1. A variable diameter wheel of the type designed to be incrementally retractable from a first diameter to a second, concentric, smaller diameter and expandable from said second diameter to said first diameter, said wheel comprising: a plurality of first spokes equidistantly attached about the periphery of a hub and extending radially outward therefrom in a common plane; a corresponding plurality of second spokes equidistantly attached in alternating arrangement with said first spokes about said periphery of said hub and extending radially outward therefrom in said common plane, said second spokes being of a predetermined lesser longitudinal dimension than said first spokes; support means comprising a circle attached to said first and second spokes at a predetermined distance from said hub, whereby said first and second spokes are supported in fixed equidistant relation to one another; a plurality of circular segments including wear plates attached to the concave sides thereof disposed in spaced apart relation along a circle of said first diameter and attached at their midpoints to the outer extremity of said first spokes; a plurality of reinforcing means attached to the outer extremity of said second spokes and abutting the concave surfaces of two of said wear plates adjacent each other; and hydraulic means connected to said first and second spokes to vary the length of said first and second spokes, whereby the length of said spokes determines which of said diameters is defined by said segments.