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Arrow spine explained

Spine is the stiffness of an arrow shaft, and matching it to the bow is what makes an arrow fly straight rather than fishtail.

Static and dynamic spine

An arrow's static spine is measured by hanging a set weight from the middle of a shaft supported near its ends and reading the deflection in thousandths of an inch. A shaft that sags 0.400 inches is called a 400 spine; a stiffer 0.340 shaft is a 340. A lower number means a stiffer arrow.

Dynamic spine is how the arrow actually behaves when the string pushes it: the shaft buckles and flexes around the bow before recovering. Point weight, shaft length and the bow's energy all change dynamic spine, which is why a spine chart reads across draw weight and arrow length, not just poundage.

Why the archer's paradox matters

A finger-released recurve arrow has to bend around the riser and recover in flight, an effect called the archer's paradox. If the spine is wrong, the arrow recovers at the wrong moment and leaves the bow off line.

Too stiff and the arrow flies left of aim for a right-handed archer; too weak and it flies right, with the opposite for a left-hander. A mechanical release on a compound reduces the sideways flex, so compound spine selection is a little more forgiving but still follows the same chart.

Reading a spine chart

Every shaft maker publishes a chart that takes the bow's draw weight and the arrow's finished length and points to the correct spine. Add point weight, and the effective spine weakens, so a heavier point can push a setup to the next stiffer shaft.

Cutting an arrow shorter stiffens it; leaving it long weakens it. Because these variables interact, the safe approach is to read the chart for your exact draw weight, draw length and point weight rather than guess, which is what the fitment tool does from the maker's own data.

Related

Sources: Easton: understanding arrow spine · Wikipedia: Archer's paradox