To be honest most of the dealers I spoke to before buying the van we did could not explain how the payload was arrived at and I was told at one place that they allow 350 kg for a single axle and 500kg for a tandem. When I asked about chassis design capacity, axle weight, structural design calculations and the like the eyes of salespeople invariably went blank.

In terms of deciding what size tug you need, you have to know the truck's unladen weight (what you have called kerb weight) and using the Mazda BT50 as an example (because that's what I have), the 2013 model had a tare/kerb weight of 2.044 tonnes, Gross Vehicle Mass (GVM - the maximum permissible mass of the vehicle) of 3.2 tonnes, a ball weight of 350 kg, Combined Vehicle Mass (the total combined weight of the tug and the van - CMV) of 6 tonnes and a maximum towing capacity of 3.5 tonnes.

This information can be combined in various ways. Firstly, the tow ball weight is deducted from the weight of what is being towed and added to the weight of the ute. The maximum weight that the towing vehicle exert of the ute is 350 kg. The ball weight noted on the compliance plate of towed vehicles only applies to the tare weight of the towed vehicle. The way it is loaded can alter that.

You can load the ute up to 3.2 tonnes which generates a paylod of 1.156 (3.2 - 2.044) tonnes but you can't actually do that when towing a caravan because the ball weight is deducted from the payload of the ute. The payload includes everything in the ute including passengers, fuel, stuff in the tray, bull bar, tow bar etc. so it can decline pretty quickly when you start putting extra fuel, generator, camp chairs, portable fridge etc into the back of the ute.

By deducting the loaded weight (and all that that includes) of the ute (providing its equal or below the GVM) from the CVM gives you your effective towing weight. In the case of a fully laden BT50 this equals 6.0 - 3.2 = 2.8 tonne and it makes no difference what it's towing capacity is. As the loaded weight of the ute includes the ball weight, this is deducted from the the mass of the towed vehicle which may make things a little easier to work with.

To take advantage of the 3.5 tonne towing weight the ute can only weigh 6.0 - 3.5 = 2.5 which gives an effective payload for the ute of 300 kg.

It's a little more complicated that it first appears and I hope this has helped a little.

Dave

We're planning to use a light truck to give us the payload we need. An Isuzu or Iveco dual cab with a fabricated utility box on the rear.
I've worked out we will have nearly 3 tonne available after the van is hitched on so we can carry additional water etc.
These trucks are available on the second hand market by the score and are much cheaper than BT50's etc.
Being a purpose build commercial machine they should be able to handle rough conditions indefinitely.


Cheers

Pejay wrote:

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Hi all,
Thanks for your information re the payload of the different types of vehicles we are considering. Whilst we were speaking with the Mazda Rep re the BT50, I made the comment that we had met a chap who had all the necessary extras fitted and when he put his vehicle over the weigh bridge he was surprised that he went 3.5ton. The rep said to both June and I that you could load the vehicle up to 3.5 ton and STILL Tow 3.5 ton. Of cause your fuel consumption would be high I would think. But I was surprised that you could according to the rep, carry and tow that amount. I wonder was this just 'Sales Talk'. Before we do take the plunge and purchase a new tow vehicle, we will need to do a lot more research.

Pete

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Sales talk Pete  you can't exceed the GVM  or the GCM under ANY circumstances (legally speaking).

Dave explained it well. 

ARB , Lovells , Iron Man there could be others do GVM upgrades for some 4x4's , it depends on the rating of the rear axle how much they can go

and like Baz said its best if done before the first rego . A Toyota HiLux for example can be upgraded from 2780Kg to 3300Kg some of the other Utes can only be upgraded about 130Kg .

But the GCM will remain unchanged , so the max towing weight will be reduced by that amount , might not be of much use if your Van/trailer is near the max .

I know my 4x4 is at least 380Kg over GVM with no trailer and believe me I'm a light weight compared to most others I see