Back in 1892, British songwriter Harry Dacre penned a little jingle about he and a woman named “Daisy” riding on a bicycle built for two, more commonly known as “tandem”. Ever seen a pair of horses pulling a trailer? Yep – a tandem. In truck world, a pair of axles is sometimes called a tandem, but sometimes it’s not. Sometimes a tandem is more than two axles. Either way, in the vast world of truck terminology misunderstandings, the tandem is not forgotten. This article will clarify the ongoing schism about tandems.
First, understand there are two kinds of tandems. The legal definition in the Illinois Vehicle Code, and the street version. The reader may use the term tandem to describe whatever he wants, but that does not mean he is statutorily correct.
In Illinois law, a tandem is two or more consecutive axles, with a minimum spacing on center of 40” and a maximum spacing of 96”. This can be found in 625 ILCS 5/1-204.3. Its plainly obvious in this definition that a tandem is not limited to only two axles.
If a vehicle manufacturer can cram three or more physical axles into the 40” to 96” window, all those axles are considered one tandem. In the weight laws of Illinois, this group of more than two axles will receive a maximum 34,000 pounds of weight (barring any special exceptions). If you are thinking about buying a truck then consider checking out this semi trailer dealer.
If two axles are so close together they do not have a minimum 40” between the two of them, they are not a legal tandem. Even though there are two physical axles (making it a tandem by the street definition), the maximum weight for the two axles is measured as a single axle, or 20,000 pounds. This is common on small landscaping trailers.
If two consecutive axles have more than 96” between them, they may very well be a street definition tandem, but again, they are not a legal tandem. They are instead two single axles, each receiving a maximum 20,000 pounds individually. Depending on their spacing, they may receive anywhere from 38,000 to 40,000 pounds under the Federal Bridge Formula.
Where the real confusion about tandems comes into play is on overweight permits issued by the Illinois Department of Transportation (IDOT) and other local agencies.
For instance, a 9-axle mobile crane which weighs 187,000 pounds gross weight obtains a permit from IDOT and some local agencies. On the vehicle, there may very well be several sets of two consecutive axles with a spacing of 40”-96”.
The confusion comes in because the permit may list a tandem, but on the scale, these tandems have weights greatly in excess of 34,000. What the term tandem really means is a “group of axles”. This same 9-axle crane may have a group of two axles, then a group of four axles, then a group of three axles. On the permit, it may list each group as the first tandem, second tandem and third tandem.
Unfortunately, many enforcement agents have applied the legal tandem definition (and weight of 34,000 pounds) to these groups of axles and issued very costly overweight citations. This is erroneous for two reasons.
First, this display of the word tandem is an alternate version, not the legal version. Second, because it is overweight, the legal definition is a moot point anyhow. The permit authority has issued a higher weight for this group, and enforcement must honor those weights.
Words matter, and keen discernment recognizing the differences between legal definitions and street definitions is paramount to truck enforcement efforts. Police officers and truckers alike must be clear and detailed when explaining these situations to guarantee correct interpretations are applied. Loose and uneducated interpretations wrongly cost people a whole lot of time and money.
