Normally the trailer will not show significant sway unless the tow vehicle does. I base that on the fact that trailers pulled with 5th wheel, Hensley/Towrite hitches rarely have any noticeable sway problems.
Rich256:
I'm trying to isolate the aspects of the problem. But if you have a front-heavy (CG ahead of the axle) trailer, you can slalom right and left quite rapidly and the trailer will obediently follow. Once you take a steady path, the trailer will fall in line without further swaying.
For the problem of losing control you can't remove the tow vehicle from the problem. It is part of the system.
As the trailer salalom's right and left the tow vehicle follows. That's when it gets scary!! The trailer begins to take over control of your steering.
As I said if you are using a 5th wheel, Hensley, or Towrite hitch the swivel point moves to the rear axle of the tow vehicle minimizing the effect of the tow vehicle.
You got that just right. It is difficult to visualize but basically the center of gravity will always try to get in front of the one wheel pivot that you describe - I like that description - it's a little easier than mine was I think...
So let's just try to take this a little farther!
If the C.G. (center of gravity) of the trailer is offset by a small distance from the single wheel, the amount of force will be less than if the C.G. is farther from that single wheel. So if we take an out of balance/rear C.G. situation and go from say 2 feet behind the exle to 4 feet behind the axle, things will happen much more rapidly as far as the trailer geting out of control. Likewise, if we have a C.G. ahead of the axle at two feet it will stabilize less quickly than if the C.G. were four feet ahead of the axle. (This is really a description of why there is tongue weight on any trailer.)
IMPORTANT NOTE: The longer a trailer is, it will usually have what engineers call a larger "radius of gyration." Simply put, it is like having a bigger flywheel that once it starts rotating is harder to stop. If heavier weaight is distributed closer to the axles, even a larger trailer will be easier to handle. That is why larger trailers on any TV will want/need all of the steering dampers and what not whereas a shorter trailer will not require those neccessarily. By integrating the two vehicles a bit more, they can both fight disturbances from straight and "normal" a little bit better. It begins to matter that the capacities of the TV are at this point. It also starts to involve the attachment point (TT vs 5R.)
Note that aerodynamic forces will play a very large role for crosswinds and attachment points. This also includes the TV into the equation. Note that most RVs have the axle placement around 60-65% rearword (or more.) That solves the C.G. problem but the aerodynamic and harmonic issues will vary from combination to combination of vehicles.
Harmonics is difficult to explain and harder yet to analyze but my guess is most folks here have had the situation where a vehicle combination "just didn't tow right" at a certain speed but was fine at higher and/or lower speeds. That is the dynamic/harmonic of that particular combination.
Uff Da! That's a lot. I actually know it helped me to think a lot about trailer dynamics although I haven't had any problems yet...
Is there less likely a chance of a vehicle on a tow bar swaying than a vehicle on a tow dolly ? I am looking into a much smaller vehicle to tow the next time I take the motorhome out and will probably flat tow it rather than putting it on a dolly.
That's what I think makes all the difference. *but* these thought experiments never work on the actual dynamic problem. It's too complicated.
Makes for some great pictures, though.
No, I think he's right. For every trailering-related crash, you can always say if you'd been pulling a half as big trailer with a twice as big tow vehicle you wouldn't be crashing. That's the important thing.
How do those semi rigs manage it? Trailer and load around 60,000 pounds, tractor weighs about 20,000. Or mobile home movers, 40,000+ pounds of mobile home being pulled by 12,000 pound truck on a ball hitch. Its being loaded correct, its having brakes on the trailer that work and are balanced to the tow rig, its not pretending your Mario Andretti when your towing, its driving with road conditions in mind, and knowing when to park the thing, like when the wind starts getting to strong.
Whitelightning
I've flat towed, and used dollies, and trailers.
Money no object I prefer trailer. But that's not an option sometimes in RV parks. I love dollies. Especially for front wheel drive which means 90% of what's out there. No messing with drive shafts or pulling half shafts if its an automatic being towed. I towed a caviler on one behind a truck from Syracuse NY to south of Tampa Bay Fl, no sway, no issues with the towing vehicles wheels goofing up on sharp turns. just like flat tow, cant back dolly. not to mention no mileage shows for the tow on a dolly. dolly does require a tag, and some maintenance once in awhile.
Whitelightning
I would say that semitractortrailers are towing their trailers differently than one would tow, say, a boat. They are attached above and ahead of the rear axels, which is more like how a fifth wheel is pulled. People who modify their pickup truck to tow a fifth wheel are more or less duplicating how a tractortrailer (semi) pulls a load. I believe it is a lot different when the pivot point (ball hitch location) is located behind the rear axel. I am not a truck driver by trade, though, so I will defer to any who are.
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