Harbor Freight bolt-together trailer frames are a great base for many home-built trailer projects. We specialize in and show people how to use them under DIY Compact Camping Trailers projects. A drawback I found with using them was their stiff riding, noisy stock springs.
So a few years ago, we started making a Harbor Freight Smooth Riding Spring Retrofit Kit. These kits have smooth riding springs matched to the loaded weight capacity of DIY Compact Camping Trailers. They have the added benefit of no annoying banging from the factory loose fit slipper style rear spring mounts.
Recently we’ve improved our Smooth Riding Spring kit by upgrading the Springs. New premium springs with a longer overall length of 31", smoother ride and available in a 1175 lb (2 leaf) and 1450 lb (3 leaf) GVWR / load weight. The springs are available in our Harbor Frame Frame Retrofit kits or separately for welding your own frame.