How to Select Hunting Arrow Spine
Photo by Carbon Express

Arrow Basics: Best Arrow Spine, and Weight for Hunting

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For those just getting started with bow hunting, the selection of your hunting arrows can seem like an overwhelming subject. This article is going to cover all aspects of arrow selection, but in an easy to follow manner. We’re going to break down the process into several smaller factors before bringing it all together and making some recommendations.

First, a Litle Background

With the advent of the compound bow, traditional wood arrows became too dangerous and prone to breaking under the higher forces of a compound bow. This led to the innovation of aluminum arrows, and then eventually the stronger and more durable carbon arrows that most bow hunters use today.

When aluminum arrows were most popular, Easton published a big chart to help hunters and target shooters figure out which arrow to choose. Every aluminum arrow has a four-digit identifying number printed on the shaft. The first two digits signify the shaft diameter, and the last two represent the wall thickness.

Example: An Easton aluminum arrow with the digits 2512 means the shaft is 0.250” in diameter with 0.012” wall thickness.

The Easton arrow charts were developed simply to make it easier for everyday shooters to figure out what arrow to choose for their bow. As the archery world has moved to carbon arrows, arrow spine deflection became the most crucial factor in hunting arrow selection.

Arrow Basics

The key to understanding arrow specifications are simple once you grasp the basics. The arrow spine, weight (gpi), and you tip weight are the most important aspects of arrow selection. Once you understand these things are all interacting with each other, and each has their tradeoffs, the complex quesiton of arrow section becomes much easier.

Arrow Spine

Arrow spine deflection is the key to the entire arrow selection process. Spine is simply how much the shaft bends under a specific load. The standardized test for determining spine deflection is taking a 28” arrow shaft and hanging a 1.92-pound weight from the shaft midpoint.

Example: Arrows that deflected 0.400 inches were classified as size 400 arrows, and a 0.340” deflection would be a size 340 arrow. The smaller the number the stiffer the arrow is.

When it comes to shooting these arrows, a 30lb draw bow may barely bend the 340 arrows at all. An 80lb bow, on the other hand, would impart much higher forces and a lot more flex into the arrow as if comes off the bow. The amount of flex directly impacts how well the arrow flies as it goes downrange.

Gold Tip Spine Chart: <315 fps
Gold Tip Spine Chart: >315 fps

Arrow Weight

The second important specification of carbon arrows is the weight, or GPI (grains per inch). Stiffer arrows will naturally have higher GPI than more flexible arrows. The stiffer shafts have thicker walls, and therefore more material, giving them higher grains per inch.

Going back to our Spine example arrows, we are left with a lighter more flexible arrow and a heavier stiffer arrow. These differences will come into play later as we get closer to selecting our arrows.

Example: Our 400 size arrow might have a 7.4 GPI, and the 340 arrow a 8.2 GPI. Over a 28-inch arrow shaft, the total shaft weights become 207 grains and 230 grains respectively.

Tip & Broadhead Weight

The third and final influence in arrow specification is the weight of your tip or broadhead. The heavier the head, the more weight you have in front of your bow. More weight in the tip contributes to more deflection in the arrow as it leaves the bow.

A 125gr broadhead is going to bend an arrow more than a 100-grain broadhead on the same shaft. So if you are shooting a heavier head, you might need an arrow with a little more spine, depending on your draw weight. For example, a low draw weight bow with a stiff arrow and heavy tip will lighten up the action of the arrow. But, the tradeoff is the arrow will drop must faster.

Further Reading:

  1. Broadheads vs Field Points
  2. Top Broadhead Choices
  3. How to Sharpen Broadheads

Other Factors Influencing Arrow Choice

Now we need to understand the factors on the compound bow that influence the arrows we’ll need to choose. Everyone is different and there are many sizes and types of compound bows. But these factors apply to everyone.

IBO Bow Speed and Cams

IBO is the International Bowhunters Organization, which has archery competitions and developed a standard for measuring bow speed. In IBO competition, you are allowed 5 grain per pound of bow draw weight. Bow manufacturers typically advertise an IBO bow speed for each model. The speed listed is how fast the bow can shoot am arrow weighing 5 grain per pound of draw weight.

Example: A 70lb draw bow must use an arrow weighing at least 350 grains when shooting in an IBO competition.

Now, a bow rated to shoot 300 feet per second with a 70% let-off will shoot differently than a bow rated to 360 feet per second with a 90% let off. You would most likely need to use different spine arrows with these bows. The faster bow has more aggressive cams, making the arrow bend a lot more as it leaves the bow.

When using a mechanical bow release, the arrow will bend straight down. Whereas a finger drawn bow bends the arrow horizontally. This is known as the Archer’s Paradox. On a compound bow, under spined arrows will deflect way too much on the release, causing the arrow to shoot poorly. You will see this on paper tuning when the nock is way too high or low from the tip. This is a sign you could need a stiffer arrow.

Draw and Arrow Length

Your draw and overall arrow length will influence the arrow selection as well. When using a compound bow, a shorter arrow will bend less than a longer arrow and draw length with the same arrow shaft and tip. Think about it in everyday terms, it makes sense that a shorter stool is more stable than a tall stool with the same weighted person standing on top of it.

Putting it All Together

The goal now is to optimize the spine and weight of the arrow with the speed of the compound bow. In general, hunting arrow will be a little heavier and stiffer, with smaller shaft diameters. The reason for this is to generate more kinetic energy that can deliver a takedown blow. The smaller diameter shaft has less surface area and less friction going through the animal, which aides with arrow penetration.

A good starting point for hunters is to have the total arrow weight, shaft and broadhead combined, to be in the 400-grain range. This is a good happy medium for most compound bows. Some shooters can handle arrows up to 500 grain, but that arrow would not work well on a lower poundage bow. With the heavier arrow, you don’t sacrifice accuracy in the preferred shots in the 20-30 yard range.

Rather than bore you or risk confusion with a bunch of complicated charts, I am going to refer you to the shaft selector websites from the top arrow manufacturers. These tools make it super easy to get arrow recommendations based on your bow speed, draw weight, and arrow cut length.

Online Shaft Selection Caluclators

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