cart wheel with no spokes
輇 refers specifically to a cart wheel without spokes, a solid or disc-like wheel used in ancient carts.
輇 combines 車 (vehicle, cart) with 全, which likely contributes the sound and may suggest the idea of completeness or wholeness, fitting a solid wheel without spokes.
A cart 車 with a complete 全 wheel: imagine a solid disc wheel with no missing spokes, making it whole.
For セン, picture a solid cart wheel rolling down a path with a sen (千, thousand) other carts: 千 -> セン, and the wheel is complete.