Arrow Spine: Stiffness, Measurement & Selection

Arrow spine is the measure of an arrow shaft's stiffness. When you release a bowstring, the string pushes the nock forward and the shaft bends around the riser before straightening in flight — a phenomenon known as archer's paradox and its effect on arrow flight path. If the spine is correct, the arrow flexes just enough to clear the bow cleanly. Too much or too little stiffness causes erratic flight and off-target hits.

How Arrow Spine Is Measured

The industry standard for arrow spine stiffness is the static spine test defined by ASTM F2031-05. An 880-gram (1.94 lb) weight is suspended from the center of a 29-inch shaft supported at two points 28 inches apart. The amount the shaft deflects in inches is its spine rating. A lower number means a stiffer arrow — a 330-spine shaft is stiffer than a 500-spine shaft. Common ratings are 300, 400, 500, and 600.

Aluminum arrows use a different system. A four-digit code describes the shaft's outside diameter and wall thickness — for example, a 2514 shaft is approximately 25/64 inch in diameter with a 0.014-inch wall. Both a larger diameter and a thicker wall produce a stiffer shaft suited to higher draw weights.

Static Spine vs. Dynamic Spine

Static spine is the controlled lab number stamped on a shaft. Dynamic spine describes how that same arrow actually flexes when shot from your specific bow, accounting for draw weight, arrow length, point weight, release type, and string material. Two archers shooting identical static-spine shafts can see completely different behavior depending on their setup. For a deeper look at how shafts flex in flight, see how arrow flex affects accuracy.

The Three Key Selection Variables

• Draw weight: Higher poundage requires a stiffer shaft.
• Arrow length: Longer arrows behave weaker — a 30-inch arrow at 70 lb may need a 250 spine, while cutting it to 28 inches could call for a 340. For more on how length is measured, see understanding draw length in archery.
• Point weight: Heavier points weaken effective spine; lighter points stiffen it. Manufacturer spine charts assume a 100-grain point as the baseline.

If you're selecting shafts for a compound setup, matching the carbon arrow shaft specifications to your bow's output is the most reliable starting point. And if your groups are drifting consistently to one side, it may be worth checking whether you're dealing with the effects of an arrow that is too stiff for your setup.

Competition archers often use a static spine tester to match shafts within a set for maximum consistency — small shaft-to-shaft variation in bow arrow spine can shift impact point across a long-distance end.