You receive a scale ticket and your drive axles are 35,700 and your trailer tandems are 30,600. Based upon a trailer with 4 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?
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 35,700 pounds on the drive axles you must move a minimum of 1,700 pounds off the drive axles. To shift weight off the drive axles and onto the trailer tandems, you slide the trailer tandems forward toward the front of the truck.
With 4 inch spacing you're moving 250 pounds per hole so you need to move 7 holes to shift a minimum of 1,700 pounds (250 x 7 = 1,750).
The precise weight of diesel fuel will vary based on temperature and humidity, but is generally around 7.25-7.5 pounds per gallon. So the safest bet is to use 8 pounds per gallon as the number you'll plug into your calculations, and that's what we're going to use here.
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.
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.
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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