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Information About a Semi Trailer or Truck Trailer

Definition of a semitrailer, semi-trailer or truck trailer:
semi-trailer is a trailer without a front axle. A large proportion of its weight is supported either by a road tractor or by a detachable front axle assembly called a dolly. A semi-trailer is equipped with legs that can be lowered to support it when it is unhooked from the tractor. When coupled together, the tractor and trailer combination is often referred to as a semi, 18-wheeler, big-rig, articulated lorry, or truck and trailer.

The purpose of a semi-trailer is to carry freight. There are several types of semi-trailers including dry freight vans, refrigerated vans (reefers), flatbeds (standard, step-deck, lowboy, double drop, etc.) and tank trailers. Trailer sizes vary, but the most common sizes today are 53' or 48' in length and 102" or 96" in width.

Semi trailers are also referred to as truck trailers and are typically listed under the Heading: Trailers-Truck in most phone directory yellow pages.

More Information about trailer types:    What is a Dry Freight Van?    What is a Flatbed?    What is a Drop Deck?

Weight and Size Limits in the United States
Although some states and localities vary, a loaded truck (tractor & trailer combination) in the U.S. is typically limited to the following size and weight restriction:
    Maximum Width: 8'6" (102")
    Maximum Height: 13'6"
    Maximum Overall Length: 80'
    Maximum Total Weight: 80, 000 lbs (including weight of equipment & load)
Shipments that exceed these limits may require special permits and other regulations may be required

Attaching a Semi Trailer to a Road Tractor
A common question asked by people outside of the trucking industry is: "How is a road tractor connected to a semi-trailer?" The attachment between these two pieces of transportation equipment is made by two connecting devices. One is the "fifth wheel" that is mounted on the rear frame of the road tractor. The other is called the "kingpin", which is located on the front end of the truck trailer. Here is a brief explanation.

What is a Fifth-Wheel?
Originally, a fifth wheel was a steering mechanism resembling a wheel that enabled the front axle of a horse-drawn wagon to rotate when making turns. Around 1910, John C. Endebrock invented a device for coupling trailers to motor vehicles. In its initial use, trailers were towed by Ford Model "T" passenger cars. This coupling scheme required three men to hook to and unhook the trailer from the automobile. In 1918, Endebrock designed a new coupling devise that allowed a single driver to easily connect the tractor and the trailer. The new fifth wheel design used a jaw with a spring locking device mounted on the fifth wheel plate that was attached to the tractor frame.

Today, the fifth wheel serves as a coupling device that attaches a road tractor to a semi-trailer. A fifth wheel is a heavy metal disc with a "V" shaped slot. It is attached horizontally to the chassis above the rear axles of the road tractor. The "V" slot is located at the rear and it contains a locking device. The fifth wheel mechanism used to connect tractors and trailers today is very similar to Endebrock's design from the early twentieth century.

What is a Kingpin?
A kingpin is a heavy metal cylindrical pin that is located underneath the front end of the trailer. The kingpin is the mechanism on the trailer that locks it to a road tractor. It is typically positioned between 18" to 48" from the trailer nose. A greased metal grid plate surrounds the kingpin. This grid plate allows the fifth wheel to slide underneath the trailer. When the tractor is backed into the nose of a trailer, the fifth wheel pivots and slides underneath the trailer's grid plate. The fifth wheel then locks onto the trailer's king pin.

When coupled to the kingpin, the fifth wheel enables the semi trailer to rotate at the point where the fifth wheel and kingpin are joined. The rotating attachment allows the tractor-trailer combination to make turns and provides stability and maneuverability on the road. When the load is delivered, the driver disconnects the tractor from the trailer by rolling down the trailer's dolly legs. He then pulls a lever to disengage the fifth wheel locking mechanism and drives the tractor away from the trailer.

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What is the Year of a Trailer?
The tenth digit in the 17-digit Vehicle Identification Number (VIN) signifies the year the motor vehicle was manufactured. This digit can be either alphabetic or numeric. The following chart identifies the model years of motor vehicles manufactured from 1980 through 2039


10th Digit & Model Year Chart

A  1980     Y  2000     L  2020

B  1981     1  2001     M  2021

C  1982     2  2002     N  2022

D  1983     3  2003     P  2023

E  1984     4  2004     R  2024

F  1985     5  2005     S  2025

G  1986     6  2006     T  2026

H  1987     7  2007     V  2027

J  1988     8  2008     W  2028

K  1989     9  2009     X  2029

L  1990     A  2010     Y  2030

M  1991     B  2011     1  2031

N  1992     C  2012     2  2032

P  1993     D  2013     3  2033

R  1994     E  2014     4  2034

S  1995     F  2015     5  2035

T  1996     G  2016     6  2036

V  1997     H  2017     7  2037

W  1998    J  2018     8  2038

X  1999     K  2019     9  2039


Brief History of the Semi-trailer

August Charles Fruehauf (1868-1930), was a blacksmith and carriage builder in the Detroit area. In 1914,  he built a trailer at the request of a merchant to carry his pleasure boat. The merchant was please with the trailer so he asked Fruehauf to build additional trailers which the merchant would use to haul lumber and wood products. Fruehauf called his product a "semi-trailers". In 1918, Fruehauf incorporated his trailer manufacturing business, and the Fruehauf Trailer Company was was formed.

John C. Endebrock began work at the Sechler Company, a horse carriage maker. Around 1910 Endebrock, believed that the future would be in  "truck trailers," and that the horse-drawn wagon would become obsolete. Endebrock developed a device for coupling trailers to a truck, which could be pulled by a Ford Model "T" passenger car. Endebrock's trailer design included angle iron chassis frames with crossmembers on spring hangers and artillery wheels.  He introduced his new design in 1915 The name "Trailmobile" was chosen because the vehicle would "trail" behind an automobile. 

In 1918, Endebrock designed a devise that allowed a single driver to easily couple the tractor to the trailer. Until this time, three men were required to hook and unhook trailers. The new  design  used a jaw with a spring locking device mounted on a fifth wheel plate that was attached to the tractor frame. The fifth wheellocked into a king pin on the bottom of the front end of the trailers. This design is very similar to the tractor fifth wheel and trailer king pin used to couple tractors and trailers today.

Sources for History Section: Fruehauf Trailer Co. and Trailmobile Manufacturing Co. 

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