Trailer having shiftable undercarriage

In an elongated trailer including a bed, an undercarriage provided with a plurality of wheels and supporting the bed, the undercarriage being shiftable longitudinally of the trailer between a roading position in which at least a portion of the bed is supported at a height lower than the tops of the wheels of the undercarriage and a loading position wherein the portion of the bed is supported at a height above the tops of the wheels, a shifting means for shifting the undercarriage, and a lifting means for lifting the bed relative to the undercarriage when the undercarriage is shifted out of the roading position and for lowering the bed when the undercarriage is shifted to the roading position, the improvement comprising:

• the lifting means including at least one beam provided on the bed and extending in the longitudinal direction of the trailer, the beam including a lower flange defining a tracking surface of a substantially constant width, a web extending upward from the central longitudinal axis of the lower flange, and first and second ramps formed in the tracking surface, the first and second ramps being spaced longitudinally from one another and extending laterally inward from opposite sides of the tracking surface, the ramps each being of a width greater than one half the width of the tracking surface so that the width of the ramps overlap one another when viewed along the central longitudinal axis and each ramp extends beneath the web of the beam;
• the undercarriage including a pair of rollers supported on the undercarriage for rotational movement, the rollers being spaced longitudinally from one another by a distance corresponding to the distance of longitudinal spacing of the ramps and being offset from one another laterally so that each roller bears against the central longitudinal axis of the tracking surface and engages one of the ramps when the undercarriage is shifted to the roading position.