A method of lightening a run-flat device for a motor vehicle wheel, in particular having a separable flat rim, the method consisting in configuring the run-flat device in a sandwich structure comprising a rigid inner annular portion, an intermediate annular portion of flexible or elastically deformable material, and a rigid outer annular portion, in such a manner that in run-flat mode, all of the forces transmitted by the rigid outer portion are distributed over the entire circumference of the intermediate portion, thereby enabling the thickness of the intermediate portion to be reduced and enabling the run-flat device to be made lighter in weight. The invention also provides a run-flat device implementing the above method.