How To Select The Correct Arrow Spine

Getting the perfect hunting arrow for you starts with selecting the correct spine. Selecting the correct arrow spine will increase your flight performance and accuracy. A common mistake I see with new and sometimes even experienced bow hunters is using the incorrect spine or using a variety of spined arrows. This is easy to do when you have several arrows laying around from a bow you upgraded from, or if you bought a pack that was on sale without looking at the specs. Arrow spine really makes a difference.

Besides the tuning of a bow, knowing how to select the correct arrow spine is a major deciding factor in accuracy and performance of your hunting arrow. If you are a bowhunter or archer that is having accuracy issues or seeing erratic arrow flight, you should make sure you have the correct arrow spine.

In this post, we will cover what arrow spine is, how to choose arrow spine, what happens if the arrow spine is too weak, what happens if arrow spine is too stiff, and how to change arrow spine with arrow length or tip weight. 

What is Arrow Spine?

Arrow spine is simply a designation of the stiffness of an arrow. Arrow spine is calculated by applying a set amount of weight (1.94lbs) to the center of the arrow while both ends are supported so that deflection of the arrow is measured. A stiffer spine will not flex as much as a lighter spine. This is how arrows are classified and tested to make sure they are up to standards. 

When an arrow is released from the bow, it immediately flexes. The flex oscillates back and forth, side to side, as the arrow continues in flight. The further the arrow gets down range, the oscillation should decrease in magnitude and the arrow flight should straighten. An arrow that over-flexes has a spine that is too weak vs a stiff spin that does not flex enough. Either way, the arrow will not correct itself quickly in flight. This will lead to decreased accuracy and lowered flight efficiency.

Unfortunately, not all arrow companies use the same spine designation for stiffness. So, one manufacturer may say an arrow is a 350 spine, and it may be as strong as a 300 spine for another manufacturer. It's critical to reference the applicable spine chart of the arrow manufacturer you have interest in. 

How to Select the Correct Arrow Spine for You

To choose an arrow spine you need three numbers, the actual peak draw weight of your bow, the tip weight you will be using, and the length of the arrow shaft. Using this information, go to the arrow manufacturers' spine specifications and find an arrow spine chart. Look for the numbers of your setup and read across to find the correct spine. If this sound complicated you can always use a custom arrow builder that will denote what spine shaft you should be considering.

There are a few things that need to be pointed out. Notice draw length is not used to calculate the correct arrow spine. A bow set at 30 inches of draw length may be shooting a 29-inch arrow. Choosing your arrows based on draw length may lead to an overly stiff arrow that will not correct itself quickly in flight. 

You need the actual draw weight of your bow when choosing arrow spine. Bows may come off the rack set for 70 lbs, but may only actually be 67-65 lbs at peak draw weight, or the weight could be higher. This could be the difference between one arrow spine and another. Make sure to use the actual peak draw weight as measured. Once you have made all your calculations don’t change the draw weight unless you plan to use different arrows.

Last, changing the point weights will effectively change the stiffness of the arrow. A heavier broadhead or fieldpoint will make an arrow flex more and a lighter point will flex less. Pick a point weight and stay with it. Mixing and matching will only lead to poor accuracy and disappointment. 

What Happens if Arrow Spine Is Too Weak?

An arrow spine that is too weak will not correct its oscillations and will over-flex. This will do a few things: it will decrease your arrow flight performance and speed, and also decrease arrow accuracy. The arrow will fly with the fletchings not trailing behind the tip (cocked to the side). 

When the arrow is first released, the acceleration is slowed by the arrow's overall weight, but also the weight of the arrow tip. This causes the arrow to flex under the weight of the tip. A heavier tip will cause the arrow to flex more. This flex continues to swing from side to side as the arrow flies. This flexing should eventually help the arrow fly straight, but if the spine is not correct the arrow will not line up.

There are a few things you can do to identify if your arrow spine is weak. If you are able to video your arrow in flight you may see that the arrow is flying “nock left” which indicates a weak arrow spine. When paper tuning, a “nock left” tear indicates a weak spine as well. And finally, for a very weak spine, you may find your arrow stuck in the target with the nock to the left.

When the bow is properly tuned, “nock left” deflection means the arrow spine is not stiff enough.

What Happens if an Arrow Spine Is Too Stiff?

An arrow that is too stiff will not be able to oscillate enough to fly straight. This leads to the arrow flying diagonally with the nock offset from the line of movement. Instead of the nock flying directly behind the point, it will fly parallel to it. This will decrease efficiency and accuracy.

Again there are three ways to identify this. You can video your arrow to see if it is traveling “nock right” which indicates too stiff of a spine. On a well-tuned bow, a “nock right” tear while paper tuning indicates a stiff spine. In extreme cases, an arrow that sticks into a target “nock right“ again indicates a stiff spine. 

How to Change Arrow Spine With Arrow Length or Tip Weight 

So far we have talked about static and dynamic arrow spine without really making a differentiation. A static arrow spine is the spine labeled on the arrow itself after testing and identifying the stiffness of an arrow. Typical static spine labels read 3 digit numbers showing the shaft's deflection, something like .400, .350, .300, .350, etc.  Dynamic arrow spine is the effective spine of the arrow after adding a tip, insert/outsert, fletchings, nock, and even cutting an arrow to a different length.

The two easiest ways to change an arrow's spine (stiffness) are to add or remove weight on the tip or to lengthen or shorten the arrow. A longer arrow will be effectively weaker, and a heavier tip will effectively weaken an arrow. Conversely, a shorter shaft and a lighter tip will make the arrow stiffer. 

How to Make an Arrow Spine Weaker

If you have an arrow that is flying “nock right” indicating a stiff spine, you can add weight to the tip (swap out 100gr for 125gr or 150gr) and you will see the arrow straighten itself out. Additional tip weight weakens the arrow. 

Another way to weaken an arrow is to use longer shafts. This assumes you are not already using a full-length arrow. If you bought a dozen arrows and didn’t cut them all to length and find you have arrows that are too stiff, try cutting one an inch longer and check your results. If it is now tearing left, try cutting half an inch off and see how it performs. This is all a part of tuning for your final setup.

How to Make an Arrow Spine Stiffer

An arrow that is flying “nock left” indicates a weak spine. In this situation, you can remove some tip weight or shorten the length of the arrow.

I have a long draw length so I shoot full-length arrows, but if I find that my new batch of arrows is weak I can remove the insert and trim half an inch off. This makes the arrow shaft shorter and effectively stiffer. 

Additionally, a lighter arrow tip will prevent the arrow from flexing as much (effectively stiffer). Pulling off your heavy tip and swapping it out for a lower weight will help fine-tune and get that arrow flying straight. 

Pro Tip | Nock Tuning

For years I had crazy inconsistency with my arrows. The spine was the same but I would get one flier that would never hit the same spot as the rest of the group. And I verified this by marking the arrow and found that it wasn’t just psychological, it was real. One arrow wasn't performing like the rest. 

I learned that nock tuning is a way to get uniform performance out of most arrows but it takes a lot of time. I say most arrows because some, though they pass manufacturer tests, still will not tune right.

An arrow has weak points in them or the point where the arrow will first react when shot. This is also known as the arrow shaft's first dynamic bend. It’s a fact, nothing is perfect in this world. Straight out of the box, the arrow you shoot may have the nock set on a weak part of the arrow. By turning the nock you can change where the nock puts pressure when the arrow is released. In the case with custom arrow builds from Exodus, each shaft has it's first dynamic bend located, marked, and built to. This eliminates the need for nock tuning and saves you a ton of time. 

Where you will see this impact your arrow flight the most is at long distances. I have literally turned a nock 90 degrees and watched an arrow’s side-to-side nock travel decrease by over half at 60 yards.

This practice can be used while paper tuning, but as I said before, for people with good eyesight launching long distances you will be able to track your nock and fletching travel. 

If you have an arrow that is inconsistent and always shoots differently than the others try taking it and shooting it through paper. Turn the nock 90 degrees each shot until it is tearing the same as the rest of your arrows. If you go 360 degrees and the arrow never straightens out, you have a reject arrow. You didn't do anything wrong, it just was defective from the manufacturer. 

This process could be done with every arrow for the best results to get them all shooting exactly the same. If you have one arrow that is not performing like you want it to, you can also just tune that one. In the end, nock tuning every arrow is very time-consuming so you may want to consider building from the first dynamic bend of each arrow shaft. 

Closing Thoughts

Using the right spine arrow for your bow setup is very important. Realize that every change you make with your arrow setup and even the draw weight of your bow will affect the dynamic spine of your arrow. Once your arrows are all set up correctly to your bow you will see better patterns and get more efficient arrow flight.

Next time you go to buy arrows make sure to grab the correct spine for your peak draw weight, arrow length, and tip weight. When you take to the woods after some practice you will know that you have your bow set up how it needs to be and only need to focus on the shot.


Author: Evan Grimm, Exodus Black Hat Team Member

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