Spring pre-assembly for a mattress foundation unit

A spring unit for a mattress foundation unit having a wire grid, a rigid base and a plurality of spring units extending between the wire grid and the rigid base, each spring unit including:

• a base portion, a pair of leg portions divergently extending from the base portion and a pair of engagement portions extending in opposite directions from the distal ends of the leg portions;
• the base portion being connectable to the rigid base;
• the leg portions each having a U-shaped deformity;
• and
• the engagement portions being generally co-planar with one another and each including a notch therein.

A mattress foundation unit including:

• a rigid base;
• a plurality of spring units, each including a base portion, a pair of divergent, operatively upwardly extending leg portions and a pair of engagement portions extending at a generally horizontal attitude in opposite directions from operatively upper ends of the leg portions;
• and
• an upper grid which includes a plurality of grid wire pairs arranged orthogonally to one another, each grid wire pair comprising a first grid wire and a second grid wire;
• the base portions of the spring units being connectable to the rigid base and the engagement portions of the spring units being engageable with adjacent grid wire pairs of the upper grid, the arrangement being such that when the engagement portions are so engaged the spring units are pre-stressed.