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Arrow Speed vs. Weight. Which Matters?

By: Stephen Hafvenstein

The Background

Speed, everything these days has to be fast, fast bows need to shoot fast lightweight arrows, or so we are told. But, does this really matter? Is it better to shoot a blazing fast arrow that will reach the target faster, or a heavier arrow that will reach the target slower? We decided to run some tests to weigh out some factors that play a role in this debate. Here is what we did.

The Test Setup

For this test, we used a Mathews NoCam HTR bow set at IBO specifications (draw weight at 70 pounds with a draw length of 30 inches). The IBO rating on this bow is 330 feet per second (fps). For the arrows, we used Black Eagle Spartan’s cut at 29 inches with a spine  of 300 and a weight of 9 grains per inch (gpi).

To change the weight of the arrow we used field points with a weight of 100, 125, 150, 175 and 200 grains, all points had the same tip design. Our insert weight was 28 grains, the bushing weight was 10 grains, the nock was 9 grains, and the vanes (3 total at 6 grains) weighed a total of 18 grains. Once the arrow was set up, we weighed the entire arrow on a digital scale to come up with total arrow weights (all components installed, lightest to heaviest) of 429.45, 454.67, 479.23, 504.82 and 529.51 grains. In addition to our test arrows, we also ran numbers from an IBO arrow with a weight of 350 grains. Our Front of Center (FOC) for each arrow (lightest to heaviest) was 10.77%, 12.82%, 14.65%, 16.31% and 17.80%. We shot each arrow three times into targets set at 10, 20, 30 and 40 yards and averaged the time the arrow took to get to the target. We used sound to determine the time it took the arrow to clear the string to the time it took the head to hit the target. By recording the shot with a Deity V-Mic D3 Pro, we were able to calculate time down to the millisecond by averaging the audio wavelengths of the shots. Finally, we purchased two archery calculation programs to compare our numbers against. We entered our specifications into the programs and ran them to calculate the results. After completing our tests, we compared our results against the two programs and found that all three sets of data were within one percent of each other. For our research we used the data from our tests due to the fact this takes into account the environmental and human aspects of shooting a bow that we felt would represent a better picture of real-world conditions.

Other Variables

We took this test a few steps further by calculating other variables that affect the flight of an arrow as well as the reaction times of game species.

DRAG:

The first calculation we ran was arrow drag. Due to air density, gravity, wind, humidity, arrow flight performance and arrow components (fletching, arrow wrap, arrow finish…etc.), an arrow will experience speed loss during flight. We calculated this via speed loss formulas for each variable and subtracted the differences in time it took the arrow to hit the target at each distance. For example, our 529.51 grain arrow exited the bow at 270.94fps, with no drag; our arrow would reach the 10 yards target in 0.1107 seconds. However, it took the arrow 0.1120 seconds to reach the 10-yard target, a difference of 0.0013 seconds. Although this number is extremely small, it represents the drag an arrow experiences during flight due to a number of speed loss variables. As a rule of thumb, as distance increases, arrow speed will decrease as it experiences a longer duration of drag. In an ideal world with no speed loss, the 529.51 grain arrow would fly a constant 270.94fps throughout its entire flight path.

SPEED OF SOUND:

The second variable we accounted for was the speed of sound. If a game animal does not see you take the shot, the only way it will know a shot was taken, is to hear it. The speed of sound is 1125.33fps. At 10 yards the sound of the shot will reach the game animal in 0.027 seconds, once again, this is an extremely small number but it represents a time variable. We took this into account when determining the time, a game animal has to react to a shot and calculated this for each distance we shot. Additionally, we marked out where I would plant my feet each time I shot to remain consistent. We then calculated the distance from where the arrows left the bow string to the microphone and the distance of each target to the microphone to account for the time it takes the sound to reach the microphone, yet another variable. Once again, as distance increases, so does the time it takes for the sound to reach the animal.

 REACTION TIME:

The last variable we calculated was the reaction time of an animal. There were quite a few variables in the reaction time, some of them cannot be calculated such as the individual response of an animal, but we calculated the variables that can. For reaction time, we took into account the time in milliseconds it takes for an animal to register noise and for the neurons to carry that to the brain. After studying many scientific articles, we felt confident that by using the audible reaction time of a rat, it would easily account for that of a game animal. Rats have one of the fastest reaction times in the animal kingdom, much faster than that of whitetail deer for example. The rat had an audio reaction time of 10 milliseconds, 0.01 seconds. We also took into account the time it takes the neurons to carry the signal from the brain back to the muscles and for the muscles to contract. For this we took the reaction time of a mid-sized dog, the reaction time was 40 milliseconds, 0.04 seconds. This is in a perfect world, it does not take into account the “surprise” factor, individual response of an animal (time it takes to recognize the sound of a bow shot as danger), or the time and speed it takes to “jump” or “duck” the arrow. We chose to use the data from a rat and a dog due to their smaller size and faster reaction times than most game animals. A rule of thumb, due to the distance of the animal’s audio receptors to its brain, the smaller the animal, the shorter distance the neurons have to carry the signal to the brain and back to the muscles, therefore smaller animals generally have faster reaction times than larger animals

Our Findings 

KINETIC ENERGY:

Kinetic Energy (KE) is the amount of energy an arrow possesses and transfers to its target. The lethality of an arrow has very little to do with the KE it possesses, however, KE is a very important variable for the lethality of a bullet. Watching a bullet hit ballistic gel at a super slow motion, gives the viewer an excellent understanding of the shockwaves a bullet sends through the gel, those waves are the KE that the gel is absorbing from the bullet. Bullets travel at such a velocity that the energy it transfers to the gel or animal is enough to send a lethal amount of energy into the tissues surrounding the path of the bullet.