Compound Bow Guide

ContentsCompound Bow Component Diagram

Compound Bow Components

A compound bow is a modern bow that uses a system of cables and pulleys to bend the limbs and store energy. This mechanical system advantages the shooter by storing more energy than a recurve bow of equivalent peak weight. This energy is transferred to the arrow which is propelled with a greater amount of energy, therefore the arrow travels faster.

Compound bows come in a variety of styles, however most bows will consist of similar components. The figure above identifies the different components as well as some of the terminology adopted in the industry.

Riser Materials

The riser forms the central portion of the bow which comprises a grip and mounts for accessories like an arrow rest, stabilizer and sight. The riser is commonly made from cast or machined 6061 T6 aluminum-magnesium alloy due to its strength, anti-corrosive properties, and light weight. High end risers may also be made from carbon fiber or 7075 aluminum alloy which is stronger and lighter, but more costly.

The overall shape of the riser typically comes in three forms, primarily to vary the bows Brace Height which we explain later in this guide. The three forms are known as reflex (lowest brace height), deflex (greatest brace height) and straight which is a compromise of the two.

Limb Design

The limb components of the bow are the parts that store energy prior to releasing it into the arrow. Limbs will usually come in either solid or split designs, which refers to a gap in the limbs center portion. This has historically been advertised as reducing hand shock and providing a more durable product, however with the development of more advanced limb materials most experts argue that the difference between the two designs is minimal.

One unique feature common to modern compound bows is what’s known as the parallel limb design. This design was released in 2002 by Bowtech and revolutionized the industry within a couple of years. The design allowed the limbs to move in an up and down vertical motion rather than front to back. During the firing process, the limbs would flex in opposite directions cancelling out each other’s force.Curved Limbs vs Parallel Limbs

Now why is that important? Well the net cancellation of force on the parallel limbs resulted in significantly reduced “hand shock” for the shooter. Hand shock is the motion felt in the gripping hand when your release the bow. In bows without the parallel limb design the hand shock is typically described as an abrupt jerking feeling which can be quite uncomfortable.

Cam Design

Cams form part of the mechanical system which is pivotal to the effectiveness of a compound bow. The number of cams and their design impact the weight and “smoothness” of the draw. The four main cam designs are single cam, dual cam, hybrid cam and binary cam.

Single Cam Design

Single Cam BowThe single cam design consists of a single round idle wheel attached to the top limb and an elliptical cam attached to the bottom limb. Single cam compound bows are very popular with archers as they are easy to maintain as they don’t require cam synchronization, are quieter, and are usually machined to provide a smoother draw. The single cam compound bow is extremely reliable however some models will be better than others with regards to maintaining level nock travel.

Dual Cam Design

Dual Cam BowA twin/dual cam design consists of two cams, either in elliptical or spherical in shape, which are synchronized to maintain accuracy and speed. If well maintained, dual cam designs offer greater speed and accuracy in comparison to the single cam design however the maintenance requirements may be somewhat greater than the single cam if the cams go out of sync.
Dual cam designs are one of the earliest forms of compound bows, however advances in technology and design each year have resulted in a more stable setup and a lowering of the maintenance requirements.
Because both cams are shaped and move at the same speed, dual cam designs will typically have level nock travel. Another benefit of having dual cam’s (as seen on the Diamond Infinite Edge Pro) is that the range of adjustment options is larger than that of a single cam.

Hybrid Cam Design

Hybrid Cam BowThe hybrid cam design features two elliptical cams as per the dual cam design however the top cam is directly connected to the bottom cam, and is slaved to its movements. The bottom cam connects to the top limb as normal. This design offers the accuracy and speed of a dual cam but require less maintenance as the directly connected cams largely remove the potential synchronization problems.

Binary Cam Design

Binary Cam BowThe binary cam design in similar to the dual cam design, but with the addition of a control cable which links the two cams together and keeps them synchronized. The dual cam and hybrid cam systems connect the limbs to the cams, while the binary cam connects cam to cam. This design is gaining momentum as the identical and self-regulating setup of each cam helps to account for any imperfections in the limbs, string or control cables.

Cam Grind – Smooth vs Hard

Regardless of how the cams are arranged, their design and the way in which they rotate on the limbs will determine how they feel when drawing the string. This is also known as their grind.

A smooth drawing cam will ramp up to peak draw weight as you start to pull the bowstring and then reduce towards the end of the draw, culminating at peak draw where you’ll be holding only a fraction of the total weight (the let-percentage). A smooth drawing cam system only requires you to hold the peak weight for a short period during the draw, before easing into the let-off.

A hard drawing cam ramps up to peak weight and stays at that level for the majority of the draw, before falling off rapidly at the end of the draw, to the let-off level. The benefit of this less comfortable draw pattern is of course, more stored energy and a more powerful bow.

For beginners and pretty much anyone who isn’t looking for the absolute highest amount of speed and force in their bow, we recommend compound bows with a smooth drawing cam. Manufacturers don’t typically provide much technical detail on their cam grind, but the published IBO speed will give you a good indication. Bows above 330 FPS are likely to have a harder cam system, whereas bows closer to 300 FPS will have a softer draw.

Let-off Percentage

Let-off percentages are one of the unique features of compound bows over their recurve bow counterparts. The quoted percentage on your bow, refer to the amount of weight drop-off you will experience when you reach the end of your draw.

For example, if you have a 50 lbs peak weight compound bow with an 80% let-off, you will find that holding the bow at maximum draw only requires 10 lbs [50 lbs x (100% – 80%)] of force.
You can see this visually at the 28” mark in the image comparison above between smooth and hard cams.

The reduced holding weight at peak draw makes it easier for you to take your time in lining up a shot and account for any variables in your shooting environment. For bowhunters, a good let-off allows them to hold the draw indefinitely while they wait for the perfect shot.

Some models such as the Genesis Compound Bow don’t actually feature a let-off percentage. This intentional design is so that a single compound bow can be used by many people with differing draw lengths. On a normal compound bow, the letoff percentage only occurs at peak draw, so the bow would need to be tuned to accommodate the person with the shortest draw, meaning that larger shooters would have an uncomfortable shooting experience.

Bowstring Materials

The bowstring is connected to the cams, and when you pull on the bowstring the cams rotate, pulling on the bow cables, and applying force to flex the limbs. When the bowstring is released the energy is released into the arrow which takes flight. Most bows for sale today will be paired with a bowstring made from Dacron or a High Modulus Polyethelene (HMPE) fiber.

Dacron B-50 is chemically similar to the material that plastic soda bottles are made out of. It’s been around for a long time has retained its popularity for its durable nature and low cost. Bowstrings made from Dacron will stretch slightly when being drawn and this wastes some of the potential energy. The result is that bows with a Dacron string will fire a slightly slower shot than the same bow with a Fast Flight string. The upside however, is that the string does snap back to its original shape when released, and the extra ‘give’ in the string reduces the stress on the limbs & riser.

HMPE strings are a newer invention and have largely replaced Kevlar strings which had a higher tendency to snap. Names such as Fast Flight, BCY 452X, Dynaflight, Spectra, and Dyneema all refer to strings made from composite HMPE fibers. As you may have guessed, these strings are lighter, stronger and faster than the Dacron equivalent, but at the cost of increased stress on the bow frame. These strings have negligible stretch when drawn which helps with performance and also helps cut down on the maintenance requirements for dual cam and hybrid cam designs, as the lack of stretch reduces the chance of the cams going out of sync.

On the plus side, their popularity around the world has meant that the cost is now very comparable to Dacron strings.

Right Handed or Left Handed Bow

Right Handed – you hold the bow by the riser with your left hand and pull the string with your right hand.
Left Handed – you hold the bow by the riser with your right hand and pull the string with your left hand.

In archery the choice between a right or left handed bow is influenced by eye dominance as well as your hand dominance. For most people hand and eye dominance is the same (i.e. if you are right handed your right eye is dominant) however it is not uncommon for them to differ, and for those few where this is not the case, it can potentially impede your archery skills.

Finding your dominant eyeTo determine which of your eyes is dominant, follow these simple steps:

Step 1 – Hold your hands at arm’s length out in front of you. Your palms should be pointing forward with the back of your hands facing you.

Step 2 – Cross over your hands to make a triangle shape between your thumbs and the part below your first finger.

Step 3 – Look at an object in the distance with both eyes through the triangle you have created with your hands. The object will need to be close enough to see clearly but far enough away to cause your eyes to focus (aim for at least 15ft away).

Step 4 – Focus on the object and slowly move your hands towards your face. Keep your head still and make sure you do not lose sight of the object. Do not think about your hands or where they are moving. Having distractions around you such as a loud TV or someone talking can help focus your mind elsewhere – so long as your eyes remain focused on the object.

Step 5 – Draw your hands until they touch your face. The triangle should end up over your dominant eye.

 

So now that you know what your dominant eye is, how does this help you with archery?

If you are right eye dominant and are using a right handed bow, your eye will line up with the bowstring and the arrow, so in effect; you will be looking straight at where the bow is aiming.

If you are left eye dominant with a right handed bow, your dominant eye will be looking off to the side of where the bow is aiming.

In this case the simple fix in this case is to close your dominant eye.

This is the preferred shooting method for many archers with mixed dominance as shooting a bow that corresponds to your hand dominance just feels more natural and will be easier to learn.

The downside to closing one eye is that you have a narrower field of view and your depth perception can be impaired.

For target archery this is unlikely to cause a problem as you will be shooting from fixed distances at a known target.

Field archery with unmarked distances should also not be too much of an issue as you can estimate the distance with both eyes before you take your shot.

Hunting is where eye dominance and hand dominance really should align.

When hunting, you target is likely to appear at varying distances, may be moving, and may only be available for brief windows of time.

Being able to acquire targets with you periphery vision, quickly aiming your shots while accounting for distance, and ensuring your firing arc is clear of other people is best achieved with both eyes open!

Keep in mind that like many things, not all bows are sold in a left handed version. If you are left eye dominant and are torn between an awesome looking bow that only comes in a right handed version, or an ok looking left handed bow, we’d suggest going with the awesome bow and firing with one eye closed.

Draw Length

Draw length is the distance between the bow string and the outer edge of the riser when you hold a compound bow at full draw.

If you want to be technical, the Archery Trade Association’s (formerly the Archery Manufacturer and Merchant’s Organization) standard definition states that the draw length is the distance between the groove of the nock on the arrow to the position 1 3/4“ forward from the pivot point of the grip when the bow is fully drawn.

This isn’t exactly the smoothest definition to read, hence the first sentence in our section is the intended approximation.

The length of your draw is directly proportional to the power of your shot, but the most important factor is that the length is comfortable for your arms.

One of the easiest ways to measure your draw length is to:

  1. Stand with your arms and hands outstretched (it’s not a competition so just stand naturally and don’t try to over-stretch)
  2. Have someone measure the distance between the tips of your middle fingers in inches
  3. Divide this distance by 2.5 or consult the table below to find your personal draw length.

Determine Draw Length

Distance (inches) 63 65 67 70 72 75 77 80 82
Draw Length (Inches) 25 26 27 28 29 30 31 32 33

 

To sanity-check this measurement, you can also make a fist and stretch out your arm to the side as if you were drawing your bow. Turn your head in the direction of your arm and have someone measure the distance between the corner of your mouth and the furthest point of your fist (excluding the thumb). This should be quite close to the estimate in the table above.

Draw length is an important factor in choosing your arrows as we discuss later in this article.




Draw Weight

Draw weight is the peak weight an archer will pull while drawing the bow, and is the key factor in determining how much power is applied to propelling an arrow forwards. With a compound bow, the weight can be set by adjusting settings on the cams. Unlike a recurve bow, the draw weight is independent and can be set separately from the draw length.

A typical draw weight table is provided below:

Your Body Weight Draw Weight
50-70 lbs 10-15 lbs
70-100 lbs 15-25 lbs
100-130 lbs 30-40 lbs
130-150 lbs 40-50 lbs
150-180 Lbs 40-55 lbs – Minimum for BowHunting
150-180 Lbs 50-65 lbs
180 lbs and up 60-70 lbs

 

Draw weight should be aligned to what you can pull comfortably and reliably. There’s no point getting the most powerful bow you can find if you need to strain your muscles every time just to pull it. That excessive strain can cause injury, but at the very least may cause your limbs to shake which will ruin your aim.

For someone who is new to Archery, we would recommend starting out with a relatively low initial draw weight (eg 35 lbs for men). This is because Archery uses a set of back muscles that aren’t typically worked out in a normal day, so on most people they won’t be at peak strength. After a few months of practice, you may find that your bow becomes very easy to draw, at which point you may wish to increase the draw weight settings. Fortunately, this is a relatively simple process for compound bows.

Keep in mind that if you are eventually going to be shooting a hardened target such as big game, or something at very long range, then a more powerful bow and the strength to draw it will be a necessity.

If you plan to go hunting with a compound bow, then we consider 50 lbs of draw weight as the minimum safe level for even close range hunting. You don’t want to be in the position where your arrow bounces off the target, or even worse, wounds the animal without killing it.

For a better indication of whether or not your bow will be suitable for hunting, follow our step by step guides in the Speed and Kinetic Energy Penetration section.

Axle to axle length

The axle to axle length is the distance between each end of the limb which connects to the cam/wheel. This is not the physical bow length as the measurement doesn’t account for the diameter of the cam or wheel.

Most compound bows for sale these days have an axle-to-axle length of 32”, however longer and shorter models are available.

Shorter models will typically be preferred by hunters as they’re more compact and easier to camouflage. Longer models will usually be found in the competition shooting space for the stability the extra length provides.

Brace Height

The distance between the pivot point on the grip (located on the bow riser) to the string. Given that your arm length (and by extension, your draw length) is fixed, a lower brace height allows you to draw the string back further than a bow with a higher brace height. This is important as greater length of draw means more energy input, which means more energy released, which means more speed.

Typically, the lower brace heights will require a greater amount of strength and archer skill to shoot accurately, whereas higher brace heights are smoother to draw but will be slower. About 7” is the starting point for most bows.

Manufacturers will vary the brace height of their bows by changing the angle of the limbs. The picture below shows some examples of Reflex, Straight, and Deflex limb styles. Reflex styles will typically have the lowest brace heights, with deflex having the highest.Compound Bow Risers - Deflex vs Straight vs Reflex

Speed! Or the International Bowhunting Organization (IBO) Speed Standards

Let’s face it, speed sells.

Apart from buying a bow that meets your personal shooting requirements (target versus hunting, weight, draw weight, brace height), the published speed is the one number that most people will compare first.
There’s a good chance that you won’t be able to notice the difference between a 340 feet per second (FPS) bow and a 330 FPS bow, but if you’re standing around with a bunch of bowhunters or archery enthusiasts, the person with the 340 FPS bow has bragging rights!

Like many things, a standard has been developed to ensure that the bow manufacturers are on the same playing field and that the buyer can be suitably informed when comparing products. The International Bowhunting Organization (IBO) has created the standard which most manufactures adopt, that requires each bow to be tested under uniform conditions.

The IBO test requires the bows to have a 70 lbs peak draw weight, a 30” draw length and they must shoot a 350 grain (22.7g) test arrow.

Given these set parameters, the speed of a bow will largely be determined by the cam grind (harder = faster), brace height (lower = faster), and how efficiently the bow transfers energy from the limbs to the arrow.

The IBO speed sounds like a pretty good set of uniform standards to compare bows against, but there is still some wiggle room and the tests are performed by the manufacturers themselves, rather than an independent body. In the quest for the highest FPS possible, some manufacturers may decide to use an arrow with no nock or fletching to reduce weight and drag, and they may try to forcibly overdraw the bow slightly.

Even without these slightly questionable methods, a manufacturer can create a bow with an exceptionally low brace height and a very hard drawing cam to create a blazingly fast bow. If that’s what you’re after then great, but such a bow will be very uncomfortable to shoot!

The IBO standards are a pretty good indication of the maximum potential speed from a bow, but do take the published numbers with a grain of salt and don’t let it be the only characteristic you look at.

Quick reference table for some of the factors that determine arrow speed:

Draw Weight Draw Length Brace Height Cam Grind Arrow Weight String Type
Faster but Harder Higher Longer < 7” Hard Light HMPE
Slower but Easier Lower Shorter > 7” Smooth Heavy Dacron

 

As we mentioned before, the IBO standards are set at 70 lbs draw weight, 30” draw length, and an arrow weighting exactly 350 grains. But what if you don’t use those settings?

As a very general rule of thumb, you can estimate what your actual arrow speed will be by adjusting the manufacturers IBO speed as follows:

  1. Drop the speed by 5% right off the bat. You won’t be shooting in a controlled lab environment and factors such as a peep sight on your string, fletching on the arrows, your arrow rest, and any number of other factors can and will marginally slow down your arrow.
  2. Add or subtract 10.0 FPS for each inch of draw length more or less than 30”
  3. Add or subtract 1.5 FPS for each pound of draw weight above or below 70#
  4. Multiply your draw weight by 5. Subtract this number from the total weight of your arrow in grains (including tip, shaft etc) and then divide by 3. Subtract this final figure from the FPS.

As an example, take the PSE Stinger X with a 316 FPS IBO speed. Let’s use a 60 lbs draw weight, a 28” draw length, and a 420 grain hunting arrow.

  1. Subtract 16 FPS for overheads (5% of 316 FPS)
  2. Subtract 20 FPS for the 28” draw length (2” less than 30” x 10.0 FPS)
  3. Subtract 15 FPS for the 60# draw weight (10# less than 70# x 1.5 FPS)
  4. Subtract 40 FPS for the heavy arrow (Draw Weight x 5 = 300, Arrow weight (420 grains) less 300 = 120, 120 / 3 = 40 FPS)
  5. Approximate Final Speed = 225 FPS (316 – 16 – 20 – 15 – 40)

The estimated feet per second can be used to calculate the penetrative force of a shot, or its kinetic energy, which is a better indicator of a bow’s power than its speed. What is kinetic energy? Glad you asked:

Kinetic Energy or Penetration

Kinetic Energy DiagramLet’s talk physics! The kinetic energy of an object is the energy that it possesses as a result of its motion. If you fire two identical arrows from two different bows, the arrow that flies faster will have greater kinetic energy than the slower flying one.

Why is this important? Higher kinetic energy allows the arrow to offset the forces of gravity and air resistance for longer which results in a straighter shot, and this helps everyone.

The kinetic energy left over when the arrow finally hits its target then becomes its penetrative force, and this is the key figure of interest for Bowhunters.

In order to determine if the energy output of your bow is appropriate for your hunting purposes, the kinetic energy of the arrow needs to be calculated.
But how do we do that?

The kinetic energy from your arrow will depend on the mass of the arrow and the speed. The kinetic energy can be calculated by:

Kinetic Energy (KE) = MV2 /450,240

Where:

KE is the energy in foot pounds

M is the mass of the arrow in grains

V is the velocity (speed) of the arrow in fps

 

For our earlier example which was a PSE Stinger X shooting a 420 grain arrow at 225 FPS, the KE would be 47.2 ft·lbs.

Kinetic Energy Equation for PSE Stinger X

The guys over at Gold Tip Arrows suggest the following KE requirements when game hunting:

KE (ft-lbs) Hunting Type
< 25 Small Game (rabbit, groundhog, raccoon)
25-41 Medium Game (deer, antelope, coyote)
42-65 Large Game (elk, black bear, wild boar)
>65 Toughest Game (cape buffalo, grizzly, musk ox)

 

So for our example with a KE of 47.2 ft-lbs, we would be able to go boar hunting!