CDL Practice Tests: Weight & Balance

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Question #657 (1 of 10)

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Where in the trailer could you place cargo so that the weight applied to the trailer tandems is greater than the weight of the cargo itself?

  • Directly above the drive axles
  • In the overhang portion of the trailer behind the center point of the trailer tandems
  • Slightly in front of the halfway point between the tractor drives and trailer tandems
  • Directly above the front axle of the trailer tandems
Because the trailer pivots up and down at the kingpin above the drive axles, the floor of the trailer acts as a lever pushing down against the trailer tandems when freight is loaded into the overhang portion of the trailer. The further back you load the freight into the overhang portion of the trailer, the more leverage it creates. The net effect of this leverage is that you're putting more weight on the trailer tandems than the cargo itself weighs, and you will actually take a little bit of weight off the drive axles at the same time.
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Question #650 (2 of 10)

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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 back 3 holes - 33,900 tandems - 32,900 drives
  • 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 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).

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Question #673 (3 of 10)

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Using the following numbers for your calculations, how much weight will come off your drive axles?

Miles per gallon: 6
Miles travelled: 360
Percentage of fuel weight on drive axles: 30%

  • 210 pounds
  • 144 pounds
  • 300 pounds
  • 200 pounds
To calculate the percentage of a value, you simply multiply the total value times the percentage you're looking for, and then divide by 100.
If you're getting 6 miles per gallon and you travel 360 miles:

360/6 = 60 gallons of fuel

60 gallons x 8 pounds per gallon = 480 pounds of total fuel burned off

Since 30% of the weight of fuel goes on our drive axles, we need to know what 30% of 480 is:

30 x 480 / 100 = 144 pounds coming off the drive axles
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Question #667 (4 of 10)

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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: 85%
Steer axle weight limit in states you're travelling: 20,000 pounds
Weight Before Fueling: Steer: 11,500, drives: 33,100, gross: 76,700

  • 117.64 gallons of fuel
  • 129.5 gallons of fuel
  • 133 gallons of fuel
  • 110.75 gallons of fuel
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 11,500 on your steer axle, which means you can add 800 pounds to your steer axle and still be legal.

Because we know that 85% 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.

800/85 = 9.41
9.41 x 100 = 941
941/8 = 117.63 gallons of fuel you can add
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Question #630 (5 of 10)

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You have steer tires rated at 6,150 pounds each, and you're in the state of West Virginia which says the legal limit for the steer axle is 20,000 pounds. What's the maximum legal weight you can carry on your steer axle in West Virginia?

  • 12,300 pounds
  • 20,000 pounds
  • 12,000 pounds
  • 34,000 pounds
When distributing weight across a set of axles, you will ultimately be limited by the most restrictive law for any given situation. For instance, if the law states that you can carry 20,000 pounds on your steer axle, but your steer tires have a load rating of 6000 pounds each, then you can only carry a total of 12,000 pounds on your steer axle
Your steer tires are rated at 6,150 pounds and you have two of them on your steer axle. Therefore you can carry a total of 12,300 pounds (6,150 x 2) on your steer axle. Because you can not legally exceed the load rating of your tires, the fact that West Virginia allows 20,000 pounds on the steer axle doesn't matter. You're ultimately limited by the most restrictive law, which in this case is related to exceeding the steer tire load rating.
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Question #644 (6 of 10)

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What affect will sliding the trailer tandems forward have on the weight distribution?

  • You will take weight off the tractor's drive axles and put more weight on the steer axle
  • You will put less weight on the trailer tandems and put more weight on the tractor's drive axles
  • You will put more weight on the trailer tandems and take weight off the tractor's drive axles
  • You will put more weight on the steer axle and take weight off the tractor's drive axles
By sliding the trailer tandems forward, you will put more weight on the trailer tandems and take weight off the tractor's drive axles.
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Question #628 (7 of 10)

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

  • Drive axles and somewhat on your trailer tandems
  • Trailer tandems only
  • Drive axles only
  • Steer axle and somewhat on 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
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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Question #679 (8 of 10)

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You're loaded heavy with 1/2 tank of fuel onboard. Using the following numbers, where should you move the 5th wheel to get it setup properly?

steer axle: 12,300 pounds
drive axles: 33,100 pounds
Weight transfer: 500 pounds per hole

  • Move the 5th wheel back 1 hole
  • Move the 5th wheel forward 1 hole
  • Move the 5th wheel forward 2 holes
  • Move the 5th wheel back 2 holes
The best way to make sure the 5th wheel is set properly is to scale the truck with a minimum of 1/2 tank of fuel and a very heavy load. Under those circumstances, you will want the 5th wheel positioned so that the steer tires are as close to 12,000 pounds as possible without going over, and the trailer tandems as close to 34,000 as possible without going over.
If you move the 5th wheel toward the rear of the tractor, you will increase the weight on the drive axles and decrease the weight on the steer axle. If we move it back 1 hole and it transfers 500 pounds per hole, then we wind up moving 500 pounds from the steer axle to the drive axles:

steer axle: 11,800 pounds
drive axles: 33,600 pounds
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Question #637 (9 of 10)

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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.

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Question #676 (10 of 10)

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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

  • steer axle: 9,800 pounds
    drive axles: 35,100 pounds
  • steer axle: 10,300 pounds
    drive axles: 34,600 pounds
  • steer axle: 11,800 pounds
    drive axles: 33,100 pounds
  • steer axle: 11,300 pounds
    drive axles: 33,600 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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About The Weight And Balance Section

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.

Types Of Weight Limits

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

Why Do These Different Weight Limits Exist?

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.

Methods Of Weight Transfer

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.

Limitations On Weight Transfer

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

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.

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.

weight

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