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Go!Sliding the 5th wheel will change the weight distribution almost exclusively between which sets of axles?

- The steer axle and the drive axles
- The steer axle is the only one affected by fuel
- The steer axle and the trailer tandems
- The drive axles and the trailer tandems

Sliding the 5th wheel will change the weight distribution almost exclusively between the steer axle and the drive axles.

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

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

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In the following picture, what are axles 2 and 3 commonly referred to as?

- "drives" or "drive axles"
- "tandems" or "front trailer tandems"
- "duals" or "tractor set"
- "drives" or "trailer drives"

#2 and #3 together are your tractor drive tandem axles (commonly referred to as "drives" or "drive axles")

You'll normally hear drivers refer to these two axles as your "drives". They'll say, "The other day I scaled out and my 'drives' were over by 800 pounds".

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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: 30%

Weight Before Fueling: Steer: 11,275, drives: 33,750, gross: 77,220

Weight Before Fueling: Steer: 11,275, drives: 33,750, gross: 77,220

- 104.13 gallons of fuel
- 112.75 gallons of fuel
- 110 gallons of fuel
- 88.5 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 30% of the weight of fuel will go to the drive axles and you have 250 pounds you can add to the weight of the drive axles and remain legal. So plugging these numbers into the formula, you get:

250/30 = 8.33

8.33x100 = 833

833/8 = 104.13 gallons of fuel you can add

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250/30 = 8.33

8.33x100 = 833

833/8 = 104.13 gallons of fuel you can add

What is the correct method of determining what percentage of fuel is distributed between the steer axle and the drive axles?

- Weigh the truck, then immediately fuel up, and then immediately re-weigh the truck to see what percentage of the added weight went to the steer axle, and what percentage went to the drive axles.
- Weigh the truck immediately after fueling, then run exactly 100 miles, and then re-weigh the truck to see what percentage of the added weight came off the steer axles.
- Weigh the truck after running 100 miles, then immediately fuel up, and then re-weigh the truck to see what percentage of the added weight went to the steer axle, and what percentage went to the drive axles.
- Weigh the truck, then drive exactly 100 miles, and then re-weigh the truck to see what percentage of the added weight went to the steer axle, and what percentage went to the drive axles.

In order to accurately determine what percentage of fuel weight goes to your steer axle versus your drive axles you can simply weight the truck, then immediately fuel up, and then immediately re-weigh the truck to see what percentage of the added weight went to the steer axle, and what percentage went to the drive axles.

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You receive a scale ticket and your drive axles are 32,200 and your trailer tandems are 35,100. Based upon the two most common tandem slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and in which direction?

- 6 inch spacing - slide tandems forward 3 holes

4 inch spacing - slide tandems forward 5 holes - 6 inch spacing - slide tandems back 2 holes

4 inch spacing - slide tandems back 4 holes - 6 inch spacing - slide tandems back 3 holes

4 inch spacing - slide tandems back 5 holes - 6 inch spacing - slide tandems forward 2 holes

4 inch spacing - slide tandems forward 4 holes

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 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).

With 4 inch spacing you're moving 250 pounds per hole so you need to move 5 holes to shift a minimum of 1,100 pounds (250 x 5 = 1,250).

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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).

With 4 inch spacing you're moving 250 pounds per hole so you need to move 5 holes to shift a minimum of 1,100 pounds (250 x 5 = 1,250).

Using 6 miles per gallon as your fuel mileage and 8 pounds per gallon for the weight of fuel, how much total fuel weight will you burn off in 600 miles?

- 800 pounds of fuel
- 480 pounds of fuel
- 600 pounds of fuel
- 850 pounds of fuel

To figure out how many gallons of fuel you'll burn off, simply divide the number of miles travelled by the miles per gallon

Number of miles travelled / miles per gallon = gallons of fuel burned off

Number of miles travelled / miles per gallon = gallons of fuel burned off

600 miles travelled / 6 miles per gallon = 100 gallons of fuel burned off

100 gallons x 8 pounds per gallon = 800 pounds of fuel burned off

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100 gallons x 8 pounds per gallon = 800 pounds of fuel burned off

What is the main factor that will limit how far back you can slide your tandems?

- The load rating of your drive tires
- The maximum legal length allowed between your trailer kingpin and your trailer tandems
- The distance between your steer axle and drive axles or "tractor length rating"
- The overall height to length ratio of the cargo in the trailer

The maximum legal length allowed between your trailer kingpin and your trailer tandems will limit how far back you can slide your tandems

Different states have different laws regarding the maximum length allowed from your kingpin to your trailer tandems and you can look up these values in the Rand McNally Motor Carrier's Atlas

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What are the Federal standards for the gross vehicle weight of a 5 axle commercial vehicle?

- 80,000 pounds
- 60,000 pounds
- 22,000 pounds
- 65,000 pounds

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

What are the two most common tandem slider rail hole spacings, and roughly how much weight is transferred per hole?

- 6 inches apart - 350 pounds per hole

8 inches apart - 550 pounds per hole - 6 inches apart - 250 pounds per hole

10 inches apart - 500 pounds per hole - 4 inches apart - 450 pounds per hole

6 inches apart - 200 pounds per hole - 4 inches apart - 250 pounds per hole

6 inches apart - 400 pounds per 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.

There will be some variance here, but generally speaking these are very good estimates and they work very well. You'll learn to recognize the spacing by glancing at them eventually, but in the beginning you might want to measure quick so you know how far to slide the 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.