CDL Practice Tests: Weight & Balance
Choose A Section:
Go!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
What are the four basic weight limits?
- double spacing, single axle, bridge forumula, overweight
- single tire, bridge formula, tri-axle, gross vehicle
- single axle, tandem axle, single tire, eight tire
- single axle, tandem axle, bridge formula, and gross vehicle
Which of the following has the greatest affect on the gross vehicle weight?
- The position of the trailer's tandems
- Moving the cargo toward the front or rear of the trailer
- The position of the tractor's 5th wheel
- None of these affect the gross vehicle weight at all
Ideally, where would you like the cargo's center of gravity located?
- Just slightly behind the tractor's drive axles
- Centered between your drive axles and your trailer tandems.
- About 10 feet to the rear the center point of the trailer
- Roughly centered between the steer axle and the trailer tandems
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?
- Centrifugal Force
- Center of gravity
- Leverage
- Momentum
Why did Congress enacted the Bridge Formula in 1975?
- To limit the weight-to-length ratio of a vehicle crossing a bridge.
- To prevent the maximum tire load rating from being exceeded by narrow axle spacing
- To prevent over-width trucks from becoming a hazard on narrow bridges
- To regulate the building of bridge structures so they can support the weight of heavy modern trucks
Using the following numbers, what would be the final result if you moved the 5th wheel forward two holes?
drive axles: 34,100 pounds
Weight transfer: 500 pounds per hole
- steer axle: 10,300 pounds
drive axles: 34,600 pounds - steer axle: 9,800 pounds
drive axles: 35,100 pounds - steer axle: 11,800 pounds
drive axles: 33,100 pounds - steer axle: 11,300 pounds
drive axles: 33,600 pounds
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
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?
- Directly centered above the trailer tandems
- A little closer to the front than to the rear
- About 5-10 feet in front of the trailer tandems
- About 15 feet behind the tractor drive axles
California allows 20,000 pounds maximum on the steer axle. The tread width of your tires is 11 inches, and the load rating on your steer tires is 7000 pounds. How much weight can you carry on your steer axle in California?
- 20,000 pounds
- 14,000 pounds
- None of these are correct
- 12,000 pounds
We have to find the most restrictive law to determine our maximum weight.
California allows up to 20,000 pounds on the steer axle
There is no requirement based upon tread width in California, so the width of your steer tires is irrelevant for this example
Your steer tires have a maximum load rating of 7,000 pounds each, which would therefore limit your legal steer axle weight to 14,000 pounds. This is the most restrictive of the laws, so 14,000 pounds is your legal weight limit in this case.
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
- 34,000 pounds
- 20,000 pounds
- 12,300 pounds
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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