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To ensure safety, it’s crucial that trucks do not exceed their maximum allowable weight. Driendl Group / Getty ImagesMany states impose taxes based on the weight of the goods transported. Weigh stations help enforce these taxes, but they also play a critical role in monitoring truck weight to ensure it adheres to safety regulations specific to each region's road system.
So, what exactly does a semi truck weigh? In this article, we’ll explore the weight of a semi truck and how weigh stations function.
The Importance of Weighing Trucks
The weight of a semi truck is crucial for its performance and safety. Every truck is assigned a Gross Vehicle Weight Rating (GVWR), which is the maximum weight it can safely carry, factoring in the truck, trailer, cargo, and even passengers.
Surpassing this weight limit can create safety risks for the truck driver and others on the road, and could result in legal penalties.
How Much a Semi Truck Weighs
You can measure the unladen weight of the truck, which is the combined weight of the tractor and trailer when it's empty. An unladen truck typically weighs between 10,000 and 25,000 pounds (4,535 to 11,339 kilograms).
Alternatively, you can measure the laden weight, which includes the combined weight of the semi-truck, the trailer, and the cargo it carries.
The GVWR also determines the vehicle's classification.
- Class 1: Light-duty vehicles weighing up to 6,000 pounds (2,722 kilograms), such as SUVs and minivans.
- Class 2: Light-duty vehicles like medium-sized pickup trucks, ranging from 6,001 (2,722 kilograms) to 10,000 pounds (4,536 kilograms).
- Class 3: Light-duty or medium-duty vehicles, like smaller box trucks or medium-duty trucks, weighing up to 14,000 pounds (6,350 kilograms).
- Class 4: Medium-duty vehicles, such as small dump trucks, with a weight range of 14,001 pounds (6,350 kilograms) to 16,000 pounds (7,257 kilograms).
- Class 5: Medium-duty vehicles like large maintenance trucks, weighing between 16,001 pounds (7,257 kilograms) and 19,500 pounds (8,845 kilograms).
- Class 6: Medium-duty vehicles like large step vans and school buses, weighing between 19,501 pounds (8,845 kilograms) and 26,000 pounds (11,793 kilograms).
- Class 7: Heavy-duty vehicles like city buses and garbage trucks, weighing between 26,001 pounds (11,793 kilograms) and 33,000 pounds (14,969 kilograms).
- Class 8: Heavy-duty trucks, including tractors and coach buses, weighing 33,001 pounds (14,969 kilograms) and beyond.
How Weigh Stations for Semi Trucks Operate
Truck scales are built to withstand significant wear and tear. They are typically made of steel, concrete, or a blend of both. The technology used to weigh fully loaded semi trucks and other large vehicles can vary.
Load-cell technology is the most widely used method for weighing. Each cell is made of a tough material like steel or concrete, with one or more strain gauges either attached or embedded in it. A strain gauge consists of one or more wires that carry a small electric current.
When the load cell is subjected to weight, the wire in the strain gauge is slightly altered or compressed. This change causes the resistance to the current passing through it to vary. The signal from each load cell is sent to a junction box where sensors detect the resistance change and calculate the weight the scale is measuring.
Load cells can have strain gauges based on either compression or tension. A compression strain gauge measures how much the cell compresses when weight is applied, while a tension strain gauge responds to slight changes in the shape of the cell caused by the applied weight.
A bending-plate system uses metal plates attached with strain gauges. When weight is applied, the plates experience stress. The strain gauge measures the stress level on each plate and calculates the amount of weight needed to produce it. The measurements from each gauge are then added together to determine the total load for that axle.
Piezoelectric systems rely on a series of piezoelectric sensors, which are embedded within a conductive material.
When weight is applied, the pressure alters the voltage of the electrical charge flowing through the conductor. The sensors detect the voltage change and compute the load. The readings from each sensor are then combined.
There are at least three methods to determine a truck's weight, aside from the different scales used:
- One-axle: The most labor-intensive method, where a truck slowly drives across a single scale, stopping each time a set of wheels is on the scale. After all axles are weighed, the total weight is calculated.
- One-stop: A series of scales are used to weigh the entire truck at once. The scales are typically connected to a single electronic controller that automatically combines the weights of the axles to determine the gross weight.
- Weigh-in-motion (WIM): This method uses a series of embedded sensors to measure the weight per axle as a truck passes over the sensor pad. Unlike the other two methods, WIM allows the truck to continue moving without stopping. Some WIM systems are even installed on highways, enabling them to monitor traffic in motion.
