Tow Vehicle/Trailer Weight Ratio

NOT a good tow vehicle to trailer weight ratio (Pic: source unknown)

NOT a good tow vehicle to trailer weight ratio (Pic: source unknown)

Introduction

The weight relationship between tow vehicle and caravan is an important one for safe towing.

When a tow vehicle and trailer combination is impacted by external forces it is the heavier of the two vehicles that will dictate what happens next.

You really do want the tow vehicle to be in control at all times, but especially in an emergency. 

You do not want the tail wagging the dog. And that necessitates a lighter tail.

The Ideal Ratio

The conventional wisdom, backed up 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 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 a legal 80 km/h.

Whilst most local rigs have caravans much heavier than their tow vehicle - and may legally travel at 100 to 110 km/h, both RV books and the buyer-oriented Caravan Council of Australia strongly advise to at least have the tow vehicle heavier -and preferably to follow UK/EU practice. 

To learn more about towing dynamics, read our technical articles on Moments along a Beam, the Double Pendulum Principle and Overhung Hitches

RV Books is also shortly to release an all-new (digital) mini-book that shows how to assess your own rigs probable stability - and what you can do to increase its vital margin of stability.

International Comparisons

There is much debate internationally (and in Germany, legislation) on the ideal safety margin that should be provided between the weights of tow vehicle and caravan.

UK

In the UK there is the “85% rule”. This is not legislated, but is a recommendation of the Camping and Caravanning Club of the UK and other UK bodies. This recommendation states that the weight of the loaded caravan should be no more than 85% of the car’s kerb (unladen) weight. You can read more about the 85% rule here.

However, according to the Camping and Caravanning Club of the UK, ‘those who are experienced at towing may go up to 100 per cent of the car’s kerb weight, but no-one should tow a caravan that is heavier than the towing limit of the vehicle it’s behind’

Germany

In Germany, caravans with brakes and hydraulic shock absorbers may not (by legislation) exceed 0.8 times the empty weight of the tow vehicle unless the tow vehicle does not exceed 80 kmph.

Australia

In Australia, there is no legislation on 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."

(for the non-mathematically-minded, 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 this 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.

Our Recommendation

RV Books recommendation is that those new to towing should tow a laden trailer 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 closer you are to the 1-to-1 ratio, the lower your towing speed should be.

Recommended Maximum Trailer ATM for Those New to Towing

We recommend that those new to towing exercise particular caution when selecting a trailer. You need to have practical experience on the road of the more cautious driving behaviour required for 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 for those new to towing. Our recommendation is that the laden weight of the trailer should be no more than 80% of the laden weight of the tow 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') - actual laden weight of the tow vehicle should be lower than this and the laden weight of the trailer should be adjusted accordingly.

Tow Matching Services

A number of organisations in Europe offer a 'tow matching' service to members, where details of the proposed tow vehicle and trailer are entered into a database and the suitability of that match is assessed. One UK-based example is here.

This might be something for Australian and New Zealand motoring and/or RV organisations to consider in future as a service to members and the wider public.

N.B. Please do not use an overseas tow matching service in Australia or New Zealand due to variations in vehicle specifications.

In the meantime, you can find out the key towing specifications of some popular Australia tow vehicles here.

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