Arrow spine has to be matched to draw weight and arrow length

The summary today

The rule holds in every model that has looked at it, each reaching it by a different route; what is missing is the proof on the target. The first, from 1990, checked against high-speed films of a Polish national-team archer and against a shooting machine, finds that a shaft of suitable stiffness leaves the string with the nock almost still sideways, while one that is too weak or too stiff leaves it with a lateral velocity, and it derives a nomogram within which the equipment of eighteen of the twenty-two best Polish archers fell. In 1997 a second model, compared with the same films, confirms the rule in both directions, since the shaft that is too weak slaps against the button and the one that is too stiff leaves the string with a stronger lateral jolt on the nock, but it questions the explanation: even with the suitable shaft the nock does not stay still sideways for long before the arrow leaves, so the rule stands while the reason for it remains open. The models of 2001 and 2012 reach the same rule and measure its window: a more powerful bow takes a stiffer and heavier arrow, changing spine by one step calls for about eleven newtons of draw weight while a bow can only be adjusted within about ten per cent of its nominal weight, so at a given draw length only one or two spines can be tuned well, and ten millimetres of shaft are worth four and a half newtons. In 2025 come two further confirmations, still indirect: a new model rebuilds the manufacturers' charts at a finer half-inch step, and on the shooting machine, with a right-handed bow, the stiffer shafts land to the left. The part on archers is still the coincidence with the equipment of those twenty-two Poles, who had in all likelihood chosen their shafts from the manufacturers' charts, and nobody has yet measured the grouping of archers with suitable and unsuitable spine.

Also phrased as: "how to choose arrow spine"; "spine depends on draw weight"; "arrow too stiff or too weak"; "arrow spine chart"; "how much arrow length affects spine"

Evidence

• 1990
  For Archers Published later

  A mathematical model checked against high-speed films, 1,000 to 2,500 frames per second, of a Polish national-team archer, plus tests with a shooting machine at 30 metres. In the simulation, with the suitable shaft the nock stays below two per cent of the reference lateral displacement for two milliseconds before it leaves the string, that is for fifteen per cent of the whole contact phase, and this makes the departure tolerant of small variations; shafts that are too weak or too stiff leave the string with a lateral velocity. The parameters that count most are the outside diameter and the wall thickness. The resulting nomogram covers eight hundred combinations of draw weight and draw length and matched the equipment of eighteen of the twenty-two best Polish archers, with four outside the band. On the machine it is also observed that a shaft which is too stiff slips sideways as it leaves the bow.

  Experimental and theoretical research in archery · Roman Pekalski

• 1997
  For Model Published later

  Finite-difference model of the arrow on a recurve, compared with the high-speed films used by Pekalski: in a sensitivity analysis on three shafts with the same wall, the softer and lighter 1514 slaps against the button, the standard 1714 clears it, and the stiffer and heavier 2114 clears the grip with more room but leaves the string with a larger jump in the lateral force on the nock. The authors do, however, question the 1990 mechanism: even with the standard shaft they find no long stretch before the arrow leaves the string in which the nock stays close to the plane of the bow.

  On the mechanics of the arrow: Archer's Paradox · B.W. Kooi, J.A. Sparenberg

• 1998
  Context Model Published later

  A fuller model of bow-arrow interaction, checked against films of a Polish national-team archer: it reproduces the shaft's flexing and proposes simulation as a tuning tool. It does not compare shafts of different spine in archers. The calculation takes one combination only, an Easton X7 1714 aluminium shaft on a 1987 recurve with a single value of button stiffness, 622 N/m, so there is no seeing what changes as spine or the button's spring vary; the softer and stiffer shafts the discussion mentions are calculations from the earlier model. What remains is the criterion the article takes up and calls still useful: a combination is well chosen when the arrow leaves the string at roughly the instant the nock crosses back over the bow's plane.

  Bow-arrow interaction in archery · B. W. Kooi

• 2001
  For Model Published later

  An independent model applied to target bows and arrows: a more powerful bow takes a stiffer and heavier arrow, and the useful range is narrow, with force at full draw from 121 to 244 N, arrow length from 622 to 826 mm, total mass from 16.6 to 23.8 g and a bending stiffness between 2.73 and 5.96 Nm². The model adds that the lateral stiffness of string and limbs weighs more on the shaft's flexing than the bow's virtual mass does. It does not compare shafts of different spine in archers. The article's strongest conclusion is another one, and it counts as a prediction to be checked: spine ought to grow in proportion to the arrow's mass, because the period of oscillation depends on the ratio of mass to stiffness. In its own data the ratio of the free periods squared, 1.23, is exactly the product of the mass ratio and the stiffness ratio. The manufacturers' charts do not index on mass, and the article does not test the point against any real set of shafts. No measurement was collected: the only experimental support is a 1992 high-speed film, cited as a film and with no data.

  Lateral deflection of archery arrows · Zanevskyy

• 2012
  For Model Published later

  A recurve model calibrated against bench measurements: to stay tuned, moving to the neighbouring spine calls for about eleven newtons of change in draw weight, while a recurve bow's draw weight can typically be adjusted within ten per cent of nominal, so at a given draw length only one or two spines can be tuned well. Lengthening the shaft from 705 to 715 mm takes the required draw weight from 164.5 to 160 N, that is ten millimetres are worth four and a half newtons. These figures are consistent with practice, but they are calculated on a single bow.

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

• 2025
  For Model Published later

  A numerical model of a competition recurve bow with Easton X7 and X10 arrows, built on a precise criterion: a combination is tuned when, once clear of the bow, the arrow no longer rotates in the horizontal plane about its own centre of mass. The resulting charts have a step of half an inch of length and four pounds of draw weight, against the one inch and four to six pounds of the manufacturers' charts, and the recommended size changes between adjacent lengths. The recommended spine for tapered arrows falls about threefold, from 1000 at twenty pounds and twenty-five inches to 325 at fifty-two pounds and thirty-two inches. It is a model, with no archers.

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

• 2025
  Context Equipment tests Published later

  Fifty-nine Easton X10 shafts in spine 325 measured one by one and shot by machine at seventy metres: with a right-handed bow, the stiffer arrows go to the left and those with a more irregular spine around the shaft go to the right. The horizontal model explains eighteen point six per cent of the variation, with a coefficient of minus 0.378 for mean spine at p equal to 0.004 and of 0.325 for the amplitude of the spine variation at p equal to 0.012. It is the direction that practice attributes to spine, measured though within a single spine category and without archers.

  Improving the arrow selection process in archery: the stiffness variation pattern matters · A. Kuch, P. Laguillaumie, F. Durand, J.-F. Debril, T. Monnet

• 2025
  Context Archers Published later

  Sixty-one elite World Cup archers, with equipment and speed data: between draw weight and arrow mass the correlation is 0.67 at p below 0.01, which is to say those drawing more pounds use heavier arrows. It says the rule is followed at the top, not that following it scores points: it is a snapshot of the equipment, not a comparison.

  Empirical analysis of the impact of bow parameters on arrow velocity: A study of World Cup archers based on energy transfer and aerodynamics · Feixue Rao, Jiasheng Lv, Haidong Chen, Peijin Li, Hao Huang

Status history

1. 1990 Reported A mathematical model, checked against high-speed films of a Polish national-team archer and against a shooting machine, shows that the suitable shaft leaves the string with the nock almost still sideways while shafts that are too weak or too stiff leave it with a lateral velocity, and that the parameters which count are the outside diameter and the wall thickness. The model yields a nomogram that derives the shaft from draw weight and length, and the equipment of eighteen of the twenty-two best Polish archers fell within it. The part on archers is that coincidence rather than a measurement of grouping, hence reported.
2. 1997 Reported A second model, compared with the same high-speed films, confirms the rule in both directions: on three shafts with the same wall, the softest slaps against the button and the stiffest clears the grip with more room but leaves the string with a stronger lateral jolt on the nock. It does, however, question the 1990 explanation, because even with the standard shaft the nock does not stay close to the plane of the bow for long before the arrow leaves the string. It is a model, with no archers compared: the status does not move.
3. 1998 Reported A fuller model of the interaction between bow and arrow reproduces the flexing of the shaft and proposes simulation as a tuning tool. It does not compare shafts of different spine against each other in archers, so it reinforces the mechanism and nothing more: the status does not move.
4. 2001 Reported A new model, independent of those built on the Polish films, reaches the same rule: a more powerful bow takes a stiffer and heavier arrow, and for Olympic competition bows the useful range of draw weight, length, mass and stiffness is narrow, from 121 to 244 N, from 622 to 826 mm, from 16.6 to 23.8 g. It also adds that the lateral stiffness of string and limbs weighs more on the shaft's flexing than the bow's mass does. Two coherent lines of work, but neither measures grouping in archers with suitable and unsuitable spine, so the status holds.
5. 2012 Reported A recurve model calibrated against bench measurements puts figures on tuning: changing spine by one step calls for about eleven newtons of draw weight, and draw weight can only be adjusted by about ten per cent, so at a given draw length only one or two spines can be tuned well; lengthening the shaft by ten millimetres is worth four and a half newtons, that is a few pounds. Figures consistent with what is done on the field, but coming from a model on a single bow: the status does not change.
6. 2025 Reported Two further confirmations arrive, independent but still indirect. A new numerical model rebuilds the charts from draw weight and length, with the recommended spine falling about threefold from a light bow with a short arrow to a heavy bow with a long one, and finds that the right size changes with as little as half an inch; it also gives the working definition of being tuned, that is no residual lateral rotation after the arrow leaves the string. On the shooting machine, with a right-handed bow, the stiffer shafts land to the left, and among World Cup archers those drawing more pounds use heavier arrows. No measurement of grouping in archers, so the status stays reported.

Related

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• Draw weight and score in the research: how much do the pounds matter?What studies have measured on draw weight and score: a small effect tied to skill level, top archers' draw weights over time, speed and control.
• Why do your groups open up with distance?Double the distance and the group doubles, or a little more: what geometry and competition scores say, and how to use two distances to find what opens your group.
• Fishtailing and porpoising: what arrow flight saysDoes your arrow wag sideways or wave up and down? What causes fishtailing and porpoising, which to fix first, and how to confirm it with a bare shaft test.

Research lines

• 29 studiesThe archer's paradox in research: from 1987 to today
• 22 studiesHow much energy passes from bow to arrow: models and measurements since 1937

Other claims in this domain

• CorroboratedAlong its flight the arrow loses an appreciable part of its launch speed12 studiesStatus since 2011
• CorroboratedAs it leaves the bow, the arrow flexes and snakes around the riser instead of leaving straight19 studies · 47 archers measuredStatus since 2012
• ReportedA camera-based scoring system misreads some arrows: the ones barely sticking out and the ones that shift the target face7 studies · 2 archers measuredStatus since 2017
• ReportedA chest band with fibre optic sensors measures breathing and mean heart rate with small errors, and an archer can wear it while shooting2 studiesStatus since 2019
• ReportedA compound bow stores between 70 and 100 joules and sheds about two thirds of the force at full draw4 studiesStatus since 2015

See all claims on equipment and tuning