Gross Combined Mass Upgrades
The Gross Combined Mass (GCM) limits the maximum laden weight of a tow vehicle and its trailer. Many owners seek Gross Combined Mass Upgrades. This article explains that this is rarely advisable, and why.
Prior to June 2018, kits were available that increased Gross Vehicle Mass (GVM). The GVM is the maximum a powered vehicle can weigh. These kits also increased the Gross Combined Mass (GCM). The GCM is the combined weight of a laden tow vehicle and trailer. Most of Australia’s caravans and tow vehicles had an original combined weight of 5400 kg to 6000 kg.
The kit suppliers main market for gross combined mass upgrades is people seeking to tow heavy caravans by vehicles that were lighter. These included the Ford Ranger, Toyota HiLux, Toyota Landcruiser, and the Isuzu D-Max and MU-X.
Pic: courtesy caravancampingsales.com
In June 2018 this situation ceased. New legislation precluded increasing the original GCM. The legislation, however, was not retrospective. Those vehicles with suspension modifications could and did continue to tow loads heavier than their makers intended.
This legislation relating to gross combined mass upgrades also confused caravan owners and dealers. Not all understand ‘towing capacity’. Most vehicles used for towing are owned by tradespeople and the military. They typically pull twin-axle trailers that rarely exceed 4 metres (about 14 ft). The typical tow vehicle claimed ‘towing capacity’ is 3500 kg. But that towing capacity relates, for example, to be able to stop and restart on a specified gradient. It does not imply 3500 kg is towable via an overhung hitch.
For most dual-cab utes towing a 3500 kg caravan, the GCM limits tow vehicle payload. Many such tow vehicles have a bull bar, winch, extra batteries etc. That, plus the caravan’s tow ball mass, driver and passenger/s results in exceeding the GCM.
If increasing GCM enables a tow vehicle’s GVM to be increased (only to carry a higher payload), it makes sense to do so. It results in a heavier tow vehicle relative to the weight of the caravan. Using a GCM increase, however, to enable towing even heavier caravans further prejudices marginal towing stability.
In 2019, the Southern Australian government did a back-flip. It announced that ‘GCM revision and towing upgrades can [also] be done on pre-registered vehicles.’
You can obtain gross combined mass upgrades for most SUVs. Depending on make and model, these add 300-500 kg.
It makes sense to do this. Carrying that extra mass results in a safer ratio of laden tow vehicle weight to laden caravan weight.
Most of Australia’s RV upgrade industry is being responsible regarding gross combined mass upgrades. It recognises that gross combined mass upgrades (if misused) prejudice suspension and handling. There are however a number of upgraded pre-2018 vehicles towing much heavier caravans. This is not a clever thing to do.
The maximum towable weight with such vehicles, while retaining a reasonable margin of safety (and tow vehicle payload) is about 3000 kg.
RV Books, and many working in this area of gross combined mass upgrades (as well as the Caravan Council of Australia) strongly recommend not to tow any laden trailer that is heavier than the laden tow vehicle. Gross combined mass upgrades can prejudice handling and reduces rigs safe maximum speed. Sadly, many do.