An annular element (3) placed between the two beads (12) of a tire (P) moed on a flat-base rim (J). In order to be able to use the tire on said rim without an inner tube, the annular element, in order to assure the tightness of the assembly, has in meridian section a contour and, in particular, two lateral sides (30) such that the clamping s between the ring and the beads, which is zero or even negative at the base of the ring, increases then slightly over a certain radial distance h.sub.1 and then more greatly over a distance h.sub.2 radially adjacent the first distance.