Towbars Help

The 12N connector needs to be connected using the correct wiring according to DIN/ISO 1724.

12N seven core cable has a black outer sheath. Each of the seven wires within should be around 0.65mm² and sometimes a larger earth wire at 1.0mm². They are usually colour coded as in the previous illustrations.

No, some dedicated and original equipment kits use the wire colours specific to the vehicle manufacturer.

The Supplementary power required by the caravan reversing lights, interior lights, battery charger and fridge are provided via the 12S connector. This is a socket on the vehicle and a plug on the caravan/trailer.

The socket cover is usually coloured grey or white and houses 7 pins which are numbered 1 to 7. The plug is also usually grey or white and has a slot that locates opposite the cover hinge of the socket. The central contact (number 7) is male in the plug.

The 12S connector needs to be connected using the correct wiring according to ISO 3732, pin connections as in the following table and picture. Please note; Make sure that when you wire the 12S socket you independently earth wires 3, 5 & 7 to the car.

If the split charge relay in the car allows it, connecting the blue wire from pin 2 to the second switched output terminal will ensure compatibility with pre and post 1998 caravans.

The relay splits the power supply from the vehicle to the 12S connector.

To avoid a flat battery on your vehicle the relay isolates the caravan from the vehicle when the engine is off apart from a permanent supply to the caravan interior lights through pin 4.

When the engine is running the relay will provide power to pin 4 and also pin 6 for the fridge and if wired for pre 1998 caravans pin 2 for the charge circuit, this can also be used as an extra power feed for larger fridges in post 1998 caravans.

Modern caravans have a built in relay that switches this power when the engine is running from the caravan interior lights to the charge system for the caravan's own battery.

All caravans built from September 1998 should have this facility. Most caravans built before then use pin 2 as a battery charge supply.

Some vehicles or caravans may come equipped with one single connector that incorporates both 12N and 12S functions, this is known as a 13pin or continental type connector.

The socket is usually coloured black and houses 13 pins which are numbered 1 to 13. It also incorporates a twist lock mechanism that secures the plug to the socket.

12N and 12S is not compatible with 13 pin, however adaptors are commercially available to interchange between both systems.

The West 13 pin socket looks like a standard 12N 7 pin socket but includes extra pins between the standard 7 pin configuration so either a standard 7 pin 12N plug can be used for trailers and light boards or the specific West 13 pin plug for caravanning.

The electrical kit for your vehicle will have a tell-tale supplied which will inform the driver of an indicator failure.

This is a device that tells the driver that the indicators on the trailer are operating correctly. It can be an audible beeper or a dash mounted warning light that operates in time with the indicators and stops if an indicator fails. Some will only operate if an indicator fails on the trailer others notify the driver of indicator failure by making the vehicles own indicators flash at a faster rate.

A full light function check should be performed every time you connect.

Ordinary electric kits will not register this however with some dedicated kits a message will appear on the dash (model dependant) to warn you of the failure.

This disables the vehicle's fog lamp when towing to reduce reflective glare from the caravan/trailer.

Some wiring kits have this function as standard otherwise you may have to request this at installation.

Modern electric kits come complete with instructions, if you have any doubts you should always employ an approved stockist to install your electrics.

In most cases the lights on the caravan/trailer will still operate, the bulb should be replaced as soon as convenient. If the vehicle has bulb failure monitoring, multiplex or CAN bus wiring, you may be notified of a bulb failure, this is model dependant.

Multiplex is a multi function power and communication cable that visits each control unit. The power is distributed by the control units to operate the chosen devices. It is also known as power-line communication (PLC).

CAN, Controller Area Network, is the name given to a system of control units that communicate and respond to each other on a CAN bus network.

Bus is a network in which all control units are attached to a line directly and all signals pass through each control unit. Each unit has its own unique identity and will only respond to those signals intended for it.

Depending on the manufacturer CAN bus will usually consist of two wires twisted together: CAN high and CAN low.

CAN high is the high speed signal network and is used to operate systems such as engine control units, safety features and ABS control.

CAN low is the low speed signal network and is used to operate systems such as lighting, seat electronics and electric windows.

If you think of the CAN bus network as a postal system, each data signal would be a message with a postcode and each control unit a mail sorter with its own postcode address. If we use the brake lights as an example, when you press the brake pedal this is registered by a control unit which then creates the message 'operate brake lights' with a postcode 'rear light control unit'. The message is then sent on the CAN bus, it will pass through each control unit and the postcode will be read. If it does not have that control unit's specific postcode, then the message is passed on. When the postcode is read by the control unit it is addressed to, the rear light control unit, the message is opened and read. The rear light control unit will then operate the brake lights as instructed. All this happens in milliseconds and so to the user it will seem instant.

The CAN protocol was created by the Robert Bosch Corp. in 1984. The first full production application was in 1992 on a Mercedes Benz.

These systems are now used in most vehicles because it is cheaper, lighter and reliable. It also makes fault diagnosis easier.

Less wire and less connections makes design and installation faster with only one loom required for each model regardless of spec.

The weight of a traditional car loom is around 40% heavier than a multiplex/CAN bus loom. Lighter cars have better performance, economy and emissions.

Most electrical faults are due to poor connections, less wire means less connections and so reliability is improved. Any post production improvements can be downloaded into registered vehicles.

All faults are registered and stored allowing the technician to interrogate the car and rectify the fault. Also the in-built diagnosis makes it possible for certain functions to work with limited functionality if a fault occurs, instead of failing to function completely.

The twisting helps to reduce interference from other power sources.

You can only see the signals with a digital storage oscilloscope or an LED test light. You can only read the signals with a laptop with specific software or a commercial CAN scanner. Multiplex may appear to be 12 volts but the signals may be disturbed if you connect to it. If you try to test CAN bus with a multi meter it will show as 4 to 5.5 volts or about 1.5 volts.

Test as close to the light units as possible, never test halfway along a loom, you don't know where it has come from or going to. Make sure you use a multi meter or an LED test light, both are no load testers.

At first glance it is difficult to tell, most cars with these systems will have fewer relays and fuses as the switching is done electronically. If a fault occurs the control unit will shut the power off until rectified. Vehicles that have a comprehensive Driver Information System or a multifunction screen and controller dial will certainly use CAN bus technology.

It has been in cars since 1992, most cars now have it in some form or another and very soon it will be used for most systems in every car.

Yes, using a dedicated wiring kit will operate with the CAN bus and multiplex system allowing many additional safety features to be activated automatically.

A bypass relay uses the power to the rear vehicle lights as a signal to switch a relay. The relay switches on the corresponding trailer function using a separate permanent power supply.

When a bypass relay is fitted the vehicle is not aware that it is towing, therefore if the vehicle has safety features such as Trailer Stability Program (TSP) it will not be activated. Please check your vehicle handbook or contact the dealer.

A dedicated kit consists of a vehicle specific wiring harness and control unit with plugs, connectors and relays designed to fit and operate specifically with your vehicle.

Yes, dedicated wiring kits are specially designed to work with your vehicle's electrical system, some will communicate with other vehicle control units to alter other vehicle settings accordingly and will activate the dash trailer indicator warning light and deactivate parking aids and rear fog lamps. These are model dependant features that will soon come as standard.

Generally not, as full and detailed instructions come with the kit. However, as the title suggests, they are specific to each vehicle and some will need more skill to install than others. Fitting is usually done within an hour and a recode may be required from the main agent on some models. The safety and user benefits make the extra cost and time worthwhile.

When you tow with a dedicated kit it can alter (model dependant) vehicle systems such as the stability control, suspension settings, traction control and anti-lock braking system. It can also provide an automatic fog cut out, trailer bulb failure warning and deactivate original equipment parking sensors.

Soon all vehicles will require a dedicated kit as they provide the safest and most reliable type of connection and allow the full function of your vehicle's towing safety features to be used.

The majority of benefits will not be immediately noticeable. However, obvious benefits include automatic fog cut out, trailer light failure warning and automatic deactivation of original equipment parking sensors. The main benefit above all else is reliability.

The C2 module used in some dedicated towbar wiring kits will not beep or operate a towing dash warning light when indicating; instead it gives a visual warning of trailer indicator failure.

In Germany, Austria and United Kingdom it is obligatory that the driver can check visually or audibly the functioning of the indicators on the trailer. The sophisticated electronics in certain vehicles means the C2 control in certain kits is standard.

Should either left or right indicator bulb fail on the trailer, the vehicle's own indicators and dash light will flash at twice the usual rate. Some dedicated kits also warn the driver with a text display on the dash panel. Other dedicated kits will only beep when an indicator light fails.

In most cases, a dedicated kit will perform the full light functions after correct installation and a recode will not be required. However, if you wish to benefit from the additional safety features and other user functions, then a recode may be required. Full recode details are included in the kit.

The colours in the dedicated kit loom may conform to ISO1724. The designation and the colours are detailed below.

Most 7 pin kits have an output to connect to the reverse pin on a 12S socket should twin electrics be required. The yellow wire is usually for this purpose.

Most dedicated kits have two or three extra wires going to a micro switch in the socket, this is to enable the Trailer Control Unit to recognise when a trailer is connected.

The switch usually disconnects the car's rear fog lights and reversing sensors when towing.

Adapters are available that will convert from a 13 pin socket into either both 12N/12S 7 pin sockets for twin electrics or from a 13 pin socket to a single 12N 7 pin socket for normal light functions.

Adapters are available that will convert from 12N/12S, 7 pin sockets into both 12N/12S functions in a 13 pin socket for twin electrics or from a 12N, 7 pin socket to a 12N, 13 pin socket for normal light functions.

To summarise a dedicated kit:

• Is quick and simple to connect to the sockets in the vehicle.

• Has reliable connections which continue to function faultlessly for many years.

• Comes complete with all required components and electronic modules.

• There are no conflicts with the other electronics of the vehicle.

• There is no risk of damage to the vehicle wiring.

• Comes with a detailed assembly manual with easy to follow 'step by step instructions'.

• Has high quality components, often original equipment.

The Normal power required by the caravan/trailer for indicators, brake, fog and sidelights are provided via the 12N connector. This is a socket on the vehicle and a plug on the caravan/trailer.

The socket cover is usually coloured black and houses 7 pins which are numbered 1 to 7. The plug is also black and has a slot that locates opposite the cover hinge of the socket. The central contact (number 7) is female in the plug.

NO.

Reg 55 was designed to create free trade. If you fit a Witter Type Approved towbar it will not invalidate the vehicle warranty. Under EC Block Exemption Rules a motor vehicle manufacturer cannot prevent a franchised outlet from supplying or fitting components deemed to be of equal quality to that supplied by the vehicle manufacture - such as a Type Approved towbar.

Only if the towbar has been submitted and tested for EC Type Approval using a 'drop plate'. Some Witter towbars for 4WD off road vehicles have been tested in this way, e.g. Range Rover, Land Rover Discovery and Defender, the new Mitsubishi Shogun and Mercedes M Class. Your local Witter specialist will be pleased to advise you.

Yes, Witter towbars are tested with a minimum spacer of 25mm between the towball and the towbar.

No. All Witter towbars have a specified towball part number which has been tested and approved as a component of the towbar; these have their own identification Z11. The identification is clearly stamped on the top flat section of the towball, set inside an outline of the Witter logo.

Although the EC Regulations are applied differently and introduced at different times in different countries, it is the country of registration that defines how and when you comply with the relevant EC Regulations.

Directive Reg 55 was introduced as part of European Whole Vehicle Type Approval to provide a common standard throughout the European Community, thus creating an open market. Irrespective of where a towbar has been manufactured, providing it has been Type Approved, a customer will know that it has been manufactured to a known technical and quality standard.

If your car is first registered on or after the 1st August 1998, you could be prosecuted for an infringement of Construction & Use Regulations or your Insurers may decline to pay out in the event of an accident claim. If you are unsure which towbar to fit contact your local Witter specialist, who can be found under National Stockists.

Yes, the towbar must be certified even if not used for towing. Otherwise it does not comply with the Road Traffic Act.

No, only Type Approved towbars can be fitted to vehicles that are registered on or after the 1st August 1998.

No, a towbar with the popular two - hole flange ball faceplate and towball made by an Independent towbar manufacturer can be EC Type Approved. Witter already has a range of over 250 covering the vast majority of vehicle applications.

No, when EC Type Approving the vehicle, the manufacturer did not declare that it was suitable for towing. It therefore has no specified towing capacity and no specified mounting points. Towing with, or even the fitment of a towbar to any such vehicle registered on or after the 1st August 1998 would make you liable to prosecution.

For a new car or model variant a definitive answer could be obtained by examining the vehicle VIN (Vehicle Identification Number) plate, which should have four figures stamped into the lower edge denoting its carrying capacity.

VIN No. (known as chassis number)

XXXX kg -(Max vehicle weight)

XXXX kg -(Max train weight i.e. vehicle and trailer)

1. XXXX kg -(Max front axle load)

2. XXXX kg -(Max rear axle load)

If no train weight is specified then the vehicle is not approved for towing, and therefore cannot legally do so. If a train weight is specified then the towing weight is this figure less the maximum vehicle weight The VIN plate is usually located under the bonnet. However it can sometimes be elsewhere.

Yes, you can fit the towbar yourself provided that it is installed in accordance with the instructions provided, but we recommend you get a Witter specialist to carry out the complete installation because of the complexity of today's vehicles and the intricacy of the electrics, you can have peace of mind it is fitted correctly by a professional.

It will have a label, data plate or stamping showing an E No. (e.g. E11*0001*00). This figure denotes that it is EC Approved in the UK (E11), has approval number (0001), of EC Document revision 0.

Insist that the towbar is immediately removed and replaced with one that is Type Approved. If you meet any resistance report the matter to your local Trading Standards Office.

Unless otherwise specified on the installation instructions all Witter towbars use Grade 8.8 high tensile bolts and nuts. (Sometimes an even higher grade 10.9 is used).

On the top face of the head of a bolt there will be a number which is the grade of the bolt e.g. '8.8' or '10.9'.

In addition to the 'grade' marking the bolt manufacturer stamps their identification on the head of the bolt. E.g. until recently all Witter Towbar bolts would have had a '3F' marking, but more recently this has changed to 'DF' and then to 'IIb'.

Note: This list is not exhaustive as on occasions it may be necessary to obtain from alternative sources.

All installation instructions produced over the past 10 years have the recommended torque settings for the fasteners which are usually as per the list below. However, if in doubt check the installation instructions.

Torque is a measure of the force applied to rotate the bolt (or nut) to tighten the fasteners and create a pre-load in the bolt. This used to be measured in 'ft lbs', and now more commonly in the metric equivalent of 'Nm'. The 'ft lbs' is perhaps easier to understand and relate to its definitions as it is simply the value in 'lbs' forces applied '1 foot' from the centre of the axis of the fasteners.

Note: most torque wrenches have scales in both 'ft lbs' and 'Nm'.

es. It is impossible to accurately and consistently estimate the applied forces using hand tools without any form of torque measurement.

No. Absolutely not! If grease or a lubricant is applied to the bolt or nuts the tensile load that can be applied to a bolt is often doubled for the same tightening torque.

A difficult one to grasp, so try this! Get an M10 bolt (1.5mm pitch) add a few flat washers to ensure that the nut is within the threaded section of the bolt and slowly tighten. Most bolts (with dry, clean threads and without grease) will typically shear at typically twice the recommended torque setting e.g. 100 Nm or more.

Repeat the same test with an identical bolt and nut with a good high pressure lubricant in the threads and under the head of the bolt and it is likely that the bolt threads will strip at a torque level around that of the recommended torque setting! The bolt is no weaker, but the grease has allowed a much higher force to be applied for the same torque.

The standard finish for Witter towbar fasteners is an 8 micron zinc electroplate coating that has been 'chromium passivated' to increase the corrosion resistance and is a golden yellow finish.

In line with the new recycling standards (see KD) all of our fasteners are changing from 'hexavalent' chromium to a more environmentally friendly 'tri-valent' chromium, which meets all EC and EU requirements. This process of change has started, and there will be no visible difference in the appearance, properties or behaviour of the fasteners.

Yes, but it must have a torque control to avoid over tightening – especially when tightening the relatively small M8 and M10 fasteners. It is also recommended that the final tightening is carried out with a hand torque wrench.

It is quite normal for towbar components to 'bed in' after initial use e.g. any small surface irregularities flatten, and therefore it is recommended that the tightness of all fasteners is checked after the first 500 miles of use, and thereafter every year (as per our warranty statement).

The holes are larger than the diameter of the bolts to accommodate anticipated normal variations on vehicle-to-towbar tolerances and help ease of alignment. Our bolted joints are designs one of the 'friction grip' type arrangement, which relies on the pre-load applied to the bolt to hold the component forces together using the friction between the components.

The thread pitch is the spacing of the threads of the fastener and is usually measured in mm. This can be measured using a thread gauge, or, by using an accurate ruler e.g. measure a convenient number of threads along the bolt, count the number of threads and divide them into distance.

For M10 bolts/nuts the standard pitch is 1.5mm and for M12 bolts/nuts it is 1.75mm. It is essential to not mix different pitch bolts and nuts. To do so would permanently damage the fastener.

Different vehicle manufacturers use different pitch threaded fasteners on the towbar to vehicle fixtures. Witter Towbars supply fasteners to match these fixtures and often together with fasteners of a standard pitch for towbar-to-towbar connections.

The finer the pitch (lower the distance between threads) the greater the resistance to loosening under vibration and greater the strength of the bolt/nut connection. However, the finer pitch is also more sensitive to damage and binding due to contaminants such as dust and debris.

There is no standard that stipulates this value, but a general guide is at least one fully formed thread should protrude past the surface of a standard nut.

No. It is good practise to always use new fasteners if there is a need to remove, adjust or replace a component. In the case of Nyloc nuts it is essential that these are not reused.

It is essential to clean out the threads using a top of the correct thread diameter and pitch. To continue to tighten the bolt would either result in too low a clamping force for the applied torque, or, even damage or strip the threads of the fasteners.

The use of a thread locking compound can be of considerable benefit in preventing loosening (and loss of clamping force) due to vibration or impact loading of the joint. However, it is essential to read and follow the guidelines of the management.

The 'nose load'is the commonly used term for the load, weight or mass applied by the coupling head of the trailer or caravan onto the towball/towbar fitted to the towing vehicle.

Yes. The static vertical mass is simply the technically correct terminology for 'nose load'. It is even sometimes referred to as the 'tongue load' in the USA!

The 'nose load' can be measured by recording the force, or weight, applied by the trailer/caravan using a set of scales or proprietary device. The force, or weight, must be measured as close to the centre of the coupling head as possible and at the same coupling height as when coupled to the intended tow vehicle. To put the scales under the A-frame away from the coupling head will not give correct measure of the 'nose load'.

Note; There are devices on the market that incorporate a nose load measuring device e.g. integrated into the jockey wheel.

The vehicle manufacturers should (legally) specify the maximum 'nose load' in the vehicle handbook, possibly using the technical terminology 'maximum static vertical load on towball', or a similar description. This may vary for different models in the range and if there is any doubt it is essential to contact the vehicle manufacturer.

The maximum 'nose load' for the towbar EC approval is specified on the type approval label as the 'S' value and is measured in kg. Please note that it is not necessarily the maximum appropriate to your model of vehicle, especially with some towbars being suitable for a range of different models and vehicles. Always refer to your car handbook for data for your specific model.

Currently it is not mandatory to EC or nationally type approve trailers and therefore it is not a legal requirement for the manufacturer to apply a rating/capacity plate or label. Many manufacturers do have rating plates, and many specify a maximum 'nose load'. This may be specified on the plate or label as the 'S' in kg.

Please note that the coupling head will almost certainly (for all modern trailers/caravans) have an EC approval marking or plate which itself will have a specified maximum 'nose load' or 'S' value. This may be in excess of the other trailer components. If in doubt contact the trailer manufacturer.

It is generally recommended that the trailer/caravan is used towards the upper range of the maximum recommended 'nose load', as research has shown that trailers/caravan stability tends to increase with increasing nose load.

However, there are sensible limits e.g. try to achieve a nose load of 150kg (for a Toyota Land Cruiser) when towing a 250kg trailer is obviously not likely to be easily achievable.

The current recommendation is that the height of the cycles and cycle carrier should be subtracted from the maximum recommended 'nose load' i.e. The combined weight of the cycles, carrier and the trailer/caravan nose load should not exceed the vehicle's maximum recommended 'nose load'.

To do so may invalidate the warranty on the towbar and vehicle, as well as being potentially damaging to both the towbar and the vehicle mounting points.

On most trailers the only effective way of adjusting the 'nose load' is to redistribute the contents of the trailer. However, on some special purpose trailers e.g. boat trailers, there is scope to change the effective 'nose load' by moving the axle(s) forward or rearward along the trailer chassis. This is best left to the trailer manufacturer or an appointed representative.

A negative 'nose load' is where it is necessary to push or force the trailer coupling onto the towball. This is potentially a dangerous situation as it significantly increases the likelihood of trailer instability or snaking.

The towball height is measured as the vertical distance between the ground and the centre of the towball. This must be measured on flat and level ground – most surfaces are far from level! Also, the laden condition of the vehicle must be known. (Note; In the case of "coupling height", this is the position of the centre of the towball location when within the coupling head).

The million Euro question! The EC Directive 94/20/EC states that the towball height should be between 350 and 420mm with the vehicle in the 'laden' condition. Laden condition may have been defined (by the manufacturer) as either fully laden (the maximum technically permitted mass of the vehicle as shown in the vehicle handbook), or, conventionally laden (defined as 68.5kg per specified passenger seat and 6.5kg of luggage per specified passenger and located in the centre of the luggage storage area).

Alternatively, the towball may be at the height or height range specified by the vehicle manufacturer in the homologation documents and possibly the vehicle handbook.

Not so straightforward!

The EC Directive states that this should be 385 to 455mm with the trailer in the 'laden' condition.

'Laden condition' in this instance is when the trailer is loaded to its maximum specified mass, with this evenly distributed over the loading area.

Not necessarily! Many think (incorrectly) that it is essential to have the trailer chassis perfectly level and go to great lengths to try and achieve this. However, from the previous two questions it can be seen that there is inherently a wide height range for both the towballs and the trailer couplings.

Also, the specifications are written to promote a tendency towards a slightly 'nose down' attitude of the trailer, with the medium of the range for the trailer being 35mm higher than the medium of the height range of the towball on the towing vehicle.

Furthermore from the permissible ranges and laden definitions it can be seen that the difference can be extreme e.g.

- Towball at 350mm (fully laden vehicle) and trailer coupling at 520mm (not untypical of an unladen trailer)……a difference of 170mm!

- Towball at 510mm (unladen long suspension travel vehicle) and 385mm trailer coupling height (for fully laden trailer)…..a difference of 125mm!

The situation is further complicated by the fact that the towbar/towball is controlled by an EC directive, whilst it is currently not mandatory for the trailer to be EC approved – and very few are.

Maybe the towball is in the correct height range and the trailer coupling height is incorrect. For some reason, this is automatically assumed to be the 'fault' of the vehicle/towbar – measure and check!

Unless there is a grounding or stability problem there is no technical reason to do anything.

However if it is necessary, then in the case of an EC type approved towbar the options are:

- Use a different towball location (some vehicles have two or more different towball fixing heights).

- Check with the towbar fitter that the towbar has been installed correctly and whether there is any scope to loosen and retighten any components to raise the height – on some towbars this can regain up to 40mm in height!

- Check the vehicle suspension. This can wear and settle over time.

- Are spring assisters or suspension changes or adjustments possible?

- Modifying the trailer coupling height should be considered.

- If the issue is the jockey wheel grounding, can this be repositioned?

- Does the vehicle have sports' suspension, often 15-50mm lower than standard models? Check against the chassis number with the Dealer.

As in the previous question, it may just be the trailer/caravan that is non-compliant – measure and check! Similarly, in the case of EC type approved towbars there are limits as to what can be changed. Options include:

- Use a different towball location (some vehicles have two or more different towball fixing heights).

- Possibly use a drop plate (some towbars are tested and EC type approved for use with drop plates – check the towbar installation instructions).

- Alternatively, if it is a commercial vehicle it is currently not mandatory to have an EC compliant towbar and it may be possible to use a drop plate (always check with the towbar manufacturer).

No! To do so could significantly increase the leverage on the towbar/towball interface, and could result in damage or failure of one of the towing system components. It would also almost certainly invalidate the EC approval of the towbar.

There are very few instances where a raiser plate can be used. Also these must be with written permission of the towbar manufacturer and use is usually limited to special applications where a pin and jaw (not towball), is raised to the same position as the towball.

A fixed flange towbar describes the shape of the towbar. First, the "flange" is fixed directly underneath the bumper of your vehicle. The towball/towbar is then attached to the plate with either 2 or 4 bolts.

Our towbars come with a 2 year guarantee.

No. A towbar involved in an accident should not be used. The metal connection to your bumper could be weakened in an accident and will likely be void.

If you have had an accident and the rear of your car was damaged it is highly likely that your towbar and towbar neck were also damaged. Damage can normally be identified visually by deformation, unnatural dents and movement on the fixing points.

Having said that, even though your towbar might not have obvious damages, we would recommend replacing the towbar and towbar neck as the impact of the accident may have caused micro-cracks that are not visible to the eye. Cracks weaken the structure of your towbar and will lead to failure. We would, therefore, recommend the full exchange of your towbar when you have been involved in an accident even if it is not visibly damaged.

The maximum towing capacity for each vehicle is set by the manufacturer. This information can be found on the Vehicle Identification Plate (VIN plate). The VIN plate can usually be found under the bonnet or on a door pillar.

It has 4 lines of numbers:

• 1st Line - 2500kg - Gross Vehicle Weight
  
• 2nd Line - 4800kg - Gross Train Weight
• 3rd Line - 1250kg - Maximum front axle load
• 4th Line - 1300kg - Maximum rear axle load

The maximum towing capacity can be calculated by subtracting the gross vehicle weight from gross train weight. If the Gross Train Weight is 0 or it is equal to the Gross Vehicle Weight, then your vehicle is not capable of towing.

A towing stabiliser is a sway control device that restricts or alters the lateral and vertical articulation between the towing vehicle and caravan or trailer. It does not increase the inherent stability of an outfit.

Commonly called pitching, this can happen on an undulating or uneven road, after a sudden dip or hump such as a bridge, or even when a pot hole or drain cover unsettles your outfit. An incorrect nose load or high centre of gravity or worn rear suspension will increase the susceptibility to pitching.

Commonly called snaking or swaying, this can happen with unexpected side winds, sudden road camber changes, the suction effect when a larger vehicle passes and the pressure effect when a much faster vehicle passes. A badly matched outfit or high centre of gravity will increase the susceptibility to snaking.

There is nothing more frightening than the feeling of your caravan or trailer starting to sway or snake behind your tow vehicle. There have been many reported accidents as a result of the loss of control of the tow vehicle steering due to the uncontrollable sway of the caravan or trailer.

Trailer 'snaking' is an instability situation that occurs when the trailer acts as a swinging mass behind the vehicle (similar to a pendulum), whereby relatively little energy is required to initiate and maintain the pendulum movement. Once the oscillations reach the natural resonance of the outfit the amount of movement increases uncontrollably prior to an inevitable crash.

The Caravan Club has put considerable effort into gaining an understanding of the phenomenon and sponsored/overseen various projects at Bath University over many years. Dr J Darling, who has been the academic mentor to these projects, is now recognised as perhaps the most knowledgeable individual on this subject.

It has been proved beyond doubt that increasing the nose load (static vertical load on the towball) reduces the susceptibility of a trailer/towing vehicle to instability. Therefore, the recommendation is to increase the nose load up to the maximum allowable on the vehicle/trailer specification (whichever is lower) and also ensure that the outfit is balanced and well matched.

The further the centre of mass from the axle, the greater the susceptibility to instability. In other words, centralise the weight above the axle both in the lateral, longitude and vertical planes to balance the outfit and achieve an acceptable nose load. An unbalanced outfit is an unstable outfit. Ideally the centre of gravity for the caravan/trailer should be low down and slightly forward of the axle.

"Matching" your outfit is a term used by caravan and towing organisations. The best caravan to match your vehicle or the best vehicle to match your caravan is calculated using the vehicle towing capacity and kerb weight, the nose load and maximum caravan weight. Caravanning clubs and organisations have detailed lists of the best matching outfits.

Single axle trailers are more susceptible to instability, with twin or multiple axle trailers being the preferred and more stable option. This is due to their inherently greater resistance to changing direction.

Having the mass in the trailer as low as possible tends to reduce the susceptibility to instability. Caravan and vehicle tyre pressures must be checked every time to the manufacturers' specifications.

Quite often trailers and caravans have existing damper mounting points but do not have them fitted as standard. Fitting dampers to the trailer/caravan has also been shown to improve stability.

Aerodynamic aids (as currently available on the market) appear to have little effect on stability.

The fitting of stabilisers does not in any way reduce the inherent tendency of a specific trailer/vehicle to instability. However, an effective stabiliser would act to dampen the pendulum motion of the trailer and in doing so, prevent or moderate potential instability. A stabiliser should not be used as a cure for an instability problem.

Changing the towball height will not have a significant direct influence on stability. The exception is when the height affects the noseload eg a low towball height when used in combination with high coupling height twin axle trailers can reduce the effective nose load.

A slightly nose down attitude on a single axle is recommended as this will reduce the amount of rear vehicle lift when under heavy braking and so reduce the possibility of a jack-knife. When towing a twin axle outfit you should endeavour to achieve a level outfit.

The towbar itself plays no direct or significant role in influencing trailer stability, as the lateral deflection or elastic deformation of the towbar are extremely small compared with those resulting from the trailer and towing vehicles' tyre and suspension movements.

On the towing vehicle, there are more variables to adjust such as:

Tyres: A tyre at the high end of the recommended pressure range will tend to be better at minimising instability.

Springs: Stiffer springs also help reduce instability. Towing vehicles also tend to perform better with leaf springs, compared with coil springs, especially when towing trailers with known stability issues.

Dampers: Stiffer dampers (shock absorbers) also tend to reduce susceptibility to trailer instability.

Unforeseen circumstances and outside forces can drastically reduce the stability of a vehicle in an instant. A stabiliser provides the driver with peace of mind and confidence that any instability will be minimised.