Run flat device

A run flat device intended to be mounted in a tyre around a wheel rim of a vehicle, the device comprising at least one assembly of two half-shells assembled axially, each half-shell being made of a composite material based on fibres embedded in a thermoplastic or thermosetting resin, each half-shell comprising a radially internal periphery a radially external periphery and a lateral wall connecting the radially internal periphery to the radially external periphery configured so as to form an internal recess within the at least one assembly and each half-shell further comprising a plurality of circumferentially distributed anti-compression ribs extending, within the internal recess, radially from the radially external periphery towards the radially internal periphery.