Why Are Semi-trailers Called Semi-trailers?

Semi-trailers are a familiar face in the domestic transportation industry. They come in many varieties, including flatbed, grid, low-bed, box, tank, container, and skeletal trucks, capable of meeting most cargo transportation needs.
So why are they called "semi-trailers"? The reason has to do with their structure. A semi-trailer's wheels are typically mounted at the rear end of the trailer, so it can't stand on its own. Instead, the front end rests on a saddle behind the tractor. This way, the tractor and trailer each share a portion of the trailer's weight. The tractor not only pulls the trailer forward but also helps support the cargo. Because the trailer only bears a portion of the cargo's weight, it's called a "semi-trailer."
The difference between "semi-trailers" and "full trailers" is easier to understand when compared to full trailers. Full trailers have wheels at both the front and rear of the trailer, allowing them to stand steadily on their own. When connected to a tractor, the trailer doesn't need to be attached to the tractor; it can simply be connected using a towing device. The tractor only pulls the trailer forward, while the trailer carries the entire weight of the cargo. Simply put, a semi-trailer "shares the weight," while a full trailer "takes it all."