Illustrated Explanation of the Chariot System in the Artificers' Record

The theory regarding the moon is identical to that found in the Record of Trades Kao Gong Ji: an ancient technical manual within the Rites of Zhou. The Record says: "The wheel has thirty spokes to symbolize the sun and the moon." Just as the sun and moon meet in conjunction every thirty days, moving through one "mansion" original: 舍, a celestial station at a time, the wheel’s circumference consists of thirty spokes. As it rolls, each spoke touching the ground in succession resembles this lunar cycle.

The part of the spoke that enters the hub is called the zi, and the part that enters the rim original: 牙 (ya), literally "tooth," referring to the wheel's wooden felloe or rim is called the zao.

Why are they called zi and zao? Both names are derived from fingers or claws. The Gongyang Commentary says in the entry for the 10th year of Duke Wen: "If one uses fingers as a metaphor, it is 'connecting the zi.'" To have "four connected zi" refers to a condition like webbed or joined fingers. In the Book of Etiquette and Rites, regarding the supports of a towel rack or clothes stand, it mentions the zao; here, zao original: 蚤, a phonetic loan for 爪 (zhua), meaning claw refers to a "claw." Ancient people frequently named objects based on parts of the human body such as the "teeth" (rim), "thighs," "thigh-bones," "chin-flap," "neck," "heel," and "belly"—all are used this way. The zi is also referred to as the "weak" original: 弱 (ruo). Because the zi tenon is hidden within the hub and is not visible, it is like the "weak" part of a rush plant that remains submerged in water. Therefore, Master Zheng Zheng Xuan, the influential Han dynasty commentator states: "People today call the base of a rush plant that stays underwater the ruo."

The width of the spoke should be equal to the width of the rim see the explanation under the "geng" geng: the structural neck or body of the spoke below. If its thickness is calculated as six-tenths, six-percent, and six-thousandths of a unit, how can we prove this? In a Great Carriage, the spoke width is three inches, and the thickness is one-third of that width. The specifications provided here are for the Small Carriage.