After reading this thread:
http://www.irv2.com/forums/f45/cant-...ph-391458.html and many like it over the years, it struck me that people just don't know what causes the problem and how to address the issue - too many responses based on emotion and not physics or data.
This is a very complex problem and I'm surprised a good mechanical engineer hasn't stepped in to provide a better explanation. I'm not a good mechanical engineer, but I'm going to take a shot and hopefully a good one will chime in.
Despite all the opinions to the contrary, there's no reason why a properly designed tow vehicle/trailer combination can't go 200 or 300 mph. There are practicalities involved, but it's not even particularly difficult – but this is an aerodynamics problem first and a mechanical problem second.
A truck/trailer combination doesn't just oscillate for some unknown reason – there are forces involved. In this case, as a trailer with a flat stern goes through a fluid (air) it creates vortices that build on each rear edge. All things being perfect, they would occur at the same time, but this is the real world, so they alternate from side to side. These exert a drag force that pulls the trailer to one side until the vortex sheds. The response to this is a movement of the trailer toward the vortex and a movement back to center when it sheds. Then a vortex builds on the other side and the process repeats.
You can see this by following a semi in a light rain/mist. The one shedding off the top will make the trailer bounce. There's too much turbulent flow off the bottom to get a strong vortex there.
The airspeed at which the movement becomes noticeable has a lot to do with CG location, suspension design and condition, and tire sidewalls. It is 100% velocity dependent. The vortex forces increase with airspeed.
A number of things resist this movement. The force acts through a moment arm from the center of gravity of the trailer back to the stern, so the trailer is attempting to rotate around the center of gravity. This is resisted by the the force exerted by the tow vehicle through the hitch (including anti-sway devices there), and by it's suspension through the road. I know that the center of aerodynamic pressure moves as speed increases and as it moves it can have a de-stabilizing effect, but I'm unable to explain that aspect well, but it can be significant. Aerodynamic lift can also reduce the load on the steer axle, especially if the combination starts with a nose-up attitude.
Obviously stiffer sidewalls are better. Torsion axles are better than springs because there is less capability for lateral movement – no bushings, no shackles, etc. Since there will be motion induced in the vertical as well as in the horizontal, shocks would help to control movement in the springs.
Don't forget that on a flat-backed tow vehicle like an SUV – the same things are happening back there but at a lower level – and when the vortexes shed off it causes the tow vehicle to rotate and wag the trailer. Those aerodynamic forces then buffet the trailer. If all that gets in sync, you are really in trouble.
At this point something has to change to maintain stability, and you can slow down or push through it, but slowing down is more predictable.
The same things that allow motion on the trailer allow motion on the tow vehicle, so reducing those has an effect – stiffer shocks to resist the roll and the sideways push on the rear axle (maybe a watts link or similar device to locate the rear axle side-to-side!), urethane bushings, stiff sidewalls, dual rear wheels increase stability by both the increase in track width and the increase in sidewall spring rate because there's twice as many of them.
The ability to control this mess and resist the movement has to do with the relative masses between the vehicles and the relative lengths between the steer axle to the drive axle and the drive axle to the hitch ball. There's also the need to have sufficient weight on the front axle for proper alignment both static and dynamic – hitting a big bump that lifts the front at the wrong time could create a moment that drives a controllable situation into an uncontrollable one – thus the need for a WD hitch to put weight back on the steer axle.
Obviously once you buy a particular trailer/tow vehicle combination there's not a lot you can change, but you need to start with controlling those things that produce the primary effects first:
1) reduce the aerodynamic forces at the rear of the trailer – about the only thing you can do here is add airtabs or some other trip strip to induce turbulent flow at the back.
2) Improve tow vehicle/trailer controllability – move mass between the trailer and tow vehicle, and in the trailer (can use water as ballast) to produce the proper tongue weight with the ball height and weight distribution to create level to no more than about a half-inch nose down attitude on the trailer while getting the nose of the tow vehicle as close to normal ride height as possible. This might require additional spring rate at the rear, and moving cargo into the tow vehicle to improve the ratio of the masses between the vehicles. Remember this will change as you burn fuel and with your passenger load, and has a big effect on half-ton vehicles and smaller SUVs. Add a cam-type sway control system to the hitch.
3) Reduce the uncontrollable mechanical movement in the suspensions, starting with the trailer – stiffest sidewalls you can get (maybe go to bias tires, which removes the speed rating concerns) go to urethane bushings, lower the trailer, and add shocks. In the tow vehicle – urethane bushings and stiffer shocks. It sounds like there's nothing to gain in the tow vehicle tires. If you have a GM product, the idler arms are known weak points and will wear quickly towing heavy loads and put slop in the system. I'd cut to the chase here and get a supersteer idler arm support, a cognito motorsports idler arm than pittman arm support kit, (I used to go though an idler arm every 20K miles and these items put a stop to that). Might also consider a PPE idler arm.
4) Consider changing the aerodynamic interaction between the tow vehicle and trailer – most likely with a small roof wing. You don't need a huge one, and small airtabs on the side of the tow vehicle might help, but that's probably not visually acceptable. The other problem with this approach is that it's just not predictable and wing angle and placement would just be trial and error.
5) Add a lateral axle location device to the rear.
If those changes don't allow you to tow at the desired speed, then you need to change up the tow vehicle (mass, length, track, wheelbase) or the trailer (airstream type, axle location, CG location through a different type or layout), because there are some things you just can't overcome technically or economically.