Drop frame trailer is a name for a trailer with a main deck which is lower than the height of the tractor unit’s fifth wheel height. The term ‘drop frame trailer’ is identical to ‘dropframe trailer’. Drop frame trailers may also be called:
- Swan’s neck trailers
- Step trailers
- Semi trailers
- Step Frame trailers
The name ‘Semi trailer’ is short for semi-lowloader. Drop 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 drop 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 legal limits for normal road transport.
The lower deck heights of drop frame trailers vary between manufacturers and models. 0.95m is common, but 0.72m is frequent, while a five axle steering drop frame trailer may have a lower deck height of 1.1m.
The lower deck height of a drop frame trailer may be wholly above the height of the trailer wheels, giving an uninterrupted flat lower deck. Alternatively the lower deck of a drop 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.
Drop 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.