Find the correct arrow spine for your bow type, draw weight, draw length, and point weight.
Arrow spine is a measurement of how much an arrow shaft flexes under a standard load. The Archery Trade Association (ATA) standard test suspends an arrow at two points 28 inches apart and hangs a 1.94-pound weight from the center. The deflection in inches is the spine rating — so a 0.400-inch deflection gives you a "400 spine" arrow. Counterintuitively, a lower spine number means a stiffer arrow: a 300 spine is significantly stiffer than a 500 spine. Selecting the right spine for your setup is critical because an improperly spined arrow will flex erratically through the shot cycle, producing inconsistent groups, erratic broadhead flight, and reduced penetration on game.
Static spine is the lab measurement described above — it is printed on the arrow box and used as a baseline for selection. Dynamic spine is how the arrow actually behaves during your specific shot: the real-world flex that happens in the milliseconds after the release. Dynamic spine is affected by your draw weight, draw length, bow cam aggressiveness, release type, point weight, and arrow length. Two archers using the same static spine arrows can experience very different dynamic flight. This is why spine charts include multiple variables, and why final tuning — paper tuning and walk-back tuning — is always necessary after building arrows to a chart recommendation.
The right spine depends on your adjusted draw weight, which accounts for your actual draw weight, bow type, release style, arrow length, and point weight. Start with your bow's peak draw weight. Add 5 lbs if you shoot fingers instead of a mechanical release. Add 5 lbs if you shoot a recurve, or 10 lbs for a traditional longbow. Then adjust for arrow length — longer arrows flex more, so add weight if your arrow is over 28 inches. Match the resulting adjusted weight to a spine chart to get your baseline recommendation. From there, paper tune to confirm flight and adjust one step stiffer or weaker as needed.
A weak-spined arrow flexes excessively as it leaves the bow, creating a fishtailing or porpoising flight pattern that is clearly visible at longer distances. Groups open up significantly past 20 yards. With broadheads, a weak spine is especially punishing because a fixed-blade head acts like an airplane wing — any lateral flex steers the arrow off course, leading to missed shots or poor penetration angles on game. Signs of weak spine in paper tuning include a tail-left tear (for right-handed shooters). If you are seeing this, try dropping one spine step stiffer (lower number) before making other adjustments.
Yes, significantly. Cutting an arrow shorter makes it effectively stiffer; leaving it longer makes it effectively weaker. This is why arrow length is one of the primary inputs for spine selection. As a rule of thumb, every inch of additional length beyond 28 inches requires approximately one step weaker on the spine chart (higher number). Conversely, cutting arrows shorter can stiffen them up if you have already purchased a batch. Many archers buy arrows slightly long, paper tune first, then cut to their final length once they know their setup is flying correctly.
Broadheads amplify any arrow flight imperfection because a large cutting head creates aerodynamic drag and steering force that field tips do not. Most experienced bowhunters use arrows one step stiffer than their field-point setup when hunting with fixed-blade broadheads. Mechanical broadheads that open on impact behave more like field tips in flight and generally require less spine adjustment, but they still fly differently than field points and should be tuned at hunting distances before the season. Always do a broadhead tune — shoot a broadhead and a field-point side by side at 20–40 yards to confirm they impact at the same point.
Start with paper tuning: shoot through a sheet of paper stretched taut in a frame from about 6 feet away. A perfect bullet hole means your arrow is leaving the bow straight. A tail-high tear indicates too much nocking point height or weak spine. A tail-left tear (for right-handed shooters) indicates weak spine or insufficient rest offset. Once you achieve a bullet hole through paper, move to walk-back tuning: shoot at a vertical line on a target from 10, 20, 30, and 40 yards. If your arrows drift left or right as distance increases, your rest needs micro-adjustment. Only after paper and walk-back tuning are you ready to confirm broadhead impact.