If you recently bought a single axle utility trailer kit, you will need to install the leaf springs and axle along the trailer's center line. Your single axle trailer kit will likely include a rectangular trailer frame, the A-frame tongue assembly, an axle, two leaf springs, two wheels, shackle hangers and bolts. The axle and leaf springs, which suspend the axle from either side of the trailer, must be centered accurately across the frame. The trailer's center line is found by taking 60 percent of the frame's total length and marking this distance along either side of the trailer, starting from the front end of the frame. The axle and leaf springs are mounted at this center line, across the width of the frame. The leaf springs must be centered across the frame to ensure proper axle alignment, and to help prevent trailer handling problems and tire wear.
Measure and note the length of your rectangular trailer frame from the front to the rear end. Measure only the length of the frame -- do not include the A-frame tongue assembly.
Determine your trailer frame's center line. Multiply the trailer length by 0.60. For example, if the trailer frame's length is 10 feet, then the center line location will be 6 feet from the front of the trailer. Measure and mark the calculated distance from each front corner of the rectangular trailer frame to a point along each side of the frame. These points, if connected, mark the center line location across the width of the trailer.
Determine the mounting location of the front leaf spring hangers. Measure the distance straight across a trailer leaf spring, from end to end. Divide this length by 2, then subtract the result from the center line measurement. This value indicates the location of the front leaf spring hangers, which will be forward of the center line mark by one half the leaf spring length. For example, if the center line distance is 6 feet from the front of the frame, and the leaf springs are 2 feet long, then each front hanger should be installed 5 feet from the front corners of the frame. Measure the calculated distance, starting from each front corner, and mark the front hanger locations along either side of the frame.
Determine the location of the rear leaf spring hangers. Add half the length of the leaf spring to the previously calculated center line distance. This value indicates the location of the rear leaf spring hangers, which will be farther from each center line mark by one half the leaf spring length. For example, if the center line is located 6 feet from the front of the frame, and the leaf springs are 2 feet long, then each rear hanger would be installed 7 feet from the front corners. Measure and mark this distance on either side of the trailer frame, starting from the front corners.
Install the front and rear leaf spring hangers on either side of the trailer, at the corresponding marked locations. If necessary, drill holes to accommodate the hangers. Mount the hangers on the sides of the frame, at their marked locations. Mark the locations of the hanger bolt holes on the trailer frame. Remove the hangers, then drill holes to fit the mounting bolts included with the kit. Reattach the hangers and secure them to the frame using bolts, nuts and a socket wrench.
Mount the leaf springs to the front and rear hangers on either side of the trailer. Attach the front ends of each leaf spring using a bolt and a socket wrench. Place the axle across the leaf springs. Raising one leaf spring at a time, attach the rear end of each leaf spring to its rear hanger. The axle should be suspended from the leaf springs. Have an assistant to hold the axle to prevent it from rolling off the leaf springs as you raise and secure them to the rear hangers, if necessary.
Fasten each axle end to a leaf spring by fitting retaining brackets over the axle at these points, and secure each bracket with a socket wrench, using two U-bolts and nuts. The U-bolts should thread through the retaining bracket and clamp the axle to the leaf spring.
Test drive the trailer to assess balance and handling. The trailer's tongue should rest firmly attached your vehicle's hitch ball. If the trailer tips backward, lifting the tongue away from its connection with the hitch ball, you might need to move the axle and leaf springs forward slightly. The trailer should not pull excessively to the left or the right.
