How to Calculate Arrow Spine

Use the spine selection matrix: find your draw weight row and draw length column, then read the recommended static spine number. A 65-lb bow at 29-inch draw calls for a 340-spine shaft with a 100-grain point. Always verify with a paper-tune test — chart lookups ship one grade wrong about 15% of the time because they cannot account for cam aggressiveness or string weight. If the paper tear shows tail-left, drop to a weaker spine (340 → 400); tail-right means step up to a stiffer spine (400 → 340).

Arrow spine is NOT a weight rating — it is a stiffness rating. The AMO/ASTM standard measures static spine as the deflection (in thousandths of an inch) when a 1.94-pound weight is hung from the center of a shaft supported on two points 26 inches apart. A 340-spine arrow deflects 0.340 inches under that load; a 500-spine deflects 0.500 inches. LOWER spine numbers = STIFFER shafts. The 'weight' you see on a shaft label (e.g., 8.5 GPI) is the grains-per-inch mass, which is a separate spec.

Static spine is the lab measurement: 26″ span, 1.94 lb load, deflection in thousandths of an inch. Dynamic spine is how the arrow actually flexes in flight — affected by static spine PLUS point weight (heavier point weakens dynamic spine), draw weight, cam aggressiveness, arrow length (shorter = stiffer), and string weight. A 340-spine shaft with a 100-gr point behaves dynamically weaker than the same shaft with a 75-gr point. Static spine is what you buy; dynamic spine is what you tune.

Yes — shorter is STIFFER. A 340-spine shaft cut from 32″ down to 27″ behaves like a ~300-spine shaft dynamically. The carbon tube resists bending more aggressively over a shorter span. General rule: every 1″ cut off the front (point end) stiffens dynamic spine by approximately one-half spine grade. This is why you should never cut to final length until after paper-tuning at full length — over-cutting forces you to compensate with a heavier point, which then shifts FOC.

Heavier point = weaker dynamic spine. The point acts as a pendulum mass at the front of the flexing shaft — more mass swings the front more aggressively around the riser, causing more flex. Adding 25 grains of point weight (100 → 125 gr) weakens dynamic spine by approximately one-quarter spine grade. This is the fastest tuning lever: if your paper tear is tail-left (too stiff), bumping the point from 100 to 125 gr often fixes it without buying new shafts.

Easton publishes spine as the literal AMO deflection number (340 = 0.340″ deflection). Carbon Express uses the same numeric scale but labels some shafts with a 'size' code (e.g., Maxima Red 250 = 250-spine class, equivalent to ~0.250″ deflection). Gold Tip uses the deflection number but rounds to fewer buckets (500/400/340/300/250). Always cross-reference the manufacturer's deflection chart, not the marketing name — a '350' from one brand is not always the same stiffness as a '350' from another.

Start with a 340-spine shaft and a 100-grain point. The spine matrix lands on 340 for the 65–70 lb row at 29–30″ draw length. Paper-tune to verify — if the tear is tail-left (too stiff), either drop to a 400-spine or bump the point to 125 gr. If tail-right (too weak), step up to 300-spine or shorten the shaft 0.5″. Aggressive single-cam bows (e.g., Bowtech, Mathews) often need one stiffer grade than the matrix suggests because they deliver energy more sharply.

Sometimes, but it is a band-aid. A 400-spine shaft on a 70-lb bow will flex too much with a 100-gr point (tail-right tear). Adding a 150-gr point weakens the dynamic spine further, which moves the tear in the WRONG direction — you would actually need a LIGHTER point to stiffen the dynamic spine. The correct fix for a too-weak shaft is shortening it (cut-to-length) or stepping up to a 340-spine. Heavy points only help when the shaft is too STIFF, never when it is too weak.

Arrow spine is simply a rating of how STIFF the arrow shaft is, and every archer has to match it to their bow's energy or the arrow will flex incorrectly around the riser, ruining groups. You can think of arrow spine like the 'action' on a fishing rod: a 250-spine shaft is the heavy broomstick-stiff action for high-energy 80-lb compounds, a 340-spine is the medium action for 65–75-lb deer rigs, a 500-spine is the whippy light action for recurve bows and kids' compounds. The AMO lab test (1.94 lb hung over a 26-inch span, measured in thousandths of an inch deflection) is how manufacturers print the number on the shaft, but in plain terms what arrow spine means for shooting is: stiffer (lower number) spine = less bend during launch, for heavier draw weights and aggressive cams; weaker (higher number) spine = more bend during launch, for lighter draw weights and smooth recurves. If you match spine wrong (either direction by more than one grade), the arrow flexes too much or too little and clears the rest at an angle — you see tail-left / tail-right paper tears and field-point vs broadhead group splits of 4–6 inches even at 20 yards.

Yes, and the 70-lb case is the most-searched compound weight class because it sits exactly on the 340/300 spine boundary. Starting base spine for 70 lbs = 340; then adjust from there: (1) Short draw 27–28″ / 100-gr point → 400 or 340 (either works, stiffen to 340 if shooting aggressive single-cam). (2) Common 29″ / 100-gr → 340 spine (the textbook 70-lb pick, ~75% of 70-lb deer rigs run this). (3) 29″ / 125-gr or 30″ / 100-gr → 340 works, but step to 300 spine if you have an aggressive cam (Bowtech, Mathews Phase4, Elite EnVision) or a heavy 29.5″+ arrow. (4) 29–30″ / 150-gr → 300 spine, period (the 150-gr point weakens dynamic spine by ~½ grade, so you need the stiffer static to compensate). (5) If you run a smooth hybrid cam (Hoyt RX/Ventum) you can usually stay one grade weaker than the aggressive-cam row. Always verify with a single uncut test-shaft paper tune before cutting a dozen.

Use the 3-step selector: (1) find your peak draw weight row in the Spine Selection Matrix above; (2) find your draw length column; (3) read the cell — that is your recommended static spine (e.g., 60–65 lb × 28″ draw = 400 spine). The matrix assumes a 100-grain point and moderate cam; aggressive single-cam bows need one stiffer grade, smooth hybrid cams one grade weaker. Always paper-tune to verify before cutting a dozen — chart lookups are wrong about 15% of the time. For brand-specific spine charts (Easton, Carbon Express, Gold Tip, Victory, Black Eagle) and a 70-lb decision tree, see the Arrow Spine Chart page; this guide teaches the calculate-and-tune math behind the number rather than just the lookup.

Choosing and determining arrow spine is the same decision looked at from two angles: 'how to choose arrow spine' is the shopping question (which number to buy), 'how to determine arrow spine' is the verification question (did I land right). The method: (1) Determine your peak draw weight with a bow scale (do not trust the limb label — it can be ±3 lb). (2) Measure your draw length (wingspan ÷ 2.5). (3) Read the Spine Selection Matrix cell at that draw weight × draw length to get the static spine starting number. (4) Apply three corrections — aggressive cam → one grade stiffer; point weight over 100 gr → one grade stiffer; draw length over 30″ → one grade stiffer. (5) Buy ONE test shaft at that spine, paper-tune it uncut, and only cut a full dozen if the tear is clean. That five-step method is how you determine spine without guessing — the matrix gives you the candidate, the corrections narrow it, and the paper tune confirms it.

Yes — traditional bows (longbow, one-piece recurve, horsebow) need a different spine logic than compounds, and it comes down to two things: finger release and arrow length. (1) A finger release loads the arrow far more gently than a mechanical release — there is no hard cam stop, so the shaft flexes around the riser in a softer, longer parabola. The practical effect: at a given draw weight, a traditional bow wants a spine ONE GRADE WEAKER than the same-poundage compound. A 45 lb longbow lands around a 500 spine where a 45 lb compound would want a 600. (2) Traditional arrows are often shot full-length (cut to the bow's draw length plus 1.5–3″ overhang past the rest), and longer arrows weaken dynamic spine — so the matrix's traditional column already bakes in both corrections. (3) If you shoot wood or bamboo-carbon traditional shafts (3Rivers / GrizzlyStik / Port Orford cedar), the spine scale is DIFFERENT — those use a 5/16″ dowel weight rating (150 / 190 / 200) instead of the AMO numeric spine, and a wood '150' is roughly equivalent to a carbon 500. The compound-vs-recurve-vs-traditional spine matrix on the Arrow Spine Chart page handles the AMO-numeric side; for wood-shaft traditional spine use that brand's dowel-weight table, not the carbon matrix.