RV Weight Terminologies Explained

RV users (especially the newbies) usually get overwhelmed by or confused at the RV weight terminologies and abbreviations associated with vehicle weights such as GVM, GCM, ATM,  axle load, etc. Why? Because there are a lot of them! Moreover, some of these are critical terms you need to know to keep your RV safe for use. That is why through this post, I’ll help you understand these terms in an easier manner.

But wait… why should you know these by the way?

The relevance of RV weight terminologies

First is that some important terms – GVM and ATM, for example – define the exact Australian standards you must comply with, how much stuff you can put in your RV, and how much you should tow.

And second, which is an effect of the first, you can avoid putting your vehicle at risk (by overloading, for instance).

So in this blog, I have presented (at least in an easier way) the terminologies related to weight ratings of RV and caravans only. Therefore, I did not include weight terminologies (and definitions) specific to other vehicles. Consequently, the term “trailer/s” here applies to caravans as well.

Aggregate Trailer Mass (ATM)

On the condition that the trailer is NOT attached, the ATM is the sum of the following:

• mass of the unoccupied trailer,
• maximum load recommended by the manufacturer, and
• any mass that may be imposed onto the towing vehicle upon attaching the trailer

As much as GVM is important for motor vehicles, so is ATM to trailers.

Axle Group

This term is relevant to determine different axle loads. Axles are grouped in the following manner:

Axle Load

An axle looks like this:

Hence, an axle load is the total load (or weight, if that helps you understand better) carried by the road as a result of the weights transferred by the tyres attached to that axle.

Also, the centres of the wheels of that axle must be less than 1 metre apart to be classed as a single axle.

Carrying Capacity

You can establish your RV’s carrying capacity by the sum of the following:

• (68kg mass + 15kg luggage) per occupant x maximum number of seating positions
• mass of water, fuel and waste tanks when full
• allowance for amenities (e.g. cooking utensils, bedding, etc.) – specifically,
  • 60kg each for the first two sleeping berths, and
  • 20kg for each additional sleeping berth

Gross Axle Load Rating (GALR)

The maximum allowable load per axle. The manufacturer defines this value.

Gross Combination Mass (GCM)

“Combination” means the setup consisting of a towing vehicle and the trailer it tows. With that in mind, you can better understand the GCM as the sum of the following:

• The GVM of the towing vehicle, and
• The sum of the axle loads of the trailer to be towed.

The manufacturer specifies the GCM.

Gross Trailer Mass (GTM)

Hitch first the trailer before calculating for the GTM in accordance to this equation:

GTM = A + B

where:

A – mass of the unladen trailer

B – the maximum trailer load recommended by the manufacturer

Gross Vehicle Mass (GVM)

The maximum allowable mass of the vehicle when it is laden. The manufacturer specifies this value.

This value is extremely important as it is the main deciding factor whether a certain rule is applicable to your vehicle.

Group Gross Axle Load Rating (GGALR)

It is the lowest limit set by ADR 38/xx Table 1, or by the GTM.

But here’s Table 1 of ADR 38/05 (the latest version of ADR 38 so far):

Laden Mass

It’s simply the mass of the vehicle and its current load – be it passengers, luggage or others.

Lightly Loaded Test Mass

The mass of an unladen vehicle equipped with the following:

• lubricating oil and coolant – full capacity
• fuel – at least 75% of the full capacity
• NO goods, occupants
• NO optional items except those needed for testing
• additional loads to be distributed in the seat next to the driver’s seat in order to satisfy this equation*:

Additional loads + Mass of driver + Mass of test instruments = 155 ± 30 kg

* In the case of a vehicle in the form of a “cab and chassis” applying for compliance to ADR 35/xx, place an additional load of at most 7.5% of the GVM, such that its centre of gravity is within 200mm of the load centre designated by the manufacturer.

Loaded Test Mass (LTM)

LTM is the mass of a laden trailer that is loaded in a manner that each of its axle groups is at its specified GGALRs.

Luggage Mass

Its value is at least 13.5kg per seating position unless otherwise recommended by the vehicle manufacturer.

Maximum Loaded Test Mass (MLTM)

It is simply the value designated by the vehicle manufacturer as the “loaded test mass” of the vehicle. However, this value must at least be equal to the sum of the masses of the following:

• unladen vehicle
• the heaviest factory-installed optional items (at least 2.3kg each)
• lubricating oil and coolant at their full capacities
• fuel filled to at least 75% capacity
• 68 kg times the number of unoccupied seating positions

In the case of trailers complying with ADR 38/xx, the MLTM is the mass of a trailer loaded to its GTM while its axle loads being within the manufacturer’s specified individual GALRs.

In the case of vehicles complying with ADR 35/xx, the MLTM is the mass of a laden vehicle that’s loaded to its GVM without exceeding the manufacturer’s specified individual GALRs.

Maximum Loaded Vehicle Mass

In the case of RVs and caravans, it is the same as the GVM or GTM

Normal Loaded Vehicle Mass

In the case of RVs and caravans, it is equal to:

4(A+B+C)-D / 3

where:

• A = Unladen Mass
• B = the sum of the masses of the heaviest regular optional items (at least 2.3kg each),
• C = 68 kg (multiply this by two if the vehicle has >1 front seating positions)
• D = Maximum Loaded Vehicle Mass (must be evenly distributed over the loading area, but if the test vehicle is a partially completed one, then the distribution must be over the rear axle or axle group instead)

Payload / Load Capacity

The payload is equivalent to the GVM minus the Tare Mass. In simple words, it is your “allowance” on how much (in terms of weight) passenger and stuff you can put in your vehicle.

The three main weights that matter the most in a vehicle are:

• Its empty weight
• The weight of its standard equipment/facilities inside, and
• The maximum weight that it is allowed to further carry (i.e. for passengers, luggage and extra equipment) – this is the payload

It is critical to ensure proper distribution of payload over all the axles of the vehicle. Under or overloading axles loads can result in poor vehicle preformance and handling characteristics, and is some case cause accidients.

 

Discussing payload distribution is important because it affects how seats, equipment and facilities must be located within your vehicle. So it’s a critical factor to consider both when modifying or building your RV and when loading it up for your trips, so be cautious.

Prescribed Transition Mass

The maximum mass by which an axle group can exert on the ground such that it causes a retractable axle to automatically go fully down.

Rated Towing Capacity

The rated towing capacity is the value that’s lesser of these two:

• The rating of the towing equipment fitted to the towing vehicle (this is determined usually by the manufacturer of that towing equipment, if not the vehicle manufacturer)
• The difference between the GCM and GVM.

In simpler words, it is the maximum weight which the towing vehicle must tow. Although data on rated towing capacities are readily given by the manufacturer, the overall towing capacity of a combination may change.

How?

When you hitch a trailer to a tow vehicle, you are actually joining 3 components – the tow vehicle, the trailer to be towed, and the tow coupling. Each of these components has its own rated tow capacities. To determine the overall tow capacity, just get the least of those individual rated tow capacities.

Reference Mass

It is the sum of the masses of the following:

• the vehicle in its operational status, equipped with all standard equipment
• fuel filled to its nominal tank capacity
• optional items – these must be the common ones (i.e. expected to be fitted to >33% of the vehicle’s engine family)
• 136 kg

In fact, the above is only one way to determine your reference mass. Another option is to use the manufacturer’s estimated mass.

Tare Mass

It is the mass of the vehicle when unladen and ready to hit the road. To be more specific, the vehicle must be equipped with all:

• standard equipment/appliances, and
• fluid containers filled to their nominal capacities, except for fuel which must be filled to 10L only

Total Combination Mass (TCM)

TCM is the mass of the towing vehicle plus that of the (laden/unladen) trailer.

Total Trailer Axle Load (TTAL)

TTAL is the sum of the forces exerted by each axle attached to the trailer, provided that the trailer is statically on a horizontal plane.

Trailer Gross Axle Load Rating (TGALR)

The sum of the GALRs of all a trailer’s axles.

Unladen Mass or Kerb Mass

The mass of the vehicle in running order unoccupied and unladen with all fluid reservoirs filled to nominal capacity including fuel, and with all standard equipment.

Unladen Trailer Mass (UTM)

The UTM is the mass of the trailer when under the following conditions:

• hitched,
• normally unladen, and
• at a condition where it establishes (or can establish) compliance with all applicable ADRs

Vehicle Maximum Load on the Tyre

Conduct the following steps in order to determine the vehicle maximum load on a specific tyre:

1. Distribute the GVM or GTM to all tyres of the vehicle.
2. Get that individual tyre’s share of the weight.
3. On the axle or axle group to which the individual tyre belongs, count how many other tyres there are.
4. Divide the value from step 2 by that from step 3. By doing this, you are distributing the individual tyre’s share of weight to the other tyres of the same axle or axle group. The result is now your “vehicle maximum load on the tyre”.

Vehicle Normal Load on the Tyre

You shall calculate this by performing the same procedure as that for the Vehicle Maximum Load on the Tyre. Although for step 1 you should distribute the Normal Loaded Vehicle Mass instead.

Closing Thoughts

Indeed, there are a handful of weight terms that must be known especially when applying for compliance to the ADRs. Nevertheless, RV owners and users can opt to master instead just the most essential terms such as GVM, GTM, GCM, laden/unladen mass, load capacity and tow-related terms.

So there, I hope I was able to serve my purpose of helping you grasp a better understanding of the term/s you wanted to know more about. But let me grab this opportunity as well to let you know that I do offer expert services (as part of CVC) on vehicle modifications and compliance certifications. So in case, you would like to verify if the change you’ve done to your RV/caravan does not affect its current compliance, then give me a call or leave a quick message.