A MUST READ FOR ANY PARTY THAT USES A PASSENGER VEHICLE OR 4X4 TO TOW A LARGE APPARATUS
CASE STUDY - GROSS COMBINATION MASS (GCM) VS. BALL MASS (BM)
BEFORE WE START, LETS EXPLAIN WEIGHT CATEGORIES
Gross Vehicle Mass (GVM)
GVM is the maximum a vehicle can weigh when fully loaded. This includes the vehicle, all of its luggage, fuel, accessories and passengers. It is specified by the vehicle manufacturer on a plate affixed to the vehicle.
Gross Combination Mass (GCM)
GCM is the greatest possible sum of the maximum loaded mass of a motor vehicle, and any vehicles that may be lawfully towed by it at one time. The GCM on a passenger vehicle is not stamped on any compliance plates - it is specified in the owners manual or upon request from the manufacturer to disclose.
Braked Towing Capacity (BTC)
BTC is determined by the vehicle manufacturer and is based on factors such as the design of the vehicle, the vehicle's rear axle load, the capacity of its tyres and the effect the fully loaded trailer will have on the vehicle's stability, durability and general safety.
Ball Mass (BM)
BM is the downward force from the a-frame of the caravan/trailer which when connected is applied to the vehicles tow bar.
Aggregate Trailer Mass (ATM)
ATM is the combined weight of the trailer and its full load when it is not coupled to a tow vehicle.
LETS LAYOUT THE LEGAL LIMITS ON THE BELOW DIAGRAMS
CASE STUDY / EXAMPLE SCENARIO / EXAMPLE VEHICLE ONLY - Toyota Landcruiser 200 series, stock standard unmodified.
Towing Capacity 3500kg
The towing apparatus (caravan)
Axles rated to 3500kg
Max ball mass 350kg
DIAGRAM 1 - vehicle + caravan on weigh bridge connected together fully loaded with driver, luggage and fuel
WHAT IS THE PROBLEM WITH DIAGRAM 1
Diagram 1 shows a vehicle + caravan on weigh bridge together, both fully loaded with luggage fuel and passengers. Total weight is 6800kg together on weigh bridge. Vehicles legal GCM is 6800kg. All legal right? Wrong. For the vehicles max GCM of 6800kg to be displayed on a weigh bridge either the car or caravan must be overloaded. Lets disconnect the caravan and weight both seperately. See diagram 2.
DIAGRAM 2 - tow vehicle from Diagram 1 weighed by itself + caravan from Diagram 1 weighed by itself
WHAT IS THE PROBLEM WITH DIAGRAM 2
The Landcrusier 200 weighs in at 2950kg (max legal 3300kg) no problem. The caravan weights in at 3850kg - big problem. The ATM of the caravan is 3500kg so the caravan is overloaded and therefore illegal. The Landcruiser 200 maximum towing capacity is 3500kg and with a caravan weighing in at 3850kg it is illegal for the vehicle to tow this apparatus deeming both the car and caravan illegal. As you can see in Diagram 1 we can clearly see that the vehicle is in its legal limits, as the ball mass from the caravan is added to the car which the cars total weight is displayed on the weigh bridge. In Diagram 1 we cannot clearly determine if the caravan is overloaded as the ball weight is not displayed. This is why the calculation of GCM is actually GVM + BTC minus ball mass of towed apparatus. The fact is you really do have 6800kg of GCM, it just means that you cannot use more than 3500kg of towing capacity and that the vehicle cannot weigh more than 3300kg once ball mass is added onto it.
Next lets look at a totally different caravan, refer Diagram 3.
DIAGRAM 3 - a totally different caravan weighed by itself
INFORMATION ON DIAGRAM 3
So lets say our caravan on a weigh bridge by itself weights 3500kg (Diagram 3), then our GCM is calculated by this: 6800kg GCM, minus 3500kg of usable
towing capacity leaves us with 3300kg. This means 3300kg is the maximum the vehicle can weigh after the ball mass is added onto it - so if the caravan
had 350kg of ball mass it means the vehicle should not weigh more than 2950kg if the Landcruiser 200 is on a weigh bridge by itself.
With this exact scenario in mind, which we can all agree on is the maximum legal scenario refer to Diagram 4 as to how this is actually displayed on a weigh bridge.
DIAGRAM 4 - tow vehicle + caravan from Diagram 3 on weigh bridge together with driver, luggage, fuel etc
INFORMATION ON DIAGRAM 4
So in other words GCM with the Landcruiser 200 and caravan together on a weigh bridge vs. GCM calculated on paper by method of weighing the car seperately (being mindful ball mass is added onto the vehicle GVM later) and weighing the caravan seperately give you two different GCM figures even though the weights in both scenarios are identical.
Some may ask well if the caravans ball mass of 350kg is added to the vehicles GVM why isn’t Diagram 1 legal, as effectively the ball mass was added onto the vehicles load? Simple, in Diagram 2 weather you like it or not the fact is the caravan by itself on the weigh bridge still weights 3850kg and the vehicle only has 3500kg of towing capacity - so in Diagram 1 the vehicle is still towing/pulling a total 3850kg of mass.
Should you be worried? Maybe. Or maybe you should just be more careful. The fact is if you want to remain legal it is not just a matter of driving your car and caravan onto the scales and taking a total weight. You must remember:
- Weigh the vehicle by itself
- Weigh the caravan total by itself
- Weigh the caravan ball mass
- Ensure your vehicle has enough payload to carry the ball mass
- The towing apparatus does not become lighter when ball weight is added to the vehicle.