Why Caravan Rollovers Happen - updated March 2019
Why Caravans Rollover - and how to prevent that
Updated May 2019
Our article listing caravan rollover reports in the media is an unfortunate reminder of how common this type of accident is in Australia.
Whilst further work is needed to determine the exact cause of these accidents, the fundamental physics involved, plus the causative factors are now reasonably understood. They explained in depth in or new book Why Caravans Roll Over - and how to prevent it.
This now-updated article provides a precis of what happens and why.
In the videos below, a caravan has become unstable with catastrophic consequences. These accidents were due to a range of factors both within and outside the driver's control.
Factors within the driver's control include speed, weight of both the caravan and tow vehicle and the distribution of that weight, particularly in the caravan.
Factors outside the driver's control include road conditions, wind resistance and side winds.
What is interesting about all three videos is that the accidents happened whilst towing on a flat road in a straight line. No bends, hills or obvious cambers. This is quite common.
Just why and how they rolled over is fully explained in our all-new book Why Caravans Rollover - and how to prevent it.
A caravan rollover rarely happens as the result of one issue. There are usually several involved. The ultimate cause however is outlined here.
The Ultimate Cause
The actual jack-knife that precedes a roll-over is caused by the rear tyres of the tow vehicle being caused to generate slip angles that those tyres cannot sustain. If the rig is travelling at above a critical (unique to each rig and its loading), they cause the vehicle to adopt a non-driver-controllable ever tightening turn that results in a jack-knife. This often causes the caravan to overturn, in many cases causing the tow vehicle also to overturn.
High Absolute Speed
Excess speed for that particular rig is virtually always involved. A caravan rollover rarely happens at low speed. The tow vehicle and trailer combination is usually travelling at high speed when a rollover happens, often in association with overtaking another vehicle. The forces involved rapidly increase with speed. A rollover at 100 km/h involves forces four times higher than those at 50 km/h.
Critical speed is the speed at and above which an unstable tow vehicle and trailer becomes literally impossible for the driver to correct if subjected to a strong enough disturbing force. Yaw (sway) builds up within a few seconds (in the manner described above), the rig typically jack-knifes and is likely to roll over. There usually little or no warning. That is why drivers often say 'it always felt so stable before the rollover'.
Every tow vehicle and trailer has a unique critical speed. Some rigs that have a much heavier trailer than the tow vehicle (or inadequate tow ball mass) may have critical speeds well below the towing speed limit.
Overweight Trailer and/or Tow Vehicle
If a trailer or tow vehicle is loaded beyond any of the maximum weight limits of trailer, tow vehicle or towing equipment manufacturers, it risks becoming unstable. It is also illegal.
Incorrect Trailer Weight Distribution
Heavy loads at the front, rear or top of a trailer causes instability. Caravans that have rolled over are often found to have heavy loads in these locations.
Tow Ball Mass
Having inadequate tow ball mass is a significant contributing factor to rollovers. Without adequate tow ball mass, the trailer will seek to deviate from the path set by the tow vehicle and may cause the trailer to sway. Sway is often the pre-cursor to a rollover.
Tow Vehicle/Trailer Weight Ratio
If the laden trailer is heavier than the laden tow vehicle (irrespective of the maximum weight capacities of each) and the combination has swaying that escalates, the trailer is likely to dictate what happens next.
With a trailer attached, an overhung hitch acts as a lever on the rear of the tow vehicle. That overhang needs to be as short as possible to minimise this leverage effect. Long distances between axle and tow ball (sometimes as a result of user modification to the tow bar or tow ball) are virtually a recipe for swaying.
These are factors over which a driver has no control. When not towing, these factors can be relatively innocuous for passenger cars, but with a trailer behind they can assume a significant role. They include:
- a wind gust, which can be natural or caused by a vehicle coming in the opposite direction
- increased or decreased (side) air pressure as a result of overtaking or being overtaken
- road camber, which can gently push a trailer off to the side of the road. (Over-) compensating for road camber can send a tow vehicle and trailer sharply towards the middle of the road
- adverse weather and road conditions
This topic is far too complex to fully explain in article form.
Our all new book Why Caravans Rollover - and how to prevent it explains (in plain English) exactly how and why caravan rollovers happen. The book is a result of Collyn Rivers' having spent much of his life studying and working in this area.
This book has a section enabling owners to obtain to estimate their own rig's stability - and suggestions re how to improve this. It also has a fully technical explanation - and is referenced.
It is now downloadable from this website. Why not do it now!
Avoiding Caravan Rollovers
Why Caravans Rollover - and how to prevent it explains all
Major ones include:
- never exceed 100 km/h (especially when overtaking)
- do not overload or incorrectly load either your tow vehicle or (especially) your trailer
- make sure your tow ball mass is correct
- do not have a laden tow vehicle that is lighter than the laden trailer
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