Thinking about buying or building a caravan has a lot to do with the towing capacity of the vehicle and the towing safety. Towing a van just below the towing capacity of the vehicle is not necessarily a save option.
Towing capacities are calculated by the vehicle manufacturer on the basis of axle loads and drive train characteristics. A maximum towing capacity does not mean it is independent from the length of a trailer. A short heavy trailer might be safe but the same weight stretched over 28′ is most likely not.
Caravans are inherently unstable and the longer the worse they fair depending on weight distribution and speed.
Many pages have been written about caravan weights vs. ball weight vs. vehicle weight and for the most part the rigs one sees on the street are beyond my comfort zone as far as safety is concerned.
There is also a lot of discussion about the weight definitions of a caravan and in my opinion the problem is actually the law. The law defines vehicle weights under certain circumstances which are mostly theoretical or not reproducible.
The only real hard definition is the ATM. This is the weight which defines the absolute maximum the van can weigh on its own.
The problem starts when we look at GTM, where the law says that it is the load on the axle group when the maximum payload is evenly distributed over the loading area. This applies to semitrailers and flat loaders, but not caravans. In a mathematical sense, when a constraint for a definition is not met the definition is false or unusable. In my view the regulations are.
ATM = Ball weight + GTM is never really the same under all circumstances with the same trailer.
The ball weight has an upper limit through the allowable down force on the vehicles coupling and/or the geometry of the draw-bar or coupling arrangement of the trailer. We can define a maximum and with a rule of thumb for safe towing we can also define a minimum.
The GTM weight specified on compliance plate is one single figure and should be the total down force on the axle group when the van is loaded at its ATM. But how is it loaded ? Where is the majority of the load ? Is the specified GTM suitable for the axle group capacity ? Is it the maximum axle group capacity ?
It gets even more complicated when we look at the regulations for load sharing and non load sharing suspensions.
To be honest – I really don’t care about the definitions in the regulations other than ATM and for me the GTM is not a single number it is a range or a maximum.
I specify and calculate the maximum of the ball weight of my van not by looking at the tow vehicle but at stability factors of the van and the weight distribution in extreme cases.
Worst case scenarios will define the possible maximum GTM and the axle group needs to be capable of dealing with it and the same applies to the coupling. We will have a maximum weight on the coupling, which defines the worst case scenario of loading and the coupling as well as the tow vehicle need to cope with it including a safety margin.
In my world GTM max and Ball max are defined by technical constraints and the sum will be higher than ATM. My design aim is an ATM of 4495kg and if I load heavy over the axle group (water tanks) I need to load lighter over the coupling and vise versa, but I can have different GTMs depending on the load situation and still have loaded to the ATM. If we define a GTM as a maximum on the compliance plate under the constraint of one specific way of loading the van, I can find a way to load the van to its ATM with the axle load exceeding the specified GTM.
It needs to be defined as the maximum load possible with the minimum ball weight for save towing. Then the formula in the regulations could have meaning, but not as long as it says evenly distributed over the loading area.