Why Caravans Roll Over - updated January 2020
Why Caravans Roll Over
Updated 11 January 2020
Why and how caravans roll over or jackknife is now totally understood. The cause is basic and readily avoided. It is substantially associated with the towing vehicle. This constantly updated article you are reading here is a brief precis of what happens, why and how to limit or prevent it happening.
Our full (plain English) book Why Caravans Roll Over - and how to prevent it explains all.
This book includes a simple Question and Answer section enabling you assess the probable stability of your own rig - and how to improve that. Reader feedback show that the Q &A pages alone has assisted many improve towing stability, and (apart from buying the book) usually at zero cost
Caravan Roll Over Videos
In these caravan rollover videos, each caravan has become unstable with catastrophic results. They are unlikely to have occurred at a lower towing speed, but once they began to sway, they could not be recovered by driver correction (no matter how skilled or experienced).
Causes outside the driver's control include road conditions, wind resistance and gusting side winds (including those from trucks passing - or being passed).
Factors within the driver's control include speed (particularly), weight of the caravan relative to the tow vehicle, distribution of weight in the caravan, tow ball mass - and tyre pressures of the towing vehicle. The use of a Weight Distribution Hitch too is involved.
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.
For the very first time, a full plain-English (plus a full technical explanation) is in our all-new book 'Why Caravans Rollover - and how to prevent it'.
The not-for-profit Caravan Council of Australia describes its as: Essential reading for all caravanners!'
Caravan Roll Over Factors
A caravan roll over is rarely the result of one issue. There are usually several involved. The ultimate cause - of the tow vehicle losing control - is outlined here.
A snaking caravan imposing side forces on the tow vehicle's rear tyres that can build up and cause them to lose all grip. This may happen if the rig is travelling at or above a critical speed (unique to each rig and its loading). These side yaw forces can trigger a violently escalating snaking that typically 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 when overtaking another vehicle, or being overtaken. The forces involved increase with speed. A rollover at 110 km/h involves forces over four times higher than at 50 km/h. It is unlikely that the accident would have occurred at less speed. Regardless of speed limits never exceed 100 km/h when towing - especially when overtaking.
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 inexplicably do this does not change the basic laws 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
Excess tow hitch overhang increases both download and side forces on the tow vehicle's rear tyres. 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.
Tow vehicle tyre pressures
Far too little emphasis is being placed on the vital importance of the tow vehicle's relative front/rear tyre pressures.
It is essential wen towing a heavy caravan to have its rear tyres at least 50-70 kPa (7-10 psi) higher than its front tyres. That alone will considerably improve towing stability and, in borderline cases, remove the need for a WDH.
The technical reason for the above is that if the tyre slip angles are ever caused to exceed the front tyre slip angles, that caravan is all but certain to be triggered into an escalating jack-knife. Such slip-angle effect is now known to be the major cause of most caravan and tow vehicle jack-knifing accidents.
Slip angles briefly explained.
A pneumatic tyre's grip on the road is partly frictional and partly molecular. Depending on tyre construction, pressure and loading, that area of tyre-tread that contacts the road is a more or less hand-shaped area that is known as its ‘contact patch.
A tyre’s ‘slip angle’ is the difference between a pneumatic wheel’s direction of travel, and where its contact patch is seemingly pointing. Steering a wheel (or subjecting that contact patch to a side force) causes the contact patch to distort, resulting in the tyre steering the vehicle. As any side force can do this, a side force exerted on the tow vehicle’s rear tyres can (and does) cause seriously unwanted rear-end steering.
It is only when subjected to a side force that is greater than the contact patches’ ability to withstand, that a tyre will slide out of control.
Understeer and oversteer
All current road vehicles (except specialised rally cars) have front tyre slip angles intended to exceed those of the rear tyres. This effect is called understeer.
Understeer ensures the vehicle will automatically veer slightly away from any disturbing side force: including if cornering too fast. That, in turn, causes that vehicle to take up a slightly wider turning radius – thus automatically reducing the imposed side forces. Without a minor degree of understeer, a road vehicle needs constant steering correction.
The extent to which a vehicle understeers depends on its relative front/rear tyre’s pressure and the weight the tyres carry. Too much weight at the rear of a tow vehicle (either within the vehicle or imposed on it by a heavy trailer) can increase the slip angle of the rear tyres (and thus sideways movement that occurs when a side force is applied). If/when this happens, that tow vehicle is literally steered by its rear tyres. This is why it is vital to have that higher tow vehicle rear tyre pressures to be substantially higher than its front tyres.
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 a wind gust (or increased or decreased (side) air pressure) as a result of overtaking or being overtaken steep road camber, which may ease 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 simply because they have not been subject to such issues. But (increasingly)many are - over 300 rigs jack-knifed in 2018 alone. Many of those also rolled over.
Police and rescue staff commonly report the driver as saying 'it always felt so stable until then .....'.
Our now top-selling 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 issues include:
never exceed 100 km/h with any rig where the laden caravan weighs more than the laden tow vehicle (especially when overtaking).
do not overload your caravan (particularly at the rear)
make sure your tow ball mass is between 8-10%.
avoid having a laden tow vehicle that is lighter than the laden trailer. If it is, keep speed lower accordingly.