Ford Forums - Mustang Forum, Ford Trucks and Cars - View Single Post - ? Tail-Heavy Trailer Sway

01-23-2006, 10:05

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.

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