Nehmo

For simplicity, let's assume one axle and a flat deck trailer. My
experience has been the sway is dependent on the relationship of the
Center of Mass (CM) to the axle, in other words, if the CM is in front
or behind the axle.

Hitch - CM - Axle = stable
Hitch - Axle - CM = unstable

Trailers are self-correcting in terms of steering, like a caster on a
piece of furniture. If the wheel "steers" to the left like this \ , the
back and the front of the wheel touching the ground get resistance from
the ground. But the resistance on the back has more leverage in
relationship to the pivot, so the force is translated into a clockwise
turning moment about the pivot. This turning moment corrects the
orientation of the caster until it is pointed parallel to the direction
for travel, like this |.

So front-heavy trailers correct themselves regarding steering like the
caster, but the corrections are small. In tail-heavy trailers, the
steering corrections are large and they may even yank at the tow
vehicle.

I still don't understand it; I'm just restating the problem.
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(||) Nehmo (||)