An improved assisted steering mechanism is described which permits an additional preload to be introduced, in a reduced space and at low cost, on the spring that centers the distributor valve, subsequent to centering that distributor valve. The mechanism that is described also permits the said additional preload to be modified depending on the vehicle speed. This mechanism is made up of two components (1) and (2) which turn on the same shaft E and which are in a relative fixed position on the steering of shaft E. The relative gyration of the two components (1) and (2) determines the opening of the distributor valve. Component (1) has axial enlargements which are inserted in notches pertaining to a spring-washer (6′) which turns solidarily with component (2). The relative gyration of the components (1) and (2) forces the preload of the washer (6′) to be overcome. Force "F" is applied on the washer by joint (16) caused by pressure in chamber A, depending on the vehicle speed. The preload can thus be varied in terms of the speed.