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How Much Do You Know About Special Rigs? Part II

Welcome back to our three Part series where we take a look at the different types of special rigs, or non-customary tractor-trailer hook ups, are out on the road today. These are the vanguards of the road, rigs that require a special touch.

In Part I we took a look at multiple articulation vehicles. Today we will dig into oversize vehicles and low-clearance vehicles. These beats of the road respond only to the most professional of hand.

Oversize Vehicles

Oversize-vehicle trailer types comprise of many different types. They could include lowboys, flatbeds, drop frames, open-top vans or extendable trailers. Depending on the weight and size of the cargo, there can be a number of different wheel and axle combinations.

Most oversize vehicles are equipped with outriggers to support oversized loads. Converter dollies can then be attached to by fifth wheel or kingpin. This distributes the weight more evenly and allows the axles to support a longer load.

Typically, the cargo that requires a vehicle to be listed as oversized is itself oversize in dimension and weight. To operate an oversize vehicle one must have special permits and use designated routes. Requirements vary depending on cargo size and by state.

Here are some examples of oversize trailer types:

  • Double drop low bed: This configuration has two axles with a bed that is at standard flatbed height over the fifth wheel, but drops close to the ground in between. These types of trailers are used to carry very large and heavy loads. At times, outriggers may be used to support the higher girth of the load.
  • Removable gooseneck low bed: Typically equipped with two, three or four axles, these are lowbed trailers with a removable goose neck. This allows the trailer to rest against the ground for easier loading and unloading.
  • Folding gooseneck low bed: This trailer type is similar to a regular goose neck, except it folds to the ground for easier loading and unloading.
  • Hydraulic sliding axle trailer: A sliding axle trailer has rear axles that can slide forward and a bed that slopes to the ground. This allows heavy equipment to be loaded and unloaded easier.
  • Hydraulic low tail bed: This type of trailer has a fixed gooseneck, but also has a hydraulically operated ramp in the rear, which is also more amenable to loading and unloading of heavy equipment.

As with all special rigs, oversize vehicles must be driven in a precise and careful manner. Always ensure you are complying with the rules of the road and the state you are operating in.

Low-clearance Vehicles

Low-clearance vehicles can also be grouped among the oversize vehicle category. They are designed to haul heavy, oversized cargo or to haul loads of a far greater cubic capacity than most normal loads. Special training and skills are required before one can operate a low-clearance vehicle.

There are essentially two types of low-clearance vehicles, double-drop and single-drop frames. Double-drop frames drop twice from kingpin to axle and hug closer to the ground. Single-drop frames drop only once, and sit right behind the kingpin.

The design specifics of both trailer types allow for enough clearance behind the kingpin that the tractor doesn’t hit the bottom portion of the trailer. The truck driver must make sure the fifth wheel is set far back enough that there is adequate clearance.

Double-drops especially are in danger of bottoming out and problems at curbs, railroad crossings and other areas where pavement is uneven. Single-drops, however, can handle taller loads than conventional trailers. Similarly, bottom clearance issues are not as significant with double-drop trailers.

Join us in the final Part to our three-part series, as we take a look at high-center-f-gravity vehicles, livestock trailers, reefers and other special cargo vehicles.

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