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Now that we have some of the common terminology out of the way let's talk about the typical causes of trailer sway. Keep in mind that trailer sway is a multivariate condition. Any one of these causes may not cause a trailer to sway. However, when more than one of these causes is present, a trailer is sure to begin to sway unless acted upon by an opposing force.
Towing Speed - Typical highway speeds have become much greater over the last 50 years. The speed at which you decide to tow your trailer may be the single greatest factor in controlling trailer sway. Unfortunately, tests show that any speeds greater than 45mph tend to allow almost any trailer to begin to sway. With traffic traveling at much greater speeds it isn't practical, and most of the time illegal, to travel at a speed that will control trailer sway.
High Winds - Winds blowing perpendicular against the side of a towed trailer push the trailer and pivot it on the hitch ball creating a tow angle. The greater the surface area of the side of the trailer, the greater the force will be at any given wind velocity. Towing in high winds is never a good idea and should be done cautiously if necessary.
Gusting Winds - Winds can gust and increase in velocity due to weather. Winds can also be described as gusting when a high wind is blocked by a tree line or a bridge overpass and you clear this blockage while towing. Your trailer goes from a relative low side force to much higher side force that pivots the trailer on the hitch ball and creates a tow angle.
Bow Wave - A bow wave is a wind produced by a large semi-truck, box truck or van pushing its way through the air on the highway. This wind travels around the truck and down the sides creating a wind disturbance in the adjacent lanes. This wind disturbance creates a sudden high to low pressure in the air traveling down the side of your trailer. The bow wave, as with the other wind disturbances, creates a tow angle by pushing the trailer and pivoting it on the hitch ball. The bow wave produces a "sucking" feeling in some combinations that feels like the entire tow combination is being pulled into the other lane.
Bad Roads - With any type of travel across this great country you'll realize there are quite a few roads that need work. These uneven, poor roads can make a trailer become unbalanced and cause it to pivot on the hitch ball creating a tow angle.
Downhill Travel - Towing downhill can allow your trailer to roll faster than the tow vehicle. The tow vehicle is geared to slow down when you let your foot off the accelerator. However, trailers are not equipped with this feature. When the trailer is traveling at a higher rate of speed than the tow vehicle it can begin to yaw.
Poor Trailer Design - The fact is that some trailers are designed better for towing than others. Trailer design includes a lot of different variables that need to be addressed to properly balance the trailer for towing. Even with a good trailer design the designer can have a hard time determining what the optimal layout of the trailer should be because of the added dimension of the trailer being used by so many different customers.
Improper Loading - Trailers are very sensitive to where you place your load your cargo. Your black and gray water tanks, your supplies, your clothing and anything else you carry with you will change the weight of the trailer and how that weight is distributed throughout your towed combination.
Poor Weight Distribution Hitch Adjustment - When a weight distribution hitch is not adjusted properly it can cause your trailer to have either too much tongue weight or not enough tongue weight to avoid inducing sway.