Here are my thoughts about heavy haul trailers. They look weird – see how they break off right after the wheels? Pretty strange.
The reason that is is so that they can care really heavy loads – they call them many things- most popular is a lowboy. This construction allows the deck to be extremely low compared with other trailers which is why they are often called lowboys or lowboy trailers. This structure give them the ability to carry legal loads up to 12 ft (3.66 m) tall, which standard trailers cannot accommodate. Lowboys are used to oil field equipment including oil rigs and bulldozers etc.
A lowboy is technically a semi-trailer with two drops in deck height: one right after the gooseneck and one right before the wheels. There are a couple of type of lowboys and I’ll post more here as I research it.