Extendible Drop / Step frame trailer is a name for an extendible trailer with a main deck which is lower than the height of the tractor unit’s fifth wheel height. The term ‘step frame trailer’ is identical to ‘stepframe trailer’. Step frame trailers may also be called:
- Swan’s neck trailers
- Step trailers
- Semi trailers
- Drop frame trailers
The name ‘Semi trailer’ is short for semi-lowloader. Step frame trailers are not low loaders proper, but have a lower deck height in common with low loaders.
The kingpin of the trailer is attached to the underside of the step frame trailer’s upper deck, enabling the trailer to be coupled to the tractor unit’s fifth wheel. The upper deck is built sufficiently high to accomodate the tractor unit.
The lower deck of the step frame trailer is built at a lower level than the upper deck, enabling high loads to be moved without the total height of the loaded trailer exceeding the legel limits for normal road transport.
The lower deck heights of step frame trailers vary between manufacturers and models. 0.95m is common, but 0.72m is frequent, while a five axle steering step frame trailer may have a lower deck height of 1.1m.
The lower deck height of a step frame trailer may be wholly above the height of the trailer wheels, giving an uninterrupted flat lower deck. Alternatively the lower deck of a step frame trailer may have ‘peep-throughs’ to enable the deck to sit within the diameter of the trailer’s wheels. The wheels protrude above (peep through) the lower deck; the deck being cut away to accomodate the wheels. Peep through covers may be bolted over the wheels.
Step frame trailers are distinguished from low loaders by having a single lower deck height. In contrast, low loaders have three deck heights: the upper deck above the kingpin, the lower load-carrying deck, and a third deck height above the wheels. See low loader trailers.