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Go!What is the Federal standard for the weight limit on a single axle?

- 22,000 pounds
- 12,000 pounds
- 15,000 pounds
- 20,000 pounds

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

Based on the following figures, how much fuel can you legally add while remaining legal on the drive axles?

Percentage of fuel weight to drive axles: 25%

Weight Before Fueling: Steer: 11,275, drives: 33,800, gross: 77,420

Weight Before Fueling: Steer: 11,275, drives: 33,800, gross: 77,420

- 108.8 gallons of fuel
- 119.75 gallons of fuel
- 122 gallons of fuel
- 100 gallons of fuel

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

We know that 25% of the weight of fuel will go to the drive axles and you have 200 pounds you can add to the weight of the drive axles and remain legal. So plugging these numbers into the formula, you get:

200/25 = 8

8x100 = 800

800/8 = 100 gallons of fuel you can add

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200/25 = 8

8x100 = 800

800/8 = 100 gallons of fuel you can add

What are the four basic weight limits?

- single tire, bridge formula, tri-axle, gross vehicle
- double spacing, single axle, bridge forumula, overweight
- single axle, tandem axle, single tire, eight tire
- single axle, tandem axle, bridge formula, and gross vehicle

There are four basic weight limits: single axle, tandem axle, bridge formula, and gross vehicle.

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What is the safe number to use in calculations for the weight of fuel?

- 8 pounds per gallon
- 10 pounds per gallon
- 7.25 pounds per gallon
- 6 pounds per gallon

The precise weight of diesel fuel will vary based on temperature and humidity, but is generally around 7.25-7.5 pounds per gallon. So the safest bet is to use 8 pounds per gallon as the number you'll plug into your calculations, and that's what we're going to use here.

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Ideally, where would you like the cargo's center of gravity located?

- About 10 feet to the rear the center point of the trailer
- Centered between your drive axles and your trailer tandems.
- Just slightly behind the tractor's drive axles
- Roughly centered between the steer axle and the trailer tandems

You'll want to have the cargo loaded into the trailer so that the center of gravity is centered between your drive axles and your trailer tandems.

If the center of gravity of the cargo is too far to the front or rear of the vehicle, it is entirely possible that you will not be able to get your axle weights legal without reloading the cargo itself.

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What is the cargo's center of gravity?

- The distance from the front of the trailer to the center point of the cargo.
- The overall length of the cargo after being loaded into the trailer.
- The center point of the cargo's weight.
- The average height of the cargo above the trailer floor.

The center of gravity location of the cargo is the center point of the cargo's weight. In other words, if all of the cargo in your trailer was one solid block, the center of gravity would be the point that the entire block would balance upon like a teeter totter.

Sometimes you will load 22 pallets that all weigh the same. That makes it simple to determine the center of gravity. But often times you'll have a mixed product and you'll have to look over what's being put on the truck so you can get the weight of the cargo roughly centered between the drive axles and trailer tandems.

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Using the following numbers, what would be the final result if you moved the 5th wheel forward two holes?

steer axle: 10,800 pounds

drive axles: 34,100 pounds

Weight transfer: 500 pounds per hole

drive axles: 34,100 pounds

Weight transfer: 500 pounds per hole

- steer axle: 11,300 pounds

drive axles: 33,600 pounds - steer axle: 10,300 pounds

drive axles: 34,600 pounds - steer axle: 9,800 pounds

drive axles: 35,100 pounds - steer axle: 11,800 pounds

drive axles: 33,100 pounds

If you slide the 5th wheel toward the nose of the tractor you will increase the weight on the steer axle and take weight off the drive axles.

If you move it forward 2 holes, then:

2 holes x 500 pounds per hole = 1000 pounds of weight transfer from the drive axles to the steer axle.

10,800 + 1000 = 11,800 pounds on the steer axle after transfer

34,100 - 1000 = 33,100 pounds on the drive axles after transfer

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2 holes x 500 pounds per hole = 1000 pounds of weight transfer from the drive axles to the steer axle.

10,800 + 1000 = 11,800 pounds on the steer axle after transfer

34,100 - 1000 = 33,100 pounds on the drive axles after transfer

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

- The overall gross weight of the vehicle and the maximum percentage of weight on trailer 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 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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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

- 119.5 gallons
- 146 gallons
- 132.81 gallons
- 121.25 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

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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

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

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

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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.