Illustrated Explanation of Carriage Systems: Volume I, Wood

What I have heard previously is incorrect: that the sou sou: the hollow interior of a wheel hub through which the axle passes is simply the empty space inside the hub original: gu.. In reality, it is the general name for both the "large perforation" and the "small perforation" that run through it.

The large perforation is called the xian xian: the wider opening of the hub's interior, usually facing toward the body of the carriage, while the small perforation is called the zhi zhi: the narrower opening of the hub's interior, facing outward. Regarding their circumferences, the Record The Kao Gong Ji (Record of Trades), a classic work on science and technology in ancient China. states: "Divide the length of the hub into five parts; remove one part to determine the xian, and remove three parts to determine the zhi."

Thus, the circumferences of the xian and the zhi are themselves the circumferences of the sou. How could there be a separate measurement for a "sou circumference" that is larger than the zhi but smaller than the xian?

Furthermore, the character zhu original: 助. Generally means "to assist." is composed of the radicals for "mound" and "strength." The Shuowen The Shuowen Jiezi, an early 2nd-century dictionary. explains it as a term related to "land terrain." Just as the word jie original: 劫. in the Book of Changes and the word le original: 仂. in the Royal Regulations are both interpreted as "remainder," the word has never carried the meaning of "one-third."

If "one-third" were truly the intended measurement for the circumference of the sou, why would the Record not simply say, "Divide the hub's circumference into three and take one part as the sou"? Why instead would it use such specialized phrasing as: "Using the grain original: zhu (助), interpreted here as wood grain of its circumference, hollow out its center (sou)"?

Yuan notes: The word zhu assistance should, according to the Shuowen, be written as jie original: 劫., meaning "the grain or pattern of wood." In the current text, it is written with the "mound" radical as zhu, which is a phonetic loan A "phonetic loan" occurs when a character is used for its sound rather than its original meaning.. The sounds for "grain" (li) and "assistance" (zhu) shifted into one another over time. All objects have a natural "grain" (li), and wood is no exception. The "Wheelwright" section of the Record says: "Where the grain is dense, the wood is strong; where the grain is sparse, the wood is soft."