Gotta share this with anyone towing anything. My rig is a '99 Expedition pulling a nice 18' steel bed dual axle trailer. Handles on the interstate with the 914 on it like it isn't even there at 70-75mph no sweat.
Sooo...helping out a friend, loaded up his Suburban on it for a 15 mile tow today. Chained and strapped down fine. Trouble came on the infamous James River Bridge at Newport News, VA. This one is 4 1/2 miles long with a high lift span that takes some momentum to keep up with traffic.
Approaching the upgrade at 55 & accelerating, the rig starts swaying--left, right, left & getting larger oscillation each time. Not wanting to lift, gentle acceleration now has me going 60 & the oscillation gets much worse, being fed I'm sure by the up,down,up,down of the concrete bridge deck spans on 100' centers. Tires squalling, using up both lanes I'm thinking "This is it....might not get out of this one alive...gonna be really messy..."
Somehow by sort of avoiding the countersteering & taking a longer arc to one side while slowing it down I came out of it....finished the other 3 miles of bridge at exactly 45mph, not one notch higher than that.
Lesson learned is that Suburban with its high CG, its suspension and the trailer suspension & tires all being fed in perfect harmonic sympathy just about killed me. Watch out if you load up anything that's 'way up high on your trailer!

You know the term tank slapper. Cool. Fun, huh? I've had that happen with low tire pressure in one tire on the trailer. Just one tire of the four, it was weird. And another time flat towing a VW with a bad tie rod end.

Also it's common here in the South and West for trucks make trenches in the lanes by continuously pounding the hot summer asphalt. If you get just the right combination of track in the tow vehicle and the trailer, then get to surfing up one truck tire groove with the truck and one with the trailer it does the same. Bad feeling, very very bad, almost impossible to drive, and makes you long for a concrete super-slab. I never would have made it as a truck driver.

Tail wagging the dog.
Your load weighed more than the truck.
Use weight stabilizer bars to eliminate the swaying.

good thing it wasn't windy too. That bridge is hairy in the van when it's blowing good

I've had that and solved it with just a bit more tongue weight.

getting out of it by using the electric trailer brakes [you do have them right!?!?] while you stay on the throttle and slow down on the trailer brakes.

also if you just strapped the axles down you might have been better off by strapping the chassis down. my jeep has a real soft suspension and there is no way to haul it with out strapping the chassis down, it would wag the trailer all over if I didn't.

I haul a 10k lb loaded toy hauler with brakes and bars [weight distribution].

towing with out trailer brakes is not a good idea and a quick way to be "that guy" that has caused the "big one" on the freeway!

That's where you need a trailer brake controller with a good hand control. You work the trailer brakes and slow the truck down using the trailer. Same thing can happen coming down a hill. Practice before it happens again, so you know how it feels

Years ago I lost control of a 22" trailer loaded with hay and thought I was going to throw the truck out of control too. Working the trailer brakes saved it, but there was hay all over the highway

Same happened to me couple weeks ago when the trailer was empty. Hit the brakes and the surge brakes transferred some weight to the hitch and stopped the motion. When we were loading the 914 on the tilt trailer we noticed the hasp on the front of the bed which secures the deck to the frame was missing- fricking rentals. So the front of the bed was just moving back and forth on the frame. Those harmonics once they get going are pretty scary.

I've saved my ass with trailer brakes in this same scenario before and I've done it more than once. The manual trailer brake option on the controller quickly straightens the rig out, stops the swaying and does it quickly.

tongue weight

What is the tow capacity on an Expedition? You had to be close, if not overweight.

I stopped towing with short wheelbase vehicles years ago for that reason. It takes quite a bit to tug my F350 longbed out of composure.

S##T.....

Lots of good input guys.....funny thing is that I did practice the manual application of the trailer brakes when I installed the controller a couple of months ago. During the episode I totally got focused on the mayhem going on in the rear view mirror with the white knuckle death grip on the steering wheel, never thought to grab the manual button down on the dash. Bet your ass I'll practice it over & over now 'til it's reflex. I know, I know--looking in the mirror also probably helped me get out of phase...."the car goes where the eyes go"...& that's the wrong place to focus!

Yep, it was overloaded, and it's short wheelbase definitely contributes, but more importantly that high C.G. really kicked butt...never again on my shift.

the high CG isn't that significant
I have an 8foot enclosed... very high

but I moved to a long bed dodge cummins and solved all of the problems..

its total weight and balance, with short wheel base

I have actually learned to lay my hand on the controller when I see a scenario that might lead to swaying. This is usually meeting a big truck, passing a large open space, like a bridge, or being passed by a big rig which usually wants to drag you toward them as they go by.

I even do it when towing with my 40' RV. I tend to load nose heavy with the trailer too, getting too much weight behind the aft axle just makes it worse.

Tongue weight is critical when hauling a heavy load. And being able to use the trailer brakes when you need them, in the event of load shift or whatever, is also critical.

All of this has already been said, but worth repeating.

I had your experience hauling a tractor and implements back from Salinas one day. The tongue weight wasn't high enough and the heavy equipment trailer I rented didn't have brakes that worked effectively. Probably the latter was more to do with my problem:

Coming down a hill the trailer started pushing the back of the truck around and I just barely got it back under control. Scared me big time, but it will never happen again.

Hauling heavy stuff is a different game than towing a light weight race car around. You can pretty much get away with anything with only a couple of tons behind you, but move that towards 4 or 5 tons and you can get into trouble fast.

Also be sure your truck and hitch are rated for the load you are carrying. A burp weighs 6,000# or there about, so with a trailer you are close to 5 tons behind you and need everything on the trailer functioning and loaded correctly.

Also I slowed down my hauling, even with only the 914 on board the trailer. All it takes is one tire exploding tire on a dual axle trailer, at 70mph, and things can get exciting fast. My limit is 65mph now and I have brand new trailer tires. Probably should be going slower.

Great discussion, every word is true! Tongue weight is critical. I've hauled many VW bugs on short trailers, makes a world of difference whether they are loaded facing forward or back (engine weght before or after trailer axles).

I was a novice when it came to towing and learned a lesson on weight placement cheap. Had too much weight to the rear of the trailer and as I approached 45 MPH on a little used country road, the rear of the truck started oscillation left to right. I slowed down real easy and got it under control and kept my speed down after that. Found out later how lucky I was in being able to slow down and maintain control. Didn't have trailer brakes either. I was really lucky!
By the way, tankslapper is a motorcycle term. The front wheel oscillates left to right in a high speed wobble, under extreme conditions, the handlebars will "slap" the gas tank. Been in a mild version of that also, scarry!!
Tom

This happens to me all the time. When I am hauling a full concrete trailer it tows great but when it is empty I have to adjust the tongue weight to the front of the now tail heavy trailer! If I forget to adjust it before I leave it will almost throw me off the road until I pull over and adjust it!

was the burbun on the trailer back wards?....or just not far enough forward on the trailer ,,,lucky no one got hurt

Ive always called it the death wobble...cause ya swear your about to die!

The Death wobble is a real thing on Jeeps!! It is very, very scary!!!

You hit a bump at 70mph and your front end starts to shake violently and keeps getting worse until you slow to below 40 mph!

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was the burbun on the trailer back wards?....or just not far enough forward on the trailer ,,,lucky no one got hurt

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It was on frontwards but as it is longer than the trailer I watched the placement carefully to judge that it wasn't too far back, noting the dip of the hitch to see that I was adding tongue weight. Frankly, I'm pretty sure it was overload plus short wheelbase then all the other factors easily set things in motion.
You're right its lucky no one was hurt...badly. I can still see the gray haired lady with the handicapped tag that I passed as I was accelerating & still wonder where she went....

I dont think you will do it again
keep on truck n