hub (of wheel)
轂 refers specifically to the hub of a wheel, the central part where spokes meet and the axle passes through.
轂 combines 車 (vehicle, wheel) with a phonetic component that historically indicated the sound. The right side likely contributed the reading while 車 specifies the wheel-related meaning.
The left side is 車 (vehicle/wheel), and the right side looks like a complex hub mechanism. Picture the central hub of a chariot wheel where all spokes converge.
For コク, imagine the hub of a wheel made from a solid block of oak: oak -> コク, and the oak hub sits at the center of the wheel.