Trailer Definitions Explained

What do GTM, tow ball weight and other terms mean? 

 You're getting ready to tow your trailer, but your car specs tell you one measurement, the tow ball specs another. You ask yourself, what does it all mean? Do I need a degree in maths to calculate maximum loads? What is it really need to know from all this information? 

 We've heard a lot of these questions from our customers and to answer helpfully there’s a couple of important things to tell you. 

  •  In Australian states and territories there are towing regulations about maximum loads. If you go over, there may be a fine, refusal of an insurance claim, or other legal action.  
  • Focus on knowing the maximum loads for your vehicle, trailer, and tow ball. Get to know them and you'll be confidently towing whenever you need. 


The main terms that apply to loads and trailers we describe for you here. We also explain where you can find these details on your vehicle and trailer, and more. 


ATM Aggregate Trailer Mass 

The weight of your trailer fully loaded and unhitched from your vehicle. 


GCVM/GCM Gross Combined Vehicle Mass 

The weight of your trailer and vehicle fully loaded and hitched together.


The weight as at manufacture. The weight of your empty trailer unhitched from the vehicle and without any payload. 


GTM Gross Trailer Mass 

The weight of your trailer fully loaded and hitched up to your vehicle. 



All the extra load you put on your trailer. The payload is the difference between your ATM and the TARE. 


T.W./Tow Ball Weight 

The load placed on the vehicle’s tow ball. The difference between your ATM and GTM is your Tow Ball Weight 



Where do I find the weight specifications? 

Your trailer 

Longlife Trailers have the ATM listed on the ID Plate. The ID plate can be found on the drawbar

If you have another brand or older trainer and cannot find the ID plate the ATM should be listed on the registration papers. 

Your towing vehicle 

Check your owner’s manual for the vehicle’s towing specifications. Sometimes the specifications can change over the years in the same model or change from model to model.  If you have any doubt, we suggest you confirm with the vehicle’s manufacturer. 

 Your tow bar 

You will find the information on the tow bar’s specification plate.  

If your tow bar is older or unmarked your towbar manufacturer or installer may be able to assist. 

 How do I make the calculations? 

Our Longlife Trailers team can help you with this.  

To do this yourself here are some tips. 

T.W./Tow Ball Weight is generally 10% of the maximum tow rating. 

Example, if your vehicle lists a 3500kg tow rating then you will need a 350kg tow ball mass rating. 

 What happens if I tow over the tow ball mass rating? 

Your trailer load could become unbalanced, start swaying and unsafe to tow.  

There are also rules in each State and Territory about this. We recommend when you are buying a trailer to talk to our Longlife Trailers team who can let you know what you need. 

 How do I weigh my trailer? 

We recommend purchasing a ball weight gauge. These can be purchased from most auto parts retailers. 

You will need to uncouple the trailer and use a trolley jack. Lower the trailer onto the gauge to weigh 

You can also take it to a weigh bridge and locations of these can be found here Weighbridge Locations – Public Weighbridges Australia ( 


When you are researching or ready to buy your new trailer, please contact one of our Longlife Trailer team who will be able to help you.