It has been a time when the archery sport was limited to a few choices of traditional bows. But things changed in 1966 when the compound bow made his first appearance into the archery world by the inventor Holless Wilbur Allen, a bow hunter from Missouri. It is only 3 years later, in 1969, that the patent was granted to the father of the invention of the bow eccentric which changed the face of archery forever.
Now, you may wonder what is a compound bow and how it affects the modern day archer? In the following article, you will get to know the differences between the compound and the traditional bow. We will also oversee this modern invention from all the basic aspects to tips on how to choose a compound bow.
Here is some help to navigate through this article:
Table of Content
- Compound Bows vs Other Bows
- Compound Bow Parts
- The Different Cam Systems
- The Importance of Brace Height
- Axle-To-Axle (ATA) Length
- RTS, RTH, RAK, DTH...What does that mean?
- The IBO Speed Rating of a Compound Bow. Probably not what you think it is...
- Kinetic Energy for Bowhunting. How much is enough?
- Tuning a Compound Bow
- Buying a first compound bow
Compound Bows vs Other Bows
As opposed to other bows, a compound uses cables and cams (pulleys) as a lever system to pull back the bowstring and bend the limbs which are stiffer than regular bows, allowing for higher draw weights with more forceful and faster shots.
One of the most interesting advantages of the compound bow is the let-off which means that a portion of the weight to be held is reduced when reaching full draw, translating into steadier hands for a longer period of time, therefore increasing accuracy.
As an example, a bow with 80% let-off at 70 pounds draw weight would have the archer hold only 20% of that poundage at full draw resulting to only 14 lbs.
Compound Bow Parts
Simply put, the limbs are the essential parts of a bow. Their purpose is to store the energy expelled when pulling back the string at full draw. Once the string is released, the energy stored in the limbs are transferred back to the string and then to the arrow.
The riser, often made of machined aluminum, is the central part of the bow on which the limbs are attached. It is also where you find the grip to hold your bow. Not only that, the riser have a huge influence on the performance of a bow. It is the piece that is contributing the most to the weight of the bow which is the reason why you will often see holes in the riser in order to reduce weight. The riser also determines measurements such as the brace height and the axle-to-axle length.
There are 4 different cam systems at the moment which will be described further later in this text. Cam or wheels are attached to the extremity of the limbs. The cams transfer the energy stored in those limbs to the string and to the arrow when the shot occurs.
4. Bow String
The string is attached to both cams and is what launches the arrow. A D-loop may be installed in its center to be able to draw the string with the help of a mechanical release.
The cables run from one cam to the other and are what puts them at work when pulling back the string.
6. Cable Guard
The cable guard has the purpose of working with the cable slide in order to keep the cables out of the line of the arrow when shot.
7. Cable Slide
The Cable slide, attached to the cable guard, serves to hold the cables and avoid them to obstruct the trajectory of the arrow when released.
8. Peep Sight
The peep sight is a device shaped like a donut and installed between the strands of the string. The peep comes in alignment with the eye of the archer when reaching full draw. By looking through its middle opening, the archer can line up his pin sight, therefore increasing substantially his accuracy. Have a look at an in depth article on Installing a Peep Sight on a Compound Bow.
The sight is used to gain a better accuracy by aiming through it and aligning the pins contained in the middle of the sight with the target. The optimal precision of the sight can be obtained by using a peep sight attached to the string.
10. Arrow Rest
Attached to the riser, the arrow rest holds the arrow in place to avoid any misalignment when drawing back and releasing the arrow. You can opt for a fixed arrow rest or a “drop away” rest which automatically moves out of the line of fire of the arrow.
11. String Vibration Arrester
The name pretty much says it all, the string vibration arrester is a silencer which kills the vibration of the string after a shot by absorbing it. The arrester is installed from the riser up to the bowstring.
Installed below the grip in front of the bow, the stabilizer is used to balance the bow when reaching full draw and helps hold a steady aim as well as keeping the bow still when the shot is executed. The stabilizer greatly improves accuracy and can sometimes be placed on the side of the bow as well as in front.
The quiver is an arrow holder mounted on the side of the bow.
Silencers are an add-on that can be installed on the bowstring and is an efficient way to reduce noise when an arrow is shot. They are especially used by hunters.
15. Wrist Sling
Installed alongside with the stabilizer, the wrist sling loosely wraps the archer’s wrist for him to avoid dropping the compound bow after the execution of the shot.
16. Nocking Point
The nocking points are installed on the string as markers where the arrow needs to be attached prior to a shot.
A D-Loop is called this way because it is shaped like a “D” once installed on the string. The arrow attaches on the string in the middle of that “D” shape. The D-Loop is used to connect a mechanical release to help draw and release the shot.
The release is a mechanical device held by the archer’s draw hand to help him pull back and release the string. Learn more about the different types of bow releases.
The Different Cam Systems
The single cam is usually easier to maintain and is less likely to have a timing issue. It is also generally a little slower but quieter than some other cam systems. The single cam system features an elliptical power cam on the bottom and an idler wheel on top.
Dual Cam (Twin Cam)
The dual cam features two symmetrical cams at the end of each limb. Overall, it delivers a very good speed and accuracy although requiring significantly more maintenance to stay in optimal condition and is noisier than other cam systems.
The hybrid cam has a power cam on the bottom while having a control cam on the top. Although requiring much less maintenance than the regular twin cam system, it still needs to be tuned to avoid synchronization and timing issues and stay in perfect shooting condition for optimal performance.
The binary cam is a Bowtech Archery (and Diamond Archery) invention which was introduced on 2005 bow model. The patent was obtained about two years later, in 2007. The binary cam system is like a dual cam modified with the differences being that the binary cam is generally faster and that both cams are slaved to each other which means that one cam will work as the equivalent of the other cam eliminating synchronization and timing issues.
The importance of Brace Height
The brace height represents the distance between the bowstring and the deepest portion of the grip.
- Shorter brace height = more speed
- Longer brace height = more forgiveness
More forgiveness means that archer’s mistakes will be minimized. This happens because the arrow is in contact with the string for a shorter period of time when releasing it as opposed to shorter brace height where the archer’s mistakes are amplified as a result of a longer contact of the arrow with the string once released.
For an Olympic archer, speed is not the dominant factor, accuracy is, so forgiveness would be more important and brace height would need to be longer.
For a 3-D target archer or a hunter, forgiveness is less an issue and speed becomes more important to get greater kinetic energy and momentum values to put down the game.
Axle-To-Axle (ATA) Length
The Axle-To-Axle Length is the measure from the center of one cam to the center of the other cam. This measurement does not matter much for most people but if you intend to make long shots (over 50 yards) and want to get optimal accuracy than you might want to consider a longer axle-to-axle bow such as for Target Archery.
Although for most bow hunting situations, a shorter bow would be more often preferred. Bowhunters want to make accurate shots but it does not require the same level of accuracy a target archer would need. A shorter bow would be lighter and easier to handle.
RTS, RTH, RAK, DTH...What does that mean?
Basically, this means that the bow is ready to shoot out of the box. A package that comes fully equipped with all the required accessories to be able to use the bow effectively with minimal adjustments, unlike bare bows for which you would need to buy all the equipment separately and will probably cost extra money. You will most likely see those packages include a sight, a peep sight, an arrow rest, a stabilizer, a quiver, a D-Loop and a wrist sling but could vary from one model to another. Below, are the meaning of those abbreviations:
- RTS: Ready-To-Shoot
- RTH: Ready-To-Hunt
- RAK: Ready-Aim-Kill
- DTH: Designed To Hunt
The IBO Speed Rating of a Compound Bow. Probably not what you think it is...
You may already know that IBO speed represents the speed a bow can shoot as per the manufacturer's testing.
Should you ask yourself, IBO stands for International Bowhunting Organization.
The speed is shown in FPS (feet per second). Below 300 fps a bow would be considered slow as opposed to a fast bow with over 340 fps which would be considered a screamer to speak the industry’s language.
Now you may wonder how this speed is measured. Since taking different poundage, draw length and arrow weight would affect that measurement, the manufacturer would probably tend to take values to their advantage in order to get a greater IBO speed.
To avoid such discrepancies between manufacturers, the industry came up with standardized settings to get an accurate rating of the IBO speed. This test should be done using 70 lbs Draw Weight at 30 inches Draw Length with an arrow weighing 350 grains.
Basically, the differences in speed from one bow to another would rely on other factors such as the brace height, the cams, the efficiency of the bow and etc.
So if you are shooting a compound bow with the same poundage (70 pounds) at a shorter draw length with heavier arrows, don’t expect shooting arrows at the same speed than what is advertised as being the IBO speed rating of the bow.
Expect to lose about 10 feet per second for every inch less of draw length and 1 foot per second for every additional 4 grains of arrow weight.
To introduce those values into an example, say you are interested in a compound bow with an IBO speed of 320 fps, your draw length is 28 inches and you intend to use the arrow at 450 grains for hunting purposes. In theory, your real shooting speed would look like this: 320 fps - 20 fps (2 inches less of draw length = 2 x 10 fps) - 25 fps (100 grains more of arrow weight = 100 grains / 4 grains x 1 fps = 25 fps) = 275 fps.
This value could also be influenced by the human factor, not releasing as efficiently as a machine does for IBO speed testing, you may lose up to an additional 5 fps.
This does not even include any additional equipment installed on your bowstring such as silencers, nock set, D-loop and peep sight which might have your speed suffer another loss of around 10-15 fps as a rough estimate, depending on the accessories you decide to install (about 1 fps lost for every 3 to 4 grains installed).
If you decide to lower your draw weight, you would bring down your speed from 15 to 20 fps for every reduction of 10 pounds.
Now you see how the IBO can be quickly affected by different settings, accessories, and physical characteristics.
So, if we take the 275 fps calculated above and remove 5 fps for the human factor and 15 fps for additional accessories installed, we now stand at 255 fps.
Kinetic Energy for Bowhunting. How much is enough?
What does this represent in Kinetic Energy value knowing that we are using 450-grain arrows?
Let's use the kinetic energy and momentum calculator to enter the arrow speed and arrow weight to find out...
70 ft-lbs of KE is what it is. Still not bad, isn't it? You can roughly take down just about any game in North America with this value.
But that’s not all, now that you have your kinetic energy value, you need to know the velocity loss over distance.
This would be an estimated loss of about 2 ft-lbs per 10 yards, this is to give an idea but is in no way the reality as there are too many factors to precisely determined the true value loss.
So if you are shooting on a target at a 30 yards distance, expect losing approximately 6 ft-lbs or even more of kinetic energy.
In other words, our value of 70 ft-lbs is now more or less 64 ft-lbs of KE which would not be as attractive as it was to take down the toughest game out there.
Now that you know more about speed and power, do you respect the kinetic energy recommendations for the game you want to hunt?
Tuning a Compound Bow
Sometimes one might think that when buying a ready-to-shoot package, there is nothing to tune on the compound bow.
This would be wrong! Packages come with certain factory settings that might not be exactly how you want it to be, draw length and draw weight for instance.
To know your specific draw weight (if adjustable) at a specific draw length, a bow scale would be required in order to know precisely how much weight you pull. For example, if a bow is set to be 50 lbs at 28" of draw length, your actual draw weight may be lighter if your draw length is shorter than 28" or heavier if your draw length is longer than 28" so the only way to know the exact weight is with the help of a bow scale. Some assume 3pounds per inch above or below 28”. So roughly, if you have 29” DL you would approximately draw 53 lbs and if you have 26” DL, you would draw 44 lbs.
The precision of those measurements (draw weight and draw length) is important to shoot arrows with the spine value offering the best fit to your settings and physical characteristics. A few pounds can make a difference on the arrow spine value needed if you want optimal accuracy.
If you have to adjust the draw length and the draw weight, some tools would be required such as an Allen wrench set following the specifications of the manufacturer.
The sight might not come in perfect alignment either, same for the arrow rest for which you can use a square to correct the misalignment.
If a peep sight is included, it won’t be installed since its location would not be the same for everyone. This installation must be done using a bow press to be more efficient and avoid it to be misplaced.
These are only some examples of basic tuning a bow may require, even if ready-to-shoot out of the box. Note that other tuning could be needed depending on the bow.
Buying a first Compound Bow
As you start, you may want to pick a versatile compound bow that offers a high level of adjustable settings according to your needs and which can grow with you as you gain experience. You would want to start shooting your bow with a lower draw weight to learn a good archery form and gain strength. Then adjust your bow to a higher draw weight and continue to raise the poundage as you go until you reach the desired poundage and be able to shoot it with ease.
If you are a young archer, you may not have reached your full adult size yet, so picking a bow that can be adjusted in draw length is also a good idea to keep the same bow a longer period of time without having to spend more money on a new bow when your draw length changes.
If you need, I highly recommend reading "Beginning in Archery" to help you out.
Something else to consider is to opt for a fully equipped compound bow package which often offers a better value for your money. You get all the necessary accessories at a very affordable price and you can be ready to shoot with your new bow in no time. To get a better idea of good quality and affordable packages, see about selecting the best compound bow and this Top 10 Compound Bows for more options.
Before you can shoot though, keep in mind that you need to determine the arrows that you will be using which might seems a little bit overwhelming at first. Choosing the right arrow spine is an important factor to improve your accuracy and gain consistency when shooting. The spine refers to the deflection of the arrow when shot. You will need to find the value you need to use by looking into arrow spine charts provided by manufacturers. Have a look at our post on what is the arrow spine.
Another thing you should look into is a release which is not included in ready-to-shoot compound bow packages. This piece of equipment is quite important and way more efficient than your fingers to draw, hold and release the string of your bow. You got many choices out there and you can get a lot of helpful information in our post on the different types of bow releases.
Tuning is something often overlook and if you are far away from a pro shop or want to minimize the cost of labor of having someone tune your bow for you, then you may want to get basic tuning tools to make simple adjustments at a fairly low cost.
Thank you for reading! Do not hesitate to ask if you have any questions or leave a comment below.
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