Arrow Spine Tester

An arrow spine tester measures a shaft's stiffness by supporting it on two points 26 inches apart and hanging a 1.94-pound weight at the center — the AMO/ASTM static spine standard. The distance the shaft sags, read in thousandths of an inch, IS the spine number: a shaft that sags 0.340″ is a 340, one that sags 0.500″ is a 500. Lower numbers mean stiffer shafts. So 'how is arrow spine measured' has one answer worldwide: 26-inch span, 1.94 lb center load, deflection in thousandths of an inch. Every commercial tester — RAM-style dial gauges, Arrow Inspector combos, digital load-cell units — automates that same measurement.

Build a two-point rig: two straight-edged supports (hardwood blocks or sawhorses) exactly 26 inches apart and level, a cord loop at the arrow's midpoint, and a 1.94 lb (880 g) hanging weight — a water jug filled to 880 g works. Lay the shaft across, hang the weight at center for 5 seconds, and measure the sag with a steel rule in thousandths of an inch. Expect about ±0.015″ accuracy — roughly half a spine grade — which is plenty for identifying unmarked shafts or confirming a spine range, but not tight enough to pair-match a dozen. Details, dimensions, and the wood-shaft variation are in the DIY build section above.

The standard load is exactly 1.94 pounds (880 grams) hung at the center of a 26-inch span. The figure dates to the original AMO (Archery Manufacturers Organization) test and was retained by ASTM F2031 for carbon shafts. It is not calibrated to anything physical — it is simply the agreed constant that makes every spine number comparable: 400 from one brand and 400 from another both deflect 0.400″ under 1.94 lb across 26 inches. Using a 2-pound weight instead over-reads flexibility by about 3%, which is enough to push a true 340 into a 350 reading.

For most home arrow builders, a mechanical dial-gauge tester (RAM-style, $95–140) is the best balance of cost and precision: ±0.005″ accuracy, direct spine-number readout, and no batteries. If you also want to check straightness, the Arrow Inspector-style combo adds a roller cradle for spin-testing in the same bench tool for around $70–110. Digital load-cell testers ($220–450) add 0.001″ resolution and target-window readouts — worthwhile for tournament builders sorting dozens to a 0.004″ band. Under $20, the DIY two-point rig identifies spine ranges within half a grade.

Yes — and the conversion is trivial: the deflection reading in thousandths of an inch equals the spine number. 0.340″ deflection = 340 spine; 0.400″ = 400; 0.500″ = 500. The chart above lists the full ladder from 200 to 700 with the draw-weight class each grade serves. Two special cases: wood shafts are graded on the same deflection scale but sold with two-number ranges (a '55/70' cedar deflects between 0.550″ and 0.700″ depending on grain axis), and some digital testers display 'adjusted deflection' when you enter a different span — always test at the true 26″ span for chart comparability.

Spine indexing (also called spine aligning) means finding each shaft's stiffest axis and orienting that axis the same way — usually toward the bow or toward the sight window — on every arrow. Carbon layups and wood grain are never perfectly uniform, so every shaft has a hard side and a soft side; a dial tester shows the difference as a 0.002–0.010″ deflection spread between axes. The workflow: measure each shaft, rotate 90°, measure again, mark the stiff axis on the nock with a paint pen, then install all nocks with marks aligned. Indexed arrows group measurably tighter at 40+ yards — the effect is worth roughly 3–5 yards of effective forgiveness on a tuned bow.

Spin testing checks straightness, not stiffness. Roll the shaft on a flat table (a bent arrow clacks and stops with the heavy side down), or use an Arrow Inspector-style roller cradle: spin the shaft while watching the tip against a fixed point — a straight shaft traces nothing, a bent one wobbles visibly. Compare the wobble to your maker's straightness spec: ±0.006″ is standard, ±0.003″ match-grade, ±0.001–0.002″ competition. Spin-test new arrows before fletching, after every steel-target impact, and after any hard hit — a bent shaft is unfixable and will group unpredictably no matter how well it is spined.

Same 26-inch span and 1.94 lb load, but three differences matter. First, wood deflects differently along the grain versus across it: test each shaft on both axes and record the stiffest — that axis faces the bow at assembly. Second, wood is graded in ranges, not single numbers: a '55/70' shaft deflects anywhere between 0.550″ and 0.700″, which is why sorting a batch of cedar by actual measured deflection improves traditional arrows far more than any other kind. Third, weigh each shaft: matched spine matched weight (±2 grains per dozen) is what makes a wood dozen fly like matched arrows.

A spine test kit bundles the tester with the accessories that make dozen-sorting practical: a calibrated weight set (so you can verify the 1.94 lb standard), a dial or digital indicator, V-block cradles that double for spin testing, paint pens for marking nocks, and a conversion card. You do not need a kit to test one shaft — the DIY rig answers that for $15. A kit earns its cost when you start sorting dozens, bare-shaft tuning, or building for multiple bows: the cradle and calibrated weights are what separate a repeatable measurement from a guess.

Victory's factory spine-aligned arrows are laser-sorted so the stiff axis is factory-marked (usually with an aligned nock or logo), saving you the measure-and-rotate pass. DIY indexing does the same thing at the bench with any brand's shafts: measure deflection, rotate 90°, re-measure, mark the stiff side, orient the nocks. The accuracy is comparable when done with a dial or digital tester — the factory's advantage is speed and consistency across large batches. If you shoot standard-grade shafts, ten minutes of DIY indexing per dozen delivers most of the same grouping benefit as buying factory-aligned premium shafts.

The tester's static reading never changes — static spine is a property of the bare shaft. What changes is dynamic spine, how the shaft behaves on your bow: cutting 1 inch stiffens the flight behavior by about half a grade, and every 50 grains of point weight weakens it by about half a grade. Pro builders exploit this: they test a dozen, cut to length, then re-test and re-sort because cutting removes material unevenly. The calculator-side of that relationship — static to dynamic corrections by draw weight, point weight, and cam type — is on the Arrow Spine Calculator; this page measures the physical shaft itself.