Confused ratings and masses confound buyers and caravan industry

July 28, 2016
Confused ratings and masses confound buyers and caravan industry

Ratings and Masses and their definitions and terminology remain the number one contentious and confusing issue for caravan buyers and the caravanning industry itself. But to amend and suitably clarify the various items involved in the Vehicle Safety Standards seems to be too difficult to allow revision of all of the legislation involved.

Many caravans cannot be loaded up to the Aggregate Trailer Mass (ATM) Rating figure without the Gross Trailer Mass (GTM) rating figure, and/or the Ball-Load Rating figure, being exceeded.

But the first practical safety aids to prevent the caravan tail wagging the tow vehicle dog are obvious and they can be applied immediately by the caravan industry and caravan buyers.

  • The tow-vehicle must be heavy enough;

  • the caravan must be professionally-designed;

  • the caravan must be correctly loaded;

  • and the driver must be competent.

The independent Caravan Council of Australia says a prime cause of trouble is the (wrong and dangerous) belief that:

Ball-Loading = ATM Rating - GTM Rating

Engineers agree you simply cannot subtract one Rating from another Rating and expect to come up with an Actual Mass.

If you fitted a Kenworth running-gear assembly to a caravan to (hideously) increase the GTM Rating, you would not change the Ball-Loading one iota!

On a weigh-bridge, you do not weigh the “ATM Rating” you weigh the "All-up Mass" of the caravan; this reading must not exceed the ATM Rating.

On a weigh-bridge, you do not weigh the "GTM Rating", you weigh the "Axle-Loading" of the caravan; this reading must not exceed the GTM Rating.

Some tow vehicles state their "GCM (Gross Combination Mass) Rating", while others state their "Maximum Permitted Tow Rating".

The combined "Actual Mass" of the loaded tow-vehicle and the loaded caravan, at any time, must not exceed the GCM Rating.

Sometimes, the "GCM Rating" is less than the sum of the tow-vehicle's "GVM Rating" plus the caravan's "ATM Rating".

The caravan Ball-Loading must be included in the actual mass of the tow-vehicle, but it is not included twice in the combined "Actual Mass" of the loaded tow-vehicle and the loaded caravan.

To express how Gross Combined Mass (GCM) is applied between the tow vehicle and the caravan it is towing care should be taken to not just state numbers and figures without clarifying that these numbers will depend on how the tow vehicle and caravan are loaded and with how much load is applied.

Industry Development Manager of Caravan Industry Association Victoria Daniel Sahlberg says most confusion comes from the application of ATM and GTM ratings.

ATM and GTM are ratings, not the actual weight of the caravan at any time. Also, the ball weight will vary depending on the loading of the caravan.

The biggest thing to be careful of is that the GCM will depend on where the weight is stored and how much, e.g. how much is in the car and how much is in the caravan.

The easiest thing to do is use the definitions as per the Australian Design Rules (ADRs) for Gross Combination Mass:

GROSS COMBINATION MASS: value specified for the vehicle by the Manufacturer as being the maximum of the sum of the Gross Vehicle Mass of the towing vehicle plus the sum of the Axle Loads of any vehicle capable of being towed as a trailer.

GROSS VEHICLE MASS (GVM): the maximum laden mass of a motor vehicle as specified by the Manufacturer.

AXLE LOAD: total load transmitted to the road by all the tyres of all the wheels whose centres may be included between two transverse parallel vertical planes less than one metre apart.

Editor's Note: A byline on a GoSee story indicates opinion.

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