Shafts have a stiffer side and a softer side: stiffness changes as you turn them around their axis

The summary today

The measurement exists, and there is only one, but it was done carefully. On 59 Easton X10 shafts of spine 325, tested on an automated rig that bends them at every sixteenth of a turn for at least four full turns, 64 measurements per shaft, stiffness almost always follows the same wave: two maxima and two minima per turn, that is a strong axis and a weak axis. The difference is small, on average 0.014 mm of deflection out of about 8.3, ranging from a thousandth of a millimetre, practically nothing, up to 0.038; a few shafts show a slower, different wave, which the authors read as a possible manufacturing or batch defect. Shot at 70 m by a machine, the shafts with the widest wave land further right, while the position of the vanes relative to the stiff side moves nothing measurable. That shafts have a stiff side is therefore documented, on a single arrow model and one of the stiffest spines, but in some of them the difference is negligible, and the idea that the anomalous wave in a few shafts is a manufacturing defect, to be weeded out in selection, remains the authors' interpretation.

Also phrased as: "stiff side of the shaft"; "spine not uniform around the shaft"; "orienting the nock to the spine"; "spine indexing"; "turning the nock 120 degrees"

Evidence

• 2012
  Context Model Published later

  In a recurve model a shaft Young's modulus 1 per cent higher moves the calculated impact 25 mm to the right at 70 m. Since the nock can be fitted at any angle, the author concludes that it matters that stiffness in the lateral plane stays the same as the arrow is turned. It is a warning, not a measurement.

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

• 2016
  Context Equipment tests Published later

  Thirty-six hunting arrows of three models, shot by a machine with a 51 lb bow at about 40 m, three times each with the nock turned 120 degrees between shots: the three impacts of the same arrow separate, with a mean distance from their centre ranging from 1.81 to 20.07 mm across arrows of the same model. That is the rotation that would put a shaft with a stiffer side to the test, but the study does not measure stiffness as the shaft is turned, and the spread may also come from straightness or from the machine: it is context.

  Position coordinate representation of flying arrow and analysis of its performance indicator · Yeongsang Jeong, Suryo Adhi Wibowo, Moonjae Song, Sungshin Kim

• 2025
  For Equipment tests Published later

  59 Easton X10 325 shafts on an automated three-point bending rig, one measurement every sixteenth of a turn for at least four full turns, 64 per shaft: stiffness follows a sine wave with a spatial frequency of 0.133 ± 0.001 per millimetre, which corresponds to two maxima and two minima per turn, that is a strong axis and a weak one. A few shafts show a slower, different wave, which the authors attribute to possible manufacturing defects or differences between batches.

  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 Equipment tests Published later

  The amplitude of the wave is small: 0.014 ± 0.009 mm of deflection, from 0.001 to 0.038 mm, on a mean deflection of 8.290 mm against the 8.255 expected for spine 325. In some shafts, then, the difference between the stiff and the soft side is practically nil, and all of them met the manufacturer's straightness tolerance.

  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 Equipment tests Published later

  The same shafts, assembled and shot at 70 m indoors by a machine over five sessions: horizontal position on the target depends on mean spine and on the amplitude of the wave, wider meaning further right, but the model explains slightly under 20 per cent of it; the group is better explained by mass, mean spine and the frequency of the wave. The angle between the vanes and the stiff side, randomly distributed because the label is printed at random, has no measurable effect on where the arrows land.

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

Status history

1. 2012 Not studied The question appears inside a recurve model, as a warning rather than a measurement: since the nock can be fitted at any angle, the author writes, it matters that stiffness in the lateral plane stays the same as the arrow is turned, and in the model a Young's modulus 1 per cent higher moves the impact 25 mm to the right at 70 m. Nobody has yet measured whether a shaft's stiffness really changes as it is turned.
2. 2016 Not studied A Korean study shoots the same arrow three times by machine, turning the nock 120 degrees between shots, and finds impacts lying from under two millimetres to two centimetres from their centre, depending on the arrow. The design is the one that would reveal a stiffer side, but stiffness around the shaft is not measured and the spread is not attributed to any cause: no one has yet measured whether a shaft's stiffness changes as it is turned, and the status stays not studied.
3. 2025 Reported The measurement arrives: 59 X10 shafts of spine 325, 64 deflections each over at least four turns, almost all show two maxima and two minima of stiffness per turn, with a mean amplitude of 0.014 mm and peaks up to 0.038, and a few shafts have a different wave that the authors read as a possible defect. Shot by a machine at 70 m, the shafts with the widest wave land further right, while the orientation of the vanes relative to the stiff side makes no measurable difference. It is a test on equipment, and so direct evidence, but it is a single test on a single shaft model: reported.

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