"Pull-behind" trailers are trailers that connect to the tow vehicle by a ball (or other attaching arrangement) on the back of tow vehicle and a trailer hitch on the tongue attached to the front of the frame of the trailer. If the trailer is improperly-loaded into a tail-heavy condition (in other words, the center of gravity is to the rear of the center of the axles) arrangement, it will sway from side to side. But why? Why would tail-heavy conditions cause such behavior? The weight on the tongue would be negative, but I still can't understand the swaying mechanism.
My thoughts are that the CG is behind the tires, they become the steering mechanism rather than the tung. The trailer axles do not have caster / camber and tow in alignment to hold it steady. The increased sway can be caused by wallowing of the tire, the more the wallow the more the sway, and the more the sway the more the wallow. One thing I do know, if you want to spread gravel just load it in the back of the trailer and get it up to speed, but watch it, the sway will be soon swaying the tow vehicle from bar ditch to bar ditch, do not try this with any one around so the can see you. They might think it is a stupid trick and it is. ;-)
Nehmo, the reason this happens is due to the wind being able to catch the underside of the trailer creating lift. It also happens because it can lift the back of the truck up off the springs, even if it is a slight amount, it will cause this problem. And then because the trailer is attached to the very rear of the truck, it causes the rear axle to act like a pendulum and causes the front of the truck to sway side to side. If the driver is experienced in pulling a trailer, he can stop the sway action. You just have to hold the steering wheel very straight, and not side to side to try to counter the sway. And yes, balancing the load on a trailer is extremely important, you dont want there to be any lift on the hitching device. This can cause an improperly hitched trailer to come loose, and possibly come completely free of the vehicle. Always use tow chains, criss-crossed under the hitch (so as to look like they are going to catch the tongue if it comes undone), and with enough slack that you can take hard corners without stretching or getting the chains tight. Happy towing, Pk
If the CG is behind the pivot point 9 (axles) the trailer will try to switch ends. This is the swaying. Much like an conventional gear airplane. Most of the weight is behind the main wheels. They have a tendency to try to switch ends.
Michelle writes: If the CG is behind the pivot point 9 (axles) the trailer will try to switch ends. This is the swaying. Much like an conventional gear airplane. Most of the weight is behind the main wheels. They have a tendency to try to switch ends.
Michelle
HUH?
Are you talking about the fact a dart flies straight because its weight is forward and the tail feathers cause a little drag to keep it straight? And, If you throw a dart backwards, it will automatically switch ends and the heavy end will lead the way?
Because the majority of the weight is behind the center of gravity and it's attempting to get to the front because it's smart enough to know that's where it belongs :-)
Just buy yourself a good hitch by Equalizer and have it professionaly installed, if you can't handle it yourself and you will be amazed at the change it makes. I have towed many trailers up to 7000 lbs and will never again tow anything besides sleds and my little boat without it.
Just having the weight behind the wheels in itself doesn't produce the sway. It allows amplification.
Many things can get the sway started (wind, bumps in the road, passing vehicles) and once going the weight unbalance makes it more difficult to control.
The trailer is like a pendelum hanging out behind the tow vehicle. The closer the weight is to the vehicle the easier it is to control. Try holding a board with a weight on it out in the wind. It's a lot easier to control if the weight is close in.
The weight and length of the tow vehicle also comes into play. When a tail heavy pendelum starts to swing it will start controlling the tow vehicle.
The flex of the tires adds more amplificaton.
No matter what you do there is always possibility of sway. The amount of weight you put on the hitch is a compromise. Only total solution is to put all the weight on the tow vehicle and elimiate the trailer.
Thinking about it a little more one could have a really long trailer that is very heavily loaded in the rear and you could pull it at very high speeds, with a short wheelbase vehicle, without any trouble. But only as long as there is no wind, no bumps or anything else to get it into a sway condition. But god help you if it begins to swing!!
Nothing more frightening than having the trailer begin to do the steering.
For simplicity, let's assume one axle and a flat deck trailer. My experience has been the sway is dependent on the relationship of the Center of Mass (CM) to the axle, in other words, if the CM is in front or behind the axle.
Hitch - CM - Axle = stable Hitch - Axle - CM = unstable
Trailers are self-correcting in terms of steering, like a caster on a piece of furniture. If the wheel "steers" to the left like this \ , the back and the front of the wheel touching the ground get resistance from the ground. But the resistance on the back has more leverage in relationship to the pivot, so the force is translated into a clockwise turning moment about the pivot. This turning moment corrects the orientation of the caster until it is pointed parallel to the direction for travel, like this |.
So front-heavy trailers correct themselves regarding steering like the caster, but the corrections are small. In tail-heavy trailers, the steering corrections are large and they may even yank at the tow vehicle.
I still don't understand it; I'm just restating the problem.
I got really curious about this, and studied it for a long time (I'm an engineer). There was a good thread on this at woodall's RV forum. I have found no frank discussion of it by anybody who has broken it down into its component forces. It's hard to explain it using ASCII here. But my opinion is this:
First off, it's clear that trailers can amplify their swaying until you crash. They don't do it all the time, but they can, and that's what interests me. They have no power of their own to bring to the table. The energy to do this comes from the tow vehicle. The trailer sways like a pendulum, sort of (I admit the forces holding a trailer back are not exactly like gravity in this analogy). A pendulum can be powered by a wide variety of motions. One of those motions is moving the pendulum's pivot point side to side. I think this is the motion we want.
The forces on the trailer come from its tires and the tow vehicle. I was not interested in any other forces. "front loaded" trailers are trailers where the center of gravity is in front of the tires. When the tires are pushing the trailer sideways (during sway), the center of gravity is in front of that force. Since the side force doesn't go exactly through the center of gravity, the weight of the trailer pushes sideways on the tow vehicle.
The inertia of the swaying trailer pushes the tow vehicle from side to side. The difference in loading is simply this. If the center of gravity is in front of the wheels, it pushed the tow vehicle one way, and if it's behind the wheels, it pushes the two vehicle the other way. One cancels the sway, and the other amplifies it.
I don't know if amplification is the correct term. Poor loading will increase the sensitivity of a tow vehicle and trailer to winds and side drafts from big trucks, but I don't understand how the forces could actually be amplified. In my experience the side-to-side swaying is amplified or increased by the driver trying to offset the swaying by steering out of it.
I do not agree. I think trailer sway issues start with proper loading. I do agree that tow vehicle stability issues can be attributed to tire and suspension problems. But they will not be the primary cause of sway, the distribution of the load will be. Even a trailer that is grossly over loaded may not sway if the CG is correct. But this is where the tow vehicle suspension and tire pressure will show up more readily. And I am no mechanical engineer or whatever scientist, but a retired transportation superintendent with a lot of miles behind me. ;-)
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.