Choosing Arrow Spine: Static vs. Dynamic Stiffness
The ultimate guide to matching your arrows to your bow.
Spine is arguably the most important factor in arrow selection. It refers to the stiffness of the arrow shaft. If your spine is wrong, your arrows will never group consistently, no matter how good your form is. But 'spine' isn't just one number; it's a balance between the physical properties of the carbon and the energy of your bow.
Understanding Static Spine
Static spine is the measurement of how much a 29-inch shaft bends when an 880-gram (1.94 lb) weight is hung from its center. This is the number you see printed on the shaft (e.g., 340, 400, 500). A lower number means a stiffer shaft. While static spine is a consistent industry standard, it's only half of the story.
The Archer's Paradox and Dynamic Spine
Dynamic spine is how the arrow actually reacts when the bow string slams into it. Upon release, the arrow must bend to clear the riser. Factors that increase dynamic spine (making the arrow act weaker) include higher draw weight, longer arrow length, and heavier point weight. Factors that decrease dynamic spine (stiffening the arrow) include shorter shafts and lighter points.
Why 'Weak' Arrows are Dangerous
If an arrow is too weak for the bow's power, it can flex so violently that it shatters upon release, potentially driving carbon shards into the archer's hand. Even if it doesn't break, a weak arrow will 'fishtail' in flight, losing massive amounts of energy and accuracy. It is always better to be slightly over-spined (too stiff) than under-spined.
Using Manufacturer Spine Charts
Most manufacturers provide charts based on draw weight and arrow length. However, these often assume a standard 100-grain point. If you use a heavy 200-grain broadhead for high FOC, you must look at the next stiffer category on the chart. Our calculator helps bridge this gap by estimating the impact of your specific components on the required spine.
FAQ
Does draw length affect spine?
Yes. A longer arrow of the same static spine is physically more flexible. If you have a long draw length, you will almost certainly need a stiffer (lower number) spine than someone with a short draw length at the same weight.
Can I use 400 spine for a 70 lb bow?
Unlikely. For a 70 lb compound bow, a 400 spine is usually too weak unless the arrow is cut extremely short. Most 70 lb setups require 340 or 300 spine arrows for safety and accuracy.
What is 'spine indexing'?
Every carbon shaft has a 'stiff side' due to manufacturing. Spine indexing involves finding this heavy/stiff line and aligning all your fletching consistently so every arrow reacts exactly the same way when fired.