CDL Practice Tests: Weight & Balance

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.