is drawn back, the wheels and/or cams rotate. This pulls the limbs together. Energy is contained within the cams, wheels and limbs of the bow. The more energy that is stored, the faster the arrow will fly and the harder the arrow will hit its target. How much energy is stored depends on 3 factors: how far back the string is drawn, the shape of the cams, and the bow’s top draw weight.
How the Compound bow Creates Let-Off
Let-off is created by levers in the cams. When the bow is at rest the lever between the string and axle is shorter than between the axle and cable. Since the cable is the stronger of the two, the string is under more stress or tension.
When you draw the compound bow the levers change. When the bow is fully drawn the cable is very close to the axle but the string has moved away. The lever arms have now reversed and the string has the advantage over the cable. This results in a decrease in the holding weight, called the let-off.
Arrows and Accuracy
The AMO (Archery Manufacturer’s Organization) is a committee that sets standards on the compound bow and other tools involved in the sport of archery. The AMO has devised a method for measuring the speed of an arrow: shooting a 540-grain arrow from a 60-pound bow with a draw length of 30 inches. All compound bows will have an AMO speed as well as all other bows available for purchase. Another organization, the International Bowhunter’s Organization (IBO) has also devised a measurement for arrow speed that seems to be becoming more popular. This speed is determined by shooting a 350-grain arrow from a 70-pound bow with a draw of 30 inches. The AMO speed will always be lower than the IBO speed.
AMO speeds from 200-215 fps and IBO speeds of 275-290 fps are considered called low performers. The compound bow with this design makes up for poor form. This bow is best used when aiming at targets. A bow with a speed of 215-235 fps or an IBO of 290-315 fps is considered a middle of the road bow. With this bow you get good speed and ease of shot. This is an excellent compound bow for hunting. When speeds go past 235 fps for AMO and 315 fps IBO stay away unless you are a very experienced archer. Use this bow for hunting only when the speed of the arrow is vital to the target.
What Affects Speed and Accuracy?
The distance from the string to the grip when the bow is at rest is a major contributor to speed and accuracy. The bottom brace height should be seven inches. With longer distances they bow is more accurate. At less than seven inches the compound bow is harder to shoot unless your form is absolutely perfect. This is because the arrow will stay on the string for a longer time so any flaws in form are amplified.
The best bow for the average hunter or archer is the middle of the road bow. You will get enough accuracy and speed even with a flawed form.