Tow hitch

A tow ball mounted on the rear of a vehicle

A tow hitch (or tow bar) is a device attached to the chassis of a vehicle for towing, or a towbar to an aircraft nose gear. It can take the form of a tow ball to allow swiveling and articulation of a trailer, or a tow pin, or a tow hook with a trailer loop, often used for large or agricultural vehicles where slack in the pivot pin allows similar movements. Another category is the towing pintle used on military vehicles worldwide.

Regional variations

Class IV receiver for up to 10,000-pound (4.5 t) towing capacity with wiring connector on the left side

North America

AAR Type "E" coupler serving as a tow hitch on a mobile crane. Pulling up on the link at the rear releases the knuckle allowing uncoupling.

In North America the vehicle attachment is known as the trailer hitch. Trailer hitches come in two main configurations: receiver type and fixed-drawbar type. Receiver-type hitches consist of a portion that mounts to the frame of the vehicle that has a rearward-facing opening that accepts removable ball mounts, hitch bike racks, cargo carriers, or other hitch mounted accessories. Fixed-drawbar hitches are typically built as one piece, have an integrated hole for the trailer ball, and are generally not compatible with aftermarket hitch accessories.

Trailer hitch classes

A trailer hitch typically bolts to the chassis of the vehicle. In North America there are a few common classes (I, II, III and IV) that are defined by the Society of Automotive Engineers (SAE). Some manufacturers market Class V hitches, but there is no such standard according to SAE J684.[1]

Class I – up to 2,000 pounds (0.91 t) – light loads
Class II – up to 3,500 pounds (1.6 t) – light loads
Class III – up to 5,000 pounds (2.3 t) – larger loads (campers, boats, etc.)
Class IV – up to 10,000 pounds (4.5 t) – larger loads (campers, boats, etc.)
Class V – up to 17,000 pounds (7.7 t) – larger loads (construction equipment, etc.)

The trailer tongue (North America) or coupling (outside North America) slips over a tow ball attached to or integral with the hitch.

Receiver tube sizes

Receiver tubes come in various sizes depending on the load they carry and the country of operation.[2]

Class I & II - 1 1⁄4 in (31.8 mm) receiver tube
Class III & IV - 2 in (50.8 mm) receiver tube
Class V - 2 in (50.8 mm) or 2 1⁄2 in (63.5 mm) receiver tube

Tow ball sizes

Tow balls come in various sizes depending on the load they carry and the country of operation:

In North America, the ball attaches to a ballmount. Receiver-type hitches use removable ball mounts, whereas the fixed drawbar type hitches have integrated ball mounts. The ball mount must match the SAE hitch class. The ballmount for a receiver-type hitch is a square bar that fits into a receiver attached to the vehicle. Removable ball mounts are offered with varying rise or drop to accommodate variations in the height of the vehicle and trailer to provide for level towing.

In order to tow safely the correct combination of vehicle and trailer must be combined with correct loading horizontally and vertically on the tow ball. Advice should be taken (see references) to avoid problems.

Outside North America, the vehicle mounting for the tow ball is called the tow bracket. The mounting points for all recent passenger vehicles are defined by the vehicle manufacturer and the tow-bracket manufacturer must use these mount points and prove the efficacy of their bracket for each vehicle by a full rig-based fatigue test.

Truck variants

Additionally, many pickup trucks come equipped with one to three mounting holes placed in the center area of the rear bumper to accommodate the mounting of trailer tow balls. The ones on the extreme left or right are often used by drivers in rural areas who tow wide farm equipment on two-lane roads. The far side mounting allows for the item (trailer, etc.) being towed to be further away from the opposite side of the road (oncoming traffic, etc.). Caution must be taken when using the bumper of a pickup truck for towing rather than using a frame mounted receiver hitch, as the bumper does not provide for as much strength and therefore is generally used to tow lighter loads. Weight ratings for both bumper-mounted and frame-mounted receiver hitches can be found on bumper of pickup trucks (for bumper-mounted tow balls) and on the receiver hitch (for frame-mounted receiver hitches). Many pickup trucks without frame-mounted receiver hitches often use the rear bumper, especially if the pickup truck is a light duty (not full size) pickup truck.

For flat deck and pickup trucks towing 10,000-to-30,000-pound (4.5 to 13.6 t) trailers there are fifth wheel and gooseneck hitches. These are used for agriculture, industry and large recreational trailers.

Front trailer hitches are also widely used on pickup trucks and full size SUVs for multiple purpose. A front mounted hitch can accommodate additional truck equipment such as front mount bike carriers, fishing / hunting gear, winches, step plates, snow plows and more. It also allows a driver to temporarily maneuver a trailer with better visibility into any convenient place. Front trailer hitches are mounted directly to the frame of a vehicle to ensure a reliable connection. Front hitches are typically equipped with standard size receiver tubes to accommodate a variety of hitch mount equipment.

Europe

A tow hook mounted on the rear of a vehicle
A trailer coupled (i.e., "hooked") onto a ball-type tow hitch

In the European Union, towbars must be a type approved to European Union directive 94/20/EC to be fitted to vehicles first registered on or after 1 August 1998.[3]

The ISO standard tow ball is 50 mm (1 31⁄32 in) in diameter and conforms to a standard BS AU 113b (replaced by BS ISO 1103:2007). The ISO standard has been adopted in most of the world outside North America.

There are two main categories of ISO tow ball: the flange fitting and the swan-neck which has an extended neck fitting into the tow-bracket. Swan-neck tow balls are often removable to avoid the inconvenience of a tow ball protruding from the vehicle when not required. Some manufacturers are introducing retractable tow balls as an option.

Across Europe around 25% of vehicles have tow balls fitted—but there are distinct regional variations, being more common in Benelux and Scandinavia. In Sweden, around 2.2 million cars of around 4.3 million (just over 50%) have tow balls.[4] In the United Kingdom the popularity of caravans is responsible for a high percentage number of 4x4 (SUV) type vehicles being fitted with tow hitches.

Trailer tow hitch

Cars can be equipped with a trailer tow hitch with a removable tow ball.

Weight-distributing hitch

A weight-distributing hitch is a "load leveling" hitch. It is a hitch setup mounted on the tow vehicle that uses spring bars and chains under tension to distribute part of the trailer's hitch weight from the towing vehicle's rear axle to the towing vehicle's front axle and to the trailer's axle(s). It can help reduce trailer sway and hop. Trailer hop can jerk the tow vehicle. Trailer sway is sometimes called "fish tailing". At high speeds, trailer sway can become dangerous. Most vehicle manufacturers will only allow a maximum trailer capacity of 5,000 pounds (2,300 kg) and 500 pounds (230 kg) of tongue weight without using a weight-distributing hitch. Tow vehicles often have square receiver sockets to accept weight distributing hitches.

NATO tow hitch
NATO tow bar
A pintle hook (top) and lunette ring (bottom), used in towing applications by the military.

Lunette ring

A lunette ring is a type of trailer hitch that works in combination with a pintle hook on the tow vehicle. A pintle hook and lunette ring makes a more secure coupling, desirable on rough terrain, compared to ball-type trailer hitches. It is commonly seen in towing applications by agriculture, industry and the military.

The clearance between the lunette and pintle allows for more relative motion between the trailer and tow vehicle than a ball coupling does. A disadvantage of that is the "slam" transmitted into the towing vehicle with each push/pull load reversal. This becomes a tradeoff between a more secure coupling, and a more comfortable towing experience.

See also

References

  1. ↑ "Trailer Couplings, Hitches, and Safety Chains -- Automotive Type (Standard: J684, Revision: A)". SAE International. 30 May 2014. Retrieved 31 July 2014.
  2. ↑ "Ball Mounts & other Towing Equipment - AutoZone.com". AutoZone. Retrieved 6 February 2015.
  3. ↑ Directive 94/20/EC of the European Parliament
  4. ↑ "Släpvagnskörning med B-körkort (SOU 2007:33)" (PDF) (in Swedish). 2007. Retrieved 31 July 2014.
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