The Trouble with Tare
As you may have read in our RV Weight Definitions article, Tare Mass is:
'the total mass of a trailer when not carrying any load but when 'ready for service'.
This is a shortened form of the full technical definition of Tare Mass which can be found in VSB 1 here and which states:
'Tare Mass is the total mass of the trailer when not carrying any load, but when ready for service, unoccupied (if relevant) and with all fluid reservoirs (if fitted) filled to nominal capacity except for fuel, which shall be 10 litres only, and with all standard equipment and any options fitted. This includes any mass imposed onto the drawing vehicle when the combination vehicle is resting on a horizontal supporting plane. (Fluid reservoirs do not include water tanks and waste water tanks fitted to caravans).'
Why care about 'Empty' Weight?
The word 'Tare' derives from the French word meaning 'empty'. It is effectively the weight of a caravan when empty. So why should you care what your empty caravan weighs?
Tare Mass is important because it determines your payload. Payload is the weight of personal contents you carry in the caravan. Maximum Payload allowance is the most that your personal contents can weigh without exceeding your trailer's Aggregate Trailer Mass ('ATM'). ATM is the maximum laden weight of a caravan when not connected to a tow vehicle.
So if your caravan has an ATM of 1500 kg and a Tare Mass of 1000 kg, your payload is 500 kg. But if your caravan has an ATM of 2000 kg and a Tare Mass of 1600 kg, your payload is only 400 kg.
So a heavier caravan might have a lower payload allowance depending on its Tare Mass. The more weight that goes into the construction materials of the caravan, the less weight is available to you for personal effects.
What's Included and What's Not?
The term 'ready for service' suggests that Tare Mass must include everything required to operate the trailer, underlined by the clarifying phrase 'and with all standard equipment and any options fitted'.
BUT Tare Mass specifically excludes water and gas and may also exclude any dealer-fitted options fitted after the caravan has been finished at the factory and weighed. Dealer-fitted options could be any number of appliances including microwaves, air conditioners, solar panels, extra batteries, TVs or washing machines. Some of these can be heavy.
All of these items will therefore eat into your payload allowance. If you have 200 kg of water in 2 x 100 litre tanks, 2 gas bottles and a dealer-fitted air conditioner and washing machine, you may be looking at 300 kg of extra weight that must be added to the notional Tare Mass established at the factory.
With our heavier caravan example above with a payload of 400 kg, that would leave you with only 100 kg for all of your personal effects. Industry recommendations are a 250 kg payload for single axle caravans under 1500 kg and a 250 kg to 350 kg payload for dual axle caravans, but there is no legal obligation for caravan manufacturers to provide such payloads.
The actual Tare Mass of a trailer should be stamped on the trailer's compliance plate. But not all manufacturers weigh every caravan they make, only samples of a particular model. There can therefore also be weight variations between individual caravans of the same model, due to variations in the quantities and quality of materials used.
How to Safeguard Your Payload
When you order a new caravan, ask the seller to calculate its likely payload based on the exact options and accessories ordered and (if satisfactory, taking into account water, gas requirements etc.) request that this payload figure be included in the purchase contract as a condition of sale.
On delivery, ensure that the caravan is weighed when 'empty' at a weighbridge (ask to see the weighbridge docket if not done in your presence) and that the difference between this (its Tare Mass) and the caravan's ATM equals the agreed payload. There may be slight differences, but you should not take delivery of the caravan if the differences are major.
RV Books recommends that you should not rely on the Tare Mass figure stamped on the caravan's compliance plate to calculate payload until you have had the caravan weighed on a weighbridge.
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