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Scaling The Truck

bridge law formula and regulations for trucks

Although the trucks we're referring to have five axles, when weighing the truck at a truckstop you will be given three sets of weights - the steer axle, the drive axles as a set, and the trailer tandems as a set. You are not weighing the drive axles or trailer tandem axles individually. They are scaled as a set and the weight limits you'll be shooting for are as shown in figure 3 to the left.

bridge law formula and regulations for trucks

The legal weight limits for each set of axles is:

  • Steer axle varies by state
  • 34,000 for the drive axles
  • 34,000 for the trailer tandems

The first image is a readout from an actual scale ticket at a truckstop (click to enlarge). As you can see, they'll list the steer axle, the drive axle(s), the trailer axle(s), and your gross weight.

The Steer Axle

There are various rules and regulations regarding the legal weight limit of your steer axle from state to state. Many states limit it to either 12,000 pounds or 20,000 pounds, but there are others that set a different limit or use a calculation based on tire tread width. For example, Kentucky has a limit of 700 pounds per inch of tread width. So if you have a 10 inch wide tire tread, you are allowed a maximum of 7,000 pounds per tire (700 pounds per inch x 10 inches wide) or 14,000 pounds total for the steer axle.

Keep in mind though - even if they allow 14,000 pounds on the steer axle, you have to have tires that are rated to carry that much weight. If your steer tires have a tire load limit of 6,000 pounds each, you can only carry 12,000 pounds on your steer axle, regardless of state laws.

You can look up the legal limits for each state in the Rand McNally Motor Carrier's Road Atlas.

Altering The Weight Distribution

The primary factors which will affect the weight distribution across a truck's axles are:

  • The position of the trailer's tandems
  • The position of the tractor's 5th wheel
  • The overall weight of the cargo in the trailer and the horizontal (front-to-back) position of its center of gravity
  • The amount of fuel onboard and the placement of the fuel tanks

In the coming pages we'll go through these one at a time and learn to apply each one individually. Later we'll put them all together and show you how to get your truck's weight distribution legal out on the highways, coast to coast, under any circumstances.

Gross Weight Remains Unaffected By Weight Transfer

The final key point to understand when it comes to weight transfer is that none of the methods of redistributing weight between the different sets of axles, including sliding your 5th wheel, sliding your tandems, or moving the cargo around will have any affect on your gross weight. The gross weight will remain the same, while the amount of weight on each set of axles will change as you make these adjustments. This is an important point to keep in mind.

The only way to affect the gross vehicle weight would be to add or remove weight from the vehicle. Adding or burning off fuel, or adding or removing cargo would be two examples of ways to alter the gross vehicle weight.

There are various weight limits imposed by the states for the steer axle. We'll cover that in a moment.
We will get into more detail about the center of gravity shortly.

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.

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".

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

California allows 20,000 pounds maximum on the steer axle. The tread width of your tires is 11 inches, and the load rating on your steer tires is 7000 pounds. How much weight can you carry on your steer axle in California?
  • 20,000 pounds
  • 12,000 pounds
  • 14,000 pounds
  • None of these are correct

Quote From The CDL Manual:

Even if they allow 14,000 pounds on the steer axle, you have to have tires that are rated to carry that much weight. If your steer tires have a tire load limit of 6,000 pounds each, you can only carry 12,000 pounds on your steer axle, regardless of state laws.

TruckingTruth's Advice:

We have to find the most restrictive law to determine our maximum weight.

California allows up to 20,000 pounds on the steer axle

There is no requirement based upon tread width in California, so the width of your steer tires is irrelevant for this example

Your steer tires have a maximum load rating of 7,000 pounds each, which would therefore limit your legal steer axle weight to 14,000 pounds. This is the most restrictive of the laws, so 14,000 pounds is your legal weight limit in this case.

Next
Which of the following has the greatest affect on the gross vehicle weight?
  • The position of the trailer's tandems
  • Moving the cargo toward the front or rear of the trailer
  • None of these affect the gross vehicle weight at all
  • The position of the tractor's 5th wheel

Quote From The CDL Manual:

The final key point to understand when it comes to weight transfer is that none of the methods of redistributing weight between the different sets of axles, including sliding your 5th wheel, sliding your tandems, or moving the cargo around will have any affect on your gross weight.

TruckingTruth's Advice:

The only way to affect the gross vehicle weight is to add or remove weight from the vehicle. The methods of weight transfer we've discussed will move weight from one area of the vehicle to another, but will not affect the gross weight of the vehicle itself.
Prev
Next
Which of the following are not a primary factor affecting weight distribution across the axles?
  • The amount of fuel onboard and the placement of the fuel tanks
  • The position of the trailer's tandems
  • All of these factors affect the weight distribution across the axles
  • The position of the tractor's 5th wheel

Quote From The CDL Manual:

The primary factors which will affect the weight distribution across a truck's axles are:

  • The position of the trailer's tandems
  • The position of the tractor's 5th wheel
  • The overall weight of the cargo in the trailer and the horizontal (front-to-back) position of its center of gravity
  • The amount of fuel onboard and the placement of the fuel tanks
Prev
Next
You're in Illinois which allows up to 20,000 pounds on the steer axle. You're heading to Indiana which only allows 12,000 on the steer axle. Your steer tires have a load rating of 6,500 pounds each. What is the maximum legal weight your steer axle can be for each of the two states?
  • 20,000 pounds in Illinois
    12,000 pounds in Indiana
  • 13,000 pounds in Illinois
    12,000 pounds in Indiana
  • 20,000 pounds in Illinois
    13,000 pounds in Indiana
  • 13,000 pounds in Illinois
    13,000 pounds in Indiana

Quote From The CDL Manual:

Keep in mind though - even if they allow 14,000 pounds on the steer axle, you have to have tires that are rated to carry that much weight. If your steer tires have a tire load limit of 6,000 pounds each, you can only carry 12,000 pounds on your steer axle, regardless of state laws.

TruckingTruth's Advice:

We have to find the most restrictive law for each state.

Illinois

Allows 20,000 pounds on the steer axle

Your tire load rating of 6,500 means your maximum legal weight is 13,000 pounds on the steer axle.

The 13,000 pounds is the most restrictive for Illinois

Indiana

Indiana allows 12,000 on the steer axle

Your tire load rating of 6,500 means your maximum legal weight is 13,000 pounds on the steer axle.

The 12,000 pound limit imposed by Indiana on the steer axle is the most restrictive for Indiana

Prev
Next
What is the maximum legal weight for the drive axles or trailer tandems?
  • 34,000 pounds
  • 20,000 pounds
  • 26,000 pounds
  • 12,000 pounds

Quote From The CDL Manual:

The legal weight limits for each set of axles is:

  • Steer axle varies by state
  • 34,000 for the drive axles
  • 34,000 for the trailer tandems
Prev
Next
In Oregon, the maximum weight on the steer axle is listed as 600 pounds per inch of tire tread width up to 20,000 pounds. Your steer tires have a load rating of 6,150 pounds each and are 10 inches wide. What is the maximum legal weight your steer axle can carry in Oregon?
  • 20,000 pounds
  • 12,300 pounds
  • 12,000 pounds
  • 15,500 pounds

Quote From The CDL Manual:

Kentucky has a limit of 700 pounds per inch of tread width. So if you have a 10 inch wide tire tread, you are allowed a maximum of 7,000 pounds per tire (700 pounds per inch x 10 inches wide) or 14,000 pounds total for the steer axle.

TruckingTruth's Advice:

You must find the most restrictive law to determine your maximum legal weight.

The maximum Oregon allows is 20,000 pounds on the steer axle

The load rating on your tires is 6,150, which would put your maximum legal weight at 12,300 pounds regardless of the fact that there are less restrictive laws.

The maximum Oregon allows by weight is 600 pounds per inch. You have 10 inch tires so that would be a maximum of 6000 pounds per tire, or 12,000 pounds on the steer axle, which is the most restrictive of the various laws. Therefore, 12,000 pounds is your legal limit in this truck in Oregon.

Prev
Next
Which of the following will alter the gross vehicle weight?
  • Sliding the tractor 5th wheel
  • Sliding the trailer tandems
  • Adding more fuel to the tanks
  • Moving the cargo toward the rear of the trailer

Quote From The CDL Manual:

The final key point to understand when it comes to weight transfer is that none of the methods of redistributing weight between the different sets of axles, including sliding your 5th wheel, sliding your tandems, or moving the cargo around will have any affect on your gross weight.

The only way to affect the gross vehicle weight would be to add or remove weight from the vehicle. Adding or burning off fuel, or adding or removing cargo would be two examples of ways to alter the gross vehicle weight.

Prev
Finish
Please select an option
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