There’s a lot of talk about bow variations and modifications, but what about arrows? The truth is, arrows can be just as specialized as bows can, and some archers don’t even know all there is to know about their arrows. One lesser-known specification is spine strength.
Spine strength refers to the stiffness of an arrow. Spine strength is measured by the number of inches an arrow bends when a certain amount of force is applied, typically ranging from about .500 inches to .300 inches.
There’s a lot to understand about the intricacies of spine strength, and bowhunters, in particular, should be concerned about choosing the correct spine strength in their arrows.
This article will break down some of the most important factors of spine strength and how choosing the right measurement can improve your archery skills.
Why is Spine Strength Important?
If you’re new to archery, it may come as a shock that the arrows you’re firing are not perfectly straight and taught.
Of course, they are when we’re not shooting them, but if they stayed that way when we shot them, it would be much more difficult to make an accurate shot.
The spine of an arrow needs to have some flex in order to correct its path in flight.
Before the invention of compound bows and special risers, arrows were forced to be in off-center positions by the bulky riser of traditional bows like longbows and recurve bows.
When the archer released the string and the arrow was freed, it would have to bend around the riser and quickly correct its flight pattern after clearing the obstacle.
Nowadays, there are cut-outs in most compound bow risers that allow the arrow a clearer path from the bow’s string, but flex is still just as important for a different reason.
The mechanical advantage of compound bows mostly come from their cams.
They create more vertical tension and allow for a cleaner transfer of energy, which puts a lot more power behind an arrow than many traditional bows are able.
This massive transfer of energy, along with any inconsistencies added by the imperfections of the human operator, must be corrected quickly if the arrow is going to make it to its target.
Both a certain amount of flex and a certain amount of stiffness are required to properly balance out the forces working on the arrow.
Just after release, it is important that the arrow is flexible enough to absorb and redistribute some of the dynamic force being transferred to the arrow.
If the arrow is too stiff, the dynamic energy will likely knock the arrow off course immediately after being released from the bow’s string.
As it heads down the course, however, the level of stiffness will become more important.
Without some stiffness to bring the arrow back on track, the flex will cause the arrow to lose control and will make it extremely difficult to recover from the chaotic release moment.
How is Arrow Spine Strength Measured?
Spine strength in arrows is not as easily measured as length or weight.
To further confuse the situation, there are two different kinds of spine strength, and they are measured in different ways.
Static Arrow Spine
Static arrow spine has the most straightforward measurement method and is the figure that most companies will display as their arrow’s spine strength if they display it at all.
The measurement of static arrow spine strength is not as regulated as, say, the tests that determine how fast a compound bow can shoot an arrow.
Ratings put out by manufacturers can sometimes be based on different factors, which is why you must be careful when searching for arrows with a specific spine strength.
Most often, however, the flex of an arrow is measured by supporting either end of the arrow and hanging a weight from the center of it.
For carbon and aluminum arrows (the popular choice for modern bowhunters with powerful compound bows), the weight hanging from the center of the arrow should be exactly 1.94 pounds.
To determine the static spine strength of a wooden arrow, a weight that is exactly two pounds should be hung from its center.
The distance between the supports at either end of the arrow is equally important.
For modern-design arrows meant to be fired from compound bows, the supports should be exactly 28 inches apart.
For wooden arrows, the supports should be exactly 26 inches apart.
This typically means that the arrow being measured is 28 or 26 inches respectively.
This can sometimes throw archers off when they go to shoot the arrow expecting a certain spine strength because, of course, not everyone shoots with 28-inch carbon arrows or 26-inch wooden arrows.
Dynamic Arrow Spine
The science of measuring an arrow’s dynamic spine strength is less exact.
This figure comes from the flexibility or inflexibility of the arrow while in flight.
Naturally, this means it has a fair amount to do with kinetic energy, or the potential energy stored in an object in motion.
Kinetic energy is a whole subject in itself, and if you want to understand more about it, I go more in-depth on the topic in my article, How Far Can a Compound Bow Shoot?
The actual process of measuring dynamic spine strength is something only the professionals and maybe a few very dedicated archers know how to do.
But even if we can’t get an exact figure like we can with static spine strength, there are a number of things everyday archers can do to adjust their dynamic spine strength and improve their accuracy.
How to Adjust Dynamic Spine Strength
Though we can’t usually detect it with the human eye, the arrow is flexing and wobbling as it flies through the air, and the amount of flexing it does while airborne invariably affects its eventual accuracy.
So, for the common target archer or bowhunter, accuracy is one sure-fire way to determine if your arrow has too much, or not enough, flex.
Granted, there are a lot of other factors that will play a part in your accuracy like draw length, brace height, draw weight, and a number of other things.
But if you’ve been shooting bows and arrows for a while and are confident that your other measurements are all where you want them to be, you can begin to fiddle with spine strength.
First, consider the weight of your arrow’s tip.
The heavier your broadhead, the weaker your arrow’s spine strength will get, resulting in increased flex that may be sending your arrow off course.
Conversely, you might adjust the weight at the other end of your arrow by swapping out your fletching or nock.
The heavier the end of your arrow is, the stiffer your arrow’s dynamic spine will become, which could help your arrow’s correction ability.
Remember when we talked about people shooting with different lengths of arrows? Arrow length is another major factor in dynamic spine strength.
A longer arrow will always flex more than a short arrow.
If your arrow seems to be off-course from the moment you release it, it could be that it is too short and therefore too stiff. Try switching to a longer arrow.
If your arrows seem to be traveling fine until they hit the target in a random spot, it could be that your overly-long arrow is having sporadic flight because of too much flex.
In this case, try switching to a shorter arrow and see if your accuracy improves.
Lastly, make sure you’re shooting an arrow with a spine strength that is compatible with your bow’s draw weight.
Many arrow manufacturers will include a range of draw weights their arrow is designed to be shot with.
If you’re not sure what they recommend, just know that faster compound bows will always require a stiffer spine strength (something like .300 inches of deflecting).
Slower traditional bows call for arrows with weaker spines (something with closer to .500 inches of deflecting).
How to Choose the Right Spine Strength
When choosing a spine strength for your arrows, definitely start by considering the kind of bow you want to use them with.
Fortunately, a lot of arrow manufacturing companies will include the static spine in the name of their model. For example, the Black Eagle Carnivore 300 was so named because it was revealed to have a static spine strength of .300 inches when tested.
If you’re unsure exactly what spine strength to go with, you can ask the professionals at your local pro shop what they recommend for your bow.
The most important thing is to test out as many different spine strengths as you can to determine what feels the most comfortable for you.
An arrow with the proper spine strength for the bow it’s being shot from with always have greater accuracy and power.
A good test to perform when sampling arrows with differing spine strengths is to shoot them from different distances.
If you’re getting particularly poor results from an arrow model at 40 yards, try shooting it from 20 yards away.
Improved results after moving indicate that the arrow likely has too much flex. From there, move to stiffer and stiffer arrows until you’re achieving the accuracy you want.
The key is finding a balance between the flex and the stiffness. As with many other aspects of archery, that balance will only come after much trial and error.