On a compound bow the draw force is a little higher than the let-down force

The summary today

The measurements exist and point the same way, but there are two of them and they come from the same author. In 2015, drawing a 55-pound round-wheel compound slowly with a winch to the bottom of the valley and then easing it back to the starting position, the force read on the draw comes out systematically higher than on the way back; in 2016 the same happens on a commercial twin-cam compound. In both cases the author puts the gap down to friction in the wheels and to hysteresis in the limbs, string and cables, but does not quantify it: the difference can be seen in the graphs, there is no figure and the two causes are not separated. The measurements were there to calibrate a model, and in 2015 the calibration used the drawing and the let-down data together; each time there was a single bow. On the two bows measured the gap is there; how much it is worth in pounds, and whether it changes with cams, strings or the number of cycles, is still to be measured.

Also phrased as: "compound bow hysteresis"; "let-down force curve"; "cam friction"; "draw versus let-down force"; "compound let-down"

Evidence

• 2015
  For Equipment tests Published later

  Force-draw curve of a 55-pound commercial twin round-wheel compound, measured while drawing slowly with a winch to the bottom of the valley and then easing back: the force is systematically higher on the draw than on the way back. The author attributes it to friction in the wheels and to elastic hysteresis in the limbs, string and cables; the difference is not quantified and the model calibration uses the drawing and let-down data together.

  An advanced model of the round-wheel compound bow · M. Tiermas

• 2016
  For Equipment tests Published later

  Force-draw curve of a commercial twin-cam compound, measured on the draw and on a slow let-down: the values on the draw are systematically higher, and the author attributes this to friction in the wheels and hysteresis in the limbs, string and cables. Here too the difference is not quantified.

  A model of the twin-cam compound bow with cam design options · M. Tiermas

Status history

1. 2015 Reported The first measurement comes from a paper calibrating a model of the round-wheel compound: the force-draw curve of a 55-pound bow, recorded while drawing slowly with a winch and then easing the bow back, lies systematically lower on the way back. The author puts the gap down to friction in the wheels and to elastic hysteresis in the limbs, string and cables, but does not quantify it. A single bow and no figure for the gap: reported.
2. 2016 Reported The same author measures the force-draw curve of a commercial twin-cam compound, on the draw and on a slow let-down, and again finds the force systematically higher on the draw, with the same explanation: friction in the wheels and hysteresis in the limbs, string and cables. The type of bow changes and the direction stays the same, but here too the gap is not quantified and there is one bow: two measurements by the same author, each on one bow, are not enough to move up, and the status stays reported.

Related

Related articles

• The compound on the draw: what the cams, the limbs and the nocking point doCompound cams, limbs and nocking point: what shapes the force-draw curve, the let-off and the path of the nock, according to what studies have measured.

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