Tractor & Construction Plant Wiki

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 swivelling 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 the same movements. Another category is the towing pintle used on military vehicles worldwide.

Regional Variations


In the U.S. 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 the U.S. there are a few common classes: I, II, III, IV, and V that are defined by the SAE.

Class I - up to 2,000 pounds (907 kg), and II - up to 3,500 lb (1,588 kg), are for designed for light loads.
Class III - up to 5,000 pounds (2,268 kg), and IV - up to 10,000 lb (4,536 kg), can accommodate much larger trailers such as campers, boats, etc.

Receiver-type hitches are typically offered with a square receiver opening of 1.25 inches / 31.7 mm x 1.25 inches (for Class I/II) or 2 inches / 50.1 mm x 2 inches (for Class III/IV/V). Some Class IV/V hitches are available in 2.5 inch / 63.5 mm x 2.5 inch opening sizes.

The trailer tongue (U.S.) or coupling (non U.S.) 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 mm (1.97 in) (ISO standard)
  • 2 in (50.1 mm)
  • 2 5/16 in (58.7 mm)

Outside the U.S. 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.

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.

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 ballmount must match the US 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 the European Union, towbars must be a type approved to European Union directive 94/20/EC to be fitted to automobiles first registered on or after 1 August 1998.

The ISO standard tow-ball is 50mm in diameter and conforms to a standard BS AU 113b. 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 with Benelux and Scandinavia having 70 to 90% of vehicles with tow-balls.

Trailer Tow hitch

Cars can include 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 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 5000 lbs. and 500 lbs. of tongue weight without using a weight distributing hitch. Tow vehicles often have square receiver sockets to accept weight distributing hitches.

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.

Towbar Wiring

Vehicle Specific Towbar Wiring

Out of the thousands of cars on UK roads with towbars fitted, a high percentage are likely to have fitted towing electrics which are ‘hidden’ from the car. This electrical installation is commonly called ‘By-pass electrics’. This means, in fact your car has no idea that it is towing.

Since the early 2000’s, vehicle technology has moved rapidly forward introducing CANbus network systems which allowed the interaction of different systems, and also the detection of a trailer or caravan. In most cases, the manufacturer's have not only designed automobiles to sense the presence of a trailer, but they have also created enhanced new features within the systems connected to the network. This actually makes it very important that the vehicle can "see" the trailer or caravan, and that it is not ‘hidden’ and invisible from the towing vehicle. Some of these new important features are for safety and stability, as well as a few convenience things like automatically cutting off the rear fog light and parking sensors. The main safety feature though is the development of the Trailer Stability Program which automatically turns on when detected in the network through the dedicated electrics.

Some of the related towing systems which use the dedicated towing electrics to detect the presence of a trailer are:

  • Lane change assistant
  • Brake electronics,
  • Adaptive Cruise control,
  • Suspension system (ASS),
  • Engine electronics,
  • Engine cooling system,
  • Parking aids,
  • Reversing camera,

Each of the above systems are affected in different ways when a trailer is ‘detected’. Vital systems such as engine cooling and brake electronics will function and react differently. TSP or Trailer Stability Program is one feature which is being added to more and more vehicles, to help correct the ‘snaking’ action of a trailer. But with such advanced technology, braking systems have even evolved further by being operated electronically, without the need for hydraulics! Braking has become more controlled with faster braking efficiency when towing! Suspension systems can now detect a trailer and allow for a more level towing adjustment when the load is applied on the towing hitch. ACC (Adaptive Cruise Control) systems are meant to ‘detect’ a trailer in order to create a greater braking distance between vehicles.

To ‘By-pass’ all the above safety systems is potentially disastrous as your vehicle is designed to behave in a different way as soon as you couple up your trailer or caravan. In addition to this, the manufacturer of the car has either, put a prepared connector in the vehicle which is a preparation on the network (Ford, Volvo) to accept a specially designed towing module, or has designed the trailer to be ‘detected’ through connections directly onto the databus (VAG, BMW). With these type of connections the vehicle will know when a trailer plug is connected to the socket.

If you accept a ‘By-pass’ solution, it is most likely the towbar installer will make connections onto the lighting harness of the vehicle. This can prove to be contrary to what the manufacturer has intended. Therefore, any connections made in this way can also result in potentially restricted warranties. Not only that, but many network vehicles will ‘switch off’ any supply to a rear lamp on the vehicle should the bulb fail. This will mean, for example, with connections onto a brake lamp wire of the car, should the bulb fail on the car, the system will shut off the supply until the bulb is repaired. This results in not having any active brake lamps functioning on the caravan or trailer!!

Universal By-pass Electrics

Universal by-pass electrics are/were used on many older cars as a way of "tricking" the vehicles light check-control systems. They work by taking a small current signal from the vehicles lighting harness to trigger a relay and send a direct power supply to the towing socket. They do not communicate with the vehicle and will not activate any safety or convenience systems. Many people are offering this type of connection (as a cheap way of fitting an installation) on new more advanced cars. This results in any modern advanced safety systems staying dormant and ineffective as they have not been activated. Many manufacturers such as Peugeot and Citroen are now attaching warning labels onto the lighting loom stating that no wires should be joined onto the rear lamps lighting circuit in any circumstances, as this may affect the vehicle's warranty.

12N, 12S or 13 Pin Sockets

12N is what the older 7 pin lighting socket is referred to as. This is used when towing just a trailer or caravan (without the need for charge or fridge functions). In the UK it has all the functions of the rear lights on a vehicle except for reverse. These sockets are not waterproof and suffer from "pin burn-out" when worn.

12S is an additional 7 pin socket mainly used when towing caravans. It consists of a permanent 12v power supply, and usually a switched 12v power supply for the fridge (uk). It also contains a feed for the reverse lights on the caravan.

13 Pin is the new Iso standard (ISO 11446) socket being fitted to all new caravans sold in 2009. It can be wired with the same functions as both the 12N and 12S sockets, or with just the lighting functions including reverse (required on all trailers and caravans from 2010). The socket has been designed to be waterproof, easy to fit/remove (twist operation), the same size as one 12N socket (ideal for detachable towbars as un-obtrusive), and with good fitting quality terminals that avoid any pin burnout or voltage failure.

See also

References / sources

External links

Smallwikipedialogo.png This page uses some content from Wikipedia. The original article was at Tow hitch. The list of authors can be seen in the page history. As with Tractor & Construction Plant Wiki, the text of Wikipedia is available under the Creative Commons by Attribution License and/or GNU Free Documentation License. Please check page history for when the original article was copied to Wikia