a buttress to be placed between beads of a pneumatic tire which in turn are clamped against the buttress by adjoining wheel rims . the buttress is a hollow unibody construction formed of plastic material by a rotary molding procedure . the buttress is first formed flat and then bent to a circular shape with the ends pinned together , or as a pair of flat members bent and joined together . it includes a plurality of hollow posts that extend between longitudinal side rail , the side rails being abutted by the rims when installed .