Article - Why Can’t I Backup Straight?

So, you cannot backup your trailer straight. Do not be concerned – nobody can.

Backing up a trailer is like herding sheep that only want to go back to the barn. Given the chance, they will circle around and head home. To keep them headed where you want them, if they circle out too much to one side of your intended path, you must chase them back to the center.

A trailer being backed up will only turn towards the side of your truck that it is already on. If allowed to turn unimpeded, your trailer will jackknife by turning back towards where you came from. To keep your trailer headed where you want it, you must steer your truck to turn the trailer back to the center of the path.

What’s going on?

It turns out (pun intended) that jackknifing is easy to deal with once you understand what is happening. In theory, if your truck were perfectly lined up with your trailer and you were to backup straight for a tiny distance, the trailer would also move back a tiny distance in a straight line.

Here is a picture to help you see the concept:

Your truck is attached by the hitch ball, so if you backup your truck a tiny distance, the hitch ball moves a tiny distance which moves your trailer a tiny distance. They all move the same tiny distance straight back.

Unfortunately, no matter how straight your truck and trailer appear to be, the trailer is never exactly straight. Even if it were straight for a moment, it would still turn to one side of your truck or the other – the road is not perfectly flat and smooth, the tires flex, the coupler will shift on the hitch ball (those are never a perfect fit), or some other thing will cause the trailer to become ever so slightly turned to one side or the other. It is going to happen, so it is easier to accept it and deal with it than to fight it.

To understand what happens when your truck and trailer are not straight, consider if your trailer were to be turned 90 degrees such that it is perpendicular to your truck and you back up a tiny distance. In this case, the trailer will only rotate about the center of its axle. Here is another picture:

With this picture and a little high school geometry, you can learn why shorter trailers are so much more difficult to backup than longer trailers (read here if you’re interested).

For any position of your trailer that is between being exactly straight back and being perpendicular, backing up will result in your trailer being both backed up some amount and rotated some amount. The rotation is what makes backing up an unstable process. There is always some rotation.

When your trailer is nearly straight with your truck, the amount of rotation will be slight. However, as your trailer turns farther to one side or the other, the more rotation you will get. When your trailer is perpendicular, the amount of rotation is at its greatest. Therefore when backing up, it is important that you correct your path before your trailer turns so far that its rotation can lead to a jackknife condition.

How to stop the jackknifing

As you backup, your trailer is always turning towards the side of your truck that it is on. To stop your trailer from jackknifing, your truck must be turning more quickly than your trailer is turning. This enables your truck to overtake the trailer’s rotation so that your trailer ends up turned to the other side of your truck.

At the moment your trailer crosses over to the other side of your truck, the process begins anew on that second side. As this happens, you must adjust your steering to the second side to again be turning your truck more quickly than your trailer is turning.

Since the rotation of your trailer is determined by how far your trailer is turned to one side, and because you must turn your truck to overtake the trailer’s rotation, how far your trailer is turned to one side is a key factor in determining how much you must steer your truck. For this reason, you typically don’t want to allow your trailer to become turned very far to one side because it will force you to make large steering corrections to control your trailer.

The best way to prevent your trailer from becoming turned too far to one side is by making continuous steering adjustments. As you overtake your trailer on the first side of your truck, you will be reducing how far your trailer is turned to that side. This allows you to begin relaxing your steering (steering back towards straight) before your trailer crosses over to the other side of your truck. Then, the instant that your trailer crosses over to the second side, you make the steering adjustment to that second side.

If you delay, your trailer will have more opportunity to turn and you will have to make a much larger steering correction to overtake the trailer’s rotation. Rather than look like a straight line, your path will be serpentine. If you delay too much, this serpentine path will grow with bigger and bigger swings as your trailer crosses from side to side until you are forced to stop and drive forward to straighten your truck and trailer and salvage the process. In doing so, you will give up much of your backing progress.

The process of backing up a trailer is actually a series of connected curves that alternate between the left side and the right side. The people who are really good at backing up make their path look straight by steering the curves very slightly, as if each curve is a small part of a very large circle – a circle so large that a small part of that circle looks like a straight line. To make your own connected curves be slight, you must adjust your steering to the second side immediately – before your trailer turns too far to that second side.

If you do this right, after adjusting on the second side, your truck and trailer will be heading in the same direction they were going in when you started and you will have progressed towards to your backing up destination. As you develop your feel for the process, you will be able to perform the same backing adjustments on paths that are other than straight. If you, like many people, do not have the opportunity to backup your trailer often enough to develop such a sense of the process, a product such as TowGo’s Trailer Backup Navigation Aid will help you know exactly when your trailer crosses to the other side and by how much.

Dan Shepard is the Founder and CEO of TowGo, LLC, manufacturer of the Trailer Backup Navigation Aid, and is an expert on trailer backup technology.

by Dan Shepard – January 1, 2019
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