GoFaster
Moderator at Large
I built a coil-and-shock suspension for my motorcycle trailer to replace the original ordinary leaf springs, for all those reasons plus more. It no longer pounds tie-down straps until they work loose or break, it no longer rearranges the contents of my toolbox, and it doesn't transmit bump impact loads into the hitch and the car.I've only owned one trailer (old garden trailer converted to utility box trailer) that didn't have any sort of suspension, and it scared me half to death -- and I've been towing boats and trailers for decades.
Trailers really need springs and/or shocks of some sort, or they go rapidly out of control at speeds over 40mph. Even torsion axles are a disaster when the trailer is empty, so should only be used on trailers that always carry weight (like teardrops).
Both the converted garden trailer with no suspension, and my empty utility trailer with torsion bars were dangerous to tow on the freeway unloaded -- they'd do 3' hops at the slightest bump. Not recommended!
KISS principle: Normal trailer axle, "truck arm" geometry (although the two arms are brought to a single attachment point - doesn't have to clear a driveshaft in the middle), Panhard rod. You have to get the pivot point heights correct in order to minimize axle steering, but I did that, and the trailer is very stable on bumpy surfaces. If I were to do it again, there are a few improvements that I would make (could use more roll stiffness), but it works well enough as is.