In the Japanese bow the bamboo chosen for the inner face is stiffer than that for the outer

The summary today

It has been measured by a single group, on the materials of one Kyoto workshop that builds kyudo bows. The bow has three layers, bamboo on the outside and the inside with a core of bamboo strips and wood between them, and the researchers tested the pieces the master bowyer had already assigned to the two faces. In bending over a 400 millimetre span the elastic modulus of the inner bamboo comes out higher, a difference that did not appear over a short 80 millimetre span, and the cross-section explains why: hard fibre takes up 18 per cent of the inner piece against 9 in the outer one, and the modulus rises in proportion to the fibre. The vibration test points the same way, with a shorter period for the inner piece, while damping is the same in both. Of each cane, about six centimetres across, only the layer near the skin is kept, where the fibre is densest, down to a 4.5 millimetre strip. The limit is plain: one bowyer, one batch of bamboo, and modulus values given only in graphs. Whether it is a rule of the craft or one master's choice, the measurement does not say.

Also phrased as: "japanese bow bamboo"; "inner and outer bamboo"; "bamboo yumi"; "uchitake and sotodake"; "bamboo stiffness in the kyudo bow"

Evidence

• 2017
  For Equipment tests Published later

  Strips of madake bamboo that the master bowyer had assigned to the two faces of a Kyoto bow (kyudo): in bending over a 400 mm span the elastic modulus of the inner bamboo is higher than that of the outer, a difference not visible over an 80 mm span. In cross-section hard fibre is 18 per cent of the inner piece and 9 per cent of the outer, and the modulus turns out proportional to the amount of fibre. The modulus values appear only in graphs.

  Materials Technology in Kyoto Bow · Hiroyuki Hamada

• 2017
  For Equipment tests Published later

  In the cantilever vibration test, with strips 23 × 3.3 mm and 140 cm long, the free end displaced by 50 cm and filmed at 250 frames per second, the inner bamboo oscillates with a shorter period, a sign of a higher modulus.

  Materials Technology in Kyoto Bow · Hiroyuki Hamada

• 2017
  Context Equipment tests Published later

  Damping, on the other hand, is the same: the amplitudes decay the same way in both pieces, so the difference lies in stiffness and not in viscosity. Of each cane, about 6 cm in diameter, only the outer layer is used, planed down to 4.5 mm, because the fibre is densest near the skin.

  Materials Technology in Kyoto Bow · Hiroyuki Hamada

Status history

1. 2017 Reported A group working with a Kyoto workshop tests the bamboo the master bowyer chose for a kyudo bow: in bending over 400 millimetres the piece meant for the inner face has a higher elastic modulus, with 18 per cent hard fibre against 9 in the outer piece, and the modulus rises in proportion to the fibre. The vibration period is also shorter in the inner piece, while damping is the same. Of each cane only the layer near the skin is used, down to a 4.5 millimetre strip. One bow and one bowyer only: reported.

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