Variable-diameter wheel

A variable-diameter wheel 1 comprising at least one rim or hub 11 having a predetermined diameter, to which at least one drive axle 14 of the wheel 1 is connected; at least one outer surface external to the hub 11, having a diameter which is greater than the diameter of said hub 11; and further comprising at least one diameter-variation mechanism 3 secured to said axle 14 and adapted to vary the actual diameter of said wheel 1 according to the torque applied to the drive axle 14 by at least one propulsion system.