Trailer 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.

Trailer stability is reduced at higher speeds. In California it is illegal to tow a vehicle faster than 55 mph.

Some vehicles are equipped with a Trailer Stability Program that may be able to compensate for improper loading.

## Trailer Stability Assist

**Trailer Stability Assist** (TSA), also known as Electronic Trailer Sway Control, is designed to control individual wheel slip to correct potential trailer swing before there is an accident. Although similar to Electronic Stability Control (ESC), TSA is programmed differently and is designed to detect yaw in the tow-vehicle and take specific corrective actions to eliminate trailer sway. Most ESC systems are not designed to detect such movement nor take the correct actions to control both trailer and tow-vehicle; so not all ESC equipped vehicles have TSA capabilities.

TSA systems detect when a trailer is starting to oscillate while under tow and corrects any dangerous trailer swing through a combination of either torque reduction and/or individual wheel braking to bring the trailer and tow-vehicle back under control. While towing heavy trailers, such as travel trailer, an unwanted wallow of the whole assembly may occur. Without the help of electronics, regaining stability requires focused attention by the driver.

## Fifth-wheel coupling

The **fifth-wheel coupling** provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. The coupling consists of a kingpin, a 2-or-3 1⁄2-inch-diameter (50.8 or 88.9 mm) vertical steel pin protruding from the bottom of the front of the semi-trailer, and a horseshoe-shaped coupling device called a *fifth wheel* on the rear of the towing vehicle. As the connected truck turns, the downward-facing surface of the semi-trailer (with the kingpin at the center) rotates against the upward-facing surface of the fixed fifth wheel, which does not rotate. To reduce friction, grease is applied to the surface of the fifth wheel. The configuration is sometimes called a *turn-table* in Australia and New Zealand, especially if it is a rotating ball-race-bearing type. The advantage of this type of coupling is towing stability.

Some camper trailers also use a fifth-wheel configuration, with the coupling installed in the bed of a pickup truck as a towing vehicle; "fifth wheel" is therefore sometimes used as a synonym for such campers.

A 'fifth-wheel' is a travel trailer supported by a hitch in the center of the bed of a pickup truck instead of a hitch at the back of a vehicle. The special hitch used for fifth-wheels is a smaller version of the one used on 18-wheeler trucks and can be connected by simply driving the tow vehicle under the trailer. Fifth wheel trailers are popular with full-time recreational vehicle enthusiasts, who often live in them for several months in one place, using their pickup truck tow vehicle for local errands. A fifth wheel trailer tows more securely than a traditional travel trailer because the hitch weight sits directly over the pickup truck's rear tires. Since part of a fifth wheel sits over the bed of the pickup, it also reduces the overall length of the vehicle/trailer package while allowing the same room as a comparable length travel trailer. Additionally, the hitch's location in the pickup's bed reduces the risk of jackknifing and allows for more maneuverability when backing. Because of the greater room available on the roads in North America, these vehicles are more popular in the United States and Canada than in Europe or other parts of the world. For uneven terrain a gooseneck hitch is an option to fifth-wheel.

The downside is that the hitch takes up room in the cargo bed regardless of whether the trailer is hitched or not. The hitch can be unbolted from the bed but this takes a lot more time and effort than the unhitch operation.