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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 you can position the tandems. 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. They accomplished this 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 the 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 off-tracking. Excessive low-speed off-tracking 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 maneuver 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 kingpin to trailer tandems.

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

Multiple-Choice Questions:

Question #632 (1 of 5)

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Why did Congress enacted the Bridge Formula in 1975?

  • To prevent the maximum tire load rating from being exceeded by narrow axle spacing
  • To limit the weight-to-length ratio of a vehicle crossing a bridge.
  • To prevent over-width trucks from becoming a hazard on narrow bridges
  • To regulate the building of bridge structures so they can support the weight of heavy modern trucks
Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge.
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Question #631 (2 of 5)

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What are the two primary laws governing the placement of the tandems?

  • 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
  • The tire load rating of the drive axle tires and the minimum clearance law
The two primary laws governing the placement of the tandems are the Bridge Formula and the maximum allowed length from Kingpin to trailer tandems.
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Question #634 (3 of 5)

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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 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 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 making a high speed turn, the momentum carried by the trailer will cause it to swing wide and possibly outside the lane of travel
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.
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Question #635 (4 of 5)

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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

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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Question #633 (5 of 5)

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What is the primary goal of implementing the Bridge Formula?

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