A lowboy or dolly in Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 is a semi-trailer

Semi-trailer

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer...

 with two drops in deck height: one right after the gooseneck and one right before the wheels. This allows the deck to be extremely low compared with other trailers. It offers the ability to carry legal loads up to 12 ft (3.66 m) tall, which other trailers cannot.

The first lowboy trailer was invented in the 1920s; it featured a rivet

Rivet

A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...

ed gooseneck and solid rubber tire

Tire

A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

s. The first detachable-gooseneck trailer was invented in 1958.

Types

The lowboy trailer comes in several types, for a wide range of tasks. Some types are:

• Fixed gooseneck: allows a longer deck length and has the lightest weight of all lowboys
• Mechanical gooseneck: second-longest deck length and weight
• Fixed-neck: the neck is fixed to trailers; offers the lightest weight, but sacrifices the ability to detach and load over the front
• Hydraulic detachable gooseneck: fastest and easiest to detach, at the expense of weight and deck length. It is the commonest and most versatile of lowboy trailers; the gooseneck is detached using large hydraulic cylinders to raise and lower the trailer and a small cylinder shores the neck to the truck, removing the neck so a large piece of equipment can be driven over the front onto the deck of the trailer for transport.
• Mechanically-detachable gooseneck: while usually lighter than the hydraulic detachable gooseneck, users sacrifice ease and speed of detaching. Used for long specialty hauls.

Suspension types

• Spring ride: lowest cost; however, it has a rougher ride and adds more stress to the trailer
• Air-ride: very smooth ride, adding to the life of the trailer frame. Air rides can also be adjusted for a higher or lower deck height.
• Hydraulic suspension: an oil-filled system, allowing wide variation of axle movement
• Independent suspension: n double wishbone, hydraulic suspension, which offers more stability, greater steering angle and more travel

Structural types

• Main structural steel
  Structural steel
  Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties...
  
   type: mill
  Steel mill
  A steel mill or steelworks is an industrial plant for the manufacture of steel.Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or...
  
   rolled
  Rolling (metalworking)
  In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling...
  
   beams
  I-beam
  -beams, also known as H-beams, W-beams , rolled steel joist , or double-T are beams with an - or H-shaped cross-section. The horizontal elements of the "" are flanges, while the vertical element is the web...
  
   save money; although being around 4000–6000 lb (1,814.4–2,721.6 kg) heavier, they also reverse camber
  Camber beam
  In building, a camber beam is a piece of timber cut archwise, or with an obtuse angle in the middle, commonly used in platforms, as church leads, and other occasions where long and strong beams are required....
  
   after a few years due to the much-lower strength of the main beams.
• T1 or A514 beams save weight and allow more versatile engineering of the trailer; they are cut to the highest-strength shape possible and welded together.