GVM (Gross Vehicle Mass)
What it is: The Gross Vehicle Mass is the absolute maximum legal operating weight of your tow vehicle, as specified by its manufacturer. Think of it as the "all-up" weight of the vehicle when it's fully loaded and parked on the road. This includes the tow ball weight if the van/trailer is hitched
Why it's important: Exceeding your GVM is a massive safety risk and a legal issue. It overloads your vehicle's chassis, suspension, brakes, and tyres, which severely compromises handling, increases braking distances, and can lead to mechanical failure. In the event of an accident, if you are found to be over your GVM, your insurance claim will almost certainly be denied.
Where it's found/calculated: The GVM limit is stamped on the vehicle's compliance plate (often found in the engine bay or the driver's door jamb) and is listed in the owner's manual. Your *actual* GVM is your vehicle's kerb weight *plus* all payload. This payload includes: all passengers, extra fuel, all luggage, any accessories (bull bars, roof racks, winches, driving lights), and—this is the critical part—the Tow Ball Mass (TBM) of your attached caravan.
How to manage it: Know your vehicle's GVM and its kerb weight, so you know your true payload limit. Be mindful of *every single item* you add to the car. The weight of accessories adds up incredibly fast. The only way to know for sure is to get your fully loaded vehicle weighed by a professional service. If you are close to or over your GVM, one solution is to remove items from the vehicle, or you might consider a GVM upgrade.
GTM (Gross Trailer Mass)
What it is: The Gross Trailer Mass is the maximum legal weight that can be supported by the *axles* of your caravan or trailer. This is the total weight of your loaded caravan *minus* the weight supported by the tow ball (when hitched) or the jockey wheel (when unhitched).
Why it's important: This rating is the structural limit for your caravan's running gear. Exceeding the GTM puts extreme, dangerous stress on your van's axles, springs, bearings, and tyres. This is a primary cause of catastrophic failure on the road, such as axle collapse, bearing disintegration, or tyre blowouts, which can easily lead to a major accident.
Where it's found/calculated: The GTM is listed on your caravan's compliance plate, which is usually on the A-frame or inside the front boot. Your *actual* GTM is measured by weighing *only* the wheels of your caravan while it is hitched to the tow vehicle. This measurement isolates the axle weight from the tow ball weight.
How to manage it: Managing GTM is all about smart load distribution. Heavy items (like tinned food, water, or equipment) should be placed low down and as close to the caravan's axles as possible. Packing heavy items at the extreme front or (especially) the extreme rear of the van can act as a lever, putting undue stress on the axles.
ATM (Aggregate Trailer Mass)
What it is: The Aggregate Trailer Mass is the "big one" for your caravan. It is the maximum legal total weight of your caravan when it's fully loaded and *unhitched* from your tow vehicle (i.e., standing on its own jockey wheel). This is the number most often referred to and is the key compliance figure for your trailer.
Why it's important: This is the all-up legal limit for your caravan. It dictates everything from your van's registration class to its required braking system (e.g., any trailer over 2,000kg ATM must have a "break-away" system). Towing a van that exceeds its ATM is illegal, voids your insurance, and means the entire structure—chassis, A-frame, and all—is being operated beyond its designed safety limits.
Where it's found/calculated: The ATM is the main number stamped on your caravan's compliance plate. It's calculated as: `GTM (axle load) + TBM (load on jockey wheel) = ATM`. The simplest way to measure it is to weigh the entire caravan on a weighbridge while it's completely unhitched from your car.
How to manage it: Know your van's "Tare" (empty) weight to understand your available payload (`ATM - Tare = Payload`). This payload is often surprisingly small. You must account for *everything* you add: water in the tanks, gas bottles, solar panels, batteries, food, clothes, bedding, and all your gear. It is essential to get your fully loaded van weighed to ensure you are under this crucial limit.
TBM (Tow Ball Mass)
What it is: Also known as "Ball Load," this is the downward vertical force that your fully loaded caravan exerts on the tow ball of your vehicle when it's hitched up and level.
Why it's important: TBM is the single most critical factor for towing stability. If your TBM is too light (e.g., under 7% of the van's weight), the caravan will be "tail-heavy." This can lift the rear of the tow vehicle, causing it to become unstable and prone to uncontrollable, terrifying "trailer sway," which is a leading cause of caravan accidents. If your TBM is too heavy (e.g., over 12%), it pushes the rear of your car down, lifting the front wheels. This reduces your steering grip, severely compromises braking effectiveness, and can overload your vehicle's rear axle and tow bar.
Where it's found/calculated: The recommended TBM is generally 8-12% of the caravan's ATM (although all caravans are different and this is only a guide). Your vehicle and your tow bar will also have a *maximum* TBM limit (e.g., "Max Ball Load: 350kg"), and you must not exceed the *lowest* of these limits. It is measured using a specialized ball weight scale, or by a mobile weighing service.
How to manage it: TBM is managed by load distribution *inside* your caravan. To *increase* TBM, move heavy items slightly forward of the axles. To *decrease* TBM, move them slightly rearward (but *never* to the extreme back of the van). Filling or emptying water tanks located at the front or rear of the van will also have a significant and immediate effect on your TBM. TBM also changes if you change the height of the hitch, especially on dual axle vans
GCM (Gross Combination Mass)
What it is: The Gross Combination Mass is the maximum legal total weight of your *entire rig combined*. It is the total weight of your fully loaded tow vehicle (without the caravan hitched) *plus* the total weight of your fully loaded caravan, all added together.
Why it's important: This limit is set by the *vehicle manufacturer* and defines what its engine, transmission, chassis, and brakes can safely handle when *pulling* (and, just as importantly, *stopping*) a heavy load. It ensures the vehicle can safely control the entire combined mass, especially when braking or navigating a steep downhill slope. Exceeding your GCM puts catastrophic strain on your vehicle's entire drivetrain and is a clear sign that your setup is dangerously overloaded.
Where it's found/calculated: The GCM is listed in your vehicle's owner's manual or on a compliance plate. *Be careful:* A vehicle's maximum "Towing Capacity" plus its GVM often adds up to *more* than the GCM. This is a common trap. You cannot simply max out both car and van; you must stay under the GCM. It's calculated by: `Your Actual GVM + Your Actual ATM = Your Actual GCM`.
How to manage it: This is the final and most important check. After ensuring you are under your GVM, GTM, and ATM individually, you must also check that the sum of your car's and van's actual weights does not exceed the GCM. If it does, you have to reduce weight from either the car or the van (or both) until your combined total is compliant.