Achieving the perfect arrow flight depends on matching your arrow’s stiffness to your bow’s power. Use our 3rivers spine calculator to determine the optimal dynamic spine for your setup, ensuring maximum accuracy and consistency in the field.
3rivers spine calculator
3rivers spine calculator Formula
Formula Reference: 3Rivers Archery Charts | Easton Shaft Guide
Variables Explained
- Actual Draw Weight (D): The measured pull of the bow at your specific draw length.
- Arrow Length (L): Measured from the nock throat to the end of the shaft.
- Point Weight (W): The total weight of the field point or broadhead in grains.
- Dynamic Spine (S): The actual stiffness required for the arrow to flex properly during flight.
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What is the 3rivers spine calculator?
The 3rivers spine calculator is a specialized tool used by traditional and modern archers to determine the “Dynamic Spine” of an arrow. Unlike static spine, which measures a shaft’s stiffness at rest, dynamic spine accounts for how the arrow reacts to the energy released by the bowstring.
By inputting variables like draw weight and point weight, archers can avoid “Archer’s Paradox” issues, where an arrow is too stiff or too weak to clear the bow riser effectively, leading to poor grouping and erratic flight.
How to Calculate (Example)
- Start with your base draw weight (e.g., 50 lbs).
- For every inch of arrow length over 28″, add 5 lbs of dynamic spine.
- For every 25 grains of point weight over 125 grains, add roughly 3-5 lbs of spine requirement.
- Adjust for string material (FastFlight vs. Dacron) if necessary.
Frequently Asked Questions (FAQ)
What happens if my spine is too weak? The arrow will whip excessively and likely impact to the right (for right-handed shooters).
Is point weight important? Yes, heavier points increase the “leverage” on the shaft, making it behave as if it were weaker.
Should I measure draw weight at 28 inches? No, you must use the weight at your specific draw length for accuracy.
Does fletching affect spine? Fletching affects drag and stabilization, but the internal spine requirement is primarily driven by the shaft and point.