Why Caravans Roll Over

Pic: NT Police

Pic: NT Police

Why Caravans Roll Over

Updated June 2019

Why caravans roll over or jackknife is now well understood. It is explained in depth in our book Why Caravans Roll Over - and how to prevent it. This constantly updated article is a precis of what happens, why and how to limit or prevent it happening.

Our article listing caravan rollover reports in the media is an unfortunate reminder of how common this type of accident is in Australia.

Caravan Roll Over Videos

In the caravan roll over videos below, each 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.

Both videos show 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.

If you have any difficulty watching these videos on our site, you can see them here, and  here .

Caravan Roll Over Factors

A caravan roll over is rarely the result of one issue. There are usually several involved. The ultimate cause however is outlined here.

The jack-knife preceding a roll-over is caused by the yawing caravan imposing side forces on the tow vehicle's rear tyres that cause them to lose all grip. This typically happens if the rig is travelling at or above a critical speed (unique to each rig and its loading). The yaw forces cause the vehicle to adopt a non-controllable ever-tightening turn resulting in a jack-knife. This typically results in the caravan overturning - and likewise the tow vehicle. 

Excess speed (for that particular rig) is almost always involved. The tow vehicle and trailer is usually travelling at high speed when a roll over happens, often in association with overtaking another vehicle, or being overtaken. 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

There is a critical speed (unique to each rig and its loading) above which an unstable tow vehicle and trailer is literally impossible for the driver to correct if subjected to a strong disturbing force. Yaw (sway) builds up within a few seconds, the rig jack-knifes and is likely to roll over. There is little or no warning. 

Rigs that have trailer heavier than its tow vehicle (and/or or too low tow ball mass) are likely to have critical speeds well below the towing speed limit. 

Incorrect Trailer Weight Distribution

Heavy loads at the rear or top of a trailer make it unstable.Keep all heavy items as close to the axle/s as possible. If possible, carry rear mounted spare wheels in the tow vehicle. 

Tow Ball Mass

Inadequate tow ball mass decreases the rig's critical speed at which jack-knifing is likely. With almost all locally-made caravans that speed is very close (or under) 100 km/h.  

Overhung Tow Hitch

An overhung hitch acts as a lever that increases the download forces on the rear of the tow vehicle. That hitch must as short as possible to minimise this effect. Excess overhang between tow vehicle rear axle and tow ball is virtually a recipe for swaying.   

External Factors

These are factors over which a driver has no control. These are mostly harmless for passenger cars, but when towing a trailer can be dangerous. 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 this 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.

That many owners of fundamentally unstable rigs have not experienced this is simply because they have not been subject to a strong enough side force. Police and rescue staff often report the driver as saying 'it always felt so stable until then .....'. 

Our book Why Caravans Rollover - and how to prevent it  explains (in plain English) exactly how and why caravan rollovers happen. A invaluable section enables owners to estimate their own rig's stability - and suggests how to improve this. It also has a fully technical explanation - and references.

The book is based on truly extensive research and Collyn Rivers' having spent much of his life studying and working in this area. (Collyn is an ex motor industry research engineer). 

Why Caravans Rollover - and how to prevent it is downloadable from this website. Do it now!

Avoiding Caravan Roll Overs

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



Buy this book now!

This article is based on content from Why Caravans Roll Over - and how to prevent it.

Our other featured RV books contain extensive information on a range of topics of interest to RV users and potential buyers. By purchasing a book, you are not only educating yourself but also supporting the work of independent RV writers. If you have found this article useful, please also visit the RV bookshop.

 

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