Understanding and Controlling Trailer Sway

Trailer sway is a dangerous effect associated with towing a trailer. Trailer sway is a side-to-side fishtail movement, and once out of control it can result in serious accidents. If you want to effectively control trailer sway you must first understand what contributes to trailer sway.

When trailer tongue weight is applied to the hitch ball and the weight is not properly distributed trailer sway can occur. Other contributing factors include tires, weight distributing bars, proper hitch adjustments, the trailer and tow vehicle suspension systems, and cargo weight distribution in the trailer. Imagine for a moment what happens when tongue weight is applied to a hitch ball. This added weight generates friction to the tow vehicle’s rear wheels, but too much tongue weight can affect the tow vehicle’s steering and handling, and too little tongue weight can result in a loss of traction contributing to trailer sway.

So, an important factor in controlling sway is the amount of tongue weight applied to the hitch ball, and how that weight is distributed to the tires on the tow vehicle and the trailer. A general rule-of-thumb is trailer tongue weight should be 10 to 15 percent of the fully loaded trailer weight, for trailers weighing over 2,000 pounds. If the tongue weight (what generates friction to the tow vehicle rear tires) is properly distributed to the tires of the tow vehicle most lateral forces you encounter will not be sufficient to start sway. But if the lateral forces do overcome the tire friction trailer sway can start.

This would be a good time to discuss tow vehicle and trailer speed. As speed increases tire traction decreases with a corresponding increase in the possibility of trailer sway. For example, traveling at 60 mph, and under wet road conditions a weak lateral wind could easily initiate trailer sway. This would imply that good tires, properly rated for the weight (on the tow vehicle and trailer) and road conditions are important considerations when discussing trailer sway.

Another consideration is the vehicle and trailer suspension. If the trailer’s vertical stabilization (associated with the suspension) is working properly it helps prevent small sways from starting. Weight distribution bars further reduce any vertical displacements (swings) and transfers the hitch weight to the vehicle and trailer tires. This results in the tow vehicle’s rear tires gripping the road better and helping avoid loss of traction caused be vertical forces. So, it is important, before leaving on a trip, that we complete a checklist of all items associated with the tow vehicle and trailer stabilization. These checks would include inspecting items like tires, weight distributing bars, proper hitch adjustments, the trailer and tow vehicle suspension systems, and that all cargo in the trailer is distributed evenly. If any of these items are not in a good shape, it will decrease the speed that we can safely travel at.

Next, we must consider the size of the trailer. The bigger the trailer is (taller and longer) increases the chance for sway. There are two reasons for this, an increase in weight and an increase in mass (the area that will receive lateral winds). The additional weight will increase the forces applied to the hitch ball, especially when traveling downhill, (due to the gravity & acceleration) and also in case of any vehicle brake activation. If there is any unbalanced item in the towing system, it can contribute to sway starting. If the trailers surface area is larger, the amount of forces due to lateral winds is increased. These forces can destabilize the tow vehicle and trailer motion, increasing the chance for sway.

There is another very important item associated with trailer sway, and that is the number of trailer axles. Compared to a single axle, a tandem trailer axle increases the number of wheels 100% over the ground, and the increased tires gripping the road decreases the chance for sway. Another potential problem with a single axle trailer is the vertical motion (front to back, referred to as porpoising) typically brought on by brake activation. In this situation the trailer’s front end will tend to push downward. This vertical downward motion affects both  front and rear wheel traction of the tow vehicle as it moves up and down, and the larger the trailer is the more unstable it is when talking about porpoising. Proper hitch work can prevent some of this from occurring.

So why is a 5th wheel trailer not prone to trailer sway? 5th wheel trailers are more stable due to the 5th wheel hitch being positioned over and close to the tow vehicle’s rear axle. In the case of a 5th wheel, lateral forces do not affect it like a trailer that is hitched at the rear of the tow vehicle. The weight at the hitch position of a 5th wheel trailer is higher and over the axle, enhancing the tow vehicle traction on the road surface.

Always keep in mind every tow vehicle has a maximum tow capacity and you must compute total RV weights including, passengers, cargo, and fluids and fuel. Also keep in mind that any weight added to the tow vehicle takes the same amount of weight from the towing capacity.

To recap, the most important items associated with trailer sways are:

a) under-inflated tires
b) improper weight distribution
c) improper hitch adjustments
d) no sway control
e) air pressure from transfer trucks passing
f) long downhill (descent)
g) lateral wind
h) towing speed
i) inadequate towing vehicle
j) overload conditions
k) hitch to tow vehicle rear axle distance
l) poor trailer design
m) mismatched tow vehicle & trailer
n) not completing trailer/tow vehicle checklist

So, to help avoid trailer sway here are some remedies:

  1. Under-inflated tires: Never leave on a trip without a tire inspection. If necessary, adjust the inflation pressure according to manufacturer’s recommendation based on weight. Always check and inflate tires when they are cold, before traveling more than one mile. Whenever you stop inspect the tow vehicle and trailer conditions to include tire conditions. Check tire and hub temperature. In case of abnormally hot tires, or hubs there is something wrong. It may be bearing problems, underinflated tires or axle overloads. Try to identify the problem and correct it.
  2. Improper weight distribution: When a trailer is manufactured it has very stringent weight balances. When you add after-market equipment or load the trailer with cargo try to balance the loaded weight from side-to-side and front to back. Bad lateral weight distribution can provoke sways and bad front to back weight distribution may generate up/down sways I discussed earlier. Remember that the weight over the hitch ball should be 10 to 15percent of the total loaded trailer weight. If the trailer has less than 10% tongue weight load more weight towards the front of the trailer.

  1. Hitch adjustments: Whenever using a weight distribution hitch it is extremely important that it is adjusted properly. If you are not sure how to adjust the hitch components take it to an authorized RV service center to have it adjusted.
  2. No sway control: Sway controls help limit the possibility of sway starting in the first place. Whenever possible have a sway control installed and make sure you understand how to adjust it. I personally use an Equalizer brand hitch with built in sway control when I tow trailers.
  3. Transfer trucks passing: Large trucks will pass you and you need to be prepared to make slight steering adjustments. When a truck passes the main factor is the air displacement. The air pressure coming off of the truck pulls and pushes against the trailer. Try to observe wind intensity and direction, as well the air moisture which is the main villain. Humid air is heavier and increases lateral trailer displacements when being passed. Being aware of the situation is the best defense. Be prepared to make steering corrections, but do not over-steer.
  4. Long downhill (descent): When descending inclines reduce your speed, use a lower gear and apply weak vehicle braking, if necessary. Never do any abrupt vehicle braking. Continued use of the vehicle brakes can cause them to fade and not work properly. It may be necessary to manually activate the trailer brakes periodically to help slow down and/or to control sway. Slowly slide the manual brake lever over to activate the trailer brakes. It is important to maintain a safe and manageable speed when descending inclines.
  5. Lateral wind: Strong cross winds and windy days in general can be very dangerous when towing a trailer. It is better not to travel on extremely windy days. If you do experience lateral wind displacement reduce your speed substantially and stay alert.

  1. Towing speed: It is difficult to state what a safe speed is when towing a trailer. There are many variables, but not exceeding a speed of 60 miles/hour is a fair assessment. Use lower speeds if the traffic or road conditions are not normal (lateral winds, high humidity, rain, road conditions, traveling downhill all contribute to sway.)
  2. Inadequate tow vehicle: Your tow vehicle might have enough power to move a trailer down the road, but a major consideration is whether the tow vehicle’s weight is adequate to support slowing down and to help prevent trailer sway. Every tow vehicle has weight limits and ratings. It is very important that these weight ratings are not exceeded. The farther you are from exceeding weight limits the safer it is for you and your family. A very general rule is the tow vehicles towing capacity should be capable of towing the Gross Vehicle Weight Rating (GVWR) of the trailer you are towing.
  3. Overload: Never overload the tow vehicle or the trailer. Overloading tires, axles, Gross Vehicle Weight Ratings (GVWR) and Gross Combined Weight Ratings (GCWR) is extremely dangerous. Make sure all cargo loaded in the trailer is secured and that all weight added is properly distributed.
  4. Hitch to tow vehicle rear axle distance: The distance from the tow vehicle’s rear axle to the hitch ball can contribute to sway. Less distance is always better. Ideally a longer wheelbase vehicle with a shorter overhang from the rear axle to the hitch ball is the best scenario. Basically, it is associated with the lever effect. The distance between the rear axle and the hitch ball position can be understood as the lever length; and the longer the distance it is the higher the lateral forces applied to the axle position.
  5. Poor trailer design: When there is too much weight behind the trailers axles causing the tongue weight to be less than 10% of the trailers weight it has a natural tendency to sway. In this case it will be necessary to load some of the weight towards the front of the trailer to increase tongue weight. Ideally tongue weight should be between 10 to 15% of the loaded trailer weight.
  6. Mismatched tow vehicle & trailer: The tow vehicle and trailer need to be properly matched to make a safe towing system. An improperly matched tow vehicle and trailer can result in trailer sway. A tow vehicle without the proper towing capacity for the trailer it is towing, too light of a suspension system for the amount of tongue weight, too short of a wheelbase for the size of trailer can all contribute to trailer sway. The tow vehicle should be rated to tow the GVWR of the trailer you purchase.

  1. Tow vehicle/trailer checklist: The tow vehicle and trailer must be in good mechanical condition to safely tow. The brakes and suspension on both the vehicle and trailer must be in proper operating condition. Prior to leaving on a trip inspect the suspension (shock absorbers, springs, bars, tires and other components on the trailer and the vehicle) and the operation and adjustment of the electric brakes for the trailer. Tires must be in good condition and inflated properly for the load. A checklist can help you remember what items to check.

Note: Adhering to these recommendations can help prevent trailer sway from starting, but keep in mind there are no guarantees you will not encounter sway. If you do experience trailer sway you should gradually reduce speed, avoid using the vehicle brakes if possible, avoid harsh steering movements if possible and manually apply the trailer brakes by sliding the lever over on the electronic brake control to help bring the sway under control.

Everyone towing a trailer, even the most experienced, is a potential sway victim. When it comes to trailer sway caution is a necessity, but not always sufficient. Safety first! Being aware of what sway is and what you can do to help prevent it can help prepare you in the event your trailer starts to sway.

Happy Camping,

Mark J. Polk

RV Education 101

RV Online Training

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