CDL Practice Tests: Weight & Balance

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Question #624 (1 of 10)

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In the following picture, what are axles 4 and 5 commonly referred to as?

  • "tandems" or "trailer tandems"
  • "back set" or "rear tandems"
  • "trailer drives" or "back tandems"
  • "trailer set" or "back tandems"
#4 and #5 together are your trailer tandem axles (commonly referred to as "tandems" or "trailer tandems")
You'll generally hear drivers refer to these axles as simply "tandems". They'll say "The load I had yesterday was loaded incorrectly and I was overweight on my 'tandems' ".
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Question #648 (2 of 10)

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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 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
  • 6 inch spacing - slide tandems back 2 holes
    4 inch spacing - slide tandems back 4 holes
  • 6 inch spacing - slide tandems forward 3 holes
    4 inch spacing - slide tandems forward 5 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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Question #677 (3 of 10)

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Using the following numbers, what would be the final result if you moved the 5th wheel back one hole?

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

  • steer axle: 12,800 pounds
    drive axles: 32,600 pounds
  • steer axle: 11,300 pounds
    drive axles: 34,100 pounds
  • steer axle: 11,800 pounds
    drive axles: 33,600 pounds
  • steer axle: 12,050 pounds
    drive axles: 33,350 pounds
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 you move it back 1 hole, then:

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

12,300 - 500 = 11,800 pounds on the steer axle after transfer

33,100 + 500 = 33,600 pounds on the drive axles after transfer
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Question #646 (4 of 10)

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

  • 4 inches apart - 450 pounds per hole
    6 inches apart - 200 pounds per hole
  • 6 inches apart - 250 pounds per hole
    10 inches apart - 500 pounds per hole
  • 4 inches apart - 250 pounds per hole
    6 inches apart - 400 pounds per hole
  • 6 inches apart - 350 pounds per hole
    8 inches apart - 550 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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Question #656 (5 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?

  • Center of gravity
  • Momentum
  • Leverage
  • Centrifugal Force
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 #650 (6 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 back 4 holes - 33,300 tandems - 33,100 drives
  • Slide tandems forward 3 holes - 33,900 tandems - 32,900 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 #627 (7 of 10)

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Sliding your 5th wheel will redistribute the weight mainly between which two sets of axles?

  • Trailer tandems and rear axles
  • Steer axle and trailer tandems
  • Steer axle and drive axles
  • Drive axles and trailer tandems
Sliding your 5th wheel will redistribute the weight mainly between your steer axle and your drive axles
Sliding the 5th wheel will generally have little or no effect on the weight of your trailer tandems
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Question #655 (8 of 10)

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If unsure of the exact location of the center of gravity of your cargo, where would you rather have the center of gravity located if you couldn't get it centered?

  • About 5-10 feet in front of the trailer tandems
  • Directly centered above the trailer tandems
  • A little closer to the front than to the rear
  • About 15 feet behind the tractor drive axles
If unsure of the exact location of the center of gravity of your cargo, you'd rather have the load a little closer to the front than to the rear. You can slide the trailer tandems forward as far as you would like to compensate for too much weight on the nose of the trailer, but the kingpin to trailer tandem length limitations will prevent you from going too far back with your trailer tandems.
A lot of times you'll be loading mixed cargo which will make it quite difficult to determine the exact center of gravity of the load. So when you're making your best guess, try to make sure it's a little forward of the center point between the drive axles and trailer tandems. That will give you the best chance of being able to adjust the trailer tandems so that the load will be legal.
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Question #673 (9 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%

  • 300 pounds
  • 210 pounds
  • 200 pounds
  • 144 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 #631 (10 of 10)

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

  • 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 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 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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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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