In Feynman's treatment of thermodynamics, The Laws of Thermodynamics, $44–14$ Heat engines; the first law, Feynman said we can use rubber bands to make a heat engine like the one below.
It consists of a bicycle wheel in which all the spokes are rubber bands. If one heats the rubber bands on one side of the wheel with a pair of heat lamps, they become “stronger” than the rubber bands on the other side. The center of gravity of the wheel will be pulled to one side, away from the bearing, so that the wheel turns. As it turns, cool rubber bands move toward the heat, and the heated bands move away from the heat and cool, so that the wheel turns slowly so long as the heat is applied.
Previously, he said rubber band contracts when heated. I think that's what he meant by stronger, but how does this explain the shifting of the center of mass from the bearing?
Contracted or not, the rubber band's mass stays the same, so each sector of the wheel containing a rubber band will have the same mass as before; thus the distribution of mass should be unaffected by the heating in one side, isn'it?