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Limitations To Axle Spacing

One of the primary methods used to change the weight distribution across a truck's axles is to slide the trailer tandems toward the front or rear of the trailer. But there are laws regulating how far forward or back the tandems can be placed. The two primary laws governing the placement of the tandems are the Bridge Formula and the maximum allowed length from Kingpin to trailer tandems.

The Bridge Formula is set by the Federal government but can be altered somewhat by the individual states. When the Interstate System axle and gross weight limits were adopted in 1956, and amended in 1975, States were allowed to keep or "grandfather" weight limits that were higher.

On the other hand, the maximum allowed length from the trailer kingpin to the trailer tandems is set solely by the individual states. So once again, we have a mix of State and Federal regulations to contend with. Let's take a look at these two laws and why they're put into effect.

The Bridge Formula

bridge law formula and regulations for trucks

Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles.

The idea here was to prevent putting too much weight on a relatively small area, causing damage to road surface and bridge structure. By requiring trucks to spread the weight across a longer distance and distributing the weight across more axles, you help prevent damage to the bridges and roadways.

Compliance with Bridge Formula weight limits is determined by using the following formula:

W = the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds.

L = the distance in feet between the outer axles of any group of two or more consecutive axles.

N = the number of axles in the group under consideration.

Kingpin To Tandem Length

trailer offtracking when negotiating a turn

When a combination vehicle makes a low-speed turn - for example a 90-degree turn at an intersection - the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle. This is called low-speed offtracking. Excessive low-speed offtracking may make it necessary for the driver to swing wide into adjacent lanes to execute the turn (that is, to avoid climbing the inside curbs or striking fixed objects like telephone poles).

This performance attribute is affected primarily by the distance from the trailer kingpin to the center of the trailer rear axle, otherwise known as the wheelbase of the semitrailer. To prevent trucks from being too long to maneuvering safely around turns encountered in cities and towns, the maximum length allowed from the kingpin to the trailer tandems is set by the individual states.

Putting The Two Laws Together

So as you can see, we can't put the axles too close together or the weight being applied to the roadway in a given area will be too great, causing damage to roadways and bridges. But at the same time, we can't make the trailer wheelbase as long as we like without making the truck nearly impossible to maneuver safely in cities and towns. So we have to work within the legal limits of the overall gross weight of the vehicle, the axle weights of the vehicle, the load rating of the tires, the load rating of the axles, The Bridge formula, and the maximum length allowed from kingin to trailer tandems.

That's a lot to take into consideration. In the remainder of this section we'll be putting all of this together and showing you what it takes to get legal under any circumstances.

You will not need to do calculations using this formula. It's purely for understanding how the numbers are derived. You will be using charts to look up legal weight and length limits.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

Tandems:

Tandem Axles

A set of axles spaced close together, legally defined as more than 40 and less than 96 inches apart by the USDOT. Drivers tend to refer to the tandem axles on their trailer as just "tandems". You might hear a driver say, "I'm 400 pounds overweight on my tandems", referring to his trailer tandems, not his tractor tandems. Tractor tandems are generally just referred to as "drives" which is short for "drive axles".

Tandem:

Tandem Axles

A set of axles spaced close together, legally defined as more than 40 and less than 96 inches apart by the USDOT. Drivers tend to refer to the tandem axles on their trailer as just "tandems". You might hear a driver say, "I'm 400 pounds overweight on my tandems", referring to his trailer tandems, not his tractor tandems. Tractor tandems are generally just referred to as "drives" which is short for "drive axles".

Interstate:

Commercial trade, business, movement of goods or money, or transportation from one state to another, regulated by the Federal Department Of Transportation (DOT).

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

What is low-speed offtracking?
  • When a combination vehicle makes a low-speed turn the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle.
  • When a trailer's tandem axles are slightly out of alignment, the rear of the trailer will track off to one side or another instead of directly behind the tractor
  • When taking off from a dead stop, the initial torque of the engine on the drivetrain will cause a slight offtracking of the tractor which could be a hazard to other traffic
  • When making a high speed turn, the momentum carried by the trailer will cause it to swing wide and possibly outside the lane of travel

Quote From The CDL Manual:

When a combination vehicle makes a low-speed turn - for example a 90-degree turn at an intersection - the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle. This is called low-speed offtracking.
Next
What is the main factor affecting the low-speed offtracking of a tractor trailer?
  • The length of the trailer, regardless of the position of the trailer tandems
  • The distance from the trailer kingpin to the center of the trailer rear axle
  • The ratio of the length of the tractor to the length of the trailer
  • The distance from the steer axle to the drive axles

Quote From The CDL Manual:

When a combination vehicle makes a low-speed turn - for example a 90-degree turn at an intersection - the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle. This is called low-speed offtracking. Excessive low-speed offtracking may make it necessary for the driver to swing wide into adjacent lanes to execute the turn (that is, to avoid climbing the inside curbs or striking fixed objects like telephone poles).

This performance attribute is affected primarily by the distance from the trailer kingpin to the center of the trailer rear axle, otherwise known as the wheelbase of the semitrailer. To prevent trucks from being too long to maneuvering safely around turns encountered in cities and towns, the maximum length allowed from the kingpin to the trailer tandems is set by the individual states.

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Next
What is the primary goal of implementing the Bridge Formula?
  • To prevent overloading the trailer tandems and exceeding the load rating of the trailer tires
  • To prevent over-length trucks from becoming a hazard on narrow bridges and tight roadways
  • To prevent putting too much weight on a relatively small area, causing damage to road surface and bridge structure.
  • To increase the minimum strength of bridges, allowing for heavier gross vehicle weights which improves efficiency

Quote From The CDL Manual:

The idea (of implementing the bridge formula) was to prevent putting too much weight on a relatively small area, causing damage to road surface and bridge structure.

TruckingTruth's Advice:

A bridge is designed to handle a certain number of pounds per square inch on the surface. By creating the Bridge Formula, Congress made sure that too much weight wasn't concentrated on a small area of a bridge, causing damage to the bridge structure.
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Next
What are the two primary laws governing the placement of the tandems?
  • The tire load rating of the drive axle tires and the minimum clearance law
  • The maximum width of 8' 6" on Interstate Highways and the maximum trailer length law
  • The Bridge Formula and the maximum allowed length from Kingpin to trailer tandems.
  • The overall gross weight of the vehicle and the maximum percentage of weight on trailer tandems

Quote From The CDL Manual:

The two primary laws governing the placement of the tandems are the Bridge Formula and the maximum allowed length from Kingpin to trailer tandems.
Prev
Next
Why did Congress enacted the Bridge Formula in 1975?
  • To regulate the building of bridge structures so they can support the weight of heavy modern trucks
  • To limit the weight-to-length ratio of a vehicle crossing a bridge.
  • To prevent the maximum tire load rating from being exceeded by narrow axle spacing
  • To prevent over-width trucks from becoming a hazard on narrow bridges

Quote From The CDL Manual:

Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge.
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