Tow Vehicle/Caravan Weight Ratio Explained - updated Sept 2019
Save caravan towing is dependent on the tow vehicle always being under the driver's control. That ability is compromised if the laden caravan weighs more than the laden tow vehicle (and also reduces the safe towing speed).
This applies not just in routine driving. That rig must especially be controllable under that rare emergency - such as the need to swerve strongly at speed to avoid a head-on collision or recover from a strong side wind gust.
In essence you must never have a tail so heavy that it wags the dog. It necessitates a lighter tail.
The Ideal Ratio
The conventional wisdom (plus Newton's Laws of Motion), is that the laden tow vehicle must be at least as heavy as the laden caravan being towed.
This has long since been accepted in the UK and Europe. There, laden caravans must not exceed about 80% of the weight of the laden tow vehicle. And that's where towing speed is mostly limited to a legal 80 km/h.
Most local rigs, however, have caravans much heavier than their tow vehicle - yet they may legally travel at 100 to 110 km/h. Both RV books and the buyer-oriented Caravan Council of Australia strongly advise to have the tow vehicle heavier - and preferably to follow UK/EU practice.
RV Books (digital) mini-book Why Caravans Roll Over - and how to prevent it explains, in plain English, exactly what happens and why. It also shows how to assess your own rig's probable stability - and what you can do to increase its vital margin of stability. The book also includes a technical explanation and extensive references.
Australia currently (Sept 2019) has no legislation on caravan/tow vehicle weight safety margins. However the Caravan Council of Australia states:
"It is strongly recommended that, at any time, the mass of the tow-vehicle be appreciably more than the mass of the caravan/trailer. It is suggested that for added safety and peace-of-mind, the laden tow-vehicle should weigh 30% more than the laden caravan/trailer."
(This is the same as saying that a trailer should not exceed about 77% of the laden weight of the tow vehicle). You can read that advice in the CCA's article, 'Selection of a Suitable Tow Vehicle' here.
It might seem that the Caravan Council of Australia is conservative in applying a 77% safety margin, however its recommendations are based on laden weights of the tow vehicle and not the unladen (or kerb) weights, they are in fact the least conservative.
Note that all the above relates to the weight relationship between tow vehicle and caravan. There is always legislation in each country on the maximum weights of tow vehicles, caravans and towing combinations.
RV Books recommendation is that those new to towing should tow a laden caravan that is no heavier than 80% of the laden weight of the tow vehicle. Furthermore to confine speed to 80 km/h for the first few uses.
Experienced towers could tow up to about 100%, but preferable keeping a 10% margin for error below the critical 1-to-1 ratio in case of emergency.
Vitally, the higher the weight of the laden caravan to laden tow vehicle the lower your towing speed should be. See also our article re tow ball mass as that too determines safe towing speed.
Recommended Maximum Caravan Laden Weight for Those New to Towing
Those new to towing need to be cautious when choosing a caravan. Then to drive extra carefully as you build up your experience of towing. Whilst you are gaining this experience, your tow vehicle should be significantly heavier than your trailer.
Below is a list of some popular tow vehicles and our recommended maximum ATM of the trailer for each vehicle. This ratio is significantly below the maximum towing capacity of each tow vehicle. During your early towing experiences (and, we suggest, even when experienced), do not be tempted to put maximum towing capacities to the test.
This table uses tow vehicle maximum permissible laden weights only ('GVM'). Note that there is also an overriding Gross Combination Mass (GCM) set by vehicle makers that places an upper limit of the combined laden weight of that vehicle and whatever it tows.
For safe towing your actual laden weight of the tow vehicle should be as high as legally possible, and the laden weight of the trailer adjusted accordingly. Never vice versa.
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