Compound Bow Basics 101

January 2, 2014 by  
Filed under Bow Tuning

Today’s bow hunters and archery fanatics want their compound hunting bows to be smok’in fast, have smooth draw stroke, lightweight, compact, quiet, easy to tune, and forgiving. To get a compound hunting bow with a certain set of characteristics, you’ll likely have to sacrifice some others. Ultimately, you will have to decide which characteristics are most important to you and choose the compound bow that best fits your needs.  The technology with today’s bow and arrows have significantly advanced  over the past five years.  It is hard to say that there are a lot of bad compound bows out there.  They all have “kill” in them.  It is important for you to take the time to shoot several different compound hunting bows to see which one fits you the best.  The content below will help you learn more about the compound bow you are shooting or help you learn more about a compound bow you may purchase in the future.

Compound Bow DiagramCompound Bow Diagram

Bow Energy Storage

When you pull the string of a compound bow, the limbs of the bow are squeezed inward. The energy you supplied to draw the bow is stored in the limbs, as potential energy, until you release the string. Upon release, the potential energy is transferred into the arrow as kinetic energy, as the limbs “spring” back into place returning the string to it’s original position. Careful examination of this process of storing and releasing energy is what gives a compound bow its performance characteristics. There are only two factors that determine how much “power” your bow will have: 1) The amount of energy that can be stored in the limbs during the draw stroke. 2) The amount of that potential energy that can be successfully transferred into the arrow upon release (efficiency).

So, how is one bow capable of a 320 fps IBO Speed, while another only shoots 280 or 305 fps?  Well, it’s all about energy storage.  The key ingredients to arrow speed are draw weight, draw length, and arrow mass. The amount of energy a bow stores also depends upon the (geometry) of the cam or wheel design, the bow’s let-off percentage, and the bow’s brace height.

The power stroke represents your effort. The power stroke begins as you pull the string back from the resting position and is completed when the bow reaches full draw. Each bow will have a different power stroke depending upon its settings and cam characteristics. Power strokes which are longer, higher, or wider will result in increased energy storage and arrow velocity.

Draw Weight  and Impact on Powerstroke

One of the major ways to increase the amount of stored energy during the power stroke is to shoot a compound bow with a higher maximum draw weight. All other things being equal, a 70 lb. bow will store more energy and shoot faster than a 60 lb. bow. The maximum draw weight of the bow is typically determined by the stiffness of the bow’s limbs. Compound bows come in a variety of maximum draw weights, but the most common are the 50-60 lb. and 60-70 lb. versions. When you may purchase a bow with 70 lb. limbs, you can generally adjust the draw weight 1-10 lbs. down from the maximum weight. Keep in mind that a 70 lb. bow, turned down to 60 lbs., will not perform as well as the same bow in a 60 lb. version operating at it’s maximum draw weight. Bows perform at their peak at or near their maximum draw weight.

Draw Weight – Impact on Arrow Velocity
High poundage bows require heavier, stiffer archery arrow shafts.  While they will generally generate more energy at the target, they may not necessarily generate much faster arrow speeds at IBO standards.  Lower poundage compound bows can use lighter, more limber hunting or target arrow shafts. IBO standards allow 5 grains of arrow weight per pound of draw weight. Forexample; a 70 lb. bow can shoot an arrow (safely) as light as 350 grains.  Now lets consider a compound bow set for 60 lb., should not  shoot no less than 300 grains.  When set for IBO minimum standards, many bows are only marginally faster in the 70 lb. version vs. the 60 lb. version.  Since a 70 lb. bow must shoot the heavier arrow, the savings in arrow weight offsets the loss of energy storage during the powerstroke.  A bow that is accurately set-up for best speed, a 60 lb. version of most bows will perform within 10 fps of the heavier 70 lb. version of that same bow.

Draw Length’s Impact on Power
The longer your draw length, the longer your bow’s power stroke will be – and the faster your compound bow will shoot. As a general rule, 1″ of draw length is worth about 10 fps of arrow velocity. So if your particular bow has an IBO speed of 300 fps, and you intend to shoot the bow at 28″ draw length – you should expect an approximate 20 fps loss in speed right off the top.

Shooting a long draw length will most certainly deliver more speed.  The trade off for shooting a bow with a long draw length is a considerable amount of control. In many cases it can be a bad trade-off. While you will have more speed with a long draw length you need to consider the fact that your accuracy could be compromised.  In the field speed is important but confidence is the most important factor.  If you are not consistently shooting well and are not 100% confident in your set up.  It could mean the difference between harvesting that animal or going home frustrated.

Understanding Brace Height- Why is it a factor?
Brace height is an important factor in the energy storage equation.  A bow’s brace height is the distance from the string to the pivot point of the bow’s grip.  You can think of brace height as how close the string will be to your wrist when the bow is at rest.  The closer the string is to your wrist, the more work you have to do to get the bow drawn back.  If you’re drawing a 6″ brace height bow back to a 30″ AMO draw length, you’ll have to pull the string back a total distance of 22.25″ before you reach full draw*.  But if the string rests farther back from your wrist to start, say the bow’s brace height is 8″, then you’ll only have to pull the string back for 20.25″.  The bow’s brace height also figures into how LONG the bow’s powerstroke will be.

As a matter of energy storage, a short brace height bow stores more energy and shoots faster than a tall brace height bow (all other things being equal).  So brace height has the same affect on total powerstroke length as does the bow’s draw length setting.  The only difference is that the brace height determines where you start and the draw length determines where you stop.  But unlike draw lengths, brace heights aren’t adjustable.  You can’t change your bow’s brace height later, should you change your mind.

(*A bow’s AMO draw length is measured 1.75″ beyond the grip pivot point.  A bow’s power stroke distance is found by subtracting the brace height and 1.75″ from the AMO draw length.)

Even though short brace height bows are faster bows they aren’t automatically favored because a bow’s brace height has a profound effect on the bow’s forgiveness and shootability. Short brace height bows are generally less forgiving and require more skill to shoot accurately. Since the arrow is in contact with the string for a longer distance and period, there is more opportunity for any glitches in your shooting form (hand-torque, trigger punching, etc.) to have an effect on the arrow’s flight. Longer brace heights have the opposite effect, limiting the effects of form glitches.

If you shoot with absolutely perfect form and technique, a short brace height bow will be just as accurate as it’s longer brace height cousins. Although, you have average skills and are prone to occasional goof-ups, a bow with a little longer brace height will yield better accuracy in most shooting situations. The average new compound bow has a brace height of approximately 7″. Bows with shorter brace heights (5-6.5″) will be faster but less forgiving to shoot. Bows with longer brace heights (7.5-9″) will generally shoot slower but will be more forgiving to your errors.
Many bow hunters tend to avoid short brace height bows (anything  below 7 inches).

Short-Draw Archers & Brace Height

If you are a short-draw archer (27″ draw length or less), you have an advantage regarding forgiveness and shootability on your compound bow. A bow which has a 6″ brace height and is set for long 30″ draw length will have 22.25″ power stroke. This means the during the shot, the arrow will remain in-contact with the string for approximately 23-24″ (including string follow-through) until the arrow finally releases. This would generally make for a rather unforgiving setup. But that same bow in the hands of the short-draw archer will be considerably more forgiving to shoot.  If a short-draw archer shoots the same bow at 26″ draw length, their power stroke will only be 18.25″ long. So the short-draw archer’s arrow gets off the string in a shorter distance providing the short draw archer some “built-in” benefits of forgiveness.

Understanding Let-Off %

80% Let-Off Graph65% let-off graph

If you’ve ever shot a heavy recurve or longbow, you’ve certainly noticed that you’re holding back the maximum draw weight just when you come to full draw, so you must aim and release the arrow quickly before your muscles weaken or begin to shake.  The original compound bow was designed to eliminate this issue, by offering the shooter more time to aim and release the arrow.  In contrast with the traditional bow, the draw weight of the compound bow decreases (sometimes dramatically) just as you come to full-draw. This is known as LET-OFF, which is controlled by the geometry of the cam system.

Early compound bows featured a 35-50% let-off.  Today’s compound bows have let-off in excess of 75%. A bow with a 70 lb. draw weight and 80% let-off will require the shooter to hold back only 14 lbs. once the bow reaches full draw. Holding back such a small amount of weight, the shooter has the luxury to take more time aiming and releasing the arrow.

A disadvantage to a high (over 75%) let-off cam is a small reduction in arrow velocity vs. a lower let-off cam system. All other things being equal, a bow with 65% let-off will shoot faster than a bow with 80% let-off. The difference in speed between a 65% let-off vs. 75% let-off is usually only a few fps. Fortunately, many cams use interchangeable modules which give you the option to easily switch between different available let-offs. Some cam systems even offer adjustable let-off right on the cam without the need for additional modules.
For more information visit our Archery Shop or if you need any Archery Equipment or Bowhunting Equipment.

Originally posted 2010-01-13 19:43:25. Republished by Blog Post Promoter

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