Towing Capacity and Weights Explained


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Why are caravan and towing weights so confusing? 🤔 Or maybe it was just me?

As long as I don’t pack too much I should be okay, right? ATM, GVM, GCM. I don’t need to worry about that stuff, do I?

Well, you might be the best driver in the world. But… what happens when you are involved in an accident that you didn’t cause?  What if someone pulls out in front of you and you have to try to stop suddenly? What if you have to swerve to miss an unexpected obstacle and get the wobbles, or are just plain unlucky and get called in over a weighbridge?

If you couldn’t be bothered learning about this stuff, you may as well not bother with paying for your insurance premiums either then.

Having a basic understanding of vehicle and towing weight definitions is a must for safety, reduced wear and tear, and for your hip pocket.

It can become overwhelming, so for simplicity sake we’ll use some rounded numbers.

The first definition to understand is a caravan’s starting point:

💡 TARE. When talking caravans, this is the weight when not carrying any load, ready for service, and with all standard equipment and any options fitted. Note well the last part of the definition, “and any options fitted”. This means the tare mass that is listed on your mass produced compliance plate may not reflect your true ‘base weight’. Even straight out of the showroom, you may find a variation between what you think your van weighs and what it shows on the local weighbridge.

For our example we’ll use our caravan, a Jayco Expanda, which is pretty much 2,000kg

The next key definition to understand is the caravan’s maximum weight:

💡AGGREGATE TRAILER MASS (ATM) is the total mass of the trailer when carrying the maximum load recommended by the manufacturer. This is the weight most people think about when talking towing capacity. The A Big Peachey Adenture mobile has an ATM of about 2,500kg.

So pretty much, the load capacity (or payload) you can carry is:

Load Capacity = ATM – Tare

This would give 2,500kg – 2,000kg = 500kg load capacity.

Right?

Well, that annex, gas, food, clothes, the water in the tanks (grey and potable) all start to add up pretty quickly.

Really, the only way you know whether or not you’re compliant is to take yourself over a weighbridge.

So, as long as you have a vehicle that’s rated for 2,500kg towing capacity and you don’t overfill the caravan you’re right now, yes?

Maybe.

Still need to consider the tow vehicle:

💡TARE. Similar to the caravan definition, this is the mass of a vehicle, unoccupied and unladen, with all fluid reservoirs filled to nominal capacity except for fuel, which shall be 10 litres only, and with all standard equipment and any options fitted. In this example we’ll use a car that conveniently has a Tare of 2,000kg.

Now we have the base weight of the vehicle, I’m sure you can guess where this is heading?

💡GROSS VEHICLE MASS (GVM). This is the maximum laden mass of a motor vehicle as specified by the ‘Manufacturer’.

This is pretty similar to Aggregate Trailer Mass, just on the tow vehicle side of things. Same way of working out the vehicles’ load capacity:

Load Capacity = GVM – Tare

If our fictional car had a max GVM of 3,000kg and a Tare of 2,000kg, that would leave us 1,000kg of load capacity… The loads that need to be taken into consideration here though include people, the rest of the fuel, things like roofracks and bumper bars, luggage, etc.

But wait, there’s more. The big one:

💡💡💡GROSS COMBINED MASS (GCM). This is the value specified for the vehicle by the ‘Manufacturer’ as being the maximum of the sum of the ‘Gross Vehicle Mass’ of the drawing vehicle plus the sum of the ‘Axle Loads’ of any vehicle capable of being drawn as a trailer (i.e. the max weight of both vehicle and manufacturer.

So, we might have a caravan with an Aggregrate Trailer Mass of 2,500kg, and a vehicle with a Gross Vehicle Mass of 3,000kg. We have a total of 5,500kg between the two.

Sounds good, as both the car and van aren’t overweight, right?

Well, the Gross Combined Mass may be less than this (e.g. 5,350k). So although neither of the elements are overweight, the combination may be.

A final point to make is on TOWBALL DOWNLOAD. Normally 10% of the trailer is transferred onto the tow vehicle.  In this scenario, that’s 250kg extra that needs to be added up with the rest of the gear. If you were already maxed out with people and stuff, putting a trailer on may make the tow vehicle overweight. It’s a bonus for the trailer though, as weight comes off the tally (and hopefully ensures you’re below the Aggregate Trailer Mass). So when the trailer is couple to the towing vehicle, it’s given up the weight off the jockey wheel and instead the car is carrying it, this is known as GROSS TRAILER MASS.

The references for these definitions are all from legislation and the ADRs. But hey, even though I used the textbook definitions, it’s my fault if I haven’t explained anything clearly so there’s still some confusion. If that’s the case, please leave a comment, and I’ll go back to the drawing board.


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