Trailer Leaf Springs come in a variety of lengths, widths, capacities and styles. Free Length is the distance from the center of each mounting hole on a double eye spring, or from the mounting hole to the tip of the radius end on a slipper spring. The Number of Leaves refers to the total sections of spring material that make up the spring assembly. Free Arch is measured from the center line of the mounting holes to the center of the top surface of the main leaf with no load applied to the spring. Capacity is measured per spring so two 1,750 pound springs would be used on a 3,500 pound axle.