The propulsive force is the force exerted on the racetrack by the drive wheels, in the x-direction. The reaction to this force, exerted by the racetrack on the car, is what propels the vehicle forward.
The origin of the propulsive force is the torque that is exerted by the wound-up spring onto the drive axle. The diameter of the wheel determines how much propulsive force this torque creates at the wheel-racetrack interface. For example, in the diagram above, suppose that the spring torque (T) is 0.75 lb-in, and the radius of the wheel (r) is 3.00 inches. The propulsive force (F) would then be:
F = (0.75 lb-in) / (3.00 in) = 0.25 lb