Why Caravans Roll Over - updated October 2019

Pic: NT Police

Pic: NT Police

Why Caravans Roll Over

Updated October 2019

Why and how caravans roll over or jackknife is explained in depth in our book Why Caravans Roll Over - and how to prevent it. This constantly updated article you are reading here is a brief precis of what happens, why and how to limit or prevent it happening.

Caravan Roll Over Videos

In the caravan rollover 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. None, however, are likely to have occurred at a lower towing speed, but once they began to sway, none could be recovered by driver correction (no matter how skilled or experienced).

Factors outside the driver's control include road conditions, wind resistance and side winds.  

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. That vitally required however is knowing how and why rollovers (or jack-knifing) occur.

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 but a particular risk is descending a steep windy hill at excess speed, and overtaking (or being overtaken by) a semi-trailer at high speed.

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 take up a non-controllable ever-tightening snaking that almost always ends in a jack-knife. This typically results in the caravan overturning - and often also 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 110 km/h involves forces over four times higher than at 50 km/h. It probable that most would have not had they been travelling at less speed. Never exceed 110 km/h when towing and especially not when overtaking.

Critical Speed

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

Be aware that a tow vehicle that pulls a trailer that is heavier (and/or has too low tow ball mass) is likely to have critical speeds well below the towing speed limit. That critical speed does not imply that an accident will happen if that speed is exceeded. It means that if travelling at our above that speed, it is subjected to a strong enough disturbing force (often a side wind gust) a jack-knife may likely result. 

The following are factors that increase your risk:  

Incorrect Trailer Weight Distribution

Heavy loads at the top and (especially) at the extreme rear of a long caravan make it potentially unstable. Keep all heavy items as close to the axle/s as possible. If possible, carry rear mounted spare wheels in the tow vehicle - not on the rear wall of the caravan. (That some makers do cannot change an immutable law of physics). 

Tow Ball Mass

Inadequate tow ball mass decreases the rig's critical speed at which jack-knifing is likely. With many locally-made caravans over about 5.5 metres with tow ball mass below about 7.5%, that critical speed is very likely to be as low as 70-80 km/h..  

Excess Tow Hitch Overhang

An overhung hitch's lever action increases download forces on the rear of the tow vehicle. Keeping that hitch must as short as possible minimises this. Excess hitch overhang is virtually a recipe for swaying. Choose a tow hitch with the shortest possible overhang, not a longer one that may be marginally easier to couple and uncouple.  

External Factors

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

The issue is not how a rig feels and acts in normal driving. It is how it behaves in abnormal driving. This may include swerving at speed to avoid a head-on crash, or a strong-enough side force on a bend and/or being passed (or passing) closely at speed by a semi-trailer.

Many owners of fundamentally unstable rigs have not experienced this is simply because they have not been subject to such issues. But (increasingly) some are - over 300 jack-knifed in 2018 alone and many of those also rolled over.

Police and rescue staff commonly 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 separate fully technical explanation - and many references.

The book is based on truly extensive research and (author) Collyn Rivers' having spent much of his life studying and working in this area. Collyn is an ex Vauxhall/Bedford 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 110 km/h (especially when overtaking). Ideally tow below that speed. 
  • 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.

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