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Section 13: Weight And Balance
A Brief History Of The Laws
The first truck weight limits were enacted in 1913 by Maine, Pennsylvania, and Washington. These laws were passed to limit damage to the dirt and gravel surfaced roads caused by the iron and solid rubber wheels of heavy trucks. Limits on length, width, and height were generally adopted somewhat later in most States. Ultimately, direct Federal involvement in regulation of truck size and weight limits did not occur until the passage of the Federal-Aid Highway Act of 1956.
Throughout the years, the States and the Federal Government have fought over who should have the right to set the standards for the size and weight of commercial vehicles. In the end, we have a system where the Federal Government issues legal minimums that the states must allow on Interestates and a set of highways commonly referred to as the "National Network", but the states also have the power to set their own standards to some degree. Hence, we have a mix of Federal and State laws to contend with.
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 the weight limits are followed in strict accordance with the law. It is incredibly dangerous to overload a vehicle or have the weight improperly distributed across the axles.
You will not be quizzed on these historical facts. This is to help you understand how and why the current laws came into being.
You will definitely need to know these. These are the basic weight limits you'll be dealing with under today's laws.
We will discuss the bridge formula soon.