CDL Practice Tests: Weight & Balance
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Go!What can be the result of too much weight on the back of the trailer?
- You won't be able to fuel up properly without being overweight
- The steering will feel "heavy" and sluggish
- 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
- The steer tires won't grip properly
What is the cargo's center of gravity?
- The distance from the front of the trailer to the center point of the cargo.
- The average height of the cargo above the trailer floor.
- The center point of the cargo's weight.
- The overall length of the cargo after being loaded into the trailer.
Using the following numbers for your calculations, how much weight will come off your steer axle?
Miles travelled: 280
Percenatage of fuel weight on steer axle: 80%
- 280 pounds
- 300 pounds
- 310 pounds
- 256 pounds
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
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 between the locking pin holes
- The distance from the drive axles to the trailer tandems
What is the main factor affecting the low-speed offtracking of a tractor trailer?
- The length of the trailer, regardless of the position of the trailer tandems
- The distance from the trailer kingpin to the center of the trailer rear axle
- The distance from the steer axle to the drive axles
- The ratio of the length of the tractor to the length of the trailer
When a combination vehicle makes a low-speed turn - for example a 90-degree turn at an intersection - the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle. This is called low-speed offtracking. Excessive low-speed offtracking may make it necessary for the driver to swing wide into adjacent lanes to execute the turn (that is, to avoid climbing the inside curbs or striking fixed objects like telephone poles).
This performance attribute is affected primarily by the distance from the trailer kingpin to the center of the trailer rear axle, otherwise known as the wheelbase of the semitrailer. To prevent trucks from being too long to maneuvering safely around turns encountered in cities and towns, the maximum length allowed from the kingpin to the trailer tandems is set by the individual states.
What is the primary goal of implementing the Bridge Formula?
- To prevent overloading the trailer tandems and exceeding the load rating of the trailer tires
- 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.
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?
- 12,000 pounds
- 20,000 pounds
- 34,000 pounds
- 12,300 pounds
Using the following numbers for your calculations, how much weight will come off your drive axles?
Miles travelled: 360
Percentage of fuel weight on drive axles: 30%
- 144 pounds
- 210 pounds
- 200 pounds
- 300 pounds
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
Which of the following will alter the gross vehicle weight?
- Sliding the trailer tandems
- Adding more fuel to the tanks
- Sliding the tractor 5th wheel
- Moving the cargo toward the rear of the trailer
The final key point to understand when it comes to weight transfer is that none of the methods of redistributing weight between the different sets of axles, including sliding your 5th wheel, sliding your tandems, or moving the cargo around will have any affect on your gross weight.
The only way to affect the gross vehicle weight would be to add or remove weight from the vehicle. Adding or burning off fuel, or adding or removing cargo would be two examples of ways to alter the gross vehicle weight.
Based on the following figures, how much fuel can you legally add while remaining legal on the drive axles?
Weight Before Fueling: Steer: 11,275, drives: 33,800, gross: 77,420
- 122 gallons of fuel
- 119.75 gallons of fuel
- 108.8 gallons of fuel
- 100 gallons of fuel
200/25 = 8
8x100 = 800
800/8 = 100 gallons of fuel you can add
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
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.
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