Why bamboo fails
A shinai is made of four bamboo slats (take) joined by leather fittings and a cord. The Japanese kendō federation (ZNKR), after an injury in which a splinter from a slat entered the eye area, found cracks on the inner side of all four slats and splinters on the surface; the shinai had not been oiled and so cracked easily in the dry season. The same analysis states that repeatedly using only one face of the shinai warps and weakens it. Dry storage and unremoved splinters, according to equipment sellers, speed up the ageing of bamboo.
Checking before, during, and after training
ZNKR calls for a check before training, during it, and after it. On the bamboo, look for splinters, cracks, warping, and deformation. A splinter can pass through the mask's mesh into the eye. Take the shinai apart regularly and check the inner sides of the slats too, for cracks or insect damage.
The nakayui (leather strap) must be firmly fixed roughly a quarter of the way along from the tip; if it is not, the manual of the International Kendo Federation (FIK) says the shinai should be replaced. The sakigawa (leather tip cap) must, under FIK rules, be at least 5 cm long and undamaged. The tsuba must be round, made of leather or synthetic material, at most 9 cm in diameter, and fitted tightly with no play; only the tsuba-dome stopper may be used to secure it, never adhesive tape. On the tsuka-gawa (leather handle wrap), look for wear and stretched leather. The tsuru (cord) must be taut with the knots tightened. FIK rules forbid any object inside the shinai other than the tip core and the chigiri plate in the handle, and forbid a shinai with large gaps between the slats or with modifications that reduce safety.
Taking it apart
Loosen the nakayui knot (a tight knot can be loosened with an awl), loosen the tsuru knot at the handle and unwind it, remove the sakigawa, take out the tip core (sakigomu), and pull off the tsuka-gawa. Note the chigiri plate at the end of the handle.
Sanding splinters
Remove splinters right away, or they deepen cracks. Use a shinai scraper or fine sandpaper and always scrape in one direction, from the handle toward the tip. Then smooth the edge of the slat with fine paper or a file. It is a good idea to run fine paper over the edges of a new shinai before first use.
Oiling
Apply a thin layer of shinai oil to the slats, especially the contact surfaces and the back between the lower nodes, wipe off the excess with a towel, and let it dry for at least 24 hours. Sources also mention linseed or camellia oil. One dōjō advises wrapping the shinai for a month in a sealed cover with oil-soaked cotton wool.
Replacing and rotating slats: sources disagree
Some teachers and sellers recommend rotating the slats by 90 or 180 degrees when reassembling so the shinai lasts longer, and replacing a cracked slat with one from a similar shinai. ZNKR, by contrast, warns that thoughtlessly rotating a shinai or reassembling its slats greatly increases risk, that warped slats may not have the needed strength once reassembled, and that a damaged shinai is better retired than repaired.
Reassembly
Put the slats together, fit the tsuka-gawa with the seam in the middle, insert the core, and fit the sakigawa. Thread the tsuru through, wind it seven to eight times around the end of the handle, and tighten it firmly. Tension a new cord a little more firmly than usual, since it will stretch, but not extremely: an overtightened cord bends the bamboo and the shinai cracks. Fix the nakayui a quarter of the way from the tip, firmly but without overtightening. Finally, wipe the shinai lightly with oil.
When to retire a shinai
Do not use a cracked shinai. Retire it or replace the cracked slat (see the disagreement among sources above), as with a shinai whose bamboo is fully split, or with splinters that could break off on a strike.
The yumi and its string
The manual of the Japanese kyūdō federation states that a bamboo bow is sensitive to moisture, especially in heat, and that it should be protected during the rainy season; wipe off moisture with a dry cloth. It also recommends coating the yumi with vegetable oil to keep it from absorbing moisture. Kyūdō teacher Dan Dayley, by contrast, advises against using oils, lacquers, or soap at all, since bamboo secretes its own wax, and recommends simply wiping the bow vigorously top to bottom with soft leather or cotton.
When the yumi is not in use, Dayley advises leaving it unstrung, standing on its lower end with its head resting against a wall, never lying flat. Protect it from damp, prolonged sun, and heat sources, since drying out, he says, leads to cracking; do not leave it in a hot car. Before training, check that the string is not deviating to the left of the bow's body, which signals danger. A bow with a crack or a delaminated layer should only be repaired by an experienced teacher, and even after repair it may not last long. Heat, according to Don Symanski, can flatten a bow; the distance from the grip to the string should be roughly 15 cm, more is fine, never less. A new string stretches and is shortened only at the upper loop. According to the kyūdō federation, a hemp string should be coated with kusune at every stringing with brisk strokes downward, following the twist; drying hemp frays and cracks.
Which weapons this applies to
Shinai for kendō and other bamboo training swords, and the yumi for kyūdō.