? Tail-Heavy Trailer Sway

Yup...too big a trailer for the vehicle and an incorrest tow bar adds up to the potential for big problems. Add to that the tendency for inexperienced drivers to go too fast for conditions, especially on the downhill and you have a recipe for a serious accident.

A family member was towing a large trailer with a Suburban and got caught in a truck sidedraft when going too fast downhill on the way to Wyoming. Rather than holding it steady he tried to compensate and ended up flipping the trailer and spinning the truck round in traffic. No injuries thanks to some fast thinking truckers.

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.

Actually a Helsley or Pullrite will "reduce" the tendency for the towed vehicle to sway the towing vehicle, but it will not eliminate it, their websites notwithstanding. They are a better solution that a standard hitch because the pivot point is moved closer to the rear axle. A fifth wheel places the pivot point between the wheels, thus further reducing but not eliminating the ability of the towed vehicle to alter the direction of the towing vehicle.

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.

And.... don't try putting the brakes on in an aggressive manner.

What do you mean reasonable response, I would stop her as soon as I could, throw the anchor out and hope for two things, it hooks on something big enough and the second thing is the chain does not break.

YOU DO NOT WANT TO VEHICLE BRAKES AT ALL WHEN THIS HAPPENS, TRAILER BRAKES ONLY TO PULL OUT OF IT.

Yes!

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

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.

Not to wake up an old thread but seems like some came close to the answer but couldn't explain in terms that were easily enough grasped by someone with out engineering/dynamics modeling experience. Here is my attemt to explain...

Take the TV out of the equation for now:

Let's go to an extreme and say that the center of gravity is located a good distance behind the axle - maybe ten feet. Also assume that the trailer is being towed by a device on a straight rail and will not leave a predetermined straight path. One of Newton's Laws says an object in motion will stay in that motion unless disturbed. Under this ideal situation the trailer would track perfectly behind the tow point regardles of C.G. (center of gravity) and maintain a straight line.

Try to imagine looking down on this situation from above in an aircraft following the direction of travel. Now, imagine there is a gust of wind (or any other disturbance) that hits the left side of the trailer and pushes the trailer to the right. Now if we release the hitch/ball at that exact moment, common sense should tell you that the trailer will swing around such that the rear of the trailer becomes the front. This is NOT stable.

Now let's look at a situation where the trailer is properly loaded with CG in front of the axle. In this case let's pull the pin on the hitch/ball right after the gust. Again, common sense should tell you that the normally loaded trailer will maybe waver right and left a bit but the front will stay in front - in the direction of travel. This is STABLE.

In a nutshell, the CG point will TRY to align itself in the direction of travel AHEAD of the wheels/axles/(any form of resistance).

When you bring in the TV, you introduce another whole set of variables including harmonics and how quickly the trailer settles down. But fundamentally a forward CG on a trailer will be more stable than that of a rearward CG. A rearward CG will always result in the back of the trailer hunting to become the front during ANY disturbance. There are other dynamics that can come into play like aerodynamic pressures that may be able to overcome the inertia of the CG but not in a travel trailer unless you have big fins... In fact the farther forward the CG the more stability there is - but this has other undesirable effects like hitch weight etc...

For simplicity, One axle trailer. Let's pretend one wheel too. The trailer is just a beam with a hitch at one end and a wheel mounted somehow near the center of the beam. The tow hitch is stationary. The trailer is on a short conveyer belt running under it to mimic road travel. We are looking down form above with the top of our head toward the front of the trailer.

The trailer sways to the right. We instantaneously remove the hitch pin.

If the CG of the trailer is behind the wheel, the wheel acts as a pivot, and the trailer tries to rotate counter clockwise.

If the CG of the trailer is in front of the wheel, the wheel acts as a pivot, and the trailer tries to rotate clockwise.

Counter-clockwise increases the sway, but clockwise opposes the sway.

Is that it? I'm still thinking about it.

Sheesh this has been beaten to death - Real easy to imagine and try out. Go to your grocery store and get a cart. Stand on the front of the cart and start riding it like a scooter pushing it backwards in a straight line and tell me what happens....(yes, the front wheels under and to the front of your feet do pivot, but that only helps to accelerate the fact that your cart is attempting to put your ass out in the front)

Scott

One problem is that the tow hitch is not stationary. When the trailer sways the tow vehicle also sways. The lighter the tow vehicle the larger the sway.

But, you are ignoring all those ratings and claims by salesmen and manufacturers who claim that incredibly light vehicles will tow incredibly heavy loads.

What's up with that?

Steve ;-)

OK, I appoligize to them. Hurt their feelings?

Also should have added if the tow vehicle didn't sway we would not be so concerned about the trailer sway.

Except for the guy in the next lane.

Normally the trailer will not show significant sway unless the tow vehicle does. I base that on the fact that trailers pulled with 5th wheel, Hensley/Towrite hitches rarely have any noticeable sway problems.