Understanding Arrow Spine and Spine Deflection for Hunting

Every serious hunter knows that the perfect shot requires the perfect arrow. But with so many arrow options on the market, it can be hard to know which one to choose. One crucial factor to consider is arrow spine and spine deflection. These terms refer to the stiffness of an arrow and how it reacts to the energy transferred from your bowstring. Understanding these concepts is essential for choosing the right arrow and achieving the best performance in the field.

In this guide, we’ll break down the difference between static and dynamic spine, explain how to measure and calculate spine deflection and give you tips for choosing the best arrow for your hunting setup. Whether you’re a seasoned pro or a beginner archer, this guide will help you make informed decisions and improve your accuracy and success in the field.

What is Arrow Spine?

Arrow spine refers to the stiffness of an arrow, or its resistance to bending. The spine rating of an arrow is based on the amount of deflection it exhibits when a specific amount of weight is applied to its center. The right spine is essential for a well-tuned setup because it affects how an arrow flexes and recovers as it leaves the bow. Too stiff or too weak, and you’ll see inconsistent arrow flight and reduced accuracy.

Static and Dynamic Spine

When shopping for arrows, you might have noticed the term “spine” thrown around a lot. Spine refers to the stiffness of the arrow shaft and is a crucial factor in determining the accuracy and performance of your arrows. But, did you know that there are two types of spine, static spine, and dynamic spine?

Static spine is the measure of an arrow’s stiffness when it’s stationary. It is what manufacturers use to sell their arrows and is typically displayed on the arrow’s packaging or specifications. It’s measured by suspending a 28-inch arrow horizontally and applying a two-pound weight to the center of the arrow. The amount the arrow bends under the weight is the static spine measurement.

However, when an arrow is shot, it undergoes dynamic spine, which is the measure of the arrow’s stiffness when it’s in motion. The dynamic spine is affected by various factors, such as draw weight, arrow length, and arrow weight. When you shoot an arrow, the bowstring applies force to the back of the arrow, causing it to flex as it leaves the bow. This flexing is dynamic spine, and it affects the arrow’s flight trajectory and accuracy.

It’s important to understand the difference between static spine and dynamic spine when selecting arrows. While static spine is a useful starting point, it’s not the whole picture. Dynamic spine can vary depending on the hunter’s setup, and a hunter must understand how dynamic spine affects their arrow’s accuracy and performance. So, while static spine is what manufacturers use to sell their arrows, dynamic spine is what hunters need to consider to optimize their arrow setup for hunting.

Measuring Spine Deflection

The ASTM F2031-05 standard is a widely accepted method for measuring spine deflection. In this test, an arrow is supported horizontally on two points 28 inches apart, and a 1.94-pound (880-gram) weight is hung at its center. The amount the arrow bends downward or deflects is then measured in inches or millimeters. To express spine deflection in thousandths of an inch, simply multiply the deflection measurement in inches by 1,000.

While this method doesn’t provide a direct formula to calculate spine deflection, it does give you a standardized way of measuring and comparing arrow spines. Most of us aren’t going to go through this process as the arrows we buy have the spine deflection advertised either on the arrow itself or on the box they came in. That being said, understanding how this static spine measurement affects you and the arrows you buy is very important.

Selecting the Right Spine Deflection for Your Arrows

Unfortunately, there isn’t a universal formula to calculate spine deflection directly based on arrow length and material. However, arrow manufacturers provide spine selection charts that take into account various factors such as draw weight, arrow length, and arrow material (carbon, aluminum, wood, etc.). These charts are based on experimental results and offer a more accurate spine deflection value for their products.

Here’s a step-by-step guide to selecting the appropriate spine deflection for your arrows:

1. Determine your bow’s draw weight in pounds (e.g., 60 lbs).
2. Measure your arrow length in inches (e.g., 28 inches).
3. Refer to the arrow manufacturer’s spine selection chart to find the recommended spine deflection value for your setup (e.g., .400″).
4. Select the arrow that has the matching spine deflection and other features you are looking for.

Keep in mind that different carbon arrow constructions and materials may affect the relationship between arrow length, draw weight, and spine deflection. So, it’s essential to consult the specific arrow manufacturer’s spine selection chart and choose them based on the specifications you selected in the steps above.

*NOTE: Some arrow manufacturers ask for your point weight and bow IBO Speed to help ensure you get the perfect arrow for your specific setup.

**NOTE: No, that number on the side of your arrow is not its weight. It normally relates directly to the spine deflection of the arrow.

Test and Fine-Tune Your Setup

Remember that factors such as the bow’s cam system, fletching type, and point weight can also influence arrow flight. It’s essential to test and fine-tune your arrow setup to achieve optimal performance in the field. Don’t hesitate to experiment with different spine deflections to find the best match for your bow and shooting style.

Remember, static spine is just a starting point for selecting your arrows. Dynamic spine is what needs to be optimized to fully tune your arrows and optimize their flight and accuracy. Just because the manufacturer’s spine chart (like the one pictured above) says you should be using a 400 spine arrow, you might need a higher or lower spine arrow based on your individual setup. There are other ways to optimize and fix this without changing arrows, like changing your field point weight and fletchings for example. So be aware that you may have to make adjustments or conduct advanced arrow tuning for optimal performance.

In conclusion, understanding arrow spine and spine deflection is crucial for bow hunters who want to maximize their accuracy and success in the field. Always consult your arrow manufacturer’s spine selection chart and don’t forget to test your setup to achieve the best results.