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If you weigh your caravan and it’s overweight, what are your options? Here’s how to get a caravan ATM upgrade, and what’s involved.
The most talked-about subject among caravan owners is weight compliance, and to many, the whole subject can be downright confusing. The caravan industry is currently experiencing a boom. As a result, many new families are getting into caravanning. However, there has never been a more crucial time to make sure you fully understand all the weight terminologies and ensure your rig is weight compliant.
Firstly, let’s have a look at all those weight terminologies and acronyms that are recorded on the caravan compliance plate. These ratings and capacities are there for a reason, ensuring the caravan is legal and able to be operated safely.
Tare Weight (TARE)
Tare weight is the total dry weight of the caravan as it leaves the factory with all accessories fitted, including empty water tanks and empty gas bottles. This does not include aftermarket items fitted after manufacture by the caravan dealer or a previous owner.
Towball Mass at Tare
The Towball Mass at Tare is measured at the tow hitch with the caravan level. This weight is added to the wheel weights to calculate the empty caravan’s Tare Weight.
Towball Mass (TBM)
Towball mass measures the weight the caravan tow hitch imposes on the tow vehicle. Most manufacturers these days do not specify a maximum allowable Towball Mass on the caravan compliance plate. If this is the case, the maximum download for the caravan tow hitch is then determined by the allowable tow ball download of the tow vehicle.
Gross Trailer Mass (GTM)
The GTM is the maximum allowable weight that the fully-loaded caravan imposes on the ground through its wheels and axles.
Aggregate Trailer Mass (ATM)
The ATM is the total weight of the fully-loaded caravan, unhitched from the tow vehicle. The ATM includes the total weight imposed on each wheel plus the weight imposed at the tow hitch. These measurements all need to be taken at the same time to ensure the result is correct. So TBM + GTM = ATM
Axle Group Loading (AGL)
The AGL records the axle capacity of the caravan and is usually determined by the chassis manufacturer or the axle manufacturer.
Weighing the van on portable scales
Get it weighed
Now that you know what all the weights mean, the next thing to do is get your caravan weighed. Remember, however, that in order to be compliant, you need to ensure you are under all the compliance ratings: Towball Mass, Gross Trailer Mass, and Aggregate Trailer Mass. We’ve spoken previously to Dave Lewis from Weightcheck, a mobile caravan weighing service and he reports that over the past three years, around 45% of the caravans he has weighed have exceeded their ATM. Many others had problems with their TBM or GTM.
In order to achieve the most accurate results, Dave points out a crucial piece of advice.
“Weighing the caravan involves measuring the weight of each wheel and the tow ball download all at the same time, yes, all at the same time,” he said. “First, the caravan must be at the same attitude or angle as it is when attached to the tow vehicle. The tow ball mass must also be measured with the tow hitch at the same height it is when attached to the tow vehicle. Anything other than this will mean the measurements, especially the TBM, will not be accurate.”
Okay, you’ve weighed the caravan and now, to your dismay, you realise that you have too much gear. It has exceeded the ATM, GTM, or TBM or maybe all three! So what are your options to get around this nightmare and ensure your caravan is compliant?
Firstly, don’t panic, they have a seriously good look at how you can reduce weight in the caravan. You have to be a hard taskmaster, and we recommend going through every cupboard and drawer and if it’s not absolutely essential that a certain item is in there, then take it out.
And don’t forget the boy’s end, open up that front boot and sort out all those tools and other stuff we all take just in case something breaks, realistically you probably don’t need it. Next, get the bathroom scales and weigh everything that comes out, no matter how small or insignificant. One of Dave’s recent customer did exactly this and was able to pull 63kg out of the caravan in the blink of an eye.
Getting more payload
So, you’ve had a cull, and the caravan is still over its ATM, so what is your next option? How do you get more available payload in your caravan? The answer may be an ATM upgrade, which involves increasing the payload of the caravan by overriding the original compliance plate ratings, in most cases without physically doing anything to the caravan.
The process is very complex and thorough and can only be completed by a licenced engineer who also needs to be an accredited person with the relevant state transport authority. If all goes to plan, a Modification Plate is attached to the caravan, beside the original compliance plate, with the relevant paperwork submitted to the State Transport Authority to reflect the new ATM on the caravans registration.
How a caravan ATM upgrade works
So let’s look at what is involved in an ATM upgrade and how it works. Most engineers will firstly supply an assessment sheet that provides the formwork to ascertain whether an upgrade is possible with the caravan in its existing state without the need for structural modifications. The details from the existing compliance, in particular the Axle Group Loading, play an important part of the assessment. There are also many ratings and sizes of various caravan components that will affect the outcome, let’s look at some of them.
Tyres: The tyre size, load rating, maximum pressure rating, and the tyre date code.
Rims: The rim size, load rating and type of construction (alloy or steel).
Coupling: The make, load rating, type (ball or off-road), and the bolt size and type of nuts.
Chains: The chain size, number of chains, and markings (Aust. Standards AS4177)
Suspension: Type of suspension (Leaf/Coil/Torsion/Airbag)
Springs: Type of spring, (Slipper/Rocker/Roller), width, thickness, and number of leaves
Axles: Type of axle (straight/drop/overlay), shape (round or square), and the load rating
Bearings: Type, size, and load rating
Brakes: Brake type (disc/drum) and size (10in/12in)
There are also a number of measurements that need to be taken, which form the basis of the assessment. These measurements allow the engineer to ascertain if the caravan’s chassis is strong enough to handle the increased payload, as well as ensuring it will handle safely and predictably when being towed. These measurements are entered into a computer program that then calculates the effect the new payload has on the tolerances of the existing, unmodified chassis.
The measurements include:
1. Drawbar length
2. Tow hitch to boot
4. Chassis overhang
5. Axle spread
6. Hanger spread
7. Crossmember length
8. Outrigger length
9. Coupling plate thickness
10. Crossmember size
11. Chassis size (length x width x thickness)
12. Chassis reinforcement, if applicable
13. Drawbar size (length x width x thickness)
14. Drawbar reinforcement, if applicable
If any of the component ratings or measurements do not meet the guidelines, the engineer will make recommendations as to what repairs or modifications would need to be carried out, in order to proceed with the upgrade. This may require work, for instance, to reinforce the chassis or drawbar, or upgrade or increase the size of a chassis component such as brakes or axles in order for the assessment to continue.
The final part of the assessment is a full inspection of the chassis and caravan that hopefully will not show up any defects that make the upgrade unachievable. It’s worthwhile noting that this could prove an advantage because if a fault is discovered during this inspection, it’s likely the owner was probably unaware of it. Dave Lewis recently referred a customer with an overweight caravan to an engineer for a potential ATM upgrade, only to discover that the caravan chassis had multiple hairline cracks and it needed some urgent repair work. Once completed, however, the caravan was able to have an ATM upgrade.
The upgrade will normally increase the compliance rating of both the Gross Trailer Mass (GTM), and the Aggregate Trailer Mass (ATM). The engineer may also suggest that the Towball Mass (TBM) be maintained at a certain weight in order for the caravan to be towed safely with the increased payload.
The legal bits
Once the assessment is complete, and the engineer is satisfied the caravan will handle the increased payload, they will complete all the relevant State Department of Transport paperwork and attach a Vehicle Modification Plate that specifies the upgraded compliance ratings. These new ratings are then recorded on the caravan’s registration certificate.
Before you order a caravan ATM upgrade, the first thing to do is get your caravan and tow vehicle weighed when they are both fully loaded. There are six compliance ratings that need to be checked, including the Gross Combination Mass, which is the total weight of the caravan and tow vehicle combined. For instance, if your rig is under its GCM, but the caravan is over its ATM, then the option is there to look into an ATM upgrade. Remember, it is a different story if the caravan is over its ATM and the rig is over its GCM, so you need to get the bigger picture first.
The best advice is to get good advice before you decide on a path to follow.
Words by Dave Lewis, Weightcheck, Brisbane