What an arrow spine number means
What an arrow spine number means: what it is and how it changes the arrow you should shoot.
Dry-fire warning
Crossbow bolt data and archery arrow spine, nock and thread data are quoted from each manufacturer's published figures, owner documentation and industry standards, cited on every page. A nock, spine or grain-weight mismatch can cause a dry-fire that damages the bow and can injure the shooter. Confirm against your own owner's manual and the arrow maker's own chart before shooting, and never drop below the stated minimum total weight.
| What it measures | How far a shaft deflects, in thousandths of an inch, under a 1.94 lb weight hung at the centre of a 28 inch span |
|---|---|
| Reading the number | A 340 spine deflects 0.340 in and is stiffer; a 500 spine deflects 0.500 in and is weaker |
| Draw weight | Higher draw weight needs a stiffer (lower-number) spine |
| Arrow length | A longer arrow flexes more, so it needs a stiffer spine |
| Point weight | A heavier point weakens dynamic spine; adjust one spine group per 25 grains of point-weight change |
Field notes
An arrow spine number is a deflection measurement, not a weight. The shaft is laid across a 28-inch span and a set weight is hung at the centre; the sag in thousandths of an inch is the spine.
That is why a smaller number is a stiffer arrow: a 340 sags 0.340 inches, a 500 sags 0.500 inches. A 340 from one brand may not match a 340 from another, so pick from the maker's own chart.
Three things drive the spine you need: more draw weight, a longer arrow and a heavier point all flex the shaft more, so all three push you toward a stiffer, lower-number spine.
Related rules and examples
Sources: Nock On Archery - Arrow Spine Chart: choosing the correct spine · Sportsman's Warehouse - Arrow Spine Charts: Compound, Recurve and Traditional