Back in time, in a galaxy far, far away, when Holden Kingswoods, Ford Falcons, and Valiant Regals roamed the earth and were used as regular tow vehicles in Australia, towing specifications looked a little different to what they do today. In some cases, they were specified by the caravan manufacturers and usually only mentioned one thing — engine horsepower, and preferably V8 horsepower at that. Weights didn’t often get a mention and were you able to travel back in time and mention things like GVM, TBM, GCM and ATM, you’d received some very puzzled looks.
About a decade ago, one of those, Gross Combined Mass (GCM), only really concerned fifth-wheeler manufacturers and owners. It was essential because tow vehicle manufacturers didn’t specify a maximum towing weight for fifth-wheelers and so the GCM figure became very relevant. To add to the confusion, not all vehicle manufacturers specified a GCM and the next best thing was to add the Gross Vehicle Mass (GVM) to the maximum tow rating.
In the current times so far as tow vehicles are concerned, tow ratings and vehicle weights are mentioned far more than tow vehicle engine performance, which is still a factor of course and Gross Combined Mass, for one, has certainly risen to near the top of the list as an item of relevance.
If confused about the jargon used so far, here’s a list of definitions.
Gross Combined Mass (GCM)
Gross Combined Mass is specified by the vehicle manufacturer and is the maximum allowable mass of the tow vehicle and the caravan.
Aggregate Trailer Mass (ATM)
This refers to the maximum legal total loaded mass of a caravan. It includes the Tow Ball Mass (TBM). It should be found on a nameplate, usually in the front boot or tunnel storage of a caravan.
Tare Mass (Caravan)
Often referred to as just tare, it is the actual unladen weight of a caravan, with water tanks and gas cylinders empty. It should include all extra fittings like awnings and batteries that are attached to the van. For new vans, the tare should be for the van as it leaves the dealer
Tow Ball Mass (TBM)
Otherwise referred to as tow ball weight or just ball weight. It’s the weight imposed on the tow vehicles tow ball by the coupling. Tow vehicle manufacturers specify a maximum TBM, usually 10 per cent of the maximum tow rating. Some tow vehicle manufacturers have a sliding scale, which doesn’t work quite the way you might expect. As the loaded weight of a caravan increases, so the maximum tow ball mass decreases.
The maximum load a caravan can carry is worked out by subtracting the tare from the ATM. Water and LPG is something the detracts greatly from payload. Two 95L water tanks, a water heater and two 9kg gas cylinders will absorb the best part of 200kg.
Payload for the tow vehicle is calculated in a similar fashion by subtracting the Kerb Mass from the Gross Vehicle Mass (GVM)
Gross Vehicle Mass (GVM) for Vehicles
The GVM for vehicles is specified by the manufacturer and is the maximum legal loaded mass of the vehicle. Some aftermarket specialists do offer a GVM upgrade but the rules on this vary from state to state, so caveat emptor depending on where you live and what your car warranty says.
Tare Mass and Kerb Mass (Tow Vehicle)
Tare Mass is similar to a caravan, except that the vehicle is in running order (i.e. all coolants and oil reservoirs are full) and there’s 10 litres of fuel in the tank. No accessories are fitted the vehicle. Kerb Mass is similar to Tare Mass, except that the fuel tank is full.
Max Tow Rating
Specified by the tow vehicle manufacturer for trailer towing (not necessarily a fifth wheeler) and should not be exceeded. It’s for the actual weight of the trailer/caravan not the rated ATM. For instance, it is quite legal to have a tow vehicle with a rating of 3000kg and a caravan with a rated ATM of 3200kg as long as the actual loaded weight of the caravan is less than 3000kg.
Why GCM Matters
GCM has become increasingly relevant in calculating safe towing weights, particularly as a number of tow vehicle manufacturers specifications for maximum towing weights aren’t always quite what they seem to be. There are several ute manufacturers, like Isuzu, Nissan, Mazda, Ford, and Toyota, who all specify a maximum tow rating of 3500kg and a maximum Tow Ball Mass of 350kg, however those same manufacturers also specify a GCM of around 6000kg, and that causes a problem.
The problem is that if a caravan fully loaded weighs in at 3500kg, then subtracting that from the 6000kg GCM gives a figure of 2500kg for the maximum loaded weight of the tow vehicle.
Let’s say a tow vehicle has a Tare Mass 2100kg, that leaves just 400kg for any payload. Once you have added in fuel (say 90kg), a bullbar (say 50kg), the all essential tow hitch (say 40kg), two adults (say 80kg each), a toolbox (say 10kg), some luggage (say 20kg) and a loaded car fridge (say 20kg) — a total of 390kg — that leaves 10kg for everything else like driving lights for instance.
In that calculation, Tow Ball Mass hasn’t been forgotten, it’s either included in the caravan figure (which I have done in my example) or as part of the tow vehicle payload.
Although many manufacturers advertise a 3500kg tow rating, there are some differences between them. I compiled a table of prospective models from various manufacturers commonly used for tow vehicles and a couple that aren't so common. Apart from anything else, it showed some differences in the available payload. In some cases, like Mazda’s BT50 and the Mitsubishi’s Triton, opting for a 2WD version, rather than a 4WD gives a better payload.
Similar vehicles from the same manufacturer can show some differences too. For instance, the Ford Ranger XL has a better payload than the Ford Wildtrak X, because it has less ‘goodies’ fitted.
Being bigger isn’t always better. The Ram 1500 has a maximum towing mass of 4500kg, yet if towing a caravan of that weight, the Ram’s payload would be just 117kg. Ahem! Naturally if towing a van weighing 3500kg, then that would look considerably more realistic.
Just for something a bit different, I threw in an Iveco Daily 50C18 Dual Cab, the smallest being just under 6m long with the uprated 115kW, turbodiesel engine. Its payload is very impressive but doesn’t include any sort of basic aluminium tray. Even with that fitted, I reckon you’d have kilos to spare, and it would make for a very stable tow vehicle.
Something that’s a bit out of the scope of this article but still relevant is that of axle loadings. Tow vehicle manufacturer specify maximum front and rear axle loadings. Unless you are an engineer whose special subject is just towing weights, then it’s hard to calculate and can really only be done by individually weighing the wheels. For most travellers, it’s probably okay but for anyone with a towing rig that’s near or right on its limit’s then it might be worth checking out.
Is My Tow Rig Safe and Easy to Drive?
If you happen to own a ute with a 3500kg tow rating and a GCM of 6000kg and this is all a bit confusing, then to give something of a safety factor I’d reckon you should be considering a maximum laden weight of 3000kg. If that sounds conservative, then a couple of motoring journos I know of reckon it should be no more than 2500kg!
Apart from the legal requirements, the other factor to consider is the stress factor. Having recently done some towing tests using both a Toyota LandCruiser and a couple of utes, there’s no doubt that the heavier LandCruiser makes for a more relaxing drive. Caravan towing that’s well within its limits is much easier than a vehicle that’s right on its limits, especially in outback areas where the roads aren’t so good. That for me, if contemplating a lengthy caravan touring trip would certainly be as equally important as making sure my towing rig was all legally within its weight limits.