A traction system for a vehicle has a support frame that defines a circular track belt support that is revolvable about an axis at its center, a flexible track belt; and a track belt stretching assembly comprising one or more track belt stretching rollers. The system has one traction configuration in which the flexible traction belt is held on said support track forming a substantially circular, wheel-like traction surface, and can change to a stretching state in which said rollers engage said belt and stretch it from its state in said one traction configuration to a stretched state to define one or more other traction configurations of the system.