Abstract: The present invention provides for a drop-in seat unit as well as a method of manufacturing the drop-in seat unit that does not require the worker to affix one end of the spring to a frame rail and stretch the spring from one of the frame rails to the other frame rail. The ends of the spring are affixed to the frame rails while the springs are in an un-stretched state. Once the plurality of springs is attached to the two frame rails, the rails are mechanically pulled away from each other, thus created a larger open area between the frame rails and thus extending the springs. Once the rails are properly positioned in this extended state, support members are positioned between the two frame rails and one end of each support member is affixed to each frame rail, traversing the space or gap between the two frame rails. The support members fix the distance between the two frame rails.