June 02, 2015
By Jon E. Silks
There are many general guidelines in life we accept without really thinking about. For example, in real estate we are told we can afford a house roughly 2.5-3 times our gross annual income. If you need surgery, hope it doesn't take more than an hour, as the rule of thumb states for every hour in surgery you will need a month of recovery.
Things aren't much different in archery, and in this installment of our Practical Bowhunting Test Series, we'll explore the rules of thumb we take for granted when it comes to arrow speed gains or losses depending on common bow adjustments.
Many of these little nuggets of wisdom have been around for several decades, so you have to wonder if they are still accurate — or if they ever were to begin with. Who came up with this stuff anyway? Did they simply notice repeating themes and make assumptions, or did they actually test their theories and produce the data to back them up?
Well, we are here to put those old adages to the test and see what happens!
Before we dive into the testing, let's go over the three pieces of conventional wisdom that inspired us to tackle this topic.
First, a brief Internet search will uncover a generally accepted belief that dropping your bow's draw weight by 10 pounds will reduce your arrow speed 15-20 feet per second. Really? For every type of bow?
Second, it is commonly said that for every inch you reduce your bow's draw length, you lose 10 feet per second of arrow speed. This is a big one, as some folks will sacrifice good shooting form for extra arrow speed by using a longer draw length than they can really handle.
Third, it is often said that for every five grains of added arrow weight, you will lose 1-1.5 feet per second in arrow speed. Of course, there are recommended minimums for arrow weight to keep you safe! Too light of an arrow will have a similar effect of dry firing your bow, which is destructive to your bow and can cause injury to you and others nearby.
Whenever BOWHUNTING embarks on a new adventure in testing, we rely on our exclusive X-Ring Shooting Machine, which offers an unmatched level of precision and consistency. For this arrow speed test, we set up four bows with a string loop and a NAP QuikTune 3000 arrow rest to tune on the X-Ring using a Beman Patriot arrow with a finished weight of 420 grains. The bows represent a cross section of those available on the market:
– Single Cam with an advertised IBO speed of 325 fps
– Dual Cam 1 with an advertised IBO speed of 335 fps
– Dual Cam 2 with an advertised IBO speed of 342 fps
– Hybrid Cam with an advertised IBO speed of 340 fps
Test 1: Draw-Weight and Draw-Length
Our initial testing focused on how variations in draw weight and draw length impact arrow speed.
To begin the test, each of the four test bows was set to a 30-inch draw length and 70-pound draw weight. Three shots were fired from each bow out of the X-Ring Machine, using the Beman Patriot arrow and recording the speed of each shot using a Pro Chrono chronograph.
Without adjusting the draw length (stayed at 30 inches), the draw weight of each bow was adjusted down to 65 pounds and three more shots were fired and speeds recorded. Then the weight of each bow was adjusted another five pounds downward to 60 pounds, with three more shots fired and speeds recorded.
Next, we repeated the same process for each bow at draw lengths of 29, 28 and 27 inches. So, when completed, our testing provided 36 data points per bow (nine shots each at 30, 29, 28 and 27 inches) and a total of 144 data points to illustrate the effect various draw weights and draw lengths have on arrow speed.
Results: Apparently our first two rules of thumb are widely accepted for good reason, as they are generally in line with our test results!
Looking at the data for all four test bows, the minimum speed change per pound of draw weight is 1.9 fps and the maximum is 2.4 fps. That is taking into account all draw lengths and all draw weights, and equals a total range of 19.3-23.7 fps.
So, the old adage that you will lose approximately 15-20 fps of arrow speed when you drop 10 pounds of draw weight is basically on the money. Maybe we can slowly change the rule to simply state, "You will lose approximately 20 fps for every 10 pounds of draw weight reduction."
For a graphical representation of the data, look at the draw-weight diagrams, which show a thin black line that represents a pure linear path while the color lines represent actual data — very close! Because the data is linear, you can closely extrapolate results anywhere along the line.
On the draw-length side, the data shows that higher performance bows (faster) have a slightly larger increase/decrease in arrow speed per inch of draw length than the slower bows. The faster models ranged from 9.8-11.7 fps per inch of draw length, while the two other bows ranged from 7.9-8.9 fps per inch. The total range of all four bows is 7.9-11.7 fps per inch of draw length.
If you round those numbers up, you get 8-12 fps, with a middle ground of 10 fps, which is the generally accepted rule of thumb. Just remember you will trend on the lower side with bows that have an IBO speed rating under 335 fps and come out a little higher on bows with a speed rating greater than 335 fps.
Again, the graphical representations will show a thin black line that represents a pure linear path, while the color lines represent actual data. With most of the data represented in a linear pattern, you can closely extrapolate results anywhere along the line.
Test 2: Arrow Weight
A single bow set to a 60-pound draw weight and 29-inch draw length was set up with a string loop and a NAP QuikTune 3000 arrow rest and tuned to the X-Ring Shooting Machine. Without adjusting draw length or draw weight, three shots were fired using a 420-grain arrow and speeds recorded for each.
An additional five grains was added to the arrow (425 grains) and three more shots were taken and recorded. This process was repeated in five-grain increments up to an arrow weight of 450 grains, for a total of 21 data points (three shots each at seven different arrow weights).
Results: Again, we found the rule of thumb to be generally reliable. Considering that the chronograph used only gives results in whole numbers, the data is very close to the generally accepted assumption that you will lose 1-1.5 fps of arrow speed for every five grains of added arrow weight.
What It Means
Our results prove that in the bowhunting and archery world, we have some solid rules of thumb that stand up under rigorous testing. We can safely say that these three statements can absolutely guide you in your own equipment setup and evaluation:
1. Dropping your bow weight by 10 pounds will reduce your arrow speed 15-20 fps. Said in a slightly modified way based on our results, "You will lose approximately 20 fps for every 10 pounds of draw-weight reduction."
2. For every inch you reduce your draw length, you will lose roughly 10 fps of arrow speed. On average, this is close. However, we found the range is actually 8-12 fps. Modifying the rule of thumb slightly to account for test results, we might say, "Expect to lose approximately 8-10 fps of arrow speed for every inch you reduce your draw length on adult performance bows rated at 335 fps and lower. On similar bows rated over 335 fps, the loss is closer to 10 to 12 fps."
3. For every five grains of arrow weight added, you will lose 1-1.5 fps in arrow speed. Yup, that seems about right!