Arrow Spine Calculator
Uses Axial Load Ratio — a physics-based method that gives a single number for your exact setup, not a chart range. Check whether your current arrows are right, or find the right spine before you buy. Full methodology →
How Axial Load Ratio works
ALR = Applied Force ÷ Shaft Buckling Force
Every carbon shaft has a static buckling point — the compressive load at which it begins to collapse sideways. A typical 400 spine arrow hits that threshold around 27.9 lb of sustained load. Your bow imposes significantly more than that — yet the arrow doesn’t explode off the string. The arrow doesn't shatter because the impulse is brief. But it does flex, dramatically, and whether that flex is consistent is what determines your groups.
Buckling force comes from spine rating and effective column length — the free carbon from nock groove to where the insert ends inside the shaft. The steel insert reinforces the front section; it does not buckle. Stiffer shafts and shorter free columns resist buckling more. Applied force is the compressive impulse the bow actually delivers: draw weight scaled by cam aggressiveness and point weight. An aggressive cam at 70 lb stresses the shaft harder than a mild target cam at the same poundage. Heavier points add front-end inertia that resists nock acceleration, which is why every spine chart recommends stiffer shafts for heavier tips. ALR captures all of this in a single number.
ALR bands
| ALR | What it means |
|---|---|
| < 1.2 | Very stiff — impractical at most compound poundages. Shafts stiff enough to land here carry high GPI; keeping total weight manageable forces a lighter point, which tanks FOC. |
| 1.2 – 1.5 | Competition. Tight, consistent oscillation. Indoor target and precision 3D. Where lighter draw weights and shorter arrows land on a precisely matched setup. |
| 1.5 – 1.8 | Hunting / High FOC. Controlled flex, forgiving to tune. Where most correctly-matched hunting builds land. High-FOC builds naturally land here when point weight is raised for penetration. |
| 1.8 – 2.2 | Recreational. Larger oscillation. Groups widen with distance. Common with long arrows or heavy broadheads on a softer shaft. |
| 2.2 – 2.6 | Too weak. Accuracy noticeably affected. Go stiffer, cut shorter, or reduce point weight. |
| 2.6+ | Dangerous. Risk of shaft failure. Do not shoot this combination. |
What moves ALR
- Higher draw weight → raises ALR (more applied force)
- More aggressive cam → raises ALR (sharper impulse delivery)
- Heavier point weight → raises ALR (more front-end inertia)
- Longer arrow → raises ALR, and this is the variable with the most leverage. Arrow length appears squared in the buckling formula — two extra inches costs more structural resistance than stepping an entire spine class
- Stiffer spine (lower number) → lowers ALR (higher buckling resistance)
- Shorter arrow → lowers ALR (shorter column is structurally stiffer)