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Severe vibration in RV towing Cherokee—engineer's opinion needed!

Until yesterday, we had flat-towed our 2015 Cherokee Limited behind our motor home (31-foot Thor A.C.E. on a Ford dual-wheel gas chassis) uneventfully for 8000 miles. We use a Sterling All-Terrain tow bar, with a frame mount on the Jeep and a standard hitch receiver on the RV. Yesterday we hit a patch of horrendously rough pavement in a construction zone on I-75 in Cincinnati that caused 2-3 seconds of severe slamming to the wheels. This started a violent vibration in the vehicles, with the RV rolling from side to side and the Jeep (which I could see in the rear-view camera) doing the same. The steering wheel of the RV did not seem to be twisting significantly, but the rate of squeaking/creaking and the rate of the oscillation of the whole rig made me think I had blown a tire, bent a wheel, bent an axle, etc. There was no shoulder in this construction zone, but by the grace of the gods of travel, it was less than 1/4 mile to a closed-for-improvements 2-lane entrance ramp set off by orange barrels, and there I was able to get off the highway onto a large empty expanse of pavement. The rolling and pitching continued until I stopped.

When I got out to survey what I expected to be kilodollars’ worth of damage, I found nothing amiss! All 10 tires and wheels on both vehicles were intact and the tow rig appeared normal and firmly attached to the Cherokee. The right rear wheelset of the RV was visibly set about 1" further back in the wheel well than the left wheelset (I can’t say that this is not how it always was), and I was afraid the axle had been damaged or displaced, but a careful check with a steel tape indicated that the right and left wheelbase measurements (center-to-center, front hub to rear) were identical. Out of an abundance of caution, I disconnected the Jeep from the RV and drove it up and down the closed on-ramp and detected no vibration, unusual sounds, pulling of the steering wheel, or any other abnormality. Then I did the same with the RV, with again no abnormal sounds, motions, or vibration. We drove both vehicles back up the (closed) on-ramp a couple hundred yards, hooked up again, and came up to speed uneventfully before getting back on the Interstate. The remaining 200 miles home were perfectly routine.

What happened?

The only transient event resulting in no apparent damage that I can imagine causing what we experienced is an oscillation caused by an asymmetric impact on one of the Jeep’s front wheels in the damaged section of pavement that caused a deviation of the Jeep’s steering far to one side, whereupon a restoring force sent it back in the opposite direction, overshooting center, thereby sustained for as long as the vehicles were moving, placing large lateral displacement and axial torsional loads on the hitch receiver that were transmitted to the RV frame. It seems to me that a violent twisting of the steering wheel/front wheels from side to side in a flat-towed vehicle at highway speed would cause the motions that we experienced in the RV, but I don’t know enough about the steering geometry of modern vehicles to know whether this scenario is dynamically possible; certainly, the geometry is self-centering in the forward direction under normal conditions, and severe oscillation originating as I propose would be, at the very least, dynamically disfavored under all but the most extreme circumstances, or we wouldn’t be flat-towing vehicles at all, would we?

Any engineers’ opinions? Or from high-mileage tow-ers? And yes, I know that the hitch receiver, its attachment to the RV frame, and the Jeep’s frame at the points of attachment of the tow bar need careful inspection.

The Jeep went into a “Death Wobble”, it’s front wheels oscillating left and right setting up an extreme vibration throughout the rig…The rough pavement started it and once started it can continue until the vehicle(s) are brought to a complete stop…Since your Jeep is new and still fairly tight, I would not worry too much about it. Hopefully a one time event. In older high-mileage 4WD vehicles this event can occur more frequently… Was the steering column in the Jeep locked?

The Jeep went into a “Death Wobble”, it’s front wheels oscillating left and right setting up an extreme vibration throughout the rig…The rough pavement started it and once started it can continue until the vehicle(s) are brought to a complete stop…

Yep, had something like this happen to me towing a car behind a pickup. All was fine until the rig went over a significant bump in the road and then it escalated into a near crash before subsiding as the rig was brought to a stop. Exactly as Caddyman described. Although the car wheels were locked, there was sufficient play in the steering linkages to allow the resonant oscillation to occur. The truck tires on one side were off the pavement at one point but fortunately the only damage was to two pairs of fruit of the looms (me and the passenger in the truck).

Knowing that this has happened to some motorcycles too I searched “Tank Slapper” and found a Wikipedia article on “Speed Wobble”. It looks like increasing some tire pressures to reduce lateral compliance can help to prevent a recurrence. If there is not enough steering damping in the Jeep afforded by the non-running power steering unit I would wonder if a steering damper on the Jeep might help.

I was a passenger one day in a mid 1950s Ford that was pulling a heavily loaded two wheel trailer that went into a “Death Wobble” on a straight road. The wobble was initiated when the driver, a young man, turned the steering wheel quickly from left to right for no good reason. The extreme wobble was frightening and would not stop until we came almost to a stop.

I have to disagree with the “death wobble” hypothesis. This SUV is a transverse primarily front-wheel drive independent strut suspension driveline and as such does not suffer from the dreaded Death Wobble as such.

That said, there are lots of undamped motions mentioned in your post like steering. Roll motions from the Jeep are pretty well damped. I would toss in another couple of motions - the torsional and beaming response of the RV’s frame to the uneven pavement (you didn’t really think that frame was rigid, did you?) and the response from the Jeep’s frame (for the same reason as the RV) and you’ve got a cornucopia (couldn’t help using that word, Thanksgiving is upon us) of undamped vibrations being fed by a seriously messed up section of I-75.

You’ve got to be pretty brave even to drive through that mess towing anything. Not much you can do about it more than you did. Once you set it off and the road feeds the vibration, it is tough settle down. Slow down pull over and make sure nothing got broken. Take a deep breath and continue.

An engineer’s $0.02

I had this happen when I was young and foolish.

A bunch of us were coming from the lake with a boat/motor in tow.

On this two lane highway we came upon a farm tractor in the same lane. Looking ahead I could see vehicles coming at us from the opposite direction. Thinking I could easily pass the tractor before they reached us I stepped on the accelerator. I didn’t take into account of the weight of the boat/motor/trailer. So it took a little longer to get up to speed to pass the tractor.

When I did finally pass the tractor I had to whip the vehicle back into my lane. With speed I was going and the sudden change of direction caused what I was towing to start rocking from side-to-side. This started the vehicle I was driving to start rocking from side-to-side. This then caused what I was towing to rock side-to-side even worse. It was like the two entities were feeding off each other. The rocking in the vehicle got so bad that I almost lost control of the vehicle.

Ever hear five guys screaming like little school girls?

And I was one of them.

I couldn’t get the vehicle under control until I stopped.

Memories…

Tester

When I went back and noticed it was a 2015 Cherokee I knew my “Death Wobble” hypothesis was probably wrong…Mustangman’s explanation is probably closer to the truth…When the tower and the towee start fighting each other, frames and hitches become energy storing springs, first absorbing and then releasing that stored energy…The vehicle driver instinctively starts trying to correct for the motion with steering input which only makes matters worse…Things can quickly get out of hand…

Thanks to all who answered. The exact mechanism is probably not important, at least not as much as knowing that this phenomenon is known to occur and does not indicate a fatal flaw in mechanicals or technique. I confess that I don’t quite understand why it could not have been a “death wobble” or just what the difference between Caddyman’s and Mustangman’s answers is, but I will add that the Jeep’s steering, to my surprise, is NOT locked when the ignition is off. I had thought that a steering lock was universal, maybe even legally required, in cars these days, but clearly this is not so. I will also point out that I did not try to correct for the oscillations with the steering wheel; I knew enough to maintain course, slow down, and get off the highway as quickly as possible. I may have only 8000 miles of towing experience, but I am a 50+ year, million+ mile accident-free driver, and have learned a few things! Not that I needed another in addition to the virtual cornucopia* of existing reasons with a rig this clumsy, but now I’ll be even more careful not to make sharp steering inputs while towing, as well as to be hypervigilant for distressed pavement.

Howard

*Obligatory expression in Car Talk Forum :slight_smile: