Lorry wrote:
Hi Erik,

From my experience I think you are right on! Trailer tires are usually
a bias belted tire which have a much stiffer side wall.

Take a look at the trailers (both travel trailer and glider trailer)
around you and in the showrooms. You will find almost all of them come
with radial tires. Bias ply tires are cheaper to buy, and that is their
only advantage. Radials are a superior tire, even for trailers. There is
an important difference between trailer rated tires and the typical car
rated tires, even with the same size: the trailer tire _tread_ deflects
less (lower slip angle) for the same side load. There are some other
differences, but that's the one we notice.

A previous
thread cited great improvement by using a 17 inch wheel with low
profile (aspect ratio) tires.

A lower profile will also yield a tread that deflects less.

One way to improve the situation you
describe is to signifigantly increase your rear tire pressure (36-38
psi) and also the pressure of the trailer tires (40 psi).

This will often reduce the "fishtailing", but don't exceed the maximum
pressure rating, shown on the side of the tire. Running a higher
pressure than recommended for the vehicle and tire will likely also
increase your tire wear, increase your braking distance, and definitely
give a harder ride.

To get a
better feel for the situation, try pushing the rear of your vehicle
sideways with a pulsing effort. When your frequency is close to the
natural frequency in that mode you will be surprised at how little
effort it takes to sustain the motion. If you watch the sidewall while
doing this you will notice that the majority, if not all, of the motion
is in the side wall.

The side to side motion you see by doing this is unimportant, and is
about the same at all speeds, unlike true swaying. As attractive as this
theory is, sidewall deflection of this sort isn't what controls the
swaying behind the tow vehicle, which is a dynamic interaction of the
tread deflections (not sidewall) on the tow vehicle and trailer, coupled
to the masses of the vehicles through the hitch.

The increased pressures will increase the natural
frequency and thereby allow a higher speed before the swaying becomes
objectionable.

THe increased pressure reduces the tread deflection for a given side
load - that's why it can help. It might help enough to be useful. I
think, instead, you should carefully consider if these new tires really
are suitable for your vehicle. Do a search using Google's "advanced
groups" search, using these:

AT least one of the words: forester
Group: rec.aviation.soaring
author: Chris

Chris solved his Subaru Forester swaying problem by replacing tires.
Quite dramatic.


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Eric Greenwell
Washington State
USA