30 years of flatdeck truckin' for money have given me some pretty strong opinions about what works. I'm quite certain that the centre of gravity (CG) of the load has to be pretty close to the middle of the trailer, over the centre of the axle group or SLIGHTLY forward. That makes for a neutral tongue weight, which yields the best handling. If, when your trailer is loaded, you unhook and 2 guys (or one very strong one) can lift the tongue, your load is positioned properly.
As to 18-wheelers, the longer the tractor, the more stable it is at high speed. Try crankin' into a snowy curve a bit fast with a SWB tractor and you'll see what I mean. Nutbar? Perhaps. But I've logged 5 million kilometres (!) and haven't killed anyone yet.
