the main distinctive feature of the new design is that the rim has an o - shape of cross - section instead of the conventional u - shape of cross - section to hold a solid of filled tire is disclosed . the rim assembly or disc type of wheels , for example as one shown in fig . 2 , consists of two half pieces , separating along a central plane . the half pieces are first molded and then are bonded to form a light , hollow and strong rim and wheel assembly . a wheel assembly having a solid tire is made by pouring formed polymer materials such as polyurethane into a rotating mold in which the rim assembly is used as an insert . use of rotating / reaction casting method can achieve a very high productivity . optionally a double shot molding may be employed to concentrate strong rigid material such as glass filled nylon in the flat cone and softer polypropylene in the rim .