Seat back structure of an automotive seat

Claim 1

A seat back structure of an automotive seat, comprising: a seat back frame having a space therein defined by its frame sections; a back support plate disposed within said space of said seat back frame; a torsion bar for elastically supporting a lower end area of said back support plate; plural pairs of tension springs which are so arranged between said seat back frame and back support plate that each pair of springs are extended in respective pair of spaces laterally between one lateral frame section of said seat back frame and one lateral edge of said seat support plate and between the other lateral frame section of said seat back frame and the other lateral edge of said seat support plate, wherein all those pairs of tension springs are disposed above said torsion bar, and adapted for elastically supporting an upper area of said back support frame; said torsion bar being at its both ends secured rotatably on the respective said lateral plate sections of said seat back plate, and so elastically bent as to give such cambered configure that its central part is projected forwardly relative to secured points where said torsion bar is secured on said lateral plate sections of said seat back plate, whereby back support force of said torsion bar is greater than that of said tension springs; and wherein to said forwardly curved central part of said torsion bar, is fixed to said back support plate.