Trailer (vehicle)
Before the advent of the wheel, early humans employed the concept of trailering by using drag sleds to transport goods.
The latter, when towed as part of a tractor-trailer or "18-wheeler", carries a large percentage of the freight that travels over land in North America.
Others range from two-axle campers that can be pulled by most mid-sized pickups to trailers that are as long as the host country's law allows for drivers without special permits.
Larger campers tend to be fully integrated recreational vehicles, which often are used to tow single-axle dolly trailers to allow the users to bring small cars on their travels.
A large proportion of its weight is supported either by a road tractor or by a detachable front axle assembly known as a dolly.
A semi-trailer is normally equipped with legs, called "landing gear", which can be lowered to support it when it is uncoupled.
Commercial freight trailers are produced to length and width specifications defined by the country of operation.
There are a number of different styles of trailers used to haul livestock such as cattle, horses, sheep and pigs.
They usually have adjustable vents and windows as well as suspension designed to provide a smooth ride and less stress on the animals.
Both stock trailers and horse trailers range in size from small units capable of holding one to three animals, able to be pulled by a pickup truck, SUV or even a quad bike; to large semi-trailers that can haul a significant number of animals.
Dollies for loose baggage are fitted with a brake system which blocks the wheels from moving when the connecting rod is not attached to a tug.
Most dollies for loose baggage are completely enclosed except for the sides which use plastic curtains to protect items from weather.
There is a limited number of manufacturers who produce these heavy-duty trailers because the market share of oversized loads is very thin when we talk about transportation industry.
There are self powered units of hydraulic modular trailer which are called SPMT which are used when the ballast tractors can not be applied.
Bumper-pull hitches and draw bars can exert tremendous leverage on the tow vehicle making it harder to recover from a swerving situation.
This coupling location allows the truck to make sharper turns and haul heavier trailers.
A fifth-wheel coupling is also referred to as a kingpin hitch and is a smaller version of the semi-trailer "fifth wheel".
A fifth wheel uses a large horseshoe-shaped coupling device mounted 1 foot (0.30 m) or more above the bed of the tow vehicle.
A gooseneck couples to a standard 2+5⁄16-inch (59 mm) ball mounted on the bed of the tow vehicle.
Standard bumper-hitch trailers typically allow a 10% or 15% hitch load while a fifth wheel and gooseneck can handle 20% or 25% weight transfer.
Many towing electrical installations, including vehicle-specific kits incorporate some form of bypass relays.
For vehicles that use the CANbus to activate towing-related safety systems, a wiring kit that can interact appropriately must be used.
The initial motion may be caused by aerodynamic forces, such as from a cross wind or a passing vehicle.
One common criterion for stability is the center of mass location with respect to the wheels, which can usually be detected by tongue weight.
Even if the center of mass is forward of the wheels, a trailer with a long load, and thus large moment of inertia, may be unstable.
