Arrow spine, arrow weight, and arrow length are all important factors in matching an arrow to a bow. These three things need to be matched so that you, the archer, have the best chance of hitting their target.
This article will explain why these factors are important when matching arrows to bows and how they can vary and complement one another.
Arrow Spine
Arrow spine is simply a measurement of stiffness for an arrow shaft or how much the shaft bends under a specific load/draw weight.
Why is Arrow Spine important?
An arrow with a weak spine will continue to correct itself throughout the arrow’s flight by oscillating side to side. This is also referred to as the Archer’s Paradox.
This effect is much stronger if you are not using a release aid but if you draw your bow by hand.
An arrow with a stiff spine won’t adjust as much, as the spine won’t allow it to oscillate properly for flight correction.
This is why target arrows typically have a weaker spine than hunting arrows.
In a target arrow, the arrow’s flight needs to be corrected because the front is lighter due to having a field point. In a hunting arrow, the broadhead is heavier and can “assist” more in correcting the flight direction.
Because of the “Extra” weight added by the broadhead, the oscillating could be too much. To counter this, your arrows will have a stronger spine.
read.. effect untuned bows have on arrow flight
Arrow Material
Modern arrows are typically made from carbon fiber. Carbon arrows are lighter and have a stiffer spine than aluminum or wood arrow shafts, which helps with the flight correction of an arrow.
The weight and length of your bow will also play into what type of arrow you use to shoot it.
How Is Arrow Spine Determined?
You have a static- and dynamic spine size.
Static Spine
For determining static spine deflection, 28″ arrow shafts are mounted with a 1.92-pound weight hanging from the midpoint of the shaft.
The deflection of the arrow is then measured. For example, the arrow deflects 0.500″. It is then classified as a size 500 arrow.
Dynamic Spine
The dynamic spine explains how your arrow behaves when it is shot from your bow. All the stored energy is transferred to the arrow and propels it forward.
Things like the weight of the broadhead, insert weight, fletching, wraps and the speed of your bow (draw weight) play a role.
To take all those factors into account and end up with an arrow that exactly meets your needs – It is almost an art!
read.. how to make a spine tester
Arrow Weight
Arrow weight is commonly referred to as GPI (grains per inch).
Stiffer arrows will naturally have a higher GPI than more flexible arrows. Because they have thicker walls, thicker shafts have more material, resulting in a greater grain density.
This also means that stiffer arrows are heavier, which helps with the flight correction of an arrow.
It is important to know the GPI of your arrow. You can then, by considering its spine, determine the total shaft weight.
For example
500 size arrow with a 7.3 GPI and 400 size arrow with an 8.2 GPI. Both 28″ arrow shaft.
- 500 size arrow = 204 grains
- 400 size arrow = 229 grains
Arrow Length
The length of a bow is measured in inches from one tip to another or in centimeters from one end to the other.
The length of your arrow will affect its weight too. A longer arrow has more weight to carry, which means it needs a stiffer spine.
You can cut an arrow matching your draw length, and doing that makes it stiffer. Remember the weight hanging from the midpoint of the arrow to measure spine deflection?
shorter arrow = less deflection = lower size arrow
Taking into account spine, weight and length, we can determine the weight of the shaft.
Overspined & Underspined
It is possible to have an arrow too stiff or too flexible. When shooting with heavy poundage bows, underspined arrows may cause erratic flight patterns because of their lower stiffness. Or worse could literally blow up!
Overspined arrows pose much less of a problem. I guess it all comes down to how much you’re overspined?
If I would have to choose between those two, I definitely would rather be using a stiffer arrow —kinetic energy matters and a heavier arrow might help with it.
What Else: Broadhead and Tip Weight
You still need to consider the weight of your broadhead or tip. This is the final factor to determine the total arrow weight.
The higher the weight of your broadhead, the heavier the head of your arrow. And consequently, this means more deflection in the arrow.
To battle this off, you need to adjust the stiffness accordingly. Either by cutting the shaft (if possible) or going for a stiffer shaft, begin with.
Putting it All Together
Even though, in theory, this is all clear, nice, and easy – The reality is that especially when you are starting, this may become pretty complicated. I addition, you also need to consider the speed of your compound bow. Draw weight and draw length.
Luckily most arrow manufacturers offer arrow charts. Here are the compound bow ones from GoldTip.
(read or hands-on review Gold Tip Velocity XT 400).
Arrow Spine Charts
Summary
To wrap it all up, the spine of an arrow is so important that most people even recommend learning to build your own arrows. It can be quite intimidating at first, but once you get the grip, it can also be a lot of fun.
And it’s not just because you can save money and are more eco-friendly – It also means being able to customize your arrows in a way that suits YOUR needs, no matter what type of archer you are or bow you use.
FAQ
Why is Arrow Spine important?
It is important for accuracy!
An arrow with a weak spine will continue to correct itself throughout the arrow’s flight by oscillating side to side. This is also referred to as the Archer’s Paradox.
This effect is much stronger if you are not using a release aid but if you draw and release by hand.
How Is Arrow Spine Determined?
You have a static- and dynamic spine size.
For determining static spine deflection, 28″ arrow shafts are mounted with a 1.92-pound weight hanging from the midpoint of the shaft. The deflection of the arrow is then measured.
For example, the arrow deflects 0.500″. It is then classified as a size 500 arrow.
