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, or paired main gears. It can take the form of a tow ball to allow swiveling and articulation of a trailer, or a tow pin and jaw 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.
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.
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 SAE. Some manufacturers market Class V hitches, but there is no such standard according to SAE J684.
- Class I – up to 2,000 pounds (910 kg) – light loads
- Class II – up to 3,500 pounds (1,600 kg) – light loads
- Class III – up to 5,000 pounds (2,300 kg) – larger loads (campers, boats, etc.)
- Class IV – up to 10,000 pounds (4,500 kg) – larger loads (campers, boats, etc.)
- Class V – up to 17,000 pounds (7,700 kg) – larger loads (construction equipment, etc.)
Receiver-type hitches are typically offered with a square receiver opening of 1.25 inches (32 mm) (for Class I/II) or 2 inches (51 mm) (for Class III/IV/V). Some Class IV/V hitches are available in 2.5-inch (64 mm) opening sizes.
The trailer tongue (North America) or coupling (outside North America) slips over a tow ball. Tow balls come in various sizes depending on the load they carry and the country of operation:
- 1 7⁄8 in (47.6 mm)
- 50 millimetres (1.97 in) (ISO standard)
- 2 inches (50.8 mm)
- 2 5⁄16 in (58.7 mm)
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 rectangular 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.
Additionally, many pickup trucks come equipped with a 1 to 3 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 2 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 (on coming 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 types of loads. Weight ratings for both bumper mounted and frame mounted receiver hitches can be found on bumper of pickup trucks (for bumper mounted trailer 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.
The ISO standard tow ball is 50 mm 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. 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 include trailer tow hitch with a removable tow ball.
A weight-distributing hitch is a "load leveling" hitch. It is a hitch setup mounted on the tow vehicle that uses spring bars 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.
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 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.
- "Trailer Couplings, Hitches, and Safety Chains -- Automotive Type (Standard: J684, Revision: A)". SAE International. 30 May 2014. Retrieved 31 July 2014.
- "Släpvagnskörning med B-körkort (SOU 2007:33)" (in Swedish). 2007. Retrieved 31 July 2014.
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