How to Build a Tandem Axle Trailer

Buy a tandem axle assembly with leaf springs, spring hangers and wheels. Design the trailer frame around this assembly. It will have two sets of leaf springs, one for each axle, with a single mounting point on the front and back spring and a center mount, called the equalizer hanger, which holds both springs. The equalizer is the center point for calculating trailer length. Figure 60 percent of the trailer to go in front of this point.

Cut a 3-by-4-inch angle iron to the frame dimensions, perhaps 6 by 12 feet, with a metal chop saw. Cut two 12-foot pieces and two 6-foot pieces; miter the corners and weld them into a rectangle with the 3-inch angle up, using a square to make sure the corners are correct. Mark the locations for the spring hangers, then remove them from the wheel assembly and weld them to the bottom of the 4-inch angle.

Set the frame on the wheels and fasten the springs with bolts, lock washers and nuts through the holes in the hangers and the spring mounts. Cut two more pieces of angle iron and weld them to the frame sides as cross-braces just ahead of and just behind the front and rear spring hangers. Cut triangular pieces of sheet steel and weld them, as gussets or supports, underneath all frame joints.

Build a yoke of angle iron to go from the front cross-brace to the hitch ball cap. The connections on that cap will determine the angle and length of the yoke. Weld the yoke to the frame at the cross-brace and the front frame member. Cut another piece of angle iron and weld it as a cross-brace onto the yoke between the hitch ball cap and the front frame.