Why your compound bow doesn't match specs (part one)
Each year I have many hunters ask the question: “Why doesn’t my new compound bow deliver the same speed performance as the manufacturer advertises for this bow?” The answer is deceptively simple, but requires an understanding of how manufacturers determine the ratings published for their bows; and an understanding of the relationships between key factors that determine how your bow will perform.
How Bow Manufacturers Determine Their Published Specs
In an effort to make ‘apples-to-apples’ bow comparisons a reality the Archery Trade Association (ATA) recommends that manufacturers use ATA standards when conducting tests for their advertised speed figures. If using a 70# draw weight, this would mean using a 30-inch draw length, a 350 grain arrow, and chronographing and recording a minimum of five shots for each test arrow. The arrow velocity ratings should come from an average of the test arrows recorded velocities. Most manufacturers still incorrectly advertise their published speeds as International Bowhunting Organization (IBO) speeds. In reality the IBO doesn’t have any guidelines for rating bows; they only define the specs allowed for IBO competitions; e.g., a maximum of 82# draw weight. The IBO does recommend for safety reasons arrows should have a weight of five grains of arrow per pound of draw weight, and this is built into the ATA’s 2009 Technical Guidelines. The transition to using the term ‘ATA speed’ will probably take some time. Hoyt now uses ‘ATA speed’; PSE uses ‘ATA/IBO speed’; and most others are still referring to the ‘IBO speed’.
The reality is that, while ATA hopes members will follow its guidelines, there are no ‘police’ to ensure that this happens. Most manufacturers do their own tests when rating bows and they make every effort to show the fastest speeds possible because ‘speed sells’. The use of such things as shooting machines, fall-a-way rests, and unfletched arrows help them do this. And you can bet your last dollar that they use perfectly-tuned bows in these tests, and no one knows how many ‘groups’ of arrows are actually shot to get the ‘best’ speed average for a bow. The bottom line is that test bows can generate speeds anywhere from 20 to 50 feet faster than you get with the same bow taken directly off the assembly line and set up for hunting with such things as proper serving, nock points, and a peep sight, etc. So this is probably the starting point with your new bow; it will most likely already shoot below the specs published for that bow. But it doesn’t stop there. Let’s look at some other important factors that will affect the specs for your personal bow.
Draw Length, Brace Height & Power Strokes
There are actually two draw lengths for a bow. The first is the ‘true draw length’ which is the measurement of your draw length from the pivot point of the bow (normally the deepest point of the handle grip facing the archer). The second is the ATA draw length, calculated as the ‘true draw length’ plus 1.75 inches. The 1.75 inch figure is the average distance from the pivot point of the bow (approximately where the shooter’s bow hand contacts the handle grip) to the back of the handle grip. Thus the ATA standard 30” draw length used for testing is equivalent to a 28.25” ‘true draw length’.
The brace height of a compound bow is the distance from the pivot point of the handle to the string of the strung bow at rest. It is dead space as far as the power stroke is concerned. To determine the ‘power stroke’ of a bow you simply take the true draw length of the bow and deduct the brace height. Thus a bow having a true draw length of 28.25” and a brace height of 7” will have a power stroke of 21.25 inches.
In any case, other things being equal, your personal draw length determines the power stroke of your bow, and this power stroke is very important. A longer power stroke means that the peak draw weight is held over a longer distance when the bow is drawn back, resulting in more stored energy in the limbs. It also means that this stored energy is delivered over a longer distance to the arrow when the bow is shot, transferring more energy to the arrow. In contrast, bows with shorter power strokes store less energy and deliver less energy to an arrow.
To visualize the effect of shorter power strokes compare Force Draw Curves 1 & 2. Force Draw Curve 1 illustrates a typical force draw curve for a high performance bow set at an ATA specs with a draw weight of 70# (line AD) and a draw length of 30”. Line AC shows the true draw length of 28.25” (30” – 1.75”). Our example bow has a brace height of 7 inches (Line AD), and Line BC shows a power stroke of 21.25 inches (28.25” – 7”). The grey shaded area represents the stored energy at full draw.
Force Draw Curve 2 shows the effect of shortening the true draw length by four inches. Note that the power stroke is reduced to 17.25”, and the reduced shaded area of the diagram now illustrates much reduced stored power.
So! IF your ATA draw length is shorter than the ATA’s 30” standard, your bow will not deliver the manufacturer’s advertised ATA speed performance figures for your bow. Generally for every inch you shorten a bow’s draw length you get a loss of arrow speed of about 10 fps. In our example the loss would be about 40 fps.
Next month, in the final part of this discussion, we will look at draw weights and arrow weights and how they can affect your bow’s performance. We will also look at hysteresis and its relationship to your bow’s efficiency, and how you might be able to somewhat increase this efficiency. Until then, Good Hunting!