"TheSnoMan" <admin@snoman.com> wrote in message
news:rY5Bf.11752$ZA2.9222@newsread1.news.atl.earthlink.net...
> William Boyd wrote:
> > TheSnoMan wrote:
> >
> >> John S. wrote:
> >>
> >>> Joe wrote:
> >>>
> >>>> I got really curious about this, and studied it for a long time (I'm
an
> >>>> engineer). There was a good thread on this at woodall's RV forum. I
> >>>> have
> >>>> found no frank discussion of it by anybody who has broken it down
> >>>> into its
> >>>> component forces. It's hard to explain it using ASCII here. But my
> >>>> opinion
> >>>> is this:
> >>>>
> >>>> First off, it's clear that trailers can amplify their swaying until
you
> >>>> crash. They don't do it all the time, but they can, and that's what
> >>>> interests me. They have no power of their own to bring to the table.
> >>>> The
> >>>> energy to do this comes from the tow vehicle. The trailer sways like
a
> >>>> pendulum, sort of (I admit the forces holding a trailer back are not
> >>>> exactly
> >>>> like gravity in this analogy). A pendulum can be powered by a wide
> >>>> variety
> >>>> of motions. One of those motions is moving the pendulum's pivot
> >>>> point side
> >>>> to side. I think this is the motion we want.
> >>>>
> >>>> The forces on the trailer come from its tires and the tow vehicle. I
> >>>> was not
> >>>> interested in any other forces. "front loaded" trailers are trailers
> >>>> where
> >>>> the center of gravity is in front of the tires. When the tires are
> >>>> pushing
> >>>> the trailer sideways (during sway), the center of gravity is in
> >>>> front of
> >>>> that force. Since the side force doesn't go exactly through the
> >>>> center of
> >>>> gravity, the weight of the trailer pushes sideways on the tow
vehicle.
> >>>>
> >>>> The inertia of the swaying trailer pushes the tow vehicle from side
> >>>> to side.
> >>>> The difference in loading is simply this. If the center of gravity
> >>>> is in
> >>>> front of the wheels, it pushed the tow vehicle one way, and if it's
> >>>> behind
> >>>> the wheels, it pushes the two vehicle the other way. One cancels the
> >>>> sway,
> >>>> and the other amplifies it.
> >>>>
> >>>
> >>>
> >>> I don't know if amplification is the correct term. Poor loading will
> >>> increase the sensitivity of a tow vehicle and trailer to winds and
side
> >>> drafts from big trucks, but I don't understand how the forces could
> >>> actually be amplified. In my experience the side-to-side swaying is
> >>> amplified or increased by the driver trying to offset the swaying by
> >>> steering out of it.
> >>>
> >>
> >> This is true and just about all trailer sway issue start with tow
> >> vehicle stabilty issues. (tires and suspension not up to the extra
> >> load on them)
> >>
> > I do not agree. I think trailer sway issues start with proper loading.
> > I do agree that tow vehicle stability issues can be attributed to tire
> > and suspension problems. But they will not be the primary cause of sway,
> > the distribution of the load will be. Even a trailer that is grossly
> > over loaded may not sway if the CG is correct. But this is where the tow
> > vehicle suspension and tire pressure will show up more readily.
> > And I am no mechanical engineer or whatever scientist, but a retired
> > transportation superintendent with a lot of miles behind me. ;-)
> >
> CG is a factor but if your TV is not up to the load, it will be
> unstable. Your TV has to effectively anchor the front of the trailer
> where you want it to be and if trailer force easily deflect the TV's
> controll of this, you will have sway. Correct CG on a poor TV can still
> be quite unstable while even some CG error can do well if the TV is
> stout enough.
>
I think you have it there. The TV is the driving force that keeps the sway
going. And the reason that a 5th wheel or trailers using a Hensley or
Pullrite hitch pretty much eliminate the problem.
Suppose you pulled a badly loaded but small trailer behind a semi tractor.
If the trailer deflected due to wind, the truck would not move and would
pull the trailer back into position. But when pulled by a small vehicle,
the rear of the TV would be pulled out of position. As it tries to pull
the trailer back into position and straighten itself out it could put energy
into the system causing the trailer to swing even further in the opposite
direction. The rear of the TV also swings even further setting up an
oscillation.
I talked to a couple that had a new 30 foot trailer and new tow vehicle and
were using a Hensley hitch. The previous year they started off on vacation
with a similar set up except using a standard platform hitch. They got
about 30 miles from home when the trailer began to sway and before they
could react it rolled.
