Arrow Spine: How to Match Spine to Draw Weight and Arrow Length
Spine is the number that decides whether your broadhead flies where you point it. Get it wrong and no rest tune, no cam timing, and no fletching helical will save the group. This page covers what static spine numbers mean, how draw weight and arrow length shift the effective spine, and the practical spine your bow probably wants.
What the spine number actually is
Static spine is the deflection in inches (times a thousand) of a 29 inch shaft supported on 28 inch centers with an 880 gram weight hung dead center. A 340 spine deflects .340 inches. Smaller number, stiffer shaft. Every carbon manufacturer uses this ASTM/ATA test, so a Gold Tip 340 and an Easton 340 are directly comparable on paper. In the field the wall thickness and stiffness taper still shift how the arrow reacts, which is why paper tune matters.
Baseline spine chart for compound hunters
| Draw weight | 27 in arrow | 28 in arrow | 29 in arrow | 30 in arrow |
|---|---|---|---|---|
| 50 to 55 lb | 400 | 400 | 340 | 340 |
| 55 to 60 lb | 400 | 340 | 340 | 340 |
| 60 to 65 lb | 340 | 340 | 340 | 300 |
| 65 to 70 lb | 340 | 340 | 300 | 300 |
| 70 to 75 lb | 340 | 300 | 300 | 250 |
| 75 to 80 lb | 300 | 300 | 250 | 250 |
Numbers assume a 100 grain field point. Add 25 grains up front and you need to jump one spine stiffer. Cut half an inch off and you also stiffen the dynamic spine by roughly one spine group.
Dynamic spine: what actually flies
Dynamic spine is what the arrow does when the string dumps energy into it. Point weight, insert weight, nock weight, and cam aggression all bend that shaft in real time. The two biggest levers are point weight (heavier point weakens the arrow) and arrow length (longer arrow weakens it). A 340 shaft at 29 inches with a 100 grain point is usually a safe start for a 65 pound bow; the same 340 at 31 inches with a 125 grain broadhead will paper tune weak.
Broadhead flight test
Sight in with field points at 20 yards, then shoot a fixed blade broadhead from the same pin. Broadhead left of field point (right handed shooter) usually means the arrow is too weak. Right of field point usually means too stiff. Shift rest a hair, or shift point weight 25 grains, and reshoot. Do not fix a spine problem with the rest alone.