Retrofitting a towbar with wiring kit – guide

Retrofitting a towbar with wiring kit - guide

Not all cars have a towbar ex-works, either because this was not considered when ordering the car or the original owner did not require one. Now you consider retrofitting a towbar. But what to look for? This guide provides you with an overview of the technology and conditions for towing a trailer.

Requirements for installing a towbar

Requirements for installing a towbarA towbar is a practical item. Yet, technology has advanced considerably for trailer couplings as well. Over the last few years, onboard wiring has made a quantum leap and the legal requirements for driving a car with trailer have become stricter.

This article describes the following topic areas with respect to retrofitting a towbar with wiring kit:

1. The right driving licence for towing a trailer in traffic
2. Different versions of trailer couplings
3. Additional technical requirements for the wiring kit
4. DIY installation of a towbar with wiring kit

1. The right driving licence for towing a trailer: what applies in the UK

driving licence for towing a trailerA full driving licence category B allows you to drive a car or van up to 3.500kg maximum authorised mass (MAM) towing a trailer of up to 750kg MAM if you passed your driving test on or after 1 January 1997. Alternatively, you are allowed to tow a trailer over 750kg MAM as long as the combined MAM of the trailer and towing vehicle is no more than 3.500 kg.

If you want to tow heavier combinations, be sure to follow the steps as described on the website with respect to towing a trailer. You can apply for a provisional licence for a medium-sized lorry and a trailer. After passing the lorry test, you can take the driving test for a C1 + E driving licence. Prior to buying and installing a trailer coupling, check your driving licence with regard to the trailer you want to tow and if necessary make an application for the required licence.

Be advised that a general full driving licence is adequately sufficient for bicycle carriers.

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2. The different versions of towbars

The different versions of towbarsA critical size reference for trailer couplings is the maximum permissible load, i.e. the load weighing on the trailer coupling. Both trailers and vehicles have a permissible load.
maximum permissible load for the carThe maximum permissible load for the car is generally mentioned in the vehicle registration, provided the car was equipped with a towbar by the manufacturer.

2.1 Matching the permissible load for car and towbar

There are exceptions: several luxury models, racing cars and hybrid cars (electric engine combined with a combustion engine).
towbar with a CE mark

  • In case the maximum permissible load is mentioned in the registration documents, you need to make the distinction between trailer couplings with or without CE-mark.
  • If the towbar has a CE mark, you just need to keep the documents about the towbar at hand.
  • Store the documents in the glove compartment. For cars and towbars without a documented permissible load, please contact an MOT or DEKRA service centre.

installing fortified suspension on the rear axleAn expert might insist upon installing fortified suspension on the rear axle. To determine this, the trailer combination is checked by measuring the distance between the trailer coupling and the ground.

It should be between 350 – 420 mm. Furthermore, additional loading of the towing vehicle has to be allowed for. The permissible load is subtracted from the maximal allowed additional loading.

2.2 Special towbars for bicycle trailers

Special towbars for bicycle trailerThere is another distinction for available trailer couplings.

  • Some trailer couplings are not intended for a real trailer, but for a bicycle carrier.
  • In case of a trailer coupling without CE mark, you might get an entry for the use of a bicycle trailer in your registration documents.
  • Manufacturers offer cheap trailer couplings, specifically suited for bicycle trailers.

3. Technical towbar versions

For the technical versions of towbars, the following distinctions are made:

– rigid towing hook
– removable towing hook
– swivel towing hook

3.1 Rigid towbars

Rigid towbarRigid towing hooks are generally the cheapest ones with the higher permissible load. The difference between very cheap and more expensive rigid trailer couplings can often not be distinguished on sight.

Price differences depend on the quality of the used steel alloy, but especially on the corrosion protection. Different manufacturers make different choices in this respect.

3.2 Removable towbars

Removable towbarRemovable towing hooks have become more common. They allow for the head to be removed, making the towing hook virtually invisible.

Depending on construction type, a part of the towing hook might be still visible under the bumper. Removable towing hooks are mounted vertically or horizontally.

  • Vertical removable trailer couplings are generally inserted hidden behind the bumper.
  • All others are shoved into the square profile under the bumper and secured.

A tip with respect to removable towing hooks: not everybody chooses to remove the trailer coupling all the time. Save from a few exceptions, there is no legal requirement for removing the towing hook when it is not used.

Nevertheless, it is a legal grey area as hitherto there have been no legal precedents. When it is left in place, the trailer coupling enhances the accident risk and the extent of possible damage considerably. Bumping into another vehicle when driving backward, or alternatively in case of a car colliding with the rear of your car, a trailer coupling could cause considerable additional damage.

3.3 Swivel towbars

Swivel towbarSwivel towing hooks are simply swiveled downwards and out of sight. This system is relatively new. Until now it has failed to establish itself.Bodywork parts for every car

3.4 Additional technical requirements for wiring kits

The type of wiring kit depends on the vehicle. The distinction occurs between older models with traditional wiring and cars with digitalised systems.
Additional technical requirements for wiring kitsThe latter has a CAN bus system, i.e. a two-core cable controlling all functions. Most differences occur between the CAN bus systems, depending on car manufacturer or model.

Cars with CAN are generally fitted with wiring for towing. Some cars require enabling the control unit after connecting a control module for the trailer and its cables. Only an authorised workshop of the manufacturer can do this. Possibly, control has to be integrated for deactivating the parking assistant.

In older cars with simple wiring, when adding the wiring kit, the flasher relay and a trailer check light must be retrofitted as well. Often, the wiring kit comes with these items.

3.5 Choosing the right power socket: 7-pin or 13-pin

correct power socket for towbarFurthermore, it is possible to order identical wiring kits with 7-pin or 13-pin socket. Extra connections are important for specific trailers such as caravans. In addition to the wiring, these can be equipped with steady plus and charging current (e.g. when loadable batteries are installed)

Only very simple trailers without any additional features are suitable for a 7-pin plug.

As requirements can change and the price difference is negligible, we generally recommend a wiring kit with a 13-pin socket. With the help of an adapter, the 13-pin socket of the car can be connected to the 7-pin plug of the trailer.

4. Towbar installation

4.1 Installing the wiring

Installing the towbar wiringEspecially with respect to the wiring kit, visiting a professional garage might prove useful. Especially for the CAN bus, faulty connections can cause heavy and costly damage. Otherwise, simple 7-pin sockets are generally connected with the rear light wiring (turn signal, brake light, rear light, rear foglamp and reversing light).

The installation kit should have an extensive installation manual with a detailed circuit diagram.

4.2 Mounting the towbar

Every high-grade trailer coupling has an installation manual, too.

Yet, installation is simple.
Mounting the towbar– A car hoist or repair pit is recommended. If using jacks, the car must be secured with axle stands.

Now, the installation is very easy.
Installation of the trailer hitch– Towbars are manufactured to match the car. The connection points are arranged in such a way that the corresponding drilling holes are already in place.

– These are found on the support frame or reinforcements of the underbody.

– In off-road vehicles and SUVs with a ladder frame, the trailer coupling is simply inserted between the ladder frame and screwed tight.

– In all other vehicles, the drilling holes are already present as these vehicles can be ordered with a towbar as well.

Foto: Bilanol, JFs Pic S. T, Gabor Tinz, anela.k, welcomia, Mikbiz, Mirko Graul, Peter Gudella, MikhailSh, BACHTUB DMITRII, sdecoret, Aleksandr Kondratov /

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