AmeriFreight Blogs

Why Semi Truck Called a Semi Truck?

Nov 11, 2025

Min READ

Lila Claybourne

Lila Claybourne has been immersed in the auto shipping industry for over 12 years, gaining extensive experienc...

A semi-truck, or a semi-trailer truck, is a vital combination vehicle for transporting goods nationwide. The term "semi" refers to its trailer, which uniquely lacks front wheels and relies on the tractor, the front power unit, for support.

This ingenious design, created over a hundred years ago, has resulted in one of today's most iconic and recognizable vehicles. Semi-trucks are a robust supply chain that assists everyday commerce.

What You'll Learn in this Guide

This guide will explore:

The origins of the term "semi truck"
The various common names for semi-trucks
The distinctions between tractor and trailer configurations
Overview of major U.S. manufacturers.

Overview

Technical Design:

The “semi” highlights the semi-trailer’s structure, which lacks front wheels and cannot stand independently. It is relying on the tractor’s fifth-wheel coupling for support and movement.

Historical Development:

Originating in the early 1900s, the semi-truck concept was popularized by innovators like August Fruehauf. It led to the term semi-trailer truck, later shortened to semi-truck.

Naming and Terminology:

Common names for these trucks include tractor-trailer, 18-wheeler, big rig, truck, and trailer. Each term emphasizes different parts or characteristics of the vehicle.

Types and Variations:

Semi-trucks come in various configurations, including sleeper cabs (with flat, mid, and raised roofs), day cabs, and aerodynamic slope-nosed tractors. Trailers vary widely, from flatbeds and reefers to lowboys, tankers, and car haulers—each built for a specific cargo type.

Wheel and Axle Configurations:

The term “18-wheeler” originates from the standard wheel setup: two front (steering axles), four mid (drive axles), and 12 rear (trailer axles)—a total of 18 wheels, which are critical for load distribution and road safety.

Industry Impact:

Semi-tractors play a pivotal role in logistics and long-distance shipping, with top U.S. manufacturers such as Freightliner, Kenworth, Peterbilt, and Volvo leading the way in innovation in design, safety, and fuel efficiency.

What is a Semi-Trailer Truck?

A semi-truck, also known as a semi-trailer truck, is a combination vehicle designed for carrying freight.

It consists of two main parts:

Tractor Unit (or Truck): This is the front end of a semi-truck, containing the engine, cab (where the driver sits), and the steering axle. It provides the power to move the entire vehicle. The tractor unit has a fifth-wheel coupling, a horseshoe-shaped device at the rear, which connects to the semi-trailer. Importantly, it can operate independently without a trailer.
Semi-Trailer: This cargo-carrying part attaches to the tractor unit via the fifth wheel. Unlike a full trailer, a semi-trailer only has rear wheels and relies on the tractor for support at the front. It cannot move on its own when detached from its base.

The word "semi" in semi-truck refers to the fact that the trailer is "semi-supported" by the tractor unit, as it lacks a front axle.

Other Names for Semi Truck

Tractor-trailer: This term emphasizes the two main components.
18-wheeler: Refers to the typical number of wheels on a tractor with a single-axle trailer (though configurations can vary).
Big rig: A general term for a large semi-truck.
Semi: A shortened version of semi-truck.
Rig: Another informal term for a semi-truck.
Trailer truck: Highlights the truck pulling a trailer.
Truck and trailer: Similar to the tractor-trailer.

Historical Context of the Name

The term “semi-trailer” emerged in the early 1900s. August Fruehauf is credited with attaching a semi-trailer to a Ford around 1914 to haul boats. The term “semi-trailer truck” came into use to describe the tractor unit pulling a semi-trailer.

Over time, the phrase was shortened to “semi-truck” for simplicity, much like the term “automobile” evolved into “car.” The shorter version has become a catch-all term for a tractor pulling one or more semi-trailers. It is now the most used term in North America and Australia, although “tractor-trailer” remains technically accurate.

Types and Variations of the Semi-Trailers

For Tractor Unit Variations (Sleeper and Day Cabs, Slope-Nosed):

The tractor unit's configuration impacts the operational efficiency and driver comfort. Different semi-trailer designs exist to meet the unique demands of drivers and the types of freight they haul.

Sleeper Cabs

Sleeper cabs are used in long-haul trucking. They are equipped with a sleeping area located behind the driver's seat. They provide a rest space for truck drivers who cover long distances. There are multiple sleeper cab types:

Flat Roof Sleeper: A basic and cost-effective option for shorter hauls requiring overnight stays. It has limited headroom but offers essential sleeping area amenities.
Mid-Roof Sleeper: This variation enhances comfort by providing more headroom and storage space. It is suitable for regional or medium-length routes.
Raised Roof Sleeper: The most spacious design. This offers generous headroom, with additional features such as compact living spaces. It’s commonly used on extended routes where truck drivers spend multiple nights on the road.

Day Cabs

A day cab lacks a sleeping compartment. These tractors are ideal for short-haul work or local deliveries. They offer improved maneuverability and better fuel economy. Since they don't require added weight from living features, they help reduce overall load and fuel consumption.

Slope-Nosed Trucks (Aerodynamic Cabs)

These trucks are designed to cut through the air more efficiently. These slope-nosed or aerodynamic tractor-trailers feature rounded hoods and streamlined cab designs. They are often paired with sleeper or day cab configurations. They are commonly used for pulling heavy loads or semi-trailers across long distances.

For Semi-Trailer Variations:

The type of semi-trailer connected to a tractor determines the kind of cargo it can haul. There’s a wide range of trailer designs built for specific needs:

Flatbeds: Open trailers without sides or roofs. They are perfect for transporting oversized or oddly shaped loads.
Reefer Trailers: Refrigerated units designed to maintain a specific temperature. They are ideal for transporting perishable goods such as food and pharmaceuticals.
Dry Vans: Fully enclosed trailers that shield general freight.
Tanker Trailers: Cylindrical trailers built to move liquids and gases.
Lowboys: Trailers with an extra-low deck height. They are used for hauling tall or heavy equipment.
Car Haulers: Multi-level trailers are explicitly designed to transport multiple vehicles securely.
Dump Trailers: Tilting trailers quickly unload bulk materials, such as sand, gravel, and debris.
Step Deck Trailers: Flatbed trailers with a lower rear deck to accommodate taller loads without exceeding legal height limits.
Extendable Flatbed Trailers: Flatbeds that can stretch in length to carry very long cargo, such as pipes, lumber, or poles.
Conestoga Trailers: Flatbeds equipped with a retractable tarp system, combining weather protection with easy loading access.
Side Kit Trailers: Flatbeds with removable side panels to hold loose materials while offering flexible loading and unloading.

Semi-Truck vs. 18-Wheeler: What’s the Difference?

While people often use "semi-truck" and "18-wheeler" to mean the same thing, they’re not identical. A semi-truck is the general term for a tractor unit attached to one or more semi-trailers, which carry cargo and don’t have front wheels. On the other hand, an 18-wheeler is a common type of semi-truck setup with 18 wheels in total:

Two on the steering axle
Four on the drive axles
12 on the trailer’s axles

All 18-wheelers are semi-trucks. However, not all semi-trucks are classified as 18-wheelers. The main difference is that some may possess a different number of wheels. They can also be designed to tow multiple trailers, such as double or triple configurations.

Semi Truck Manufacturers in the U.S.

Trailers and trucks are used for car shipping. That's why choosing the right truck is necessary for specific transportation purposes. Here are the well-known semi truck manufacturers in the trucking industry:

Freightliner: Holds the largest market share in the U.S. and offers a wide range of models.
International: Manufactured by Navistar. Produces a diverse range of trucks renowned for their advanced technology.
Kenworth: Known for its durability, innovation, and fuel-efficient aerodynamic designs.
Mack Trucks: One of the oldest American brands, known for rugged construction and dependability, especially in vocational applications.
Peterbilt: An iconic American brand known for classic designs, quality, and driver comfort.
Volvo Trucks North America: Although a Swedish company, Volvo has a strong presence in the U.S. It is recognized for its commitment to safety and driver comfort.
Western Star: A premium brand under Daimler Truck North America, specializing in heavy-duty, customizable trucks.

Final Thoughts: Understanding Tractor Trailers

Semi-trucks are the backbone of global freight movement, especially in the United States. These powerful vehicles are essential for efficient logistics. They support numerous industries by reliably and safely delivering goods across vast distances. Understanding their fundamental design highlights their crucial role in modern commerce.

Need to Ship a Car?

AmeriFreight Car Shipping provides auto-transportation services tailored to meet your needs. We are a top-rated company with more than 20 years of experience. We offer various services to ensure you have an option best suited for your vehicle.

Get your free quote now, and partner with us.

Frequently Asked Questions (FAQs)

What is the difference between a semi-truck and a full truck?

A semi-truck consists of a tractor unit and a detachable trailer, while a full truck has a single, rigid frame with the cargo space permanently attached.