GROSS MISTAKES


How well do you know your van and vehicle weights?

WORDS BY JOSEPH VAN WOERKOM PHOTOGRAPHY BY CARAVAN & MOTORHOME

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If you believe the marketing hype, there are a lot of vehicles around now that have the capacity to tow trailers of up to 3.5 tonne (3500kg) weight. But what does this really mean and can the average person really take advantage of this headline towing capacity?

The reason I ask this question is that, in my experience, most people expect to be able to tow up to the rated limit of their tow vehicle, known as the Maximum Braked Towing Capacity or MBTC and, at the same time, have the vehicle loaded to its maximum capacity, known as the Gross Vehicle Mass or GVM. I know I did when I first got into this caravanning caper.

Unfortunately, determining a vehicle’s ‘real’ towing capacity is often not a simple matter and involves close inspection of the vehicle’s weight ratings. One of the main weight ratings that catches people out is the Gross Combination Mass (GCM) which is the maximum allowable mass (weight) of the tow vehicle and trailer combined.

Now most people expect the GCM would simply be the sum of the GVM and the MBTC but often it is far less than this. For example, one very popular turbo diesel dual cab ute on sale here in Aus has a GVM of 3200kg and a market leading MBTC of 3500kg. However, its GCM is only 6000kg, which is a huge 700kg less than you probably expect!

This means that to stay under the GCM limit, the loaded mass of this vehicle must not exceed 2500kg when towing a 3500kg trailer. Given that the kerb mass is just shy of 2100kg, this translates to a payload of just over 400kg – not counting the trailer’s ball weight. Compare this to the advertised payload of just over 1100kg and you can see that you can’t load this vehicle anywhere near its maximum if you want to tow to its maximum capacity.

121003 Mazda BT-50 Tow Test-MazdaBT50Tow-50

Another way of looking at this issue is to ask the question “what is the maximum weight of trailer that this vehicle can tow when the vehicle is fully loaded?” To me, the answer to this question gives the ‘real’ towing capacity of the vehicle.

Crunching the numbers we again find that GCM limits the maximum axle loading of the trailer to GCMGVM or 6000-3200 = 2800kg. If we assume a ‘standard’ 10% ball weight, this translates to a maximum aggregate trailer mass of just over 3000g. This is a long way short of the advertised 3500kg.

Continuing the calculations we determine the ball weight to be 310kg and the payload that can be put into the vehicle as 1100-310 = 790kg, almost double the 400kg available when towing a 3500kg trailer.

The message I’m trying to convey here is that you need to do your homework to ensure that your tow vehicle and caravan combination stays within all the legal weight limits.