Arrow Selection: Some Considerations and Choices

When it comes to nerding out about archery gear and archery science, I’m guilty as charged. One of the most important considerations in putting together a bowhunting rig (and a topic that stimulates a lot of conversation) is arrow selection. Everything comes into play in choosing an arrow: what are you hunting? What kind of bow are you shooting? How much speed do you want? How much kinetic energy do you need? The answers to these questions all depend on your priorities and your hunting situation.

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Foreward: What is Really at the Heart of the Speed vs. Power Issue?

The debate around arrow selection often revolves around one main question: to be fast or not to be fast? Or more specifically, do you want a fast arrow or a powerful arrow? Most of the rest of your decisions come from your answer to this question. I imagine that archers have been engaged in this conversation for thousands of years, at one point having lively debates into the night over the type of wood to use for their arrows. Arrow weight is the single most important characteristic that determines the results you are going to get for speed and power.

Generally speaking, I think the race for the fastest arrow is a bit moot. However, manufacturers need an easy way to assign some kind of value to their bows, and people are drawn to fast things. It’s easy to want a bow that shoots at 370 feet per second (fps) over one that shoots at 280 fps.

But let’s back up. There are a couple other important considerations besides speed, so let’s look at the science and see how this need for speed plays out.

I’m going to focus on three main points for this discussion:

Act 1: Ensure you have the correct spine.
Act 2: In the bowhunting world, weight kills.
Act 3: I think there is a specific, but important, difference between seeking seeking a fast arrow and seeking a fast bow.

Of course, there are many things to think about when choosing arrows, but I think these 3 points will give you a good start when first tackling arrow selection. You will need to spend time doing plenty of research, doing some calculations, and figuring out what works best with your bow. I hope reading this article is one part of that research.

Act One: Arrow Spine

The first thing you need to know when selecting an arrow is what spine you need. Most choices in arrow selection come down to personal preference, but not spine. Arrow spine refers to the stiffness of an arrow, and it’s critical for the safety of your bow and for accuracy that you choose the correct arrow spine.

Imagine you drive a truck into the end of a horizontal telephone pole. Imagine  you drive the same truck into a tooth pick. Finally, imagine you drive it into a 2×4. While perhaps a little rough around the edges, this analogy is meant to explain what happens if you shoot an arrow that is too stiff or too weak for your bow. When the bowstring pushes the arrow off the end of the bow, the arrow flexes, stabilizing throughout the course of its flight. Simply put, if your bow is too weak, the arrow can`t flex enough; if your bow is too heavy, the arrow flexes too much. At worst, these situations could damage your bow, but they will certainly reduce the consistency and accuracy of your shooting. The rule is that heavier bows require stiffer arrows.

Every arrow manufacturer has a way of designating the spine of their arrows, and while the system used to measure spine stiffness is standardized, the systems used by manufacturers to represent spine is not. Easton’s system is generally the easiest to understand, because they use the direct measurement for “arrow deflection“, which is the way spine stiffness is calculated. With Easton arrows, the lower the number, the stiffer the arrow. 20151218_232210So a 70# bow might shoot a 340 spine arrow, while a 60# bow might shoot a 400 spine arrow. All arrow manufacturers will have charts (Easton, Carbon Express, Gold Tip) to help you identify the correct arrow spine based on your bow specifications (e.g. poundage, draw length, and arrow length). Be sure you understand how your specifications affect spine selection. Click here for a great resource to explain some of the finer points in arrow spine.

Act Two: The Science Part

Here’s the science in bowhunting: arrows kill by haemorrhaging, doing internal damage by cutting. Ideally, you want both an entrance wound and an exit wound. Therefore, an arrow’s ability to kill depends on effective and powerful penetration. The ability of an arrow to penetrate depends on kinetic energy (KE) and momentum. Kinetic energy is the energy an object possesses as it moves. Momentum is the relationship between speed and mass of an object. We could get into the differences between KE and momentum, but this is meant to be an introduction to the topic, so for simplicity, I’ll use KE to refer to the ability of an arrow to penetrate effectively.

Imagine that you have two cups. Fill them both half full with water. The first cup is speed; the second cup is the arrow weight. You can pour water from one cup to the other, gaining more speed and reducing weight, or vice versa, but there’s a trade off either way. Together, the interaction of the water in the two cups determines your KE.

As a rule, light arrows travel faster than heavier ones. Think of the difference between throwing a golf ball and a bowling ball. We will all be able to throw the golf ball faster. Speed is great in getting an arrow to the animal quickly, but as soon as the arrow touches the animal’s hide, it stops. At this point, the energy required to penetrate is stored in the arrow as KE, meaning that you need to optimize the amount of KE in the arrow. In the equation to calculate KE, the speed of an arrow has less influence over its killing power than the mass of that arrow. Therefore, the heavier an arrow is, the more KE it will have, and the more penetrating power it will have at the animal. I would rather be hit with a golf ball than a bowling ball, because even though the bowling ball is going slower, it is more powerful.

Some people will say that the faster the arrow is, the less time the animal has to react to it and jump the string. That’s true; however, sounds travels at 1,116 fps. The fastest bow in the world is still pushing an arrow slower than the speed of sound, meaning that the sound of the bow shooting will reach the animal long before the arrow. In other words, no matter what, the arrow can never be fast enough. So again, the real focus is on the ability of that arrow to kill the animal efficiently and effectively when it does reach its target.

To calculate the KE of your arrow, here is the equation:
KE=(mv²)/450,240

m = mass. The mass of modern arrows are described in grains per inch (gpi). Most arrows will have the gpi marked right on the shaft. To calculate total arrow mass, take the length of your arrow in inches and multiply that by the gpi weight of the arrow, and then add everything else on the arrow: vanes, nock, insert, arrow head.
v = velocity. You will need to shoot your arrow through a chronograph. There are calculations you can do to estimate your arrow speed using bow poundage, draw length, and arrow weight, but to be absolutely accurate, you need to use a chronograph.

Here’s an example using my old bow and arrow setup. I was shooting an arrow that weighed 9.0 gpi, was 28.5″ long, and had a total arrow mass of about 400 grains. I put it through a chronograph at 282 fps. So if we plug in those numbers, here’s what I was shooting for KE:

KE=(mv²)/450,240
KE=(400×282²)/450,240
KE=71 foot pounds

Here’s a handy resource for some other useful calculations.

So if KE kills animals, the next question is, how much KE do I need?

We don’t really know precisely how much KE is needed to kill every animal. Every bowhunting situation is different, but generally speaking, larger animals require more KE. Easton provides an estimated range of recommended KE for animals of different sizes:

Small game (rabbits, groundhog, etc.): 25 ft. lbs.
Medium game (deer, antelope, etc.): 25-41 ft. lbs.
Large game (elk, black bear, etc.): 41-65 ft. lbs.
Toughest game (buffalo, grizzly bear, etc.): >65 ft. lbs.

So that gives you an idea of what you should aim for depending on your hunting scenario. My position is that there is no such thing as “overkill”; you can’t have too much power. The more the merrier.

Check out this video comparing penetration between two different bow and arrow combinations.

It is worth spending some time doing the calculations, checking the numbers on different arrows, and thinking about what you will be hunting and what you need. Think about how much poundage you can handle in your bow and what kind of arrow weight you need to get the KE needed for quick kills. This brings me to my third point.

Act Three: The Real Need for Speed…Bow Poundage

Having said all that about arrow speed, I would bet that deep down, we all still want a fast arrow. Plus, there is some scientific basis for this: recall that velocity is a factor in KE, so it is important to have speed.

The IBO speed rating on a bow is determined using a 70# bow with a 30″ draw length and a 350 grain arrow. By hunting standards, 350 grains is on the lighter end of the spectrum. So what can the IBO rating tell you about what you can expect from the bow once you start shooting your hunting arrow? Well, since IBO ratings are standardized, it gives you some indication about the efficiency and power of the bow itself. A bow shooting IBO speeds of 350 fps is still going to be faster than one shooting 320 fps, even at lower hunting arrow speeds.

Of course, I’m not suggesting that the IBO rating should be your most important consideration when choosing a bow. On the contrary, it’s one of the last things I look at. My point is that if you want to get more speed out of your arrow, I would suggest you focus on increasing the power of your bow, rather than decreasing the weight of your arrow. So you can look for a bow with a higher IBO rating, more aggressive cams, and increase the poundage. I was in this exact situation with my Hoyt Charger. I was already shooting a heavy arrow, so if I wanted to get more KE, it had to be from increasing speed. So I worked the poundage on my bow up to its max of 60#. My new bow, a Prime Rize, is in 70#. This is specifically because I want more KE, but I also want to maintain some good speeds.

Afterward: Where To Start

Ok, so where should you start if you’re picking out arrows for your brand new bow? Or, if you want a more efficient hunting arrow?

First, do some research on different arrow manufacturers. Read some reviews, talk to pro shops, and do plenty of other research. Use the selection charts to determine which arrows fit your bow specifications. Look at the arrow weights. Determine your budget (I didn’t go into this, but basically, spend as much as you are able on arrows…generally, the more expensive they are, the more consistent and precise they are). IMG_3044 If possible, try shooting a few of them through a chronograph and do some calculations to compare the KE you’re getting out of each arrow. There are some other factors and calculations you will want to think about to maximize the efficiency of your bowhunting arrow as well (e.g. arrow length, F.O.C., broadhead weight, etc.). As with anything to do with archery, don’t discount your own intuition about which arrow feels most comfortable shooting, but certainly be confident in the numbers you are getting from your tests and trust those.

This is the fun part. Enjoy the shooting!

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