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How About Them Axles?

Let’s start at the very beginning. Why?  Because it’s a very good place to start. Truck 101. Sometimes you need to drill down to the basics to understand a bigger concept that causes confusion. No house, no matter how ornate and expensive the finishes, is worth anything if the foundation is bad. The article this week will focus on what defines an axle. Sounds simple doesn’t it? Well maybe there is more to it than you thought.

The legislature in Illinois was wise to create Chapter 1 of the Illinois Vehicle Code. It is here where definitions used throughout the IVC are located. 625 ILCS 5/1-105.6, gives us the definition for an axle load.

“The total load transmitted to the road by all wheels whose centers may be included between 2 parallel transverse vertical planes 40 inches apart extending across the full width of the vehicle.”

Two key concepts here. First, there is actually no definition of an “axle” in the IVC. Motorcycles and bicycles have axles. If they didn’t, the singular wheels would not spin. This definition refers to what a legal “axle load” is. In order for there to be an axle load, there has to be a plurality of wheels extending across the full width of the vehicle.

Here is an example. A semi-tractor trailer combination is tooling down the road. The trailer has two axles, more commonly referred to as a tandem (more on that later in this article). However, the first of the two axles is missing one wheel on the driver’s side. Is there physical “axle” present? Yes. Is there an axle load? No there is not, because there is only one wheel.

The second concept to understand in the definition of an axle load is the measurement of 40” on-center. While the author probably could have explained this a little more clearly, the intent is obvious. An “axle load” may have more than one physical axle. However, it is only a singular axle load if the distance, on-center between the physical axles, is less than 40”.

Look at the picture below. There are two physical axles, however they are only about 35” apart on center. Since the this distance is less than 40” for purposes of weight law in Chapter 15 of the IVC, this is only a single axle, meaning the entire weight of all four wheels for both axles only is allowed 20,000 pounds gross weight.

2014-10-09 09.14.50 (500x375)

These are typically found on smaller landscape and enclosed trailers. In reality, most of these axles would never survive being loaded to the legal weight of 20,000 pounds or they would snap. The purpose of the multiple axles is more for balance and load sharing on smaller wheels and tires.

In order to be considered a tandem, this definition found in 625 ILCS 5/1-204.3 must be met: “Any 2 or more single axles whose centers are more than 40 inches and not more than 96 inches apart, measured to the nearest inch between extreme axles in the series…”

There are two key points to consider here also. First, this definition is more plainly written, but the key phrase is “2 or more single axles”. This means there could be three, maybe even four single axles in a tandem. Think of those tiny wheels under an oversized manufactured housing trailer.

The second part is the spacing. In order to be a tandem, the distance must be a minimum 40” on center between the first and last axles in the group, but no more than 96”. The number of axles is irrelevant. If there are four axles, and the on-center measurement is 95” like the one in the picture below, it is a tandem and receives 34,000 pounds legal weight.


Unfortunately, there has been some poor and short-sided interpretation of this language taught to police officers in Illinois. Apparently the 40”-96” part of the text has been forgotten, and the “2 or more single axles” means everything. Wrong.

What has happened is police officers being taught any series of three consecutive axles, regardless of the distance, is a tandem. This is wholly incorrect. Many times on a lowboy trailer or an intermodal chassis there is a series of three axles. The improper instruction has taught truck officers this is always a tandem and only receives 34,000 pounds. That’s dangerous teaching.

Maybe a group of three axles is a tandem, maybe it is not, but it is surely not an absolute. If a group of axles is more than 96” on center, then what is to be done? Next week, the article will explore the correct application of the Federal Bridge Formula and expose more erroneous teachings.

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