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Static spine is the deflection in inches when a 1.94 lb weight hangs from center of a 28-inch supported arrow. Lower deflection number = stiffer arrow (e.g., 340 spine is stiffer than 500).
Spine rating = deflection × 1000 / arrow length factorHigher draw weight requires stiffer (lower spine number) arrows. Compound bows with high let-off and aggressive cams may need spine adjustment from traditional charts.
Peak draw weight → base spine from manufacturer chartHeavier broadheads and field points flex the arrow more at launch, requiring stiffer spine. Add ~5 spine units (stiffer) per 25 grains above 100 gr reference point.
Adjust 1 spine step stiffer per ~25 gr point weight increaseUpdated: July 2026
Chart recommends 340 spine carbon arrow. At 29-inch length, drop to 300 spine (stiffer). With 125 gr broadhead, stiffen further to 300 or 250 spine.
40 lb recurve, 31-inch arrow, 120 gr point: select 500–550 spine aluminum or 400–450 carbon per Easton chart. Finger release adds slight side load vs release aid.
Light draw needs weak spine: 600–700 spine carbon at 26 inches with 85 gr point. Overspined arrows fly left (right-hand bow); underspined fly right.
Measure peak draw weight with a scale on your specific bow setup. Limbs rated 50 lb may pull 52 lb at your draw length — spine charts need actual measured weight.
A 150 gr broadhead requires a stiffer spine than 100 gr field point at the same draw weight. Always include total point weight (insert + tip) in spine calculation.
Arrow spine — stiffness under load — must match bow draw weight, arrow length, and point weight for accurate flight. This calculator recommends spine deflection (static spine rating) for target and hunting setups.