4X4Dodger wrote:E&J push'n wind wrote:4X4Dodger wrote:
Let me clarify that when I wrote about the distance between the pivot point and the AXLES I mean the TRAILER axles NOT the tow vehicle axles. It is this distance that truly affects how a trailer tows. I question whether the distance from pivot to TV axle has any great affect at all. This is a weight issue.
One reason that the axle distance matters is overhang on the rear. The greater the overhang beyond the rear axles the more the Airflow works against the sides of the trailer...with a longer lever arm than if the axles were farther to the rear. This would cause sway.
Lets use your example. Lets place the wheels on the trailer all the way at the rear like on a tractor trailer combo. Looking at the pivot point on a 5th wheel trailer and a conventional hitch or a pintle hitch. Now lets place lateral force against the pivot point on the 5th wheel hitch and lateral force at the pivot point on the conventional hitch. That wind force that you speek of works forward of the trailer axles as well. Difference is with a 5th wheel hitch, that leverage I spoke of earlier doesn't have as great an effect on the pivot point on the 5th wheel hitch as it does on the conventional merely because there is virtually no leverage at the pivot point because the pivot point on a 5th wheel hitch is directly over the center line of the rear axle. With a conventional hitch that force exerted by the wind forward of the axle (on TT) has a compound effect because of the leverage created by the distance of the pivot point behind the rear axle (on TV).
Now, if you will. Move the trailer axles to about 2/3 the distance form the pivot point like on many TT's. That wind again that you spoke of has a multiple effect in that it works against you in one direction as a passing vehicle begins to pass and pushes against the back of the trailer aft of the center line of the axles. As the passing vehicle reaches the trailer axle center line it begins to work in the opposite direction as it continues to push against the side of the trailer, once past the axles, that push is opposite it was before it "crossed" the center line of the trailer axles. This is where "sway" begins to occur. Again only because of the leverage created by the distance of the pivot point behind the center line of the rear axle of the TV.
That lateral force is eliminated with a 5th wheel hitch setup. The placement of the hitch directly over the center line of the TV rear axle makes it so that there is absolutely no leverage on the TV. Not so with a conventional hitch, again.., that lateral force against the hitch is working against the TV in as much as it has leverage to move the TV by the later force working against the hitch. The longer the distance behind the center line of the rear axle of the TV the greater the leverage. Can you see it? I hope I was able to explain what I am trying to get across.
Now with the traditional friction sway control, if the friction is overcome problems can begin. With the "premium" hitches (and this is where the firestorm begins) the pivot point is "changed." I'm not an engineer so I can't and wont attempt to explain the math. The only one I can explain is the Pull Right where the pivot point is literally and physically moved forward to the center line of the rear axle (or as near as possible).
Either way, tested by an independent organization or not. You can't ignore the physics of it. With the friction type of control, if you make it "harder" for the trailer to pivot. It accomplishes what it designed to do, create tension so as not to allow it to pivot so easily. With the "engineered" type, the possibility of sway is engineered out.
The question you ask about it masking a bad setup. I'm sure beyond a doubt that there are setups out there like that where someone maybe happy as a lark (with there sway control) all the while they are a ticking time bomb. I am equally sure that there are folks out there that need a sway control of some type (and are without) that are an accident waiting to happen. And there are those that have it that perhaps don't need it and so on and so forth. You get the idea!
I was speaking of AIRFLOW along the side of the trailer and didnt make that clear I am afraid.
You are right however that a force = wind from the SIDE, MAY have the effect you say IF it was localized, which it isnt. It tends to be fairly evenly spread over the entire side. This should not cause the sway you might think. However it would tend to RESULT in sway as the driver tries to compensate for the force of the wind.
Truth be told, the wind force developed from a passing vehicle looks very much like the wake developed in the water from a boat. Some of difference is that it is not only on a “flat plain” as on the surface of water but rather is reflected by the height of the vehicle as well as the speed of the passing vehicle. We’ve all felt that wake as semi’s passing us while we are standing still. Whether we are in a solo automobile or van or pickem up or what have you. It is not “evenly spread out along the entire side of the trailer” as you say, by a passing vehicle (semi or motor coach or any other vehicle for that matter). I would agree with you that this in and of itself may not necessarily be the cause of sway but it could be the cause of that “Harmonic Oscillation” rather. I would also agree that it might be aggravated by driver input.
I had an occasion to experience a “mild” sway condition on one occasion when towing a small single axle utility trailer that was clearly overloaded. I was in the slow lane taking it easy when the trailer began to oscillate, mildly mind you but it got my attention none the less. The trailer had no brakes and I know about accelerating to attempt to alieve the conditions. Problem was I was already going faster than I would like to have been going and speeding up I’m sure while it may have momentarily cured it, I believe ultimately it would have gotten worse as I believe it was the speed itself that contributed to the oscillation. I knew better than to hit the brakes as this would have been disastrous as well. I gently eased off of the throttle and prayed. Thankfully it worked its way out and I kept my speed much lower after that.
To claim as you do that in a fifth wheel set up there is "NO LEVERAGE" by the trailer on the tow vehicle is a misunderstanding of the physics. There is significant leverage. Leverage = force; There are forward, rearward and side (lateral) forces as well as Yaw forces put on that fifth wheel. There is also friction between the surfaces (even greased) that play into this. A simple example of the forces upon your fifth wheel: drop one side of your trailer axle into a low ditch; the forces on your fifth wheel may be enough to loft your opposite rear TV axle OFF the ground. This is an extreme example but it clearly illustrates the forces at work. The other extreme example of forces on the fifth wheel is jackknifing. Being over the TV axles may help stability but it DOES NOT eliminate the forces involved.
Please do not misunderstand what I mean when I say that there is no leverage on a 5th wheel setup. The examples of the forces you describe are understood and are a given. The same forces are exhibited on a conventional hitch as well with perhaps yaw as an exception (provided hitch is setup without sway control as this can effect yaw in a conventional hitch with spring bars attached depending on the type of hitch). Where you say ”leverage = force,” force that would be created by a lever. In this case, where there is no lever, there is no leverage. What I mean by leverage is that there is no (virtually if you will) lateral “leverage” on the pivot point of the tow vehicle with a 5th wheel setup in the same way as a conventional hitch. The word leverage implies that there is a lever. Again, if you’ll be intellectually honest, the length of the overhang behind the centerline of the TV axle plus the length of the hitch itself will absolutely affect how the TV responds to lateral forces placed against the pivot point. Again, not so with a 5th wheel hitch.as there is no point of leverage. With a conventional hitch, the center line of the rear axle of the TV becomes the fulcrum point. Where is the fulcrum point with a 5th wheel hitch setup? The only 5th wheel hitch setup that I can think of where this could come into play is an auto transporter where the 5th wheel hitch is placed aft of the center line of the TV axle. I really can’t see this as a problem though because the lateral forces placed on a hitch pivot point are usually caused by wind. Auto transporters are usually open and “airy,” not affected by blowing wind
The REAL issue is WHY is the trailer swaying in the first place. Is it driver induced sway? Improper design of axle placement vs trailer weight? or a TV that is less that 1/3rd the overall weight of the fully loaded trailer? If sway is induced by weather conditions or being passed it is often made worse by driver response (reflexively slowing or braking when in fact a slight acceleration is the right move and will usually result in the sway disappearing fast)
If the sway is DESIGN induced this can probably be fixed with different loading in some cases. (of the trailer) or a different TV.
BTW: most tractor trailers do not keep their axles all the way rearward this overloads the Drive axles and may not be legal in some states due to "bridge" laws. Where having the axles all the way to the rear really helps you is in parking in reverse in very tight spots. This allows the trailer to pivot at the far rear end and it can be spotted very precisely. I know to some this may sound counter intuitive but it's true.
