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Originally Posted by Bill
A perennial topic on this board is the addition of a bike rack on the rear bumper. Several people have experienced sway when they do this, and the question is, WHY does it cause sway? The usual answer is that the weight of the bikes, mounted so far behind the wheels, reduces the hitch weight, which in turn causes sway. I admit I’ve had some trouble with this explanation.
Bill
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I missed this discussion when it first started and only recently came upon it. I'm surprised that Bill, my friend, doesn't understand the mechanics of swaying, but then, I've excused him because he's merely an electrical engineer. ;-) Now, Bill, you don't need to remark about my total ignorance concerning electrical devices and circuitry.
I've attached a drawing that may help most of you to understand why swaying occurs. First, let me define some engineering terms:
"Moment" is the product of an applied force and the "Moment arm" through which the force is applied. The "Moment" of a force is the force's tendency to cause rotation of an object around a point, such as a hinge pin.
"Fulcrum" is the term used for the point of rotation such as a hinge pin.
"Frictional resistance" refers to the force that resists the sliding of one object over another.
The attached drawing depicts the trailer's original center of gravity (CG) as a circle. This CG is located 2.4 from the hitch ball, or fulcrum (the numbers don't represent real distances, but are distances in inches of my model) and the tires meet the road's surface at 4.8 from the fulcrum.
When this trailer is towed straight behind the tow vehicle, the lateral forces exerted on the trailer are zero. A lateral force can be exerted on the trailer, however, from wind gusts or the drivers swerving quickly (due to a directional change of "Momentum"). A strong gust of wind striking the trailer's right side, for example, will result in a lateral force that exerts itself through the trailer's CG. This force creates a moment that attempts to rotate the trailer clockwise around the hitch ball. The main force resisting this is that due to the frictional resistance of the tires against the roadway. The moment created by this frictional resistance is the product of the frictional resistance and its moment arm. This counterclockwise moment attempts to keep the trailer from rotating clockwise around the hitch ball. Since frictional resistance is created in reaction to the lateral force, its moment can only equal that of the lateral force's moment but can never exceed it.
With properly inflated tires, sufficient tire tread and a dry roadway, there is a high value of frictional resistance. In a well-designed trailer such as our TMs frictional resistance's moment is more than adequate to counterbalance the lateral force's moment trying to swing the trailer to the left. No swaying occurs in this instance.
The two moments are equal and
Lateral Force X 2.4 = Frictional Resistance X 4.8
Notice that the frictional resistance's force in this instance is only half that of the lateral force.
Now if one loads heavy objects rearward of the axle, the trailer's CG shifts rearward. Assume that loading bicycles and batteries on its rear bumper pushes the CG to its new position, 3.6 from the hitch ball. That same gust of wind now creates a moment equal to:
Lateral Force X 3.6, or one that is 50% greater than the previous moment.
The frictional resistance of the tires on the roadway will increase to counterbalance the clockwise turning moment. If there is sufficient frictional resistance to generate a moment equaling the lateral force's moment, no sway will occur. If the lateral force's moment exceeds the frictional resistance's moment by just a small amount, the driver may notice only a slight swaying motion. If the wind gust strikes the trailer while it is on a wet, icy or oily roadway, however, the moment of the diminished frictional resistance may not be able to significantly counterbalance the clockwise turning tendency. In this instance the swaying will be severe; it can result in the driver's loss of control and overturning of trailer and tow vehicle (this happened to friends of ours several months ago). You can see that continuing to put too much load on the rear part of the trailer will move the CG even farther rearward. Then towing becomes dangerous as even a slight swerving to the right or left can result in severe swaying.
You are correct, Bill, in stating that we cannot easily determine the location of the CGs of our loaded trailers. We can, however, use the load on the trailer's tongue as a rough indication of the altered CG's location. I'd venture to guess that the tongue load should increase by at least 10% of the weight added to a trailer. This means that if our empty trailer's tongue load is 400 lb. and we add 1,000 lb. of stuff to a trailer, the tongue load should increase by 100 lb. on a properly loaded trailer. Isn't this a good argument for purchasing a tongue scale - such as the ones that Camping World sells?
My explanation may be a simplified one but I believe that I've covered the major forces in play that can result in a trailers swaying.