Tech Talk

THE LOWDOWN ON PAYLOADS

By necessity, you become very familiar with weight when caravanning. I'm not suggesting that the caravan touring lifestyle leads to extra kilos around your waist, but you do need to pay attention to how heavy your rig is. There are many safety, legal and insurance implications if you exceed prescribed figures for heaviness.

Vehicles and caravans are engineered to perform while carrying only a certain amount of weight. When manufacturers develop a product, they set a safe weight that the vehicle or van can reliably carry without causing component failure or damage. This has other implications, such as the tyre chosen by manufacturers for the car or caravan. As an owner, you have to make sure replacement tyres adhere to manufacturer's load rating requirement.

Let's start with the tow vehicle. The first figure you'll probably hear is'kerb' weight. This is a vehicle's weight when empty, and many car manufacturers publish this weight. What 'empty' means to carmakers is a standard vehicle added with fuel (some manufacturers weigh with a nominal amount of fuel, some with half a tank, some with a full tank), all fluids filled to specification (engine oil, coolant, etc) and, sometimes, the weight of a driver - so no passengers or load. Obviously, it can be open to interpretation, so a manufacturer's kerb weight is never really an ironclad figure.

You need to pay more attention to the manufacturer's quoted GVM - Gross Vehicle Mass. The GVM is a set figure that represents the maximum all-up weight a vehicle is designed to cope with, including everything in it and on it, from passengers, luggage, roof racks and fuel, to the weight of the trailer pressing down on the towball. You'll find this figure in the owner's manual or on the vehicle's compliance plate.

Weighing your tow vehicle loaded up to see if you have exceeded its GVM is the way to really know if you're legal. A public weighbridge will give you the definitive GVM weight. We're not talking about trailers yet, but TBM (Tow Ball Mass) must be considered when calculating vehicle payload, because this weight is borne by the vehicle. Check with your specific vehicle's requirements, as many can impose weight trade-offs. For example, at maximum GVM, a Nissan GU Patrol wagon's 350kg maximum permitted TBM is reduced to just 150kg.

Reduce the Patrol's payload by 290kg and TBM returns to its 350kg permitted maximum.

How much weight you can tow behind your vehicle is decided by either a) the towbar rating or b) the vehicle manufacturer's maximum tow rating (whichever is lower). This applies to vehicles that have a GVM figure of 4500kg or less, which is the majority of cars and 4WDs. You can find out what maximum towing weight is specified by the manufacturer by checking the owner's manual or by contacting the manufacturer directly.

Vehicle manufacturers quote two maximum recommended figures for towing weights - the unbraked trailer maximum and the braked trailer maximum. The statutory unbraked maximum weight is 750kg, and most vehicle specs prescribe this figure (be aware though, some vehicles have a lower unbraked maximum - the superseded Triton ute, for example, had a maximum of 500kg unbraked).

Most 4WDs have one figure for braked trailer maximum weight. Yet, to make life interesting, some 4WDs and cars have more than one - the new Ford Ranger, for example, has three different maximum capacities spanning from 2250kg to 3000kg, according to your engine and transmission choice.

In some cars, the tow capacity will depend on what towbar is fitted - the maximum Commodore VY tow capacity is 2100kg, but there are also factory 1200kg and 1600kg-rated towbars. Obviously, you can't tow 2100kg legally or safely with towbars with lower ratings.

Towing capacity can also be restricted by payload. For example, the Chrysler 300C has a 910kg towing limit with five occupants and 7kg of luggage each; it goes up to 1725kg with only two people and 7kg of luggage each.
- PL

INITIALS THAT MATTER
Caravans have four types of weight you need to know about: ATM, GTM, TBM and Tare. The Aggregate Trailer Mass (ATM), Gross Trailer Mass (GTM) and Tow Ball Mass (TBM) should all be stated on the trailer's compliance VIN (vehicle identification number) plate, usually found in the boot or on the A-frame.

Similar to the GVM for a vehicle, Tare Mass is the weight of the unladen trailer with all the fittings and equipment it had when it left the factory.

Aggregate Trailer Mass (ATM) is the total weight of the trailer loaded to its maximum possible weight, as recommended by the manufacturer. This includes any weight imposed onto the towing vehicle (ie, the TBM) when the combination is resting on a horizontal surface. In other words, if you were to unhitch the trailer and park it, on its jockey wheel, on a weighbridge, you would obtain the ATM.

Gross Trailer Mass (GTM) is the weight transmitted to the ground by the axle or axles of the trailer, when hooked up to a vehicle and carrying its maximum permitted load distributed over the load-bearing area. Again, if you were to load up your trailer and park it, hooked up but with only the trailer wheels on the weighbridge, this is the GTM weight.

The Tow Ball Mass (TBM) is the weight pressing down on the towball. If you add GTM to TBM, you should have ATM.

Add vehicle and van together and you have the GCM, Gross Combination Mass. This is the total laden weight of the combination - that is, your vehicle and trailer's total weight pressing down on the road, measured by adding the Gross Vehicle Mass (GVM) and the trailer's weight, the GTM. A vehicle manufacturer will often quote a maximum permitted GCM figure.

Another thing to bear in mind is that even if you stay within the limits defined by the manufacturer, your GCM can easily restrict where you drive. For example, towing your 2000kg GTM van with a 2200kg GVM Land Rover Discovery (thus with a GCM of 4200kg) means you can not legally drive on weight-restricted roads - the 3.5-tonne-limited bridges found in many country areas are a good example.

With these theoretical numbers, it behoves any caravanner to go to a public weighbridge and do the exercise of weighing your rig.

It will then be clear in your mind where you stand in respect of the safe loading of your rig and where you can drive it.

This info is, of course, of vital interest to newcomers to caravanning and towing, and at the risk of boring you experienced and capable vanners out there, the basic facts bear frequent repetition. This is because when it comes right down to it, no-one else is going to help you with getting it right - it's up to you, the individual caravanner, to find out what your tow vehicle's specs say, and up to you to establish the weight of your loaded trailer and combination.

Guesswork and the word of others (who may or may not know what they're talking about) are no substitute for a bit of legwork and investigation on your own car and caravan.
- PL

LACK OF SERVICE
Readers may be interested to learn that RACV Total Care might not live up to its name.

I experienced a total breakdown while towing my caravan. RACV towed my vehicle to a service centre but my caravan, left without a tow vehicle, was not covered. I had to arrange towing the stranded caravan.

Luckily, I was able to do so, but I wonder how many people with Total Care erroneously believe it will ensure movement of the broken-down vehicle, occupants and caravan under the terms of the insurance. RACV will only tow a caravan that is disabled mechanically.

Clearly there is a gap that RACV needs to provide coverage for, and I would be happy to pay a reasonable premium on top have towing the van included.

DIFFERENTIAL GAIN
I read an enquiry in issue 437 (CW January 2007) from Greg Short from Bendigo regarding the altering of the differential ratio on his Nissan Patrol.

Judging by the power output that would exist in his vehicle and the reasonably light weight of his caravan, I very much doubt if by increasing the ratio he would gain any better fuel consumption with his current power-to-weight ratio, though there would be a noticeable increase in power at the wheels (but that comes at a cost at the fuel bowser).

The other issue that exists and one that a lot of folk overlook is the fact that if you do alter the final drive ratio, you then end up with an incorrect speedo/mileage readout (not at all helpful if you are wanting to document your fuel consumption accurately).

Maybe that can be overcome by an instrument repairer making some adjustments, but I would think that by the time you went through what you had explained to Greg and this additional alteration cost, I'm not sure if there would be any significant saving in the long run.

SINGLES ON THE ROAD
I am thinking of travelling Australia, but would like to know of a club or other group that caters for single people of 40 to 60 years of age travelling solo, so for safety reasons we could get together as a group.

Is there such a group that you know and can recommend?

HATCH PROBLEMS
With regard to Barry James' complaint of sun-degraded hatch covers (CW February 2007), I had the same problem with the toilet hatch on my Jayco caravan, which at the time was 14 months old! It was so degraded that I have seen icing on a cup cake that was stronger. When I contacted the supplier of my caravan, Caravan World of Cannington, WA, they were very surprised about what had happened and they assumed that it was a'bad batch' of covers.

They said they would supply me with a new cover and as I have two of these hatches and to make sure I was prepared if it ever happened again, I ordered a spare myself. Since that incident the hatches have remained intact for my journey from Secret Harbour, WA, through the top end, and Qld to Eden, NSW. Along the way I have enquired at various caravan shop/suppliers and have discovered that this fault is prevalent with these sorts of hatches.

Each place at which I made an enquiry had an ample supply, each costing $45 or more!

DUMP POINT LOCATIONS
Just wondering if there is a publication in Australia that lists all dump points for motorhomes? We have heard that there is one, but we can't find it.

TREG FITTING ON NS PAJERO
To correct the totally wrong impression given by your reporter Tom Olthoff in his tow test of the Mitsubishi Pajero (CW May 2007), I would recommend that he look at this picture of my new NS Pajero VRX Di-D.

Without any alteration whatsoever and only a short drive down the street to ARB Brighton, I bought the proper tongue and had our Coromal offroad Silhouette campervan, complete with Treg coupling, hooked onto the back of the NS ready to start the next trip.

Even the rear door opens now against the soft tyre without doing any damage to the car.

NEW DECALS FOR OLDER COROMAL
I have a Coromal 11ft (3.4m) pop-top 360 and would like to put burgundy/charcoal stripes on the sides because the current decals are peeling. Can you advise where I could purchase these?

TERRITORY STABILITY
We have been considering buying a new Ford Territory AWD for some time. It will be used to tow our Millard pop-top caravan. We have been told that there is a problem with the stability control system when towing a caravan. Apparently it tries to take over control, for example, when going down hills and around corners, as the weight of the caravan pushes the towing vehicle. This tends to push the towing vehicle towards the guardrails.

Have you heard of this problem? I also understand the stability control can be partially or fully turned off, which should alleviate the above mentioned problem. Do you agree?

I spoke to a person who had experienced the problem with the stability control in his Territory AWD. He said that when the brakes were applied the vehicle would "spear off anywhere".

This meant that he could only travel slowly on a trip towing his van from Pambula to Gerringong, NSW, but at the same time the vehicle consumed fuel at a rate of 29L/100km. He sold the vehicle and bought a LandCruiser!