Abstract: An assembly for mounting tubeless tires on tube type rims. The assembly includes a pair of cooperating annuluses that define respective seating surfaces each of which are adapted for receiving one of the companion beads of the tire. One of the annuluses is telescoped onto the rim and positioned adjacent the rim flange integrally formed with the rim proximate one of its axial end portions. A locking annulus secured to the opposite axial end portion of the rim is adapted for receiving the companion bead and can be readily secured to or removed from the rim for purposes of mounting or demounting the tubeless tire without forcing the beads over a flared flange. The locking annulus may be mounted on a variety of conventional tube type rims while maintaining the bead seating surface at a predetermined spaced location from the rim wall. A suitable pneumatic inner tube inserted in the tire serves to prevent air from escaping the confines of the tubeless tire which may have damaged sealing properties.