PURPOSE: To facilitate the fitting of a tire on a rim and prevent air leakage, by convexly shaping the bottom of the bead in accordance with the form of the bead seat.

CONSTITUTION: The bead seat 5 of a rim 4 comprises an outer part 2 sloping down from the outside of a tire toward its inside in tight contact with the bead heel 6 and an inner part 3 sloping down from the inside toward the outside in tight contact with the bead toe 7. The bead seat 5 has such an approximate V shape that the surface of the seat is lowest at or near the point P of meeting of the inner and the outer parts 3, 2 and slopes up inwards and outwards from the point. The bottom of the bead of the tire is provided with an approximate V shape corresponding to that of the head seat 5, to fit the bead on the seat.