A radial motor uses a single throw crankshaft that carries the no 1 connecting rod. The no 2 conrod is fixed at the root of no1 conrod, and so on for the three-, five-, seven- or nine-piston air cooled configurations.

You end up with a narrow, star-shaped engine that fits inside a motorcycle wheel, where the crank acts as the axle, and the complete engine rotates with the wheel.