The calculator recommends an arrow spine deflection (in thousandths of an inch) and a common spine rating (300–1000). Heavier points and longer arrows require a stiffer shaft.
Recommended spine
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What is arrow spine?
Spine describes how much an arrow bends under a standard 1.94 lb (880 g) weight hung from the shaft's centre. Lower numbers mean stiffer arrows.
Too weak
A limber arrow flexes excessively at release, causing porpoising and erratic left-right drift.
Too stiff
An overly rigid arrow resists flexing around the riser, flying off target with poor clearance.
Just right
The shaft flexes evenly as it passes the riser, giving clean clearance and tight groups.
Factors that change your spine
Always re-check spine when you change equipment — these variables all matter:
- • Draw length & weight — longer draws and heavier bows need stiffer arrows.
- • Arrow length — a longer arrow acts weaker than a short one of the same rating.
- • Point weight — heavier broadheads or field points pull the shaft's dynamic spine down.
- • Bow type — compound bows with higher let-off tolerate slightly different spines than recurves.
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