Tow Vehicle/Caravan Weight Ratio Explained
Having the right weight relationship between tow vehicle and caravan is vital for safe towing.
When a tow vehicle and caravan is exposed to external side forces it is the heavier of the two vehicles that will dictate what happens next.
You do want the tow vehicle to be in control at all times, and especially in an emergency.
You do not want the tail wagging the dog. That necessitates a lighter tail.
The Ideal Ratio
The conventional wisdom, confirmed by Newton's Laws of Motion, is that the tow vehicle must be at least as heavy as the caravan being towed (i.e. a 1-to-1 ratio between tow vehicle and trailer weight).
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 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.
From the above you might think that the Caravan Council of Australia is being the most conservative of all these countries in applying a 77% safety margin. But in fact, because the CCA’s 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.
Experienced towers could tow up to about 90%, always keeping a 10% margin for error below the critical 1-to-1 ratio in case of emergency.
Just as importantly, the higher the weight of the laden caravan to laden tow vehicle the lower your towing speed should be.
Recommended Maximum Caravan Laden Weight for Those New to Towing
We recommend that those new to towing take particular caution when buying a caravan. You need 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.
Note that this table uses tow vehicle maximum permissible laden weights only ('GVM') - the actual laden weight of the tow vehicle should be lower than this and the laden weight of the trailer should be adjusted accordingly.
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