At 13:57 07 October 2016, Dave Martin wrote:
Don.
You need to update your driving knowledge - the UK speed limits
for towin
a trailer is 60mph

https://www.gov.uk/speed-limits

I know that, as I should. My point was that adding 20psi to the tyre
pressure changed the handling of the trailer from unstable at
50mph to steady at 20mph faster. I suspect that a faster speed
would be possible but it was unnecessary to test as a 10mph
excess over the limit of 60mph was enough to show that just
increasing the tyre pressure was a partial answer to the problem we
had, and give a reasonable margin.
Being able to tow at 60mph means that I was able to avoid being
overtaken by large Artics governed to 56mph, avoiding the main
condition that triggers swaying, being restricted to 50mph did not
allow that.
Police generally ignore excess speed less than 10mph on
motorway/dual carriageways in the UK anyway, which is why most
traffic on those roads is doing 80mph.

At 17:13 05 October 2016, Don Johnstone wrote:
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
th
trailer
axle would help in most situations. Granted, proper weight is
best, bu
the
anti sway bar may widen the margin.
Thoughts?

Weighting is important, no doubt. Reducing the weight and
therefore th
inertia on the anchored end of the trailer does reduce the
possibility o
any swing increasing by increasing the ability of the towing
vehicle t
damp the oscillations. The trigger for these oscillations is
aerodynami
caused by the low pressure area between two vehicles as they
overtake/ar
overtaken. The situation is made worse if the vertical surface is
the sam
or more than the area in front of the axle. Cobras and Komets
are designe
to reduce the side area behind the axle.
I towed a trailer, containing a Grob 103 back from Holland. The
nos
weigh
was close to the maximum limit allowed for my car, theoretically
the bes
situation to improve stability, however it swayed readily. I have
jus
towed the same combination over 300miles. Before leaving I
increased th
tyre pressures on the trailer from 35psi to 55psi (Max 65psi).
The traile
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 bein
overtaken by large trucks, the condition which is most likely to
trigge
swaying. Tyre pressure appears to have a large influence on the
stabilit
of the trailer, greater than loading. Vertical instability is now th
majo
problem, easily fixed by a stabiliser.