Seat back rest with adjustable spring rate

Claim 1

A SEAT STRUCTURE COMPRISING A MAIN FRAME AND AN AUXILIARY FRAME, A PLURALITY OF ELONGATED SPRING ELEMENTS HAVING END PORTIONS SUPPORTING SAID AUXILIARY FRAME IN SPACED RELATIONSHIP TO SAID MAIN FRAME, SAID ELONGATED SPRING ELEMENTS BEING DISPOSED IN SUBSTANTIALLY PARALLEL RELATIONSHIP TO EACH OTHER AND HAVING THE CENTER PORTIONS THEREOF DISPLACED OUTWARDLY FROM THE PLANE OF SAID AUXILIARY FRAME, AT LEAST ONE ADDITIONAL ELONGTED SPRING ELEMENT INTERPOSED BETWEEN A PAIR OF SAID FIRST MENTIONED SPRING ELEMENTS AND HAVING ITS CENTER PORTION DISPLACED OUTWARDLY FROM THE PLANE OF SAID AUXILIARY FRAME, SAID ADDITIONAL SPRING ELEMENT BEING SUPPORTED AT EACH OF ITS ENDS ON SUPPORT MEMBERS DISPLACEABLE IN THE PLANE OF SAID MAIN FRAME, AND MEANS OPERATIVE TO DISPLACE SAID SUPPORT MEMBERS TOWARD OR FROM EACH OTHER TO VARY THE DISPLACEMENT OF THE CENTER PORTION OF SAID ADDITIONAL SPRING ELEMENT, THE DISPLACEMENT OF THE CENTER PORTION OF SAID ADDITIONAL SPRING ELEMENT WHEN SAID SUPPORT MEMBERS ARE AT A MAXIMUM DISPLACEMENT FROM EACH OTHER BEING SUBSTANTIALLY THE SAME AS THAT OF THE CENTER PORTIONS OF SAID FIRST MENTIONED SPRING ELEMENTS, SAID ADDITIONAL SPRING ELEMENT CENTER PORTION BEING DISPLACED OUTWARDLY FROM THE PLANE OF SAID AUXILIARY FRAME MEMBER FURTHER THAN THE CENTER PORTIONS OF SAID FIRST MENTIONED SPRING ELEMENTS WHEN THE DISPLACEMENT OF SAID SUPPORT MEMBERS RELATIVE TO EACH OTHER IS LESS THAN SAID MAXIMUM DISPLACEMENT.