What in the heck is the Offset Law? You may be asking yourself. Also, if I break it, can I go to jail?

No, it’s not that kind of law. What I’m referring to here is a scientific law. One that hopefully will be as familiar to you as the laws of gravity.

If you hold an apple at arm’s length and you open your hand, it will fall to the ground. The laws of gravity are very familiar to us. We don’t need to know the complex mathematical formulas that relate to them. We can count on that apple and any subsequent apples behaving the same way time after time.

Likewise, the Offset Law is an observable law related to backing up a trailer that can make your life less stressful. It would be beneficial to take the time to understand and incorporate its principles into your knowledge base.

I have observed this law many times over the years and needed a name to refer to it by when explaining it to students. I gave it the term “The Law of Increasing Trailer Offset,” or “The Offset Law” for short. It relates specifically to the behavior of your trailer while traveling in reverse.

If there is such a thing as a “secret” that experienced drivers know that beginners don’t, this is it.

It is my opinion based on my experience that having a solid understanding of this fundamental law sets apart the experienced truck driver from the beginner. A skilled driver that is exceptionally proficient in alley-dock backing has instinctively become aware of these principles even if they don’t have a conscious awareness of it.

You cannot be good at backing into an alley-dock without using these principles to your advantage.

I would like to see this law taught in truck driving schools.
I believe learning it earlier rather than later can eliminate weeks and months of frustration caused by trial and error. From my experience, over-steering is a sure sign that a student lacks a solid understanding of these principles.

I once heard a mentor comment about a student that was practicing a backing maneuver say something to the effect of,
“You know? If he would leave the steering wheel alone, that thing would just about park itself.”
He was referring to what I call, “The Offset Law.”

Over time, I have performed repeated experimental observations and
I have narrowed down the essential elements of this law. I would encourage you to memorize these principles so that you will know what to expect when backing up your trailer. Knowing what your trailer is going to do ahead of time is a game-changer. This predictability will allow you to maintain complete control over it. We want to eliminate guessing and over-steering.

How the Offset Law works:

The necessary foundation for this requires the steer tires to be straight. Try to develop an awareness for where your steer tires are pointing at all times. I suggest looking out your open window down at your steer tires before starting back. When the steer tires are straight the tractor will continue on its current trajectory. You will need to develop an awareness of where your tractor is pointing at all times. This awareness is especially crucial during an alley dock backing maneuver. You will be traveling down a narrow corridor with obstacles nearby.

Any offset or angle between tractor and trailer will increase at a predictable rate until a jackknife occurs.

This principle is what makes “straight-line” backing a challenge. When the tractor and trailer are perfectly straight or in-line with each other, the pressure exerted against the kingpin by the fifth wheel is at the very front of the kingpin. Since you have a curved surface pushing against a round cylinder, nothing is locking that pressure to that center-most point on the kingpin. Any movement off of that center point to either side will create a sideways pressure that will start to push the front of the trailer around its pivot point. The pivot point is at the rear of the trailer at the center of the tandems. The trailer will start to rotate and point away from its original position.

The above diagram illustrates how the Offset Law effects the trailer if the tractor goes from point A to point B with NO turning of the steering wheel.

In a real backing situation, you should have the tractor gradually follow the trailer into the “hole” and avoid jackknifing.

There are two essential properties to understand the rate at which offset occurs:

✓ The speed at which the offset happens does not follow a linear progression. It starts slowly and gets increasingly faster as the offset increases. By the time the tractor and trailer form a 90-degree angle, it will jackknife very quickly and cause damage to both the tractor and the trailer.

✓ Trailer length vs. offset speed. The shorter the trailer— the faster it offsets. The longer the trailer— the slower. A 53-foot trailer with the tandems (rear tires) slid forward offsets faster than one with its tandems at the rear. The trailer tires are the pivot point. What matters is the distance from the kingpin to the pivot point.

These two properties are critical to understanding so that you can become familiar with the timing of the process from start to finish.
You must make a conscious effort to develop new awareness. You must be continually aware of where your entire trailer is currently pointing. The tractor’s “position” relative to the front of the trailer (kingpin). And how fast both of these are changing.

Your steering wheel’s only influence on the trailer tires is through directional pressure against the kingpin.

There is no direct connection between the steering wheel and the back of your trailer.

The only connection you have to the trailer is at the kingpin. The front end is where your primary focus should be. Watching the Offset Law in action is the best way to prove this to new drivers. Anytime there is offset between tractor and trailer the Offset Law is working either for or against you.

The trailer rolls in the direction it is currently pointing. The only time your trailer is not changing direction is when the pressure is against the very front of the kingpin— when the tractor and trailer are perfectly in-line.

Controlling the Speed of the Offset:

Once you get a feel for the speed of the Offset Law with the steer tires straight, start to experiment with speeding it up and slowing it down by turning the steering wheel.

I’m hesitant even to use the terms, “Right” and “Left” because new drivers are quick to cling to some “trick.” They will try to memorize which direction does the speeding up and slowing down without taking the time to understand what’s going on. Then later, they will mix the two up and get it wrong.

You must know what’s going on at the kingpin. In other words, you need to develop an awareness for and pay closer attention to the front part of the trailer.

From my observations, most new drivers over-focus on the rear of the trailer.

How is your tractor interacting with the front part of your trailer? Exactly which direction is it pushing the front part of the trailer? You must be constantly aware of the changing angle between the tractor and trailer so that you can make adjustments and control where the trailer points.

Moving the pressure from the side of the kingpin towards the front of the kingpin will slow the offset. Turning the wheel in a direction that gradually moves pressure from the front of the kingpin to the side of the kingpin will make the trailer offset faster.

Refer back to the diagram where tractor B is pushing against the kingpin at a 90-degree angle relative to the trailer.

For the tractor to follow the trailer into the “hole,” the steering wheel must be turned counterclockwise. This turn will gradually move the pressure from the side of the kingpin to the front of the kingpin— slowing down the offset.
Counterclockwise slows the offset.
Clockwise speeds up the offset.
Unless you live in a country where the steering wheel is on the right—in that case, everything would be the opposite.

Note: In this position, waiting until the tractor and trailer are at 90 degrees before turning the steering wheel counterclockwise is too late. It must be done gradually and started earlier. It will take time to get your tractor from the side of the kingpin back to the front. During that time the tractor will be applying sideways pressure which will cause the whole trailer to rotate clockwise around its pivot point at the back tandems. By the time your tractor gets to the front, the trailer it would no longer be pointing at the hole.

I believe it is a mistake to think of your tractor and trailer as one unit. The two are separate units connected at the kingpin. You control the tractor with your steering wheel—not the trailer. Where your tractor pushes against the kingpin determines where the trailer points—which determines where the trailer rolls.

Summary:

With the steer tires straight, any offset between tractor and trailer will increase until eventual jackknife occurs. The speed of the offset increases as the offset increases.
Longer trailers offset slower than shorter trailers.
The trailer tires are the pivot point. A trailer with the tandems slid to the rear is longer than the same with the tandems moved forward.

Terminology:

“jackknife”: move into a bent or doubled-up position: (think of the folding blade of a pocket knife). When a tractor and trailer offset or “fold” in against each other causing damage.

“the box,” or “the hole”: refer to spaces designed to park a trailer but are currently empty. An empty parking space. An open dock space.

“tandems”: the complete set of a trailer’s tires and axles. Some have a fixed position. But most can be slid forward or backward to help distribute weight.

If you found this post helpful, please share it with fellow students.
Be safe out there!

Recommended reading :

Arrow Awareness: The First Key to Mastering Your Trailer’s Movement.