Tom Ivar Helbekkmo

"Joe" <Joe@dontspam.net> writes:

> 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 think you're on to the right mechanism here. However, I think that
while the twisting of the tow vehicle is part of it, the real problem
is the twisting of the trailer around its own wheels.

Picture the trailer as it's nearing one extreme of the swaying cycle.
It's being pulled back in toward center by the forces exerted on it by
the tow vehicle and the trailer's own tires. This sideways force is
opposed by the inertia of the trailer's weight, which pulls outward.
The intertia is acting at the center of gravity (CG) of the trailer.
If the CG is behind the wheels, it will tend to twist the trailer, so
as to *increase* the correcting force of the wheels. This will, when
the trailer starts to swing inbound, cause it to do so *faster* than
it otherwise would, and will cause it to overcorrect, swaying over to
the other side. The twisting of the tow vehicle, which you describe,
will, of course, allow the trailer to twist more, making things worse.

The problem is well known in aerodynamics. A control surface, like
the aileron on a wing, has its CG well behind the pivotal point. At
high velocity, the aileron can start to flutter. The deflection of
the aileron causes an opposite movement of the wing, and, as the wing
starts to spring back, the aileron gets thrown the other way by
inertia. At sufficient speed, the oscillation is self-amplifying. A
common protection is to add weights on arms connected to the control
surfaces and extending forward of the hinge points, thus moving the CG
forward far enough to be safe at higher speeds.

-tih
--
Don't ascribe to stupidity what can be adequately explained by ignorance.

 

Rich256

"Tom Ivar Helbekkmo" <tih@hamartun.priv.no> wrote in message
news:864q3sviyu.fsf@athene.hamartun.priv.no...
> "Joe" <Joe@dontspam.net> writes:
>
> > 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 think you're on to the right mechanism here. However, I think that
> while the twisting of the tow vehicle is part of it, the real problem
> is the twisting of the trailer around its own wheels.
>
> Picture the trailer as it's nearing one extreme of the swaying cycle.
> It's being pulled back in toward center by the forces exerted on it by
> the tow vehicle and the trailer's own tires. This sideways force is
> opposed by the inertia of the trailer's weight, which pulls outward.
> The intertia is acting at the center of gravity (CG) of the trailer.
> If the CG is behind the wheels, it will tend to twist the trailer, so
> as to *increase* the correcting force of the wheels. This will, when
> the trailer starts to swing inbound, cause it to do so *faster* than
> it otherwise would, and will cause it to overcorrect, swaying over to
> the other side. The twisting of the tow vehicle, which you describe,
> will, of course, allow the trailer to twist more, making things worse.
>
> The problem is well known in aerodynamics. A control surface, like
> the aileron on a wing, has its CG well behind the pivotal point. At
> high velocity, the aileron can start to flutter. The deflection of
> the aileron causes an opposite movement of the wing, and, as the wing
> starts to spring back, the aileron gets thrown the other way by
> inertia. At sufficient speed, the oscillation is self-amplifying. A
> common protection is to add weights on arms connected to the control
> surfaces and extending forward of the hinge points, thus moving the CG
> forward far enough to be safe at higher speeds.
>

That happens. Ever see a small trailer "wagging" all over the road when
being pulled by a big truck?

I still think the problem with real amplification comes when the tow vehicle
gets into the act as well. Now we have the tail wagging the dog. It can
add even more energy, to the point where it causes them to roll.

If it were only due to the trailer 5th wheels and those using Hensley
hitches would not be almost immune to the problem.

As I said I have had it happen to me. Going perhaps too fast down a steep
grade when a gust of wind out of a canyon hit us. The trailer started to
take control of the steering. A quick flip of the brake control manual
lever pulled it right out.