by Collyn Rivers

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 (GCM 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 travel trailers and tow vehicles had an original combined weight of 5400 kg (11,900 lb) to 6000 kg (13,000 lb).

The kit suppliers main market for gross combined mass upgrades is people seeking to tow heavy travel trailers by vehicles that were lighter. These included the Ford Ranger, Toyota HiLux, Toyota Landcruiser, and the Isuzu D-Max and MU-X.

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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 travel trailer 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 (7700 lb). But that towing capacity relates, for example, to be able to stop and restart on a specified gradient. It does not imply 3500 kg (7715 lb) is towable via an overhung hitch.

For most dual-cab utes towing a 3500 kg (7715 lb) travel trailer, the GCM limits tow vehicle payload. Many such tow vehicles have a bull bar, winch, extra batteries etc. That, plus the travel trailer’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 travel trailer. Using a GCM increase, however, to enable towing even heavier travel trailers 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 (660-1100 lb).

It makes sense to do this. Carrying that extra mass results in a safer ratio of laden tow vehicle weight to laden travel trailer weight.

Summary

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 travel trailers. 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 (6600 lb).

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.

Collyn Rivers is a semi-retired automobile research engineer.  He is the author of seven books, five of which are about making RVs and solar work optimally.

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Any bookshop, whether online or bricks and mortar, can order copies of Why Caravans Roll Over.  Just ask.
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The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Solar That Really Works!  Just ask.
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eBook versions

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The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of The Caravan & Motorhome Book. Just ask.
ISBN: 978-0-6483190-5-4.

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Caravan and Motorhome Electrics

The essential handbook for anyone with an RV of any kind.  Everything you need to know to make the electrics in your rig work for you.

Cover of Caravan and Motorhome Books by Collyn Rivers

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Caravan & Motorhome Electrics.  Just ask.
ISBN: 978-0-6483190-8-5

Solar Success

Whether you’re building a new home or adding solar to an existing structure, this is the one book you can’t afford to be without.

eBook versions

Paperback version

Any bookshop, whether online or bricks and mortar, can order copies of Solar Success.  Just ask.
ISBN: 978-0-6487945-4-7

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The Caravan & Motorhome Book covers every conceivable aspect of campervan and motorhome usage. If you own a camper van or motor home, you'll want this book.

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of The Caravan & Motorhome Book. Just ask.
ISBN: 978-0-6483190-5-4.

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Solar that Really Works! covers the design, installation and use of solar in cabins, camper trailers, travel trailers, fifth wheelers, campervans and motor homes.

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Solar That Really Works!  Just ask.
ISBN: 978-0-6483190-3-0

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and how to prevent it

Explains in detail every aspect of tow vehicle and travel trailer stability. This is what you need to know to be safe on or off the road.

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Why Caravans Roll Over.  Just ask.
ISBN: 978-0-6483190-6-1