chris wrote:
...This had no impact on the swaying issue.
The swaying issue begins with a steering input, does not appear/feel
to be aerodynamic in nature. [And even if it were the Forester does
not seem to have much steering response on these tires to counteract
the motion].

Also the swaying starts at about 45mph [72km/h] which is low for this
much force to be generated aerodynamically.

I owned a Forester for a short time a few years ago and I think the
steering is either over-assisted or inadequately self-centreing (or
both!). I suppose you'd say the force gradient is too flat. Whatever
the cause, it always seemed to me to be too sensitive to small inputs.
Unless I rested at least one elbow, the car would readily set up a
slight wander from side to side. It was easy to set up a rhythmical
roll which had very little natural damping even with no trailer
attached. It would drift off line easily with a moment's inattention.

The tyres were probably Bridgestones but I can't be sure now. They
certainly weren't low profile - probably 75-80 (this was the unblown 2
litre - not the GT)

I towed a Blanik about 800kms with the Forester. I certainly didn't
have chris's problems - I drove happily at speeds up to 110km/h with no
trailer instability but I remember it wasn't a relaxing drive and winds
did affect it. I recall carefully keeping my elbows on the armrests to
damp the steering. The drawbar load would have been about 60-70kg and I
followed the book on tyre pressures for max load.

My thought would be that the main problem is the trailer but it's
exacerbated by a car with sensitive steering and inadequate roll
damping. Check the drawbar load and suspension of the trailer first.
It's probably cheaper to move the trailer axle back than to change cars!

I'd agree with the guy who said boost the tyre pressure at the rear but
not the front. Stiffer rear roll bar would be useful. The GT version
probably has lower profile tyres and rims which would help - check if it
has a stiffer roll bar.

The Forester steering is similar to a SAAB I once owned. Audi are a bit
like it too. I think largish FWD cars tend to have dead, over-assisted
steering and Subaru is a 4WD which comes from a FWD tradition. 4WDs
which come from the other end - Land Cruiser, Land Rover - seem to have
better weighted steering and are less sensitive and more stable.

GC