 A large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer.The other portion of the semi-trailer's weight is semi-supported (half supported) by its own wheels, which only support the rear of the semi-trailer.A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.