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Pic: The Wanderer's first trip to Scotland in 1885, showing what can happen when a 2 tonne caravan goes off road.

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Gross Combined Mass Upgrades (update)

by Collyn Rivers

PIc: courtesy caravancampingsales.com

PIc: courtesy caravancampingsales.com

In June 2018 legislation was passed that prevented second stage manufacturers (workshops modifying vehicles with engineered suspension and tow kits) from making changes that precluded increasing a vehicle’s original manufacturer-specified Gross Combination Mass (GCM).

The GCM is the legal maximum weight of the fully laden tow vehicle and the fully laden weight of the trailer. For most of the more popular tow vehicles the GCM is typically 5400 to 6000 kg. It is a rating set by the vehicle manufacturer.

Increasing the GCM via suspension modifications etc allows vehicles to legally tow a heavier load than originally stipulated by the manufacturer. 

Prior to June 2018, a few aftermarket specialist suppliers provided kits intended to increase both the vehicle’s Gross Vehicle Mass (the maximum a powered vehicle can weigh when not towing) and also the GCM. The main market was people seeking to tow heavy caravans by vehicles such as the Ford Ranger, Toyota HiLux, Toyota Landcruiser, and the Isuzu D-Max and MU-X.

RV Books has mixed feelings regarding this - not least that many owners misunderstand the industry term 'towing capacity'.

Many tow vehicle manufacturers claim a ‘towing capacity’ of 3500 kg. But that claim does not typically advise that the criteria primarily relates to that towable on the end of a rope. It specifies, for example, the ability to stop and restart on a specified gradient.

For most of the more popular dual-cab utes, if a 3500 kg caravan were to be towed the existing GCM drastically limits that tow vehicle’s payload – to  the extent that vehicle is virtually unusable.

 In practice, the maximum towable (whilst retaining usable payload) is about 3000 kg. Whilst currently legal to do so, many working in this area (as well as the Caravan Council of Australia) strongly recommend to not tow any laden trailer that is heavier than the laden tow vehicle. It prejudices handling and reduces the rig's safe maximum speed.

If the GCM were to be lifted to enable that tow vehicle’s GVM to be increased, and that increase used only to carry a higher payload, there seems every argument for doing so. If however a GCM increase were to be used to enable still lightly laden utes to tow even heavier caravans that will inevitably further prejudice already marginal towing stability.

The upgrade industry is seemingly being responsible regarding this. To RV Books knowledge, it recognises the issue (that RV books raises above): that increasing GCM (if misused) can prejudice suspension and handling.

There are good reasons for GVM upgrades and such upgrades are not affected by the regulations.


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