Choose A Section:

Go!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 prevent over-width trucks from becoming a hazard on narrow bridges
- 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

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

Previous
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
- 12,000 pounds
- 15,500 pounds
- 12,300 pounds

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.

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.

Your trailer has a 6 inch hole spacing in the tandem slider rail and your trailer tandems are 900 pounds overweight. What is the minimum number of holes you will have to slide the tandems to get the tandems within legal limits?

- 3 holes
- 2 holes
- 4 holes
- 1 hole

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

6 inch hole spacing means you'll move approximately 400 pounds per hole. In order to move a minimum of 900 pounds, you have to move at least 3 holes (400 x 3 = 1200).

Previous
Next
You weigh the truck immediately before and after fueling. Based on the following, determine what percentage of the weight goes to each set of axles:

Before fueling:

steer: 11,450, drives: 33,100, gross: 76,700

After fueling:

steer:11,850, drives: 33,300, gross: 77,300

steer: 11,450, drives: 33,100, gross: 76,700

After fueling:

steer:11,850, drives: 33,300, gross: 77,300

- 73% went on the steer axle

27% went on the drive axles - 88% went on the steer axle

12% went on the drive axles - 67% went on the steer axle

33% went on the drive axles - 75% went on the steer axle

25% went on the drive axles

To determine the percentage of fuel weight added to the steer axle, take the weight added to the steer axle, divide it by the total fuel weight added (determined by the change in the gross weight), and then multiply that times 100

To determine the percentage of fuel weight added to the steer axle, take the weight added to the steer axle (400 pounds), divide it by the total fuel weight added (600 pounds), and then multiply that times 100

400/600 = .67

.67 * 100 = 67% fuel weight to the steer tires

100% - 67% = 33% went on the drive axles.

Previous
Next
400/600 = .67

.67 * 100 = 67% fuel weight to the steer tires

100% - 67% = 33% went on the drive axles.

What factor has the greatest affect on how much weight is moved per hole in the tandem slider rail?

- The distance from the drive axles to the trailer tandems
- The distance between the locking pin holes
- The amount of fuel you have onboard
- The center of gravity of the cargo

The distance between the locking pin holes on the trailer will be the main factor affecting how much weight is moved per hole. This distance varies between the different trailer manufacturers, and the larger the distance between the holes, the more weight will be shifted with each hole.

Previous
Next
Where will the weight of fuel be distributed?

- Entirely on your drive axles
- Entirely on your steer axle
- Mostly to your drive axles, with some going onto the steer axle.
- Mostly to your steer axle, with some going onto the drive axles.

The weight of the fuel in your tanks will be distributed mostly to your steer axle, with some going onto the drive axles.

Previous
Next
Adding or burning off fuel will mainly change the amount of weight on which axles?

- Drive axles and somewhat on your trailer tandems
- Drive axles only
- Steer axle and somewhat on your drive axles
- Trailer tandems only

Adding or burning off fuel will mainly change the amount of weight on your steer axle and somewhat on your drive axles

Most of the weight of the fuel will go on your steer axle. The percentage of weight distributed between your steer and drive axles with regard to fuel will depend on the placement of your fuel tanks.

Previous
Next
You receive a scale ticket and your drive axles are 31,700 and your trailer tandems are 35,100. Based upon a trailer with 6 inch slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and what would the final weights be?

- Slide tandems forward 3 holes - 33,900 tandems - 32,900 drives
- Slide tandems forward 4 holes - 33,300 tandems - 33,100 drives
- Slide tandems back 3 holes - 33,900 tandems - 32,900 drives
- Slide tandems back 4 holes - 33,300 tandems - 33,100 drives

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

With 35,100 pounds on the trailer tandems you must move a minimum of 1,100 pounds off the trailer tandems. To shift weight off the trailer tandems and onto the drive axles, you slide the trailer tandems back toward the rear of the truck.

With 6 inch spacing you're moving 400 pounds per hole so you need to move 3 holes to shift a minimum of 1,100 pounds (400 x 3 = 1,200).

Previous
Next
With 6 inch spacing you're moving 400 pounds per hole so you need to move 3 holes to shift a minimum of 1,100 pounds (400 x 3 = 1,200).

What will be the ultimate limitation that prevents you from putting the trailer tandems as far to the rear of the trailer as possible?

- The load rating of the steer tires
- The kingpin to trailer tandem length limitations
- The hole structure of the trailer tandem slider rail
- The center of gravity of the cargo

The kingpin to trailer tandem length limitations will prevent you from going too far back with your trailer tandems

Previous
Next
Based on the following figures, how much fuel can you legally add while remaining legal on the steer axle?

Load rating of steer tires: 6,150 pounds

Percentage of fuel weight to steer axle: 80%

Steer axle weight limit in states you're travelling: 20,000 pounds

Weight Before Fueling: Steer: 11,450, drives: 33,100, gross: 76,700

Percentage of fuel weight to steer axle: 80%

Steer axle weight limit in states you're travelling: 20,000 pounds

Weight Before Fueling: Steer: 11,450, drives: 33,100, gross: 76,700

- 132.81 gallons
- 121.25 gallons
- 119.5 gallons
- 146 gallons

Once you know your current axle weights and the percentage of fuel weight that goes on the steer axle, you can use this formula you can calculate the amount of fuel you can take on:

Because your steer tires are rated at 6,150 and the legal weight limit is 20,000 pounds for the steer axle in the states you're travelling in, the most restrictive law would be to not exceed the tire load rating. In this case, you can have 12,300 on your steer axle. Right now you have 11,450 on your steer axle, which means you can add 850 pounds to your steer axle and still be legal.

Because we know that 80% of the weight of fuel will go to the steer axle and you can add 850 pounds to your steer axle, we can plug the numbers into the formula above.

850/80 = 10.62

10.62 x 100 = 1062

1062/8 = 132.75 gallons of fuel you can add

Previous
Next
Because we know that 80% of the weight of fuel will go to the steer axle and you can add 850 pounds to your steer axle, we can plug the numbers into the formula above.

850/80 = 10.62

10.62 x 100 = 1062

1062/8 = 132.75 gallons of fuel you can add

This section was created by Trucking Truth to help people understand how to load cargo, scale the truck, and understand the laws about truck weight limits. These materials will not be on your written CDL exams but it is critical that every truck driver knows these materials.

There are four basic weight limits: single axle, tandem axle, bridge formula, and gross vehicle. The Federal Standards are as follows:

- 20,000 pounds single axle weight
- 34,000 pounds tandem axle weight
- 80,000 pounds gross vehicle weight
- Bridge Formula Calculations

There are a long list of critical safety issues which require putting limitations on the gross weight, axle weights, the weight distribution across the length of a vehicle, and the weight distribution across a minimum number of axles.

- Putting too much weight on a small area of the road surface can cause ruts, cracks, and potholes
- Putting too much weight on a small area of a bridge surface can cause structural damage to the bridge
- Too much weight on your steer axle can lead to a "heavy steering" feel and may cause the truck to react improperly to steering inputs
- Not enough weight on your steer axle can lead to a loss of traction for your steer tires
- Improper weight balance between your tractor drive tires and trailer tandems can lead to poor traction and an increased risk of jackknifing
- Too much weight toward the back of the trailer can lead to a "pendulum effect", causing the rear of the trailer to sway back and forth while driving down the highway or jackknife going around a curve
- Overloading a tire beyond it's maximum tire load capacity can cause tire damage and blowouts
- Overloading the suspension system of the truck can cause damage to the suspension system which could easily lead to loss of control of the vehicle

So as you can see, it's critical in so many ways to make sure that we follow the weight limits in strict accordance with the law. It is incredibly dangerous to overload a vehicle or to have the weight improperly distributed across the axles.

There are several ways to distribute the overall weight of the vehicle across the different sets of axles. You will affect weight distribution by:

- Changing the weight distribution of the cargo along the length of the trailer, or moving the cargo around after loading the truth. This will distribute the weight mainly between your drive axles and trailer tandems
- Sliding your trailer tandems forward or backward will redistribute the weight mainly between your drive axles and trailer tandems
- Sliding your 5th wheel will redistribute the weight mainly between your steer axle and your drive axles
- Adding or burning off fuel will mainly change the amount of weight on your steer axle and somewhat on your drive axles also, depending upon the placement of your fuel tanks.

There are several factors that will reduce the amount of weight we can transfer between the different sets of axles on the truck:

- Federal laws limit the maximum weight on any set of axles and the gross vehicle weight - 20,000 pounds single axle, 34,000 pound tandem axles, 80,000 pounds GVW
- The bridge law formula limits the maximum amount of weight you can carry across any set of axles based upon the number of axles and the spacing between them (we'll discuss this formula soon).
- The maximum legal length allowed between your trailer kingpin and your trailer tandems will limit how far back you can slide your tandems
- The load rating of the tires you have will determine the maximum amount of weight allowed on any particular tire
- The load rating of the suspension system will limit the amount of weight you can have on any axle

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.

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.