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Staying within the law isn't helped by inaccurate tow vehicle and trailer weights.

Nationally accepted trailer towing regulations make it theoretically easier for owners to comply with the legalities of towing, but there’s much that’s not covered by legislation.


Since 1989, when the Australian States and Territories agreed on common national towing regulations, the starting point for 4WD owners wanting to tow trailers has been relatively easy to understand.

The national position is that a passenger vehicle, light commercial vehicle or a 4WD with a GVM of less than 4.5 tonnes can tow a trailer with an ATM that weighs the lesser of the tow vehicle manufacturer’s recommended maximum trailer towing mass (including the tow ball load), or the tow vehicle’s tow bar rating (including the tow ball load). Clear? It becomes clearer if we look at the GVM and ATM acronyms.


Legal Weights

Gross vehicle mass (GVM) is a rating given to a 4WD by its manufacturer and is the maximum the loaded vehicle can weigh. Modified vehicles may have a higher GVM rating than originally provided, but this is granted by State and Territory registration authorities only upon presentation of an accredited engineer’s certificate.

Gross trailer mass (GTM) is the maximum permitted weight imposed on the trailer’s suspension, wheels and tyres, when coupled to a towing vehicle.

Aggregate trailer mass (ATM) is the total weight of trailer, including the tow ball weight imposed on the towing vehicle.

Every trailer must carry a vehicle plate, providing clear indication to the State and Territory registering authority, and to owners and the general public that the trailer is ready for road use.

The vehicle plate must be durable, non-corrosive metal and must be affixed to the vehicle by pop rivets, hammer drive screws or welding, in a position where it may be readily examined.

The vehicle plate must show at least the following information:

• Manufacturer’s or Importers Name (whichever party takes responsibility for the certification statement);

• Trailer model;

• Vehicle Identification Number (specified in Section 8);

• Date of manufacture (month/year, e.g. 02/08);

• Aggregate Trailer Mass (kg); and the Certification Statement: This trailer was manufactured to comply with the Motor Vehicle Standards Act 1989.

Trailers with a GTM not exceeding 3.5 tonnes must also have a tyre placard made of a durable material affixed to it in a prominent position and showing at least the following information:

• The manufacturer’s recommended tyre size;

• Tyre load rating;

• Speed rating;

• Cold inflation pressures; and  either the statement: ‘The tyres fitted to this vehicle shall have a speed category not less than ‘L’ (120km/h)’; or  if the recommended maximum vehicle operating speed is less than 120km/h, ‘The tyres fitted to this vehicle shall have a speed category at least equal to the recommended maximum vehicle operating speed,‘…’km/h.’, where ‘…’ is the vehicle manufacturer’s recommended maximum vehicle operating speed.

All this sounds comprehensive, but it isn’t. We’re sure that the trailer vehicle plate should also be required to show an accurate tare (empty) trailer mass of the actual, fitted-out van and its empty ball weight rating. It should also have to display the maximum permitted ball weight.

Some trailer plates do volunteer tare weight and ball weight figures, but nearly all of them are wrong, our research indicates. In fact, we know of towbar fitment companies who reckon they’ve never, ever seen a trailer plate with truly accurate empty weight figures on it.

The fact that trailer makers don’t have to display accurate tare weight and unladen ball weight figures leads to a great deal of trouble for most trailer owners – particularly camper trailer and caravan buyers. Many new camper and caravan buyers discover – all too late – that the tare weight figures they’ve been given are wrong.

This situation is compounded by 4WD makers’ tare weight figures that aren’t even close to what we’ve found at certified public weighbridges. Typically, a road-registered, fuelled 4WD weighs around 200kg more than the spec’ sheet states. Add two people – which is how you’ll be weighed by road authorities if they pull you over – and that ’empty’ weight goes up by another 150-200kg.

We’ve spoken to hundreds of camper and caravan owners who’ve discovered that their tow vehicles exceeded their rated GVMs and their trailers were also well overweight, because the empty weight figures supplied by vehicle and trailer manufacturers are inaccurate.


GVM, GTM and ball weight

Factors that can influence GVM and GTM include changes to wheels and tyres on 4WD and trailer. Tyres with lower load ratings than the originals effectively reduce the plated GVM and GTM.

Inoperative trailer brakes also reduce GTM, from the trailer maker’s figure back to 750kg, which is the upper figure allowed for unbraked trailers.

GVM is the total weight of the 4WD and its contents: fuel, water, people, cargo, accessories and spares – everything. In the case of a towing vehicle coupled to a trailer the GVM figure also includes the tow ball weight imposed on the vehicle by the trailer.

When you consider that most 4WD wagons have a real-world payload figure of between only 450kg and 750kg it’s easy to see why many 4WDs towing heavy trailers exceed the manufacturers’ GVM ratings. Any vehicle operated in excess of its rated GVM or GTM is illegal and the driver faces heavy fines and, in the event of an accident, cancellation of insurance.

All tow bars made after July 1, 1988 should have a plate that lists the maximum load, the maximum tow ball load and the make and model of vehicle the bar was designed for.

The vehicle’s towing rating remains the maximum the vehicle can legally tow, even if the tow bar is rated for a higher load. However, if any of the tow bar specifications are lower than those of the vehicle, the tow bar limits the load that can be towed.

It is common to find tow bar ratings that differ from those of the vehicle, most often where the bar is made for a number of different models in the range, or where light and heavy duty tow bars are offered.

Among all these very precise guidelines is one major variable: tow ball load. There’s a convention that sets tow ball load at around 10 percent of the trailer ATM, but this rule of thumb developed in the days of small trailers.

It’s still the practice by many North American and Australian trailer makers to specify 10 percent loadings for heavy trailers, but in Europe, where there has been considerable testing done, the tow ball load range is specified in EC E94/20 regulation as a minimum figure of 25kg and a maximum of 100kg, for trailers up to 2000kg ATM. Even above that figure the heaviest gazetted coupling for cars, light commercials and 4WDs is rated at 120kg tow ball load.

It’s easy to check the tow ball weight of a disconnected trailer by sliding a set of bathroom scales under the trailer coupling, using a block of wood between the scales and the underside of the coupling. It’s important that this measurement be done at the height of the tow ball.

If you want check the safety and legality of your towing combination, check out our calculator.


Safety Chains

All States and Territories require the use of safety chains. Safety chains must be strong enough to hold the trailer should the trailer coupling disconnect and must comply with the appropriate Australian Standard. Every second link in the chain must have a load rating stamped into it.

Trailers up to 2500kg ATM are required to have one safety chain and trailers from 2500kg to 3500kg must be fitted with two safety chains.

The shackle used to connect a safety chain to a vehicle’s tow bar must have a stamped load rating equivalent to that of the safety chain. Safety chains must attach to the main frame of the tow bar, not to a removable part such as the tow bar tongue.


Legal Dimensions

It’s unlikely that most 4WD owners will run foul of the maximum legal dimensions for a 4WD and trailer combination – 19 metres overall and 4.3m high – but there are some internal dimensional restrictions that need noting.

The distance from the tow ball to the axle centre line (or the centre of a group of two or more axles) must not exceed 8.5m and the rear overhang, including any attachments to the trailer, must not exceed 3.7m.

The overall width of the trailer and any attachments must not exceed 2.5m, unless a permit has been granted by the State or Territory authority. Note that permits allowing legal travel in one jurisdiction will not be legal in another State or Territory.

In addition to these nationally accepted dimension limits there are local exemptions, such as a special 5m rear overhang allowance for trailerable yacht masts in Victoria. Always check with local authorities to find out what rules may apply to your 4WD and combination.


Trailer Brakes

The nationally approved upper limit for unbraked trailers is a GTM rating of 750kg.

Trailers not over 2000kg GTM must have brakes that operate on at least one axle.  Trailers over 2000kg GTM must have brakes on all wheels.

Trailers over 2000kg GTM are required to have brakes that apply if the trailer becomes detached from the towing vehicle.

Brakes other than override systems must be able to be operated from the driver’s seat.


Trailer tyres

It’s vital that tyres on any trailer must have load and speed ratings that match the trailer’s GTM. Apart from the obvious consequences of overloading or overspeeding a tyre, incorrect selection of tyres can contribute to poor handling and instability.

Trailer tyres are often under-rated, or operate close to their rated maximum load capability, when the trailer is fully loaded. Under-rated tyres are dangerous and illegal, and those operating at their upper limit have no reasonable safety factor.

The opposite situation occurs when trailers are fitted with tyres that match those on a large towing vehicle, such as the large-section tyres fitted to 4WDs. Many camper trailer and caravan owners like to have a common tyre size on the tow vehicle and the trailer.

In many cases, these tyres are grossly over-rated, such that when the trailer is fully loaded, the tyres are loaded to only 50-70-percent of their capacity. If the tyre pressure of these large tyres is matched to the load they’re carrying it’s likely that this relatively low pressure will lead to the trailer swaying or wallowing. If the pressure is raised to control that issue the trailer may skip or bounce badly – even on relatively smooth roads.

Yet another factor is tyre age. Tyre manufacturers generally agree that around seven years of age is the safe operating limit. Many trailer tyres are used only a few times each year and reach that safety limit long before tread wear dictates a tyre change.

Owners are understandably reluctant to replace tyres that have ample tread depth, but old tyres become brittle and are prone to failure without warning. Tell-tale signs of age are visible fine cracks in the sidewalls and tread area and tyres exhibiting any such cracks must be discarded.

While opinions of tyre specialists may vary a bit, the general consensus that the Caravan Council of Australia has found is that caravan tyres should be loaded to around 85-90-percent of their maximum rating, when the ‘van is fully loaded.

The Handbook of the Tyre & Rim Association has a detailed listing of the recommended inflation pressures, for various tyre sizes and loadings. All too often on trailer plates the “recommended inflation pressures” for the empty and loaded cases, do not correlate with the recommended pressures provided by the Tyre & Rim Association.

Often the recommended inflation pressure on the trailer plate is simply that embossed on the tyre sidewall, for when the tyre is loaded to its maximum rating, although the actual tyre load is far less than the tyre’s rating.

The CCA has a simple Excel sheet freely available for ‘van owners to check their own situation regarding tyre loadings and pressures.



Speed Limits

The speed limit when towing is generally the posted speed limit, however in Western Australia the maximum speed for trailers over 750kg GTM is 100km/h.

It’s important to realize that when towing, your vehicle is heavier and will be harder to start off, accelerate and stop, so you will need to drive accordingly.

If you can travel at lower than the legal maximum you and the trailer will have an easier time. You’ll reduce stress and fuel consumption. However, use polite road etiquette and allow faster vehicles – particularly trucks – to get by when possible.




























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