Junji Miyata summarized the report on the results of his original research on Kyudo equipment and shared the information with the participants of the training camp for the World Archery Championships 1967. Let's get into the detail for the bow part of the research.
Kyudo Equipment researched:
Bow (only bamboo bow at that time):
・Number of bows used: 40
・Bow length: Nami-sun (221 cm) to 6 shaku 6 sun zumari (200 cm)
・ Draw weight: 18 kg to 31 kg
- Incompatibility between long shot and hitting accuracy
Basically, long shot and hitting accuracy are incompatible. With short bow such as 5 sun - zumari, Junji Miyata could shoot long distance, but hitting accuracy was not good. Junji Miyata found that the best balance o bow length was Nami - sun (221cm) or 3 zun - zumari (212cm), in terms of hitting accuracy.
To shoot at the target 90m distance, Junji Miyata had to increase the draw weight. With the bow of the draw weight 30kg, the arrow could reach the target, but the hitting accuracy was not good. At the distance of 90m, Junji Miyata judged the ideal bow strength would be 23 to 25kg, to increase the hitting accuracy for Junji Miyata. However, the draw weight of 23 -25kg of bamboo bows were not enough to deliver the arow to the target of 90m distance.
2. Durability and Weatherability
From the viewpoint of weatherability, in the archery competitions where matches were continued even in rainy weather and humid environments, bamboo bows were naturally made of bamboo and wood, so they were greatly affected by cold, hot, dry and humid weather. It would become a very big problem. Also, many of the bamboo bows were already made with synthetic glue, which were already mainstream at the time, but Junji Miyata felt that the bamboo bow made with the traditional adhesive, nibe, had better performance. However, it cannot be used in hot and humid environments due to its nature of solidification /gelation due to temperature differences. In any case, Junji Miyata felt the need to make the bamboo bow a highly weather-resistant lacquered bow, or to coat the bow with a water-repellent paint.
3. Uniformity
From the viewpoint of uniformity (related to weatherability), in the early morning when the temperature was low and the high temperature in the daytime, the draw weight of the bamboo bow changed, and it was not possible to obtain a uniform performance even if Junji Miyata would have made each shot identically. Also, at that time, strings made of chemical fibers had not yet become as popular as they are today, and although with strings made of hemp string Junji Miyata made good performance, from the standpoint of durability, hemp strings were more likely to break than strings made of chemical fibers. If the strings were broken in shooting, bamboo bows recovered elasticity with the shock. The effect was good to bamboo bows in ordinal Kyudo practice, but in the competition the effect was a risk that the draw weight and shape of the bow would change.
Finally, Junji Miyata decided to use Fiber Reinforced Plastic on Kyudo bows, which had already been used in archery bows, to solve the problems above.