What a Prime Mover and a Semi-Trailer Are

Much of the world's heavy freight moves on articulated vehicles. An articulated vehicle has two parts that bend at a joint. The front part is the prime mover, also called a tractor unit. It has the engine, the cab and the driven wheels, but no load space of its own. The rear part is the semi-trailer. It carries the load, but it has wheels only at the back. Its front end rests on the prime mover.

The two parts are joined by a coupling called the fifth wheel, a round steel plate on the tractor, and a steel pin under the trailer called the kingpin. Air and electrical lines run between them, so that the driver can brake the trailer and light its lamps from the cab.

What "6x4" means

Trucks are often described by two numbers. The first number counts the wheel positions, and the second counts how many of them are driven by the engine. A 6x4 prime mover has three axles: one steering axle at the front and two drive axles at the rear. That gives six wheel positions, four of them driven. Twin tyres on one end of an axle still count as one position. A 6x4 is common for heavy work because the two drive axles share the weight and give grip on poor roads. A 4x2 tractor, with one drive axle, is common for lighter road freight.