Tire Stiffness & trailer swaying

Several people have had good luck towing a glider with a Subaru
Forester.
I do not know why mine is different.
I just bought a 2004 Forester tried to tow with it. Mine is a
nightmare. It is unstable above 50mph. It has a lot of side to side
sway above 50mph if there is any steering input. It is almost
harmonic in nature – it does not dampen out quickly. If feels like
you are driving on Jello. It feels like the problem is much more the
tires than the suspension. Though that is hard to prove.

Even when parked if you push on the hitch with your foot the Forester
will sway side to side [right and left] a lot. You can watch the rim
move right and left in and out of the tire. I think the sidewalls are
just really weak. The tread is probably relatively soft also.

The Forester's tires are Yokohama Geolander G900 P215/60 R16 94H.
Even in normal driving the steering response and cornering are poor.
[Other Foresters may be equipped with 15" tires, and other models of
Geolander tires – rather than the G900].

My Mazda MX-6 [much lower car] has Pirelli P4000 P205/55 R15 87H – it
is stable, and if you push sideways it does not sway side to side
anywhere near the degree that the Forester does. Steering response
and cornering are good.

The Geolander G900 has a tall soft sidewall, it is 4.500" above the
rim rather than 3.625" for the P4000 [24%higher].

My best idea right now is to change to a shorter and stiffer sidewall
tire.
I am considering changing from P215/60 R16 94H
- an "H" rate tire with 60% width/height ratio, to
Bridgestone Turanza LS-V 225/50R16 92V - this is a V rate tire so the
sidewall is stiffer and at a 50 or 55 ratio it is shorter.
This is the V rated version of the tire that some other Forester
owners have [LS-H]. The tread should also be a harder sports car like
compound.



Some of these tires are rated and discussed on www.tirerack.com. It
seems that Steering response and cornering stability are good
expressions of my issue.
On tirerack there are a mass of good comments on the Turanza tire –
though I doubt anyone is towing. As far as I can tell everyone on
"tirerack" hates the geolander 900 like on mine.


Does anyone else have any Experience changing to a shorter V rated
tire for better stability?


My trailer does great and stable to 85+mph behind my Mazda MX-6, so I
do not think it is the trailer. The trailer is a 1979 Komet with a
Mosquito. It weighs ~1800lbs [816kg]. The tongue weight is 153lbs
[69kg]. The Forester's manual says to keep the tongue weight between
8-11% of the trailer. 8%=144lbs [65kg] 11%=198lbs [90kg].
[the max allowed is 200lbs on the tongue] The manual transmission
version of the Forester is rated for 2400lbs [1088kg] towing capacity.


I have already tried raising the tire pressure on the rear tires to
41psi [2.8bar] as recommended by the manual. The trailer tires are
about 40psi.


Chris Ruf