Anybody ever pull a trailer on Interstate 20 between Meridian, MS and Jackson, MS? That section of Interstate is worse than any roller coaster ride I've ever been on. The "Yazoo Clay" in the ground underneath the road causes roads all over MS to look like ocean waves but it's extremely bad in this section I'm speaking of. I doubt there's any trailer hitch system that could ever tame that section of road.
My truck or car without a load will make you sea sick driving through there. My shocks on my truck had only 40,000 on them and the first time I crossed that section of Interstate I would have hit my head on the roof of the truck cab had I not had a seat belt on. I change to Bilistine Shocks and it made it better but it's still bad.
When I get my TT in a few days I will have to cross that section of road going from my home in Atlanta to Yazoo City, MS on my first trip since I'm going to be starting work there next month. I'm looking at getting the "Equal-I-Zer" brand sway control hitch. Anybody had any experience with that brand? I've been reading up on them the past couple of days and it sounds like it's a good hitch.
The following is just my opinion. I think what you have is a cyclic or resonant situation. It can be de-tuned by either increasing WD hitch load or decreasing it. But there is a steady state tow that supports it. It may be set off by a road that is timed right to match natural resonance.
I think the reason it happens is due to air pressure initially pressing against the TT front and unloading the hitch weight. This is what the difference is between speeds that encourage the porpoising and other speeds, including stop, that don't. As one poster stated, it's too bad they don't offer a damper for porpoising. The WD springs, which are now a major part of suspension, can spring vertically back and forth freely without any damping at all.
Part of the "air" forces might be further periodically modified by air getting under the front of the TT and lifting from the bottom when the TT porpoises up. When the front lifts slightly, the airstream may be breaking away from the TT roof, creating a sudden vacuum. Perhaps an imaginary person riding in the TT would experience a periodic sudden ear popping from the top vents, were they open. The same might be said of sway regarding an open window able to periodically pop ears.
In other words, I believe porpoising and most instances of TT sway are probably forms of slow speed flutter, the same forces that cause a flag to wave. The forces of air can be very powerful and hold large jet liners in the air, so porpoising and sway are a piece of cake.
Theories are cheap. What to do? Short of a "porpoise damper", I think new shocks. Other than some friction sway dampers, these are the only dampers available in the oscillating assembly. As some have stated, creating a different load can also break the porpoising by jamming weight to one end or the other. It's quite possible porpoising occurs because everything is too much in balance like a cat sitting on a fence post, leaning into the wind just right. But when the wind changes, look out.
Last fall, a fellow with an F-150 had the same problem. Since the truck was under warranty and well within it's tow rating, his local Ford dealer even removed the OEM shocks and they tested OK. In the end he installed the Bilstein shocks that the shop foreman had originally recommended ...and the truck was fine, more than fine actually. You can find the thread here. It's an interesting read, about 7 pages in all.
For those interested in further exhaustive speculation as to what may cause instablity in our TT's, there is a thread called Towing stability physics.
More tounge weight keeps the TT from rocking like a teeter totter with the axles being the fulcrum. More tension on the WD bars also helps. I found the dual cam sway control not only eliminates just about all sway, but also inhibits porposing too. I ran into some roads in MS also that no matter what would almost bounce the front wheels off the ground. Only thing to do then is to slow down.
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