Towing Terms and Acronyms Explained

Do you know your GVM from your ATM? There are a lot of towing terms and acronyms that are tricky to get your head around, especially if you’re starting out on your towing journey! It’s important to stay on top of them, particularly from a towing safety and legal perspective. It’s not just about speaking the language – they help you understand and stay compliant with the rules and regulations that help us all tow safely. 

We’ve put together this Speedy Towbars guide to some important concepts, acronyms and terminology in the towing world that might have you confused. If you still have questions, our expert team will be glad to answer them for you! 

Let’s start with GVM. 

What is Gross Vehicle Mass (GVM)?

Gross Vehicle Mass (GVM) refers to the maximum weight a vehicle can legally carry. This total weight is specified by the manufacturer and includes:

• The vehicle’s kerb weight.
• All accessories and modifications.
• Fuel and fluids.
• Passengers.
• Cargo and luggage.
• The ball weight of anything being towed

The last point is crucial when towing – the ball weight of your trailer or caravan must be included in your GVM calculation. Let’s look at an example: if your vehicle has a GVM of 2900kg and currently weighs 2750 kg with passengers and cargo, the maximum ball weight you can legally accommodate is 150kg. If you go over that, you exceed your GVM.

What happens if you overload your vehicle and trailer and exceed your GVM? Well, it’s illegal for a start, and it can also create dangerous driving conditions that endanger you and the other drivers on the road. 

If you’re confused by what ball weight is, we’re about to clarify that for you! 

What is Ball Weight?

Ball weight (also referred to as tow ball weight or tow ball mass) is the weight pushing down on the tow ball from directly above by the trailer or accessory (like a bike rack). On average, the ball weight is about 10% of the total weight of the trailer being towed. 

This is an important measurement that impacts both towing stability and vehicle handling. Here are some key points about ball weight:

• It’s the weight sitting directly on top of the tow ball.
• This is not affected by any modifications or accessories on the towing vehicle. 
• It is affected by what is in the trailer, and how it is loaded. 
• Must be accounted for in GVM calculations.
• Different vehicles and models have different ball weights, e.g. a Mitsubishi Triton may have a maximum ball weight capacity of 310kg. However some models will have a lower rated ballweight from the manufacturer, You must always use the lower value.

If you’re using accessories like bike racks or motorcycle carriers, the combined weight of the accessory and its load must be under your vehicle’s ball weight. So, a bike rack weighing 20kg with three bikes totalling 58kg creates a ball weight of 78kg. This is well within most vehicles’ capacities. However, a motorcycle carrier of 37kg carrying a 209kg sport bike would create a 246 kg ball weight. This is significantly heavier, and could potentially exceed the ball weight of many vehicles. 

What is Gross Trailer Mass (GTM)?

Gross Trailer Mass (GTM) is the total weight of a trailer when it’s loaded, resting on its wheels and hitched to the vehicle. It includes the weight of the trailer plus its load, but importantly doesn’t include the ball weight described above. 

GTM is calculated as:

GTM = Total Trailer Weight – Ball Weight

Trailers and caravans will all have a GTM recommended by the manufacturer. It’s important that when you’ve loaded your trailer, it doesn’t exceed this GTm capacity. 

What is Aggregate Trailer Mass (ATM)?

Aggregate Trailer Mass (ATM) is the total weight of a fully loaded trailer when it is unhitched from the vehicle. It’s basically the max weight your trailer can be when loaded with all your gear and equipment.

The simple formula to work out ATM is: 

ATM = GTM + Ball Weight

There should be a compliance plate on your trailer stipulating the maximum ATM your trailer is rated for. Don’t exceed this when loading it up and towing! 

What is Gross Combined Mass (GCM)?

Gross Combined Mass (GCM) is also referred to as Gross Combined Vehicle Mass (GCVM). It refers to the combined weight of the fully loaded trailer and the fully loaded vehicle towing it. 

Your towing vehicle will be rated to a specific GCM. This is the total permissible weight of your entire setup, and it’s important you don’t exceed it. 

GCM is where a lot of towing setups run into issues as it is not commonly discussed when it comes to choosing your towing vehicle and trailer. Understanding the GCM of your vehicle is arguably one of the most important things you can do. Exceeding this weight poses significant safety risks. It can also render your insurance null and void, while having further legal ramifications should something go wrong. Please take the time to understand what GCM is and what the GCM of your setup is.

To put it clearly, your GCM includes:

• The total weight of your loaded vehicle
• The total loaded weight of your loaded trailer
• The ball weight (which is counted once as part of the vehicle’s weight)

Let’s break this down into a practical example of a vehicle towing a caravan: 

• Your vehicle has a GCM of 5885kg. This is the most your vehicle and caravan setup can weigh. 
• Your vehicle alone weighs 2500kg.
• This leaves 3385kg of capacity for your trailer. 
• The ball weight must be added as a separate weight. In this instance, it’s 200kg. This leaves 3185 kg of capacity for your trailer. 
• Add a family of two adults and two children (say 200kg of weight) and max capacity is now 2985.
• Add 100kg of cargo in the vehicle and you’re down to 2885kg available towing capacity. If your vehicle is rated to tow 3100kg, you legally won’t be able to tow up to that amount. 

It’s important to understand towing capacities and how they are affected by your load. If you’re confused, please check in with our Speedy Team and they’ll gladly clear it up for you! 

What is the TARE Weight of a Trailer?

TARE weight is the weight of your trailer when it is completely empty. You should be able to find the TARE weight on your compliance plate next to the VIN number. 

The TARE weight is a baseline figure from which you can calculate how much payload your trailer can safely carry. What’s payload you ask? Let’s take a look!

What Does Payload Mean?

The payload is the total load weight that’s being carried on the trailer. It includes all your cargo, extra fuel, any trailer accessories and so on.

For trailers, payload represents the maximum additional weight you can add to an empty trailer. It’s simple to calculate:

Payload = ATM – Tare Weight

Get Towing Today

If you’re joining the adventure that is towing, having an understanding of the above terms will give you clarity around the rules and regulations that are in place to keep you safe. We all have an obligation to tow responsibly, and understanding your vehicle and trailer capacities and your load dimensions is a huge step to achieving that. 

At Speedy Towbars, we’re here to install the best quality towbars available on your vehicle. Equally important to us is that you tow safely and look after yourself and others on the roads. So if you ever need support relating to the above terms or towing safety in general, we’re here for you. 

Contact us today for all your towbar installation bookings and safe towing questions!