Matteo Campagna
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Spine measured at rest is not enough to predict how the arrow behaves when shot

6 studies

ForContextAgainst2000200520102015202020252001, for2012, context2012, for2014, context2019, context2025, for
Each circle is a study: its position gives the year, its row whether it is for, against or context, its size how many participants it measured. Light circles are studies on other populations or without archers.

The models have said consistently for twenty-five years that the number on the packet does not tell the whole story; a test that shoots the same shafts and compares the static ranking with the ranking when shot does not exist yet. Static spine is a deflection: 880 grams hung from the middle of a shaft resting on supports 711 mm apart, and a 380 bends by 9.65 mm. In 2001 a model of the bow and arrow system finds that the static test gives critical forces a third lower than those the arrow meets on the string, that two shafts whose static stiffness differs by a ratio of 1.38 behave when shot as if the ratio were 1.47, and that the rule setting draw weight at two and a half times the critical force picks the shaft the dynamic analysis rejects, because neither mass nor the compression from the string enters the static test. In 2012 a second model calculates that an X10 550, 4 per cent stiffer than a ProTour 570, needs less draw weight to leave cleanly, because the two shafts have different profiles, and in 2025 a university project reaches the same conclusion for all tapered shafts. Laboratory measurements confirm only the starting point: stiffness is not uniform along the shaft, four arrow models shot from the same bow flex differently, and a rig that shakes shafts instead of loading them cannot tell a 500 from a 600, but none of these tests actually compares the static measurement with behaviour when shot. For now, then, the statement rests on models.

Also phrased as: "static spine and dynamic spine"; "the spine on the packet is not enough"; "dynamic spine"; "the arrow behaves differently from what its spine says"; "static spine test"

In practice

It is said that the spine printed on the arrow is only a starting point, and that the "dynamic" spine, the one that really counts, can only be found by shooting, with bare shafts or tuning.

Evidence

  • 2001
    For Model Published later

    Model of the bow and arrow system, no arrow shot: the critical forces of the static bending test come out a third lower than those the arrow meets in the bow and arrow system during the shot. For two 660 mm shafts on a 202 newton Olympic bow, the second is 1.38 times stiffer than the first in the static test (EJ 5.12 against 3.72 Nm²), but the ratio of the squared periods is 1.47 in the bow and arrow system and 1.23 in free oscillation; the author attributes the difference to the longitudinal compressive force.

    Lateral deflection of archery arrows · Zanevskyy

  • 2001
    For Model Published later

    In the same example the classic rule, draw weight at two and a half times the static critical force, points to the first shaft (211 newtons against a 202 newton bow), while the dynamic analysis points to the second (290 newtons): the first leaves after three quarters of a cycle and its tail brushes the riser, the second completes the cycle and bends around it. In the static test spine does not depend on the arrow's mass, when shot it does.

    Lateral deflection of archery arrows · Zanevskyy

  • 2012
    Context Equipment tests Published later

    Three Easton ProTour shafts measured with a laser vibrometer and two numerical models: diameter, mass and bending stiffness vary along the 710 mm of the shaft, and stiffness rises at both ends because of the nock insert and the point shank, more at the front than at the back. The static test, by contrast, hangs 880 grams from the middle of two supports 711 mm apart and reads a single deflection: the ProTour 380 bends by 9.652 mm, that is 380 thousandths of an inch. The paper does not compare static spine with behaviour when shot.

    Modelling the three-dimensional vibration of composite archery arrows under free–free boundary conditions · Marianne Rieckmann, James L Park, John Codrington, Ben Cazzolato

  • 2012
    For Model Published later

    In the recurve model an X10 550 shaft needs a draw weight of 175.5 newtons to leave without rotation and a ProTour 570 needs 180, even though the X10 is 4 per cent stiffer: the stiffness and mass profiles along the shaft matter, and the author warns that shafts of the same nominal spine but of different types should not be assumed to behave alike. The more flexible rear of the X10 also gives it more clearance past the button, 11 mm against 9.

    Arrow behaviour in the lateral plane during and immediately following the power stroke of a recurve archery bow · James L Park

  • 2014
    Context Equipment tests Published later

    Three frames crossed by laser sheets in front of a bow fixed on a stand measure the arrow's flex in flight: four arrow types 75.4 cm long, six shots each, give on average 4.942, 5.527, 4.693 and 4.634 mm at 80 cm from the bow, a maximum spread of 0.893 mm. The spine of the four arrows is not reported, and the journal is regional.

    Measurement of Archer's Paradox Size using Multiple Frames · Jonggeun Kim, Yeongsang Jeong, Moonjae Song, Sungshin Kim

  • 2019
    Context Equipment tests Published later

    Course project on arXiv, not peer reviewed: shafts clamped at both ends and shaken at the centre by an actuator driven with a random signal. In carbon the measured frequency lies between 4 per cent below and 2 per cent above the value expected from factory data; in aluminium the 2219 was expected at 31.9 hertz and measures 40.8. Carbon 500 gives 34.5 hertz and 486 newtons per metre, the 600 gives 35.3 and 491, against expected stiffnesses of 680 and 566: two neighbouring spines cannot be told apart. Within a single spine stiffness varies by 14 to 22 per cent, and the authors cannot say how much comes from manufacturing and how much from the rig, which is not calibrated.

    Dynamic Characterization of Arrows through Stochastic Perturbation · Ryan Fish, Youzhi Liang, Kyle Saleeby, Jonathan Spirnak, Mingxiu Sun, Xiang Zhang

  • 2025
    For Model Published later

    Park's model rewritten in Python and solved by finite differences to build shaft selection charts: in non-parallel shafts such as the X10, the dynamic spine of a statically stiff shaft can turn out weaker than that of a statically weak one. The reference static spine is the deflection in thousandths of an inch under 8.63 newtons over a 28 inch span, under the ASTM F2031 standard. A university project in a student journal, not checked by shooting.

    Modelling Arrow Dynamics for Optimised Recurve Archery Arrow Selection · Alex Williams

Status history

  1. 2001 Transferred A mathematical model of the bow and arrow system raises the question and answers it in three steps: the static test gives critical forces a third lower than those the arrow meets on the string; two shafts whose static stiffness differs by a ratio of 1.38 behave when shot as if the ratio were 1.47; and in the worked example the rule setting draw weight at two and a half times the critical force picks the shaft that the dynamic analysis rejects. No arrow was shot to check it, and a model on its own leaves the statement at transferred.
  2. 2012 Transferred The first laboratory measurement close to the question arrives, together with a second model. The measurement, with a laser vibrometer on three competition shafts, shows that stiffness is not uniform along the shaft, because it rises at both ends where the inserts sit, while the static test reads a single deflection at the centre; it does not, however, compare static spine with behaviour when shot. The model calculates that an X10 550, 4 per cent stiffer than a ProTour 570, needs less draw weight to leave cleanly. Two models in agreement and no direct test: transferred.
  3. 2014 Transferred A second measurement, with three laser frames in front of a fixed bow, finds that four arrow models flex differently in flight, from 4.63 to 5.53 mm at 80 cm from the bow. The spine of the four arrows is not reported, so the test cannot say whether the difference is the one static spine predicted: it remains context, and the status does not change.
  4. 2019 Transferred A university course project builds a rig that shakes shafts instead of loading them and estimates their frequency and stiffness. In carbon the frequency falls within a few per cent of the value expected from the factory data, in aluminium it exceeds it by up to 28 per cent, and a 500 and a 600 in carbon cannot be told apart. The rig is not calibrated, the variability within a single spine reaches 22 per cent, and no arrow was shot: the result is mixed and remains context.
  5. 2025 Transferred A third model, which rewrites the 2012 one to build shaft selection charts, concludes that in tapered shafts an arrow that is stiffer statically can behave like a softer one when shot. It is a university project published in a student journal and not checked by shooting: three models in agreement over twenty-five years, and still no test comparing the two measures on the same shafts: transferred.

Related

Related articles

Research lines

Cite
APA

Campagna, M. (2026). Spine measured at rest is not enough to predict how the arrow behaves when shot (Version 1). In Evidence Registry. Retrieved 16 October 2026, from https://matteocampagna.com/en/registry/equipment/static-and-dynamic-spine

BibTeX
@misc{campagna2026_spinestaticodinamico,
  author = {Campagna, Matteo},
  title = {Spine measured at rest is not enough to predict how the arrow behaves when shot},
  year = {2026},
  howpublished = {Evidence Registry, matteocampagna.com},
  url = {https://matteocampagna.com/en/registry/equipment/static-and-dynamic-spine},
  note = {Version 1; accessed 2026-10-16}
}

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