CDL Practice Tests: Weight & Balance

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Question #656 (1 of 10)

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If you are able to exert 200 pounds of force against an object but you are only exerting 50 pounds of force to do so, what method are you applying to make this happen?

  • Momentum
  • Centrifugal Force
  • Center of gravity
  • Leverage
Using a lever on an object allows you to exert more force upon the object than the amount of force you're actually exerting against the lever itself. In other words, you might rig a lever that can lift a 250 pound boulder but requires only 50 pounds of force against the lever itself.
Loading the cargo behind the center point of the trailer tandems exerts leverage against the trailer tandems. You will actually put more weight on the trailer tandems than the cargo loaded behind the trailer tandems actually weighs. At the same time, you'll be taking weight off the drive axles.
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Question #651 (2 of 10)

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You receive a scale ticket and your drive axles are 35,700 and your trailer tandems are 30,600. Based upon a trailer with 4 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 6 holes - 33,750 drive axles - 32,550 trailer tandems
  • Slide tandems forward 7 holes - 33,950 drive axles - 32,350 trailer tandems
  • Slide tandems back 6 holes - 33,950 drive axles - 32,350 trailer tandems
  • Slide tandems back 7 holes - 33,750 drive axles - 32,550 trailer tandems
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 35,700 pounds on the drive axles you must move a minimum of 1,700 pounds off the drive axles. To shift weight off the drive axles and onto the trailer tandems, you slide the trailer tandems forward toward the front of the truck.

With 4 inch spacing you're moving 250 pounds per hole so you need to move 7 holes to shift a minimum of 1,700 pounds (250 x 7 = 1,750).

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Question #663 (3 of 10)

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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

  • 67% went on the steer axle
    33% went on the drive axles
  • 75% went on the steer axle
    25% went on the drive axles
  • 88% went on the steer axle
    12% went on the drive axles
  • 73% went on the steer axle
    27% 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.
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Question #633 (4 of 10)

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What is the primary goal of implementing the Bridge Formula?

  • To prevent over-length trucks from becoming a hazard on narrow bridges and tight roadways
  • To increase the minimum strength of bridges, allowing for heavier gross vehicle weights which improves efficiency
  • To prevent putting too much weight on a relatively small area, causing damage to road surface and bridge structure.
  • To prevent overloading the trailer tandems and exceeding the load rating of the trailer tires
The idea (of implementing the bridge formula) was to prevent putting too much weight on a relatively small area, causing damage to road surface and bridge structure.
A bridge is designed to handle a certain number of pounds per square inch on the surface. By creating the Bridge Formula, Congress made sure that too much weight wasn't concentrated on a small area of a bridge, causing damage to the bridge structure.
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Question #645 (5 of 10)

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

  • The amount of fuel you have onboard
  • The center of gravity of the cargo
  • The distance from the drive axles to the trailer tandems
  • The distance between the locking pin holes
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.
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Question #672 (6 of 10)

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Using the following numbers for your calculations, how much weight will come off your steer axle?

Miles per gallon: 7
Miles travelled: 280
Percenatage of fuel weight on steer axle: 80%

  • 300 pounds
  • 256 pounds
  • 310 pounds
  • 280 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 7 miles per gallon and you travel 280 miles:

280/7 = 40 gallons of fuel

40 gallons x 8 pounds per gallon = 320 pounds of total fuel burned off

Since 80% of the weight of fuel goes on our steer axle, we need to know what 80% of 320 is:

80 x 320 / 100 = 256 pounds coming off the steer axle
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Question #650 (7 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 forward 4 holes - 33,300 tandems - 33,100 drives
  • Slide tandems back 3 holes - 33,900 tandems - 32,900 drives
  • Slide tandems forward 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).

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Question #661 (8 of 10)

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Where will the weight of fuel be distributed?

  • Entirely on your steer axle
  • Mostly to your steer axle, with some going onto the drive axles.
  • Mostly to your drive axles, with some going onto the steer axle.
  • Entirely on your 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.
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Question #654 (9 of 10)

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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 hole structure of the trailer tandem slider rail
  • The kingpin to trailer tandem length limitations
  • 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
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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: 10,300 pounds
    drive axles: 34,600 pounds
  • steer axle: 11,300 pounds
    drive axles: 33,600 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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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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