Semi-Trailer

What is a semi-trailer?

A semi-trailer is a type of trailer designed to be pulled by a tractor unit. Other common terms include “trailer” or simply “semi.” Unlike traditional trailers for rigid trucks, semi-trailers are not connected via a drawbar and tow hitch, but instead via a kingpin and fifth-wheel coupling.

The kingpin is a small and rather inconspicuous component on every semi-trailer. Despite its size, it plays a crucial role: it provides a secure connection between the tractor unit and the trailer. This coupling allows the two parts to remain safely connected while still being able to move independently. In a very literal sense, the kingpin is the pivotal point of the articulated truck.

Another key difference from conventional trailers lies in the chassis design: a semi-trailer has no front axle of its own. The tractor unit takes over this function, supporting the front of the trailer – hence the name “semi-trailer.”

This design brings several advantages:
Semi-trailers are easier to maneuver than rigid trucks with full trailers. They also offer higher payload capacity, as part of the weight is distributed over the tractor unit’s axles. Additionally, trailers can be swapped out quickly, without changing the tractor unit. In contrast, rigid trucks form a single, fixed unit.

The most commonly used type in practice is the curtain-sided trailer (also known as Tautliner or Curtainsider). However, there are also box trailers, refrigerated trailers, and tipper trailers for various transport needs.

Key characteristics of a semi-trailer:

• Part of the weight is supported by the tractor unit

• Kingpin and fifth-wheel coupling enable flexible connection

• Higher payload compared to conventional trailers

• Available in various designs for different transport requirements

• Some models are suitable for intermodal transport (road and rail)