Abstract: The inertia wheel contains a left piece having its circumference extended rightward into a first ring side wall with a left-high-right-low step between a left side of the left piece and the first ring side wall, and a right piece having its circumference extended leftward into a second ring side wall. The second ring side wall wraps the first ring side wall and a left end of the second ring side wall tightly interfaces the step of the first ring side wall. An axle tube is inserted in a first through hole and a second through hole of the left and right pieces, respectively. The inertia wheel as described is simple, robust, low-cost, low-carbon, appealing, and adaptable.