A collapsible strut with controlled deformation for the frame of a road vehicle, the strut consists of an internally hollow tubular body, which presents a closed transverse section, extends between an inner end and an outer end, which are axially opposite to one another, is suited to be fixed in a projecting manner to the frame of the road vehicle at the inner end, and fulfills the function of deforming in a controlled manner in case of crash, thus collapsing axially, furthermore the strut presents a thickness which axially varies from the inner end to the outer end.