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.
