4X4Dodger wrote:
E&J Pushnwind:
I am not sure why you think there is no lateral force on a fifth wheel. There certainly are..in fact because there is a round pin that fits into the fifth wheel you can say that there are forces at any given time in almost any direction of the compass.
I'm not saying that there arent lateral forces on a fifth wheel hitch, I'm saying there isn't leverage in the same manner as a conventional hitch.
Your point about car carriers is not really complete. The STINGER type of fifth wheel arrangement you see on Auto carriers has all the same forces as any other. However it does exert different leverage upon the Tractor.
My point exactly, because it is aft of the center line of the rear axle assembly. This is where the lever is and the center line of the rear axle assembly is the fulcrum point.
As for wind and auto carriers I am afraid that you have overstated the case somewhat. When an auto carrier is loaded about 75-80% of the area on the side is blocked by cars and trailer structure meaning that side winds can affect them quite a bit. Add to that the fact that the cars on top will sway on their own due to being on their suspensions means there is an added lever arm way up high oscillating at a different rate than the trailer making things even worse.
But to cut to the chase here. The ball type hitch is a proven design that has worked well for many many years. There is nothing wrong with it. A 5th wheel has some advantages but only if the design of the hitch and the plate and pin on the trailer are up to the job (not always the case) The same forces are present no matter which hitch you are using. HOW those forces are transmitted to the TV is different.
The distance from the average p/u truck rear axle to the ball hitch pivot point is so close as to be mathematically insignificant in thinking about which forces are the most likely to cause sway. SIDE WINDS in and of themselves are not the cause of sway...it is the driver CORRECTION that causes the sway.
All due respect 4X4Dodger, this is where I strongly disagree with you. You make somewhat of a blanket statement. I know you stated pickup truck but everybody is not towing with a pickup truck and that overhang will vary from MFG. to MFG. and in that, from vehicle to vehicle.
I stated earlier that I am no engineer so I can't show you the math however I ask the question. How long does a lever need to be to be effective? Follow me if you will. Lets say that you have 36" from the hitch ball to the center line of the TV rear axle. You can grab that hitch ball with your hand and begin to rock the TV from side (on a lateral plane), with each push and pull you will get a little more movement with each "rocking" so to speak. Now if you will place a pin of some sort into a 5th wheel hitch and attempt to do the same thing. If you were able to grab hold of it, you wouldn't be very effective in moving the TV because of the placement of the hitch over the center line of the axle.
Now place a 25' to 35' trailer with all of its weight and the lever it has between the center line of the axle assembly and the tongue coupler. I wish I knew how to place illistrations on here, I will try to paint a word picture. Picture an arrow pointing at the pivot point in a latteral fashon at both a 5th wheel hitch and a conventional hitch. This of course is latter force. Sure 5th wheel hitches have lateral force exerted against them. The difference is that because of its placement over the center line of the rear axle (or slightly forward of it) you lose the lever effect that a conventional hitch would exert against the TV because of its distance from the center line of the rear axle.
Lastly, a sway control works for a conventional hitch because it counters leverage. It is able to meerly because of the fact that there is a lever to work against, length of hitch and the center line of the rear axle being the fulcrum point. Again, I ask the question. Where is the "lever" with a 5th wheel hitch? I'll say again, where there is no lever, there is no leverage. How can there be?
As for passing vehicles. Tractor trailers pass me all the time when I am towing my 30ft tt. It doesnt move.
My guess is that some drivers, slightly spooked by that huge truck and its Air Wake zooming by them instinctively let off the accelerator pressure and/or make a sudden move to the right side. THIS is SURE to cause sway as the Big Truck goes by.
As for your analogy of the boat wake...It's not completely accurate. If you look at what really happens to the air there is a period of force as the air wake hits and then a SUCTION as the truck goes by.
When your trailer is going down the road there is a similar wake coming off the front as it passes thru the air. this air pushes out from the side of the trailer near the front and comes back at some point to slide along the side of the trailer. For all intents and purposes this air flow is even on both sides. therefore it isnt really a cause of sway.
I agree with you partially on this point, however the initial wake that "brakes off" the front is commonly called a bow wave. That bow wave that is created from motor coaches, semi's, large vans and yes our TT's is what affects other vehicles as they or we pass by. That bow wave is what "upsets" or may cause that harmonic oscillation we spoke about earlier. The vaccume or airmovement along the sides of our trailers as we pull them down the highway does not affect us while we are moving down the road. I was never refering to that air movement but rather the air movement caused by similarly large vehicles as they pass us.
However when you have a dramatic rear overhang as some trailers and RV's do this does present a lever arm for a greater amount of air to hit behind the trailer axles on one side or the other.
Remember, as passing semi's and motor coaches reach the rear of your TT that bow wave will begin to push against the back (overhang) of your TT. As that bow wave passes the center line of your TT it will still be pushing against the side of your TT only as it passes the axle assembly the force is in the opposite direction. That is where the harmonic oscillation may begin.
The best way to avoid this unequal force is to keep a steady acceleration on when being passed by any larger vehicle.
