At 14:59 05 October 2016, Charlie M. UH & 002 owner/pilot wrote:
I've wondered if adding a small (1/4" - 3/8") anti sway bar to the
trailer
axle would help in most situations. Granted, proper weight is best, but
the
anti sway bar may widen the margin.
Thoughts?

Weighting is important, no doubt. Reducing the weight and therefore the
inertia on the anchored end of the trailer does reduce the possibility of
any swing increasing by increasing the ability of the towing vehicle to
damp the oscillations. The trigger for these oscillations is aerodynamic
caused by the low pressure area between two vehicles as they overtake/are
overtaken. The situation is made worse if the vertical surface is the same
or more than the area in front of the axle. Cobras and Komets are designed
to reduce the side area behind the axle.
I towed a trailer, containing a Grob 103 back from Holland. The nose weight
was close to the maximum limit allowed for my car, theoretically the best
situation to improve stability, however it swayed readily. I have just
towed the same combination over 300miles. Before leaving I increased the
tyre pressures on the trailer from 35psi to 55psi (Max 65psi). The trailer
now tows with no swaying up to the maximum speed limit in the UK (70mph).
It does allow towing at just below 60mph which means I can avoid being
overtaken by large trucks, the condition which is most likely to trigger
swaying. Tyre pressure appears to have a large influence on the stability
of the trailer, greater than loading. Vertical instability is now the major
problem, easily fixed by a stabiliser.