A trailer is generally an unpowered vehicle pulled by a powered vehicle. Commonly, the term trailer refers to such vehicles used for transport of goods and materials.
Sometimes recreational vehicles, travel trailers, or mobile homes with limited living facilities, where people can camp or stay have been referred to as trailers. In earlier days, many such vehicles were towable trailers.
United States
In the United States, the term is sometimes used interchangeably with travel trailer and mobile home, varieties of trailers and manufactured housing designed for human habitation. Their origins lay in utility trailers built in a similar fashion to horse-drawn wagons. A trailer park is an area where mobile homes are placed for habitation.
In the United States trailers ranging in size from single-axle dollies to 6-axle, 13 ft 6 in high, 53 ft 0 in long semi-trailers are commonplace. 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.
Hitching a trailer
A trailer coupler is used to secure the trailer to the towing vehicle. The trailer coupler attaches to the trailer ball. This forms a ball and socket connection to allow for relative movement between the towing vehicle and trailer while towing over uneven road surfaces. The trailer ball is mounted to the draw bar, which is usually removable. The draw bar is secured to the trailer hitch by inserting it into the hitch receiver and pinning it. The three most common types of couplers are Straight Couplers, A-Frame Couplers, and Adjustable Couplers.
Another type of hitch is known as a “fifth” wheel. Fifth wheel coupling is when trailers attach to a “fifth wheel” mounted within the bed of the tow vehicle. Though a fifth wheel and a gooseneck trailer look the same, their method for coupling is different. A goosneck trailer couples to a trailer ball mounted in the tow vehicle, usually a pickup truck. This type of hitch is used for larger trailers and provides considerably more stability than does a traditional bumper-pull hitch.
A trailer hitch, fifth wheel coupling or other type of tow hitch is needed to draw a trailer with a car, truck or other traction engine. The basic function of a trailer jack is to lift the trailer to a height that allows the trailer to be hitched or unhitched to and from the towing vehicle. Trailer jacks are also used for leveling the trailer during storage. The most common types of trailer jacks are A-Frame Jacks, Swivel Jacks, and Drop-Leg Jacks. Some trailers, such as horse trailers, have a built-in jack at the tongue for this purpose.
Electrical Components
Many older cars took the feeds for the trailer’s lights directly from the towing vehicles rear light circuits. As bulb-check systems were introduced in the 1990s “by-pass relays” were introduced. These took a small signal from the rear lights to switch a relay which in turn powered the trailer’s lights with its own power feed. Many towing electrical installations, including vehicle-specific kits incorporate some form of bypass relays.
In the US trailer lights usually has a shared light for brake and turn indicator. If such a trailer is to be connected to a car with separate lamps for turn indicator and brake a Trailer light converter is needed.
Nowadays some vehicles are being fitted with CANbus networks, and some of these use the CANbus to connect the tow bar electrics to various safety systems and controls. For vehicles that use the CANbus to activate towing-related safety systems, a wiring kit that can interact appropriately must be used. Without such a towbar wiring kit the vehicle can not detect the presence of a trailer and can therefore not activate safety features such as Trailer stability program which can electronically control a snaking trailer or caravan.
By-pass systems are very cost effective but are not appropriate on cars with interactive safety features.
Brakes
Larger trailers must be fitted with brakes. These can be either electrically operated, air operated, or in some countries overrun brakes are permitted.
Stability
Trailer stability can be defined as the tendency of a trailer to dissipate side-to-side motion. 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. If the center of mass of the trailer is behind its wheels, therefore having a negative tongue weight, the trailer will likely be unstable. Another parameter which is less commonly a factor is the trailer moment of inertia. 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.
Some vehicles are equipped with a Trailer Stability Program that may be able to compensate for improper loading.
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