Loosing control of your towing rig while driving is a nightmare that you don't want to come true. A common control problem with travel trailers is the sway that occurs with wind gusts or passing big rigs. You can minimize the effects of trailer sway and make for a more enjoyable driving experience with just a bit of planning and preparation. Know how to evaluate equipment options and how to maintain and trim your rig by gaining an understanding of what causes sway and how it can be controlled. See the page on maintenance for more about what you can do to minimize sway problems with your rig.
People have been trying to move boxes full of stuff from one place to another for ages. To make it easier, things such as sliders and rollers have been put under the box to make it move easier. After reducing friction, the next biggest problem to solve is making it easy to change direction.
Putting a wheel under each corner of the box is one of the most common solutions. It provides good stable support. In order to change direction, though, two things have to happen. One is that a pair of wheels have to be able to turn underneath the box in unison and the second is that the other pair of wheels need to be able to turn at different rates. Most cars and trucks now generally using a steering mechanism to turn the front wheels and a differential to allow the rear wheels to rotate at different rates.
To avoid the complexities of wheels doing more than just rotating, most travel trailers put wheels only under the center of the box. This means that some support is needed at one end or the other to keep it from tipping back and forth. Each wheel is also independent of the others so no differential is needed to supply drive power and allow different rotation rates. It is this configuration of wheels in the center with a support at one end that leads to problems with sway. The same situation that makes it easy to change a trailer's direction of travel also means that it takes some effort to control its orientation towards the desired direction of travel. Sway is what happens when the control isn't up to the job of keeping the trailer in proper alignment with the tow vehicle.
One issue that seems to generate some heat in discussions about sway is that of making sure people are talking about the same thing. In automotive engineering, sway is the back and forth roll of the vehicle that occurs in maneuvers. This happens because the center of gravity is higher than the place where forces are bing applied to move it around – where the rubber meets the road. This makes for a leverage that tries to roll the car over.
Those towing trailers often think of sway more in terms of trying to steer a straight course while the trailer is trying to change their direction. The worst case for trailers is the jacknife scenario where angle between the trailer and the tow vehicle becomes uncontrollable.
Sway can be caused by an inherent problem with the rig, in which case it will cause steering difficulties at speed without anything else happening. Or it can be caused by winds or other big rigs passing that push the trailer around. Or it can be caused by the driver trying to dodge something in the road, which can be particularly hazardous when braking.
When you drive your vehicle, your goal is to have the vehicle go in the direction you steer it. When the road is slick or you try to corner too fast or something pushes your vehicle around, it doesn't go the way you want it to. A trailer can be that 'something' that pushes your vehicle around. When that happens, it is called sway.
The hitch point is where your trailer and tow vehicle meet. This hitch point is usually the trailer ball but some hitches, such as the Pullrite and Hensley Arrow will use mechanical linkage to move the hitch point forward of where the ball is located. Forward (pull) and backward (brake) forces on the hitch point when the rig is lined up straight are are fairly easy to handle because the hitch point is always lined up with the middle of the vehicle looking from side to side. The trailer is only pulling against the engine or pushing against the brakes to nothing is trying to steer the tow vehicle.
When you have sideways forces in the hitch point, though, the trailer will be trying to steer the tow vehicle.. How much and what kind of steering influence it has will depend upon where the hitch point actually is. This sideways force can come from push or pull forces when the rig is at an angle or they can come from something that pushes on the trailer. Since a trailer has large surface areas on its sides, it can easily be pushed around by gusts of wind such as those from passing trucks or from strong crosswinds on the road. Even irregularities in the road surface when coupled with suspension and tires can push a trailer around.
How the sideways forces on the hitch point influences tow vehicle steering will depend upon where it is. Your tow vehicle rear axle acts like a trailer axle in steering. It doesn't resist any rotation about the middle of the axle. This means the front tires provide the sideways forces to change vehicle direction. When the hitch point is behind the rear axle, the rear axle acts as a fulcrum in a lever between the front wheels and the hitch point. The longer the distance from the hitch point to the rear axle compared to the wheelbase of the tow vehicle, the more mechanical advantage the trailer has to steer the tow vehicle. This is why longer wheelbases and shorter hitch point to rear axle distances provide better trailer handling.
The Pullrite and Hensley Arrow hitches move the hitch point to very near the rear axle. This doesn't give the trailer much leverage on the tow vehicle so control is improved.
A fifth wheel hitch puts the hitch point between the tow vehicle's front and rear axles. In this case, the lever configuration is completely changed and sideways forces get shared between the tow vehicle's front and rear tires.
Note that a pull, caused by trailer braking and tow vehicle acceleration will tend to straighten a rig while push, cased by tow vehicle braking more than a trailer, will tend towards jacknifing. If you grab a sheet of paper and pull the edges away from each other, it will straighten out. If you try to push the edges together, it will buckle. As long as your rig will bend in the middle, it will tend to behave the same way that sheet of paper will.
One white knuckle scenario dreaded by some trailer drivers is being passed by a big rig. Sometimes it seems the big rig drivers like to pass real close and fast just to tease the trailer driver. The big rig (and all moving vehicles for that matter) pushes air aside as it goes down the road. This air push will also push your rig sideways as the big rig passes you. First it pushes your tail over and then the front of your trailer and then your car. This wiggling and moving around can cause a driver to try to compensate and that compensation, due to understeer, may actually make the control situation worse rather than better.
A common cause of disaster is a drastic avoidance maneuver. This is trying to steer around something on the ride while braking at the same time to slow down. Good trailer brakes are absolutely necessary in this scenario and even then your tow vehicle may not be able to change the direction of the trailer at speed. This kind of sudden maneuver is what often causes jacknifed rigs.
Other sources of sideways forces that can cause instability and potential loss of control include side winds, tire blowouts, or evasive maneuvers. Any situation that requires the driver to take action to keep the rig going straight may lead to successive corrections that can then lead to loss of control if not properly managed. Hitches that better control the pivot point or dampen movement around the pivot point will assist the driver in managing these disturbing events.
Perhaps the most important consideration in trailer sway is leverage. The weight or mass involved might seem to take the prize but increases in the weight of a trailer usually mean longer trailers. So it is the leverage, or the weight at a distance (also known as a moment arm), that is perhaps most important in maintaining control.
The ideal hitch point on a tow vehicle would be in the exact center of all four wheels. This would spread the vertical load on the hitch evenly and would make sure that any sideways motion was also countered by all four wheels. The fifth wheel trailer hitch comes closest to this ideal. A simple bumper hitch is perhaps farthest from this ideal.
Another leverage component to watch is the location of the hitch point. As you move the hitch point from the center of the four wheels towards the rear axle, more and more of the sideways control is put on the rear tires. When the hitch point starts to move behind the rear axle towards the rear bumper, there will be a stronger and stronger tendency to rotate the car rather than just shove it sideways.
After leverage - distance and weight - the characteristics of the control mechanism become important. These characteristics involve the design of your tow vehicle, the tires you use, and the design of the trailer hitch.
When travel trailers weighed under a ton and were under fifteen feet long, a bumper hitch was all that was really needed because the weight and strength of the tow vehicle just overwhelmed any influence the trailer might have (as long as the trailer weight balance provided good hitch weight). Now, with trailers running over four tons and thirty feet, they are larger and heavier than their tow vehicles and some accommodations may need to be made for safe towing.
The choice of the method used to attach the trailer to the tow vehicle determines how and where the sway forces are handled. Four of the more common hitches used for travel trailers to control sway include the following.
Pullrite - http://www.pullrite.com/principle.html - This moderately expensive hitch extends the trailer frame up to a point at the center of the rear axle using a special track on the tow vehicle to help support the vertical hitch load. The result is trailer towing with fifth wheel characteristics.
Hensley Arrow - http://divtune.com/hensley/ - http://www.hensleymfg.com/arrowdetails.htm - This expensive hitch uses a standard 2 inch receiver on the tow vehicle. Instead of a simple pivot point, this hitch uses a trapezoidal linkage that creates a virtual pivot point well forward of the hitch point for small angles.
Dual Cam - http://www.reesehitch.com/sway_control.html - This hitch uses a damping mechanism derived from a cam system.
Equalizer - http://www.equalizerhitch.com/ - The spring bars on this hitch provide friction damping as the trailer turns around the pivot point.
The running gear includes tires, suspension, shock absorbers, brakes, bearings, and the other parts that are vital to keeping your rig rolling down the road. These must be chosen to support and manage the loads you will put on them. Maintenance is important to assure reliable operation. Tires must be properly inflated and checked before each run. Check the torque on lug nuts. Make sure the bearings don't run hot by checking wheel temperatures after twenty minutes or so on the road.
Besides the obvious fact that you should not put more stuff in your rig than it is specified to handle, you also need to make sure that this load is evenly distributed so that each tire carries an even share of the load. The trailer should have about fifteen percent of its weight towards the front to be supported by the hitch. A load leveling hitch uses levers to move this weight to all four tow vehicle tires and the trailer tires by using spring bars that work against hitch point drooping.
It is also a good idea to try to keep leverage in mind when you load your rig. Keep the heavy part of the trailer load over the axles so that any sway has minimal mass at larger distances.
Trailer Towing - Hitch & Sway Control at disney.com - good pictures and illustrations of the various brand name hitches
Sunline Coach Controlling Sway - a Bill Estes column
Sway – Natasha's Camping site has good rigging and driving hints,
The government guide to towing a trailer -
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