Tractor unit

These fall into two categories: heavy- and medium-duty military and commercial rear-wheel-drive semi-tractors used for hauling semi-trailers, and very heavy-duty typically off-road-capable, often 6×6, military and commercial tractor units, including ballast tractors.

The tractor–trailer combination distributes a load across multiple axles while being more maneuverable than an equivalently sized rigid truck.

Drawbar couplings are also found, particularly in dedicated exceptionally heavy-duty ballast tractors and as a means to connect intermediate fifth-wheel dollies for pulling multiple semi-trailers.

Most 6×2 units allow the undriven rear axle to be raised when lightly loaded, or running without a trailer, to save tire wear, save toll road fees, and increase traction on the driven axle.

In certain countries (such as Switzerland), a certain amount of weight must be spread over driven axles, which led to heavier varieties having six-wheel drive, otherwise, another tractor unit would have to be used.