Abstract: The disclosure is of a tractor and trailer combination in which the tractor has the fifth wheel structure and the trailer carries, embedded in its floor, an assembly adapted to be coupled to the fifth wheel. The assembly includes two pins adapted to be detachably coupled to the fifth wheel and connected each to a plunger of an pneumatic cylinder.In operation, when the trailer tends to turn with respect to the tractor and the fifth wheel, one of the pins and its piston are driven to cause compression of its pneumatic cylinder which thus opposes the turning of the trailer and prevents jackknifing.