Have you ever wondered what all the cables and wheels on a compound bow are for? You’re not alone. It’s the most widely used type of bow in archery today but due to it’s complex design, many people have asked, “how does a compound bow work?” In this article, I will briefly cover the history of the compound bow and also simplify the interesting way a compound bow works.
The simple answer is that a compound bow works by allowing you to draw a bowstring attached to a pulley system of cables and cams which exert force on the limbs.
That force, then transfers an extreme amount of energy to your arrow when the bowstring is released. This energy is so strong that arrow fired from a compound bow fly at speeds upwards of 350 ft per second.
The Design of A Compound Bow
First it needs to be said that since the invention of the compound bow, it has undergone many changes. Most of these changes have been improvements to the design and also the materials used.
Today, there are a lot of different compound bows available and many variations in the designs. This article will not be long enough to look at every variation but will rather try to answer our question in the simplest way possible. Thankfully, even though there are many different designs, they all share the same basic principles that make a compound bow work.
Basic design of a compound bow
- A pulley system
- Rigid limbs
- Strong riser
Compound Bow History
W.H. Allen’s First Compound Bow Patent Drawings
For thousands of years, man made use of bows for hunting and war. The earliest bow design was the straight-limbed bow.The only major improvement on the traditional bow design was the recurve bow, which has limbs which curve forward. The recurve was a big improvement because the limb design stores and transfers energy much better than the straight limb bow.
The straight limbed and recurve bow were the only bow types used until 1966 when a gentleman named Holless Wilbur Allen revolutionized archery when he invented the compound bow. He received a patent for this design in 1969.
Allen’s first design simply cut the ends off a recurve bow and attached pulleys to each end. This didn’t work great. Since the limbs were cut short, you couldn’t draw it back very far. Allen spent several years working on his design and eventually settled on a system that used cams and eccentric wheels.
This “system” has undergone countless changes and improvements in the years since then but however many changes there are, it’s still just one thing that makes a compound bow work; mechanical advantage.
Construction of a Compound Bow
The main section of a compound bow is the riser. This is what the limbs and rest, bow sight and stabilizer are attached to. Because of the extreme amounts of tension exerted on the limbs, the riser must be extremely ridge and durable.
The challenge was finding a material for the riser that meet these qualifications and yet, was as light as possible. The first compound bows were extremely heavy but since have become unbelievably light. The materials now used for risers are aluminum, carbon fiber, 7075 aluminum alloy.
Draw weight for a compound bow can be as low as 8 pounds on youth bows and as high as 70 pounds on adult models. Most commonly fiberglass composite materials are used for the limbs.
In all honesty, I have yet to find the perfect video explaining how compound bows work. (especially when it comes to the physics) However, the video below is worth your time for a basic understanding of how compound bows work as well as seeing how a compound bow is made.
Compound Bow Pulley System
The basic design of a compound bow pulley system is having either cams or wheels on each limb. There are a large number of cam shapes because this is a key component of determining the draw force curve. For example, some cam designs place the peak draw weight in the middle of the draw, while another cam causes the peak draw weight to come near the beginning of the draw.
Bow eccentrics are the different concepts for how the cams store energy in the limbs. The four major concepts are dual cam, hybrid cam, single cam, and binary cam. “Let off” is the term used to describe the force necessary to hold the bow at full draw.
Compound Bow Cam Design
For instance, if you have a 60-pound bow with %50 let off, this means only 30 pounds of force is needed to hold the bow at full draw. Most compound bows are advertised with this let-off percentage which is usually between 60%-80%.
Let off is possible because of the bow eccentrics, which includes things like the shape of the cam. Cams are typically oblong , and as they rotate back, less force is needed to hold the draw weight.
As you can see in the video example of my bow, when I begin to draw the cam starts to rotate and compress the limbs. At the beginning, the draw weight is heaviest, and there is some mechanical disadvantage.
But as the draw continues, the outer cam rotates to a position where the smaller, inner module is taking some of the draw weight and lessening the force needed to hold the draw. In full draw, the outer cam and internal modules are at their best position for mechanical advantage.
Check out this video of a compound bow in slow motion
The genius of the compound bow is its use of mechanical advantage. A bow with a single bowstring, like a recurve bow, is easy to understand. As you pull back on the bowstring, the limbs flex and are storing energy which is released through the arrow when you let go.
Many people wander how a compound bow works because it’s so different from the traditional bow design. By the way, when researching the topic, how does a compound bow work, this Wikipedia article was very helpful.
Traditional Bow Physics
You may recall,(or not if you’re like me!) that energy is equal to force x distance. Wit h a traditional bow, this is a draw force curve which is linear. This means that the further back you draw the bowstring, the more draw weight is required and the more energy is stored (potential energy). Additionally, the energy needed to hold this type of bow at full draw is directly related to the draw distance. Meaning much easier to hold the bowstring back 5 inches than it is 15 inches. If you are interested in learning more about recurve bows, you can read this article.
Compound Bow Physics
The compound bow physics are related to the pulley system. The cams and pulley system on a compound bow functions like a block and tackle system which multiplies the energy over a distance. A compound bow does not have the linear draw force curve of a traditional bow. While you are drawing the force needed will peak at a certain point, possibly at the start of your draw, and then “let off.”
Both of the larger, outer cams are connected by a single bowstring. There are two bus cables, and each wraps around a small inner cam and then attaches to the opposite cam. When you draw the bowstring, it rotates the larger outer wheels or cams, which multiplies the smaller wheels force.
Asking how a compound bow works is truly a good question because it is a complex set up. You can look at a traditional bow and you quickly have an idea of how it’s working but the compound bow is not as easy to understand. The great thing is that you don’t have to completely understand all the physics of the compound bow to enjoy shooting one. This article can get you started shooting a compound bow in no time. I hope this article simplified how a compound bow works for you.
Author: Kasey Jones
Published: July 17, 2018
Category: Compound bow