We’ve worked with David from Dwnunder Towing to give you an updated definition on some of these terms in this updated article.
Glossary
Please see explanations of key terms below for reference, which you may find on the compliance plate of your 4X4, camper trailer, caravan or other trailer. Note that these are interpretations.
GVM – Gross Vehicle Mass – The maximum amount of weight the vehicle can apply down to the ground through the tyres. It includes all cargo, the weight of passengers, fuels and oils, tyres and wheels and all components on the vehicle. When towing, the tow ball down weight is also counted as part of the GVM. No matter what, the vehicle must always weigh less than or equal to the GVM.
GCM – Gross Combined Mass – The total combined weight of the vehicle and everything being towed, including all cargo, and people. This is set by the manufacturer of the tow vehicle, and can’t be altered or increased. In modern vehicles, the GCM is generally less than the sum of the GVM and towing capacity, which means you can’t tow the maximum specified towing weight while the vehicle is loaded to GVM.
Towing Capacity – The maximum amount that the vehicle can tow, specified by the manufacturer. It factors the total weight being applied through the tow ball and tow hitch, and includes the tow ball down weight (TBM or TBW).
GTM – Gross Trailer Mass – The maximum legal weight a trailer is allowed to weigh fully loaded when hitched to the tow vehicle and in transit (the weight applied to all wheels of the caravan). Keep in mind this DOES NOT mean your caravan DOES weigh this amount, it just means it cannot exceed that weight. The GTM will always be lower than the ATM due to part of the caravan’s weight being applied to the tow vehicle. The maximum GTM ensures that the load is safe to be towed at manufactured specified max speeds and conditions.
ATM – Aggregate Trailer Mass – The combined maximum total weight of the trailer, including tow ball down weight. This is the maximum legal weight a is allowed to weigh fully loaded when not hitched to a tow vehicle (the weight applied to all wheels and jockey wheel). Keep in mind this DOES NOT mean your caravan DOES weigh this amount, it just means it cannot exceed that weight.
Tare Mass: Tare weight is your caravan’s starting weight at time of delivery, with all inclusions and extras you have ordered at build time. The tare weight will be different on even the same models of caravan depending on what optional extras have been added. For instance, if you and your best mate have brought the same caravan but you have added an outdoor kitchen but your mate has not, your van’s tare mass will weigh more than theirs. The tare weight is the one number that will never change and also bares no legal consequences.
Payload – This is a trailer’s Tare Mass subtracted from its ATM, indicating the amount of extras it can carry, including gear and supplies.
TBM or TBW – Tow Ball Mass or Weight – Also referred to as tow ball down weight this is the weight applied down onto the tow bar (and vehicle) by the trailer. The maximum tow ball weight of a tow vehicle is normally 10% of what it can legally tow e.g. a vehicle that can tow 3500 kg will often have a maximum TBM of 350 kg.
Individual axle loads – Set by the manufacturer of both the vehicle and trailer, and can’t be exceeded.
As a quick aside on axle loads and TBM, weight distribution is very important, as you can see from the diagram above courtesy of DwnUnder Towing.
For example, if you have a bike rack on the back of your caravan or behind the axles, it will take weight off the tow hitch, but if you add the bike rack to the front or in front of the axles, it will add weight to the tow hitch. Think of a trailer as a seesaw and the centre is an axle; whichever side of the axle you add weight to, the other side loses weight.
This is an interesting one and the slightest thing can change it drastically.
Anyway, on to…
The situation
John* (not his real name), owns a slightly older 4WD with a 2500 kg towing capacity. He is interested in purchasing a new caravan, but it has an ATM of 3000 kg. In reality, he doesn’t carry that much stuff, so loaded as he intends to use it, the actual ATM it comes in at is 2700 kg, with 250 kg of that on the tow ball.
Summary below:
Tow Vehicle:
GVM: 2800 kg
Max. tow weight: 2500 kg
Max. allowed tow ball down weight: 250 kg
GCM: 5300 kg
Caravan
ATM: 3000 kg ATM as loaded: 2700 kg
GTM: 2700 kg GTM as loaded: 2450 kg
Tow ball down weight as loaded: 250 kg
John’s questions:
- If I’m towing a Caravan with a stamped GTM and ATM that exceeds my vehicle’s capacity, but the actual GTM and ATM as I’ve loaded it are lower and stay within the vehicle’s towing capacity, within my GVM and within my GCM, can I tow that van legally?
- Given the tow ball down weight is counted as part of the GVM on the vehicle, does this mean that I’m only really towing the GTM, and therefore I can I tow the van legally because if I count the tow ball down weight in what I tow as well, I’m counting it twice?
The Answers
Before I go into this, I want to make it clear that I’m not an engineer, so while I’ll go into detail in my answers based on plenty of reading, the truth is that there is no clear answer provided by any authority on this matter. I’ll do my best to identify the challenges and issues, and allow you to interpret that. Please know that I did make several phone calls to industry experts for their opinion on this…
1. Legally towing a trailer that has a GTM and ATM much greater than your vehicle can tow
The reality is that the GTM and ATM figures stamped onto your van or trailer are a measure of the maximum that a trailer can weigh, but whether what you tow is legal depends on the actual loads, not the theoretical maximum. So long as the following conditions are met, you can legally tow a van or trailer with a GTM or ATM that exceeds your vehicle’s own towing capacity:
- Total trailer weight, including tow ball down weight, doesn’t exceed the towing capacity of the vehicle, as specified by the manufacturer.
- The actual ATM and GTM of the trailer are under the maximum amounts the trailer is allowed (a given in this case, but still important)
- The individual axle loads on trailer and vehicle are not exceeded
- The GVM of the vehicle (inclusive the tow ball down weight) doesn’t exceed the amount stamped on the compliance or mod plate.
- GCM doesn’t exceed the manufacturer set amount.
- Towball mass doesn’t exceed the maximum amount set by the tow vehicle manufacturer (usually 10% of the max towing capacity, but listed on the tow bar of most vehicles)
Example of how the above might work: If you were towing a car trailer that had an ATM of 4495KG, but a tare (empty) weight of 800KG, and it was towing a 1500kg vehicle, with a 250KG tow ball down weight, and your vehicle had a 2500KG towing capacity with 250KG max towball download, you are OK because the total weight being towed is 2300KG (under the 2500KG limit) and the tow ball down weight is within limits, as long as the 250KG tow ball down weight added to the vehicle keeps it under GVM, and you don’t exceed individual max axle loads. GCM is not an issue because you would be under that too in this case. The key here is keeping under all of the maximum limits allowed on both the vehicle and trailer at all times…
2. Given the tow ball down weight is counted as part of the GVM on the vehicle, does this mean that I’m only really towing the GTM, and therefore can I tow the van legally because if I count the towball downweight in what I tow, I’m counting it twice?
This is the hard question, and one no-one really wants to answer. The logic here makes sense – surely you don’t count the towball down weight twice?
Unfortunately, the towball downweight does need to factor into both weights because the vehicle wears the down weight by virtue of it ‘sitting’ on the vehicle, but the towbar and tow hitch also has this weight applied to it, in addition to the GTM.
To summarise, to legally tow, you must remain in compliance with all of the following:
Towing capacity – In this case the vehicle is over, because 2700KG of weight is being applied through the tow hitch where the capacity is listed as 2500KG.
GVM – In this case, he is under GVM
Gross Trailer Mass –In this case he is well under the 2700KG GTM – it is at 2450KG
Aggregate Trailer Mass – This is listed at 3000KG, and he is at 2700KG, so well under.
Axle loads – We’ll assume that the loads are OK – the Caravan max axle load would be higher than the maximum GTM which the Caravan is under, and we’ll assume he’s packed the vehicle to work within these limits given he is under GVM for the vehicel by 100KG in this case.
Gross Combined Mass – In this case, the vehicle weighs 2700KG, plus 2450KG for the GTM when attached, so the total is 5150, when the allowed GCM is 5300KG. He’s clear here.
In the above case, the vehicle combination is actually illegal and overweight. Not because of GVM, GCM, ATM, or GTM, but because the towing capacity has been exceeded. That is, that the total weight being put onto the towbar and hitch exceeds the manufacturer specifications. Remember, the towing capacity is 2500KG, although in reality 2700KG of weight is being applied through the towbar.
Now, there is nowhere that provides absolute clarity on this situation, but the failure point here is that the towbar has 2700KG applied to it, even though the down weight is calculated as sitting on the vehicle. If the vehicle had a towin capacity of 2700KG, then you’d be legal even with the 3000KG Caravan, as long as you kept the weight to that level.
Summary
If you are questioning whether your vehicle is legal from a weight point of view, you need to be on or under all of the following limits – remember – being over in one area will mean that technically you are not legal:
- GVM
- GCM
- Tow ball Down weight (max as specified by manufacturer of vehicle)
- Towing Capacity as stated by the manufacturer (and remember that most modern vehicles can’t be at GVM and tow at the full listed capacity)
- GTM
- ATM
- Individual axle loads
Hope you find this useful! If you are an expert in this area, or have a different point of view, please comment below – it isn’t an easy thing to understand!
Aiden
