shaft
輈 centers on a shaft, specifically the central pole of a carriage or cart that connects the vehicle to the draft animal.
輈 combines 車 (vehicle) and 舟 (boat), likely suggesting the long central shaft of a cart that resembles the keel of a boat. The exact historical development is uncertain.
A cart 車 needs a central shaft, and the right side 舟 looks like a boat's keel—imagine a long, straight shaft running under a cart like the keel of a boat.
For チュウ, picture a long shaft that you choose for your cart: choose -> チュウ, and the shaft is the central piece you select.
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