This page is a work in progress. It is now starting to take shape. Feel free to read, ask a question, highlight an error or suggest a way to make it easier to understand.
There are several things that you must know to see if your weights are legal
You must know all your vehicle and caravan (5th Wheeler) specifications
GVM The total weight of the tow vehicle including passengers, fuel, luggage and ball or pin weight. This weight must not be exceeded.
Front Axle weight limit. You need to stay under the front axle limits as well.
Rear axle weight limit. You need to stay under the rear axle limits as well. This weight limit is one of easiest limits to exceed.
Tow bar weight (pin weight) This the limit of the weight that can be placed on towbar or towing hitch.
GCM This is the maximum weight that your rig and van can weigh loaded up ready to head off (fully loaded). This weight must not be exceeded. This is the easiest limit to exceed, especially if you listen to sales reps trying to sell you a new vehicle. Once you have signed on the dotted line and they have your money, you are on your own.
ATM This is the maximum weight that your caravan (5th wheeler) can weigh. This weight must not be exceeded.
GTM This is the maximum weight that can be placed on the caravan (5th wheeler) wheels. This weight must not be exceeded.
Towbar or pin weight. The maximum weight must not be exceeded on the towbar (hitch). All towbars and 5th wheeler hitch should (must) have a compliance plate that shows maximum towing capacity and maximum ball (pin) weight These weights must not be exceeded.
You must visit a weighbridge.
Once you know all your ute and van specifications, it is now necessary to visit a weighbridge. You must be all loaded up ready to travel. If you leave anything out, you will not get an accurate result. You may need to make more than one visit if you exceed any one of your weight limits
Picture 1 above GTM is achieved by weighing just the caravan wheels when fully loaded
Picture 2 above ATM the weight of the whole caravan with the wheels and jockey wheel on weigh bridge, disconnected from car
Picture 3 above Tare weight is the weight straight from factory, no accessories or water or gas. This weight is usually less than an empty van because of accessories that have been added since the van left the factory.
Picture 4 above GCM is the whole unit weighed on one long weighbridge or by adding up weights from weighing car and van seperately, preferably all hooked up. GVM is the ute weighed all loaded up with the caravan hooked up, with just the ute on the weighbridge.
The specification terminology below is the maximum allowable weights allowed for each area mentioned. If your actual weight goes over these specifications, you need to adjust your load as you are illegally loaded.
GCM Gross Combination Mass = Total weight of tow vehicle and towed vehicle
GVM Gross Vehicle Mass = Total weight of tow vehicle including loaded mass and tow ball weight
ATM Aggregate Trailer Mass = Weight on trailer axles + ball weight or total weight of trailer unhooked
GTM Gross Trailer Mass = Weight on trailer axles and does not include ball weight
The picture below tries to show how the weight transfer from the hitch to show how your weight is measured to stay below your GVM and GCM weights
GCM = GVM + (ATM – ball (pin) weight)
GCM = (GVM – ball (pin) weight) + ATM
GCM = GVM + GTM
GCM = GVM + ATM is incorrect as ball (pin) weight is counted twice
The video below is a lot of the same but it is worth watching