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.
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.
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.
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 (800 pounds), divide it by the total fuel weight added (1000 pounds), and then multiply that times 100
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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