Arrow spine is the measure of a shaft's stiffness — specifically how much it deflects under a standard load. Matching spine to your bow setup is one of the foundational decisions in archery. Get it right and your arrows fly straight and recover cleanly. Get it wrong and you'll fight inconsistent groups, erratic broadhead flight, and performance that never quite comes together no matter how much you adjust your rest or sight.
Spine is expressed as deflection in thousandths of an inch. A 340 spine deflects 0.340" under a standard weight load. A 400 spine deflects 0.400". This means a lower spine number = a stiffer shaft. A 340 is stiffer than a 400, which is stiffer than a 500. This trips up a lot of new archers — stiffer arrows have smaller numbers.
Common spine ratings for compound hunting setups: 250, 300, 340, 350, 400, 500, 600. The range you'll likely be in as a typical bowhunter at 55–72 lb is 300–500.
The biggest driver. More draw weight means more force at release, which means the arrow deflects more dramatically. Higher draw weight generally requires a stiffer spine (lower number) to recover cleanly. At 50–58 lb, most setups run a 400–500. At 59–65 lb, 340–400. At 66–72 lb, 300–340. See the full arrow spine chart for your exact draw weight and arrow length.
Longer arrows flex more than shorter ones — they act weaker (higher effective spine number). If you cut 1 inch off your arrows, you effectively stiffen them by roughly one spine rating. This is why manufacturers publish charts based on arrow length, not just draw length. Always use the length of the arrow after cutting, not the raw shaft length.
Heavier points add front-of-centre weight, which increases the arrow's tendency to flex at release. 125 gr broadheads require a stiffer spine than 100 gr field points used at the same draw weight and length. A good rule: for every 25 gr increase in point weight, go one step stiffer in spine selection.
The spine number on a shaft is its static spine — measured at rest under a fixed load. Dynamic spine is how the arrow actually behaves during the shot, which is affected by release style, cam timing, nock point position, and arrow rest type. Two setups with identical static spine arrows can behave differently because of dynamic differences.
This is why paper tuning matters — it reveals how the arrow is actually flying through dynamic spine effects, not just what the chart predicts. Use the chart to start, then paper tune to confirm.
If you're between two spine options, choose the stiffer one (lower number). An arrow that's slightly too stiff forgiving more than one that's too weak. Underspined arrows are responsible for the vast majority of broadhead flight problems and erratic field-point groups. Overspined arrows still fly well with field points at hunting distances — the error shows up less dramatically.
Switching to heavier broadheads before season? Re-check your spine selection. Moving from 100 gr to 125 gr mechanical broadheads is often fine, but 125 gr fixed-blades on an arrow that was already borderline-weak can cause serious flight issues. When in doubt, go one step stiffer.
After selecting a spine, shoot through a sheet of paper stretched tight in a frame, from 6–8 feet away. A bullet hole (round entry with clean fletching slits) confirms the arrow is flying straight. Any tear tells you there's a problem. Left/right tears typically indicate spine issues or rest position. Up/down tears point to nocking point height. See paper tuning vs walk-back tuning for the full diagnostic process.