The present invention concerns nonpneumatic wheels, notably, those capable of being used in substitution for tires on vehicles.
It has been tried for a long time to design such nonpneumatic wheels, that is, operating without compressed air, in order to overcome every problem raised by flats or reduction of inflation pressure of tires.
Among very numerous proposals, the one described in U.S. Pat. No. 3,234,988 can be cited. That patent describes a nonpneumatic deformable wheel containing a disk, an internal element fastened to the disk, an annular external element, flexible and appreciably inextensible, intended to come in contact with the road, and a plurality of spokes arranged between the internal and external elements. The external element has a length such that it stresses the spokes under radial compression. In other words, they are prestressed (that is, preloaded). Beyond a certain stress threshold, since the spokes are stressed on end, the radially oriented reaction force that each of those spokes can develop remains constant. The wheel also contains means of stabilization of the relative positions of the external and internal elements. The spokes bend in a meridian plane and the means of stabilization limit the relative axial displacements of the internal and external elements.
This deformable wheel uses as connection between the internal and external elements spokes prestressed beyond their buckling load. Thus, in case of increase of the load supported by the wheel, that increase is compensated only by an increase in number of spokes actually supporting the load. This results in an increase of length of the contact between the wheel and the road. Such behavior is very close to that of a tire.
This wheel presents, however, one major disadvantage. The different spokes buckle in their meridian planes, but have practically no possibility of circumferential deformation, for their section presents great inertia in the circumferential direction. Now, on rolling, considerable longitudinal forces are undergone by the external element in contact with the road, notably, in the area of contact, which leads to rapid deterioration of the previous nonpneumatic wheel.