RV Weight Definitions

Visit a weighbridge often

Visit a weighbridge often

Introduction

The following weight definitions are the vital ones you need to know to tow safely and legally in Australia.

The wording of each is that of RV Books. They have  been simplified from the (often technical) original so that all readers can focus on safety rather than trying to workout the meaning of technical terms or complex (and often incorrect) formulae used by some in the RV industry.

Under Australian state and territory towing legislation, weight limits set down by tow vehicle, trailer or towing equipment manufacturers MUST NOT be exceeded.

Tow vehicles and trailers have additional weight limits such as tyre and axle loadings, but these are generally taken into consideration by the manufacturer when setting a tow vehicle’s primary weight limits.

For towing,you need to know manufacturer-stipulated maximum weight limits relating to your own tow vehicle and trailer. These can normally be found on compliance plates or instruction manuals. If in doubt consult the relevant manufacturer.

Do not confuse maximum weights with actual weights. Maximum weights are allowances set by manufacturers. You need to check if you are under or over a maximum weight allowance - usually on a weighbridge.

Find your nearest weighbridge here and pay regular visits.

There are many reasons why you should always tow well within maximum weight limits. Find out more by reading our technical articles on towing dynamics.


Tow Vehicle Weight Definitions

1. Gross Vehicle Mass ('GVM')

Definition

This the maximum allowable laden weight of a tow vehicle.

Comments

The GVM (Gross Vehicle Mass) applies to the tow vehicle (only). It is set by the tow vehicle manufacturer. GVM is a maximum weight rating. You can  check if under it on a weighbridge. It includes the weight of all tow vehicle passengers, accessories, luggage, fuel and the weight of the coupling load of a trailer (if attached). Exceeding the GVM is illegal.

How to Check

Drive a fully laden tow vehicle and coupled trailer onto a weighbridge but weigh the tow vehicle only.

2. Gross Combination Mass ('GCM')

Definition

The maximum laden weight of a tow vehicle and trailer combination.

Comments

Applies to a tow vehicle and trailer combination and is set by the tow vehicle manufacturer. It is a maximum weight rating and cannot itself be weighed – check if under GCM on a weighbridge. It includes the weight of all passengers, accessories, luggage and fuel of the tow vehicle plus the weight (including payload, which includes gas and water) of the trailer when connected to the tow vehicle. Exceeding GCM is illegal.

How to Check

Drive a fully laden tow vehicle and coupled trailer onto a weighbridge and weigh both at once.

3. Maximum Towing Capacity

Definition

The maximum laden trailer weight a tow vehicle may tow.

Comments

Applies to a tow vehicle (only) and and is set by the tow vehicle manufacturer. It is a maximum weight rating and cannot itself be weighed – check if below maximum towing capacity on a weighbridge. It includes the weight of all accessories, luggage, gas and water in the trailer. It is usually broken down into maximum towing capacities for both braked and unbraked trailers. Exceeding maximum towing capacity is illegal.

How to Check

Tow a fully laden trailer onto a weighbridge and (whilst coupled) weigh the trailer only.

4. Maximum Tow Bar Mass

Definition

The maximum weight a tow bar attached to the tow vehicle may tow. The maximum downforce on the tow bar may also be stipulated.

Comments

Applies to a tow vehicle (only) and and is set by the tow bar manufacturer. The tow bar manufacturer may or may not be the tow vehicle manufacturer. It is a maximum weight rating and cannot itself be weighed – check if below maximum tow bar mass on a weighbridge. Exceeding maximum tow bar mass is illegal.

How to Check

Tow a fully laden trailer onto a weighbridge and (whilst coupled) weigh the trailer only.

5. Maximum Tow Ball Mass

Definition

The maximum downforce that may be exerted by a trailer on a tow ball (or other form of coupling).

Comments

Applies to a tow vehicle (only) and and is set by the manufacturer of the tow ball (or other form of coupling). The tow ball manufacturer may or may not be the tow vehicle manufacturer. It is a maximum weight rating and cannot itself be weighed. Exceeding maximum tow ball mass is illegal.

Note: a recommended tow ball mass will be stipulated by the trailer manufacturer. Find out what this is from your trailer manufacturer.

How to Check

Place tow ball scales directly under the coupling of a fully loaded trailer.

Trailer Weight Definitions

1. Aggregate Trailer Mass ('ATM')

Definition

The maximum laden weight of a trailer when not coupled to a tow vehicle.

Comments

Applies to the trailer (only) and is set by the trailer manufacturer. It is a maximum weight rating and cannot itself be weighed – check if under ATM on a weighbridge. It includes the weight of all trailer accessories, luggage, water, gas and jockey wheel downforce. Exceeding ATM is illegal.

How to Check

Tow a fully laden trailer onto a weighbridge, lower the jockey wheel, disconnect the trailer and weigh the trailer only (including weight on jockey wheel).

2. Gross Trailer Mass ('GTM')

Definition

The maximum laden weight of a trailer when coupled to a tow vehicle.

Comments

Applies to the trailer (only) and is set by the trailer manufacturer. It is a maximum weight rating and cannot itself be weighed – check if under GTM on a weighbridge. It includes the weight of all trailer accessories, luggage, water and gas but not jockey wheel downforce. Exceeding GTM is illegal.

How to Check

Tow a fully laden trailer onto a weighbridge and, whilst still coupled to the tow vehicle, weigh the trailer.

3. Maximum Coupling Load

Definition

The maximum weight a trailer’s hitch mechanism can tow. As well as a horizontal maximum force, a vertical maximum downforce may also be stipulated.

Comments

Applies to the trailer (only) and is set by the trailer hitch manufacturer. It is a maximum weight rating and cannot itself be weighed – check if under maximum coupling load on a weighbridge. It includes the weight of all trailer accessories, luggage, water and gas. Exceeding maximum coupling load is illegal.

How to Check

Tow a fully laden trailer onto a weighbridge and, whilst still coupled to the tow vehicle, weigh the trailer.

4. Tare Mass

Definition

The total mass of a trailer when not carrying any load but when ‘ready for service’.

Comments

Applies to the trailer (only) and is weighed by the trailer manufacturer prior to sale. It is the actual weight of the trailer excluding Payload and can only be measured accurately on a weighbridge. For trailers, Tare Mass excludes water, gas and any non-manufacturer-fitted accessories, so Payload allowance will be reduced by the weight of these items.

How to Check

Weigh the uncoupled trailer with jockey wheel down and no Payload on a weighbridge.

5. Maximum Payload

Definition

The maximum allowable weight of a trailer’s contents.

Comments

Applies to the trailer (only) and is calculated by subtracting a trailer’s Tare Mass from its ATM. Payload is the term used for a trailer's contents. Maximum Payload is a formula-derived maximum weight allowance for these contents. It includes all luggage and personal items stored in or on a trailer as well as the  weight of water, gas and any non-manufacturer-fitted accessories.

How to Check

To calculate you Maximum Payload allowance, subtract actual Tare Mass (as weighed above, not as per the trailer's compliance plate) from ATM. To check your actual Payload weight, check on a weighbridge as per ATM above - the weight of your Payload is actual laden weight of the trailer less actual Tare Mass.

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