Basics : A Guide to Compound Bows

In this article we're going to introduce you to compound bow basics. You might know a little or you might know a lot. We'll cover everything from the basic principles of the compound bow through all the specification figures you'll often see printed on bow literature. How are they calculated? What do they mean? Why does it matter to you?

If you're just looking to simply identify the parts of the compound bow​, check this out instead.

Since I was just a boy I have loved and respected archery. As a child I remember watching my father shooting his beautiful wooden compound bow in our back yard. He also had a recurve bow, but he rarely used it, and I always wondered why?. As I got older, I had the opportunity to draw back both bows.

I instantly understood why the recurve stayed in the closet.

Any bow works by storing energy and then releasing it. Long bows and recurve bows work by using the arms of the bow as springs. Pulling the bowstring back creates energy which is then released as the arrow flies. Compound bows are designed in more of a ‘C’ shape. The limbs are pulled toward each other instead of back towards the archer. This allows for several advantages.

How it Works

The compound bow was designed based on a mechanical system called the Eccentric System. This system always has a string, one or two cams, and one or two cables or harnesses. In a conventional two cam compound bow, the harness is attached to the cam on one arm and the axle on the other. As you draw the bowstring, you wrap the harness around the cam and pull the arms towards each other. In this case, there are actually two harnesses that work to create energy.

The shape of the cams is what determines the speed of your arrow along with how the draw feels. By adjusting the shape of the cams, you can increase the energy stored which makes for a faster arrow speed. It can also adjust the let-off allowing you to hold more weight longer as a target comes into view.

When hunting, these two factors are huge in determining accuracy and arrow penetration.

The Let-off Principle

To explain how let-off works, I want you to imagine a teeter-totter (see-saw). This is a simple lever rested on an axle with an opposing weight on each side. If one weight gets closer to the axle, it becomes easier to lift that weight. If the weight moves further from the axle, lifting becomes more difficult. The same is true for a compound bow. In this case one branch of the teeter-totter extends from the harness to the axle and the other one extends beyond that from the axle out to the bowstring.

If you watch the draw of a compound bow, you will notice that the harness starts out away from the axle and moves towards it, while the bowstring starts close to the axle and moves away. This explains why you feel more weight when you first draw the bow, but feel let-off when you get to full draw. It allows you to store more energy in the bow with less strain on your shoulder. Let-off occurs when you have the best mechanical advantage giving you maximum leverage.

Fore and Draw Curve for a Compound Bow

Fore and Draw Curve for a Compound Bow

Here's our guide to finding your ideal draw weight.

The Back Wall

One of the first things you will notice when testing out compound bows is that each bow feels differently as you come to full draw. Some feel like they are locked in place and cannot be pulled back any further. Others are softer and allow you to keep pulling if you desire. This is all determined by the shape of the cam where it meets the harness when you are at full draw. A more flat shape will make the bow feel like it has a hard back wall. A round shape will give you more of a mushy back wall that allows you to keep pulling.

Sometimes the bow designer includes draw stops to intentionally give the bow a rock solid back wall. These are pegs that are designed to come in contact with the arm of the bow or the harness. This stops the motion, giving the bow a solid back wall. This is a very good thing for the archer. When a bow designer creates a solid wall, it encourages a consistent draw length. Archery is all about consistency. Your stance, draw length, and anchor point all need to be the same with every shot if you want to become more accurate.

Arrow Speed (IBO)

When comparing compound bows, we use a standard measurement called IBO speed.

This is the speed of a 350 grain (gr) arrow released from a 70 pound (lbs) bow with a 30 inch draw length through a certified chronograph, measured in feet per second (fps).

Of course these specifications do not necessarily represent the arrow weight, draw weight, and draw length you will actually use. They do however, give us a standard measurement by which to compare bows. For hunting we suggest an IBO speed of at least 300 feet per second.

Over 320 feet per second is considered a fast bow.

The speed of any bow is determined by four factors. This includes the shape of the cam, the distance from the drawstring to the back of the handle (brace height), the draw weight, and the efficiency of the bow design.

As mentioned before, the shape of the cam can drastically affect the speed of the arrow. The more energy that the cam stores, the harder the bow will feel. The less energy that the cam stores, the more smooth and comfortable the bow will feel.

It is important that you find the right balance that delivers accuracy and comfort.

Brace Height

Brace height is defined as the distance between the bowstring and the back of the handle. As stated previously, this can greatly affect the speed of your arrow. The shorter your brace height, the faster your arrow will travel. If you look at the physics involved, energy equals force times distance. A short brace height means you create more distance between the draw and the handle. This creates more energy.

Most hunters consider a brace height of seven inches to be the perfect distance.

Anything less is hard to control, and anything more sacrifices power.

ATA and Brace Height

(A) ATA Legnth, (B) Brace Height

Axle to Axle Length (A2A, ATA)

Axle to axle length is defined as the distance between axles when the bow is at rest. This measurement should be taken with the draw weight bottomed out for the most accurate measurement. Conventional wisdom has stated that a longer A2A length is more accurate than a shorter one. However, this means carrying a heavier bow. Recent tests have shown that at normal hunting and target distances, the difference in accuracy is barely noticeable. Unless you are trying out for the Olympics, it will not make a huge difference.

Draw Weight

Draw weight is the resistance that you will feel when you draw back your bow string. It also helps to determine how much force with which your arrow will leave your bow. Compound bow draw weights can range from 10 pounds for a child to 70 pounds for a strong adult. You typically need at least 40 pounds for hunting big game (such as whitetail deer) to penetrate into the vital organs. When in doubt, you are best to go with a lighter draw weight. Too much weight will make it hard to take the time needed to aim accurately.

Efficiency

The efficiency of a bow design is considered the ability to reproduce the energy put into a draw. To understand efficiency you must look at it as a comparison between the force you apply and the force released by the bow. For the actual equation, divide the kinetic energy of the arrow leaving the bow by the energy stored by the arms of the bow at full draw. The equation will always leave you with a figure less than one. This is due to friction and any energy left in the bow after the arrow is released. This leftover energy is the vibration noise you hear at the end of a release. Vibration can be reduced with the use of string silencers and limb dampeners.

The most efficient bows range between .8 and .85 in efficiency. Bows that are more efficient will give you more arrow speed with less effort on your draw. If you increase your draw weight or arrow weight, your efficiency will typically drop. However, a heavier arrow will use more energy and create less noise upon release.

Cams

When I picked up my first compound bow, all cams were the same. Since then, there have been several advancements in the world of compound bow cams. Now you have two cam, single cam, hybrid, and binary compound bows.

Two Cam

Two cam systems were the standard for decades, but there was one problem. If the harnesses were slightly different in length or if they stretched, the arrow would have an irregular release. This would result in kicking the tail of the arrow either higher or lower than the point.

The most common issue was with warm temperatures causing the wax found in the harness to soften. This drastically affected the flight of the arrow. If you had a bow press and checked the bow every few days, you could stay on top of it. However, who wants to guess when you rely upon your bow to be accurate. This is why the single cam system was created.

Single Cam

The single cam system is different in that it has a cam on the bottom arm and an idler wheel on the top limb. The idler wheel is simply there to unroll the bowstring as you draw the bow, while the cam does all the heavy lifting. The bottom cam has two jobs. It takes up the control cable as you draw back the bowstring and also lets out the other end of the bowstring at the proper rate to keep the nock moving straight back. The biggest advantage to this system is that the length of the cable does not affect accuracy. If it stretches a bit, you should still fire an accurate arrow. The downside is that some single cam bows are not designed to work with all draw lengths. When these bows were released, many people blamed all single cam systems. This is why the hybrid was designed.

Hybrid Cam

A hybrid bow has two cams, but the top cam is directly connected to the bottom one. This prevents any change in bowstring length from affecting arrow flight. The hybrid is designed to give you the best possible forwards and backwards motion for your nock. A good hybrid bow is about on par with a good single cam bow, but definitely better than the single cams that have issues with arrow flight due to draw length.

Binary Cams

A binary bow has identical cams on the top and bottom to maximize efficiency, but the harness from each cam is attached to the other cam instead of the bow arm. By having the two cams linked together, you get the best of both worlds. You can store a huge amount of energy by gaining a mechanical advantage on each cam, but the cams cannot have their timing thrown off. If the length of the bowstring changes, the bow automatically adjusts.

Cams

Binary Cams

Draw Length

Each archer has a specific draw length that must be matched when firing a bow. If your bow has a draw length that is too short or too long, you will not get the maximum efficiency out of your cams. Most compound bows either have adjustable cams which let you adjust draw length using a hex wrench or modular cams that let you swap out modules for different lengths. However, some bows have draw length specific cams that must be replaced for different draw lengths. These should be avoided if possible.

The Riser

On a compound bow, the riser is the middle portion of the bow upon which the handle is mounted. Modern bows typically have several cutouts that are used to make the bow lighter while still maintaining structural integrity. Most risers are made of aluminum to keep the bow light and prevent rust. However, some modern bows now have risers made of fiberglass or carbon for an even lighter finished product. Most of the bows accessories are attached to the riser such as sights, quivers, stabilizers, arrow rests, and wrist slings.

Riser

Riser

The Limbs

The limbs on a bow are the arms that attach to the riser and are typically made of fiberglass. They store all the energy to give your arrow flight. Some limbs are solid while others have a split design. Solid limbs are more likely to break, while some people feel that split limbs are more likely to warp or affect the flight of the arrow. Either way, most modern bows have parallel limbs. This creates less noise from vibration as the limbs are moving in opposite directions.

Strings and Related Accessories

Most people already know that the bow string is the part of the bow that sends the arrow flying. What most people do not know is that modern bow strings are typically made of Dyneema. This is a fabricated material that is designed to be incredibly strong and is used in bulletproof vests. I have a pair of cut-proof gloves that are made of Dyneema.

There are several ways that you can reduce the noise created by the vibration of the bowstring. One option is a silencer. This is typically a rubber piece that is attached to the string to absorb vibration. You can also use a string suppressor. This is a metal rod that faces back towards the archer with a rubber bumper on the end. The bumper is designed to both stop the forward movement of the string while also protecting the arm of the archer. By stopping the forward movement of the string, the vibration is greatly reduced.

Stabilizers can help reduce bow vibration and noise. However, they are also designed to keep the bow more stable as the bowstring is released and the arrow starts its flight. By adding weight in front of the bow, you have less chance of jerking the bow as you release the arrow. All archers deal with some level of anticipation as they fire the arrow. A natural reaction is to flinch as you release the bowstring. This tool helps reduce that movement.

Sights

There are several different styles of bow sights available on the market. Most have two primary parts. There is a pin or crosshair setup attached to the riser designed to allow you to adjust the height and direction of your aim based on the distance at which you are firing. You will have more arc to deal with as you increase the distance of your shot, so you must aim the bow higher to compensate. You also typically have a peep-sight which is attached to the bowstring. This gives you a small hole through which to view your sights giving you a more accurate aim. It also forces you to use a more consistent anchor point when you pull back the bowstring. These two parts combined can make a modern compound bow accurate up to distances of around 70 yards, though I rarely take shots over 50 yards.

7 pin sight aperture

7 pin sight aperture

Grip and Wrist Sling

The key to an accurate shot is a loose grip. In fact, I often shoot with my hand loosely open. Because of this, narrow grips tend to work better than fatter grips. The loose grip also makes a wrist sling that much more important. You do not want to spend several hundred dollars on a top of the line bow only to drop it while target practicing. The wrist sling wraps around your wrist to catch the bow in case it comes loose while firing. It is a nice safeguard to have.

In Conclusion

The compound bow is a wonderful innovation that takes a tool used for thousands of years and makes it even more efficient. With modern cam systems, you can hold the pin on a target several times longer than you ever could with a recurve bow or long bow. Just since I was a boy, the designs of compound bows have made even more advancements. These bows are lighter, more efficient, and more accurate than ever before.

Archery is a wonderful hobby and an excellent way to hunt, so give it a try if you have not before. Pulling back and releasing arrows has a calming effect on me. It is pure silence and focus until you hear that light thud from releasing the bowstring. If you hunt with a bow, it is a very personal experience. You can actually hear the animal breathing instead of staring at it through a scope at a few hundred yards. Whatever your reason is for using a compound bow, I can assure you that you will not regret it.

About the Author David James

Hi there! I'm Dave, the founder of targetcrazy.com. I'm a passionate bowman and a fan of all target sports in general. I created this site to share my knowledge with you and help you make more informed choices! Please let me know what you think of my work, comment, like, rant, speak up!

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1 comment
HARRY ELLIOT says June 15, 2017

The compound bow took little more than the shape and string from the long bow to become an entirely new animal. If you ask a bow hunter to draw you a picture of a bow, this is most likely what you’ll see.

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