"TheSnoMan" <admin@snoman.com> wrote in message
news:Ck7Bf.5206$vU2.5042@newsread3.news.atl.earthlink.net...
> TheSnoMan wrote:
> > Rich256 wrote:
> >
> >> "TheSnoMan" <admin@snoman.com> wrote in message
> >> news:rY5Bf.11752$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.
> >>
> >>
> >>
> >
> >
> > Bet is was a 1/2 ton P/U or SUV pulling it too.
> >
>
>
> I also want to add if you are quick on your feet and apply trailer
> brakes when this happens (not vehicle brakes because it can make it
> worse) you can usually get it under control if you catch it in time.
>
Been there. Did it. Coming down a steep mountain road, going too fast,
encounted a strong cross wind out of a canyon. Trailer brakes pulled it
right back into position.
