Why caravans roll over is long known – here is why they do – and how to avoid it happening. Collyn Rivers explains.
There are many reasons why caravans roll over. In almost every instance the tow vehicle’s rear tyres loose all grip, causing the rig to jack-knife. By understanding how and why tyres behave as they do, caravan behaviour becomes clear. In 2013 one of Australia’s several caravan insurers (alone) reported 135 caravan roll overs. http://caravanbuyersguide.com.au/caravan-accidents/
A sad ending (pic courtesy of the Caravan Buyers Guide)
Most caravaneers – if they think about it – reasonably (but wrongly) assume that, when steered, today’s tyres normally cause their vehicle to travel where its tyres are pointing. Or and rarely, if/when side/cornering forces became too great, they loose all grip and slide. Further, that only a vehicle’s front tyres have any steering effect: but the rear tyres can and do!
Pneumatic tyres and their effect on vehicle handling is very different. Once that is understood, why caravans roll over becomes clear.
Why caravans roll over – how tyres really behave
Here’s how to visualise what happens. A partially inflated balloon has one person (the ‘driver’) press the sides together (to simulate wheel rims). Then have another (the load) mimic loading by pressing the balloon’s top onto a hard surface. Steering is mimicked by turning the balloon from its ‘wheel rim’ sides. That force causes the balloon’s surface footprint to distort. It becomes an elongated oval-shaped. It has a partly frictional, partly molecular bond with the surface it rolls over. Unless grossly stressed that footprint does not slide.
A vehicle tyre acts just like that balloon. Turning force, exerted on its footprint by steering the wheel (rims), causes the car to turn. It does so – but it follows a wider radius than that of the steered wheel rims.
The angle between where a car’s (if steered) front wheels are pointing and where the car is actually going is called (misleadingly) the ‘slip angle’.
A tyre’s ability to steer increases more or less linearly for the first few degrees of slip angle: then decreasingly so before dropping more sharply away. The transition to where all grip is lost is typically sudden. Tyre cornering ‘power’ also varies with load and (tyre) air pressure. Up to a point it increases with load assisted also be air pressure stabilising the tyre side walls.
Why caravans roll over – unsteered tyres develop slip angles too
Slip angles can and are imposed on otherwise non-steered tyre’s footprints. A yawing caravan causes the tow vehicle’s rear tyre footprints to form slip angles. Yawing literally causes those rear tyres to steer vehicle. Worse: it is steered into the curve so formed – thereby increasing the effect. Yaw affects front slip angles too, but to less extent.
How and why caravans roll over – understeer (good)
The load and side force effects on a moving tow vehicle’s tyre slip angles do not remain equal. They constantly vary front/rear. The difference between those slip angles (front/rear) determines how that vehicle behaves when cornering hard, hit by a wind gust, or a sudden yaw. Pitching too causes front/rear tyre loading to vary considerably and suddenly.
If the front slip angles are always greater than the rear, that tow vehicle has desirable understeer. It attempts to travel in a straight line. Understeer also causes the vehicle to run slightly wide if cornered too fast, automatically reducing cornering forces. It is a subtle effect: few drivers are aware it happens. Understeer results in the vehicle remaining straight on a cambered road.)
How and why caravans roll over – oversteer (bad)
If the rear tyre slip angles exceed the front, they literally steer a vehicle into an ever-tightening curve. If not towing, a skilled driver can correct this by steering. This is not possible when towing as such correction worsens the effect.
Never reduce understeer
Excepting only for (rare) sports cars all modern cars and 4WDs have inbuilt understeer. At their maximum intended loading and correct tyre pressures, their front slip angles always marginally exceed their rear slip angles. Understeer, particularly when towing, must never be reduced.
Understeer is reduced by owners fitting stronger springs, or adding air bags at the rear alone. This causes the front springs to resist roll less, and the rear springs to resist roll more. This varies the front/rear forces on the tyres. Whilst cornering , yawing or subject to side wind gusts, this causes rear slip angles to increase and front slip angles to decrease. This, in turn, reduces understeer.
Apart from making the steering slightly more ‘responsive’ (wrongly seen as a plus) this effect is not apparent in normal driving. It does, however, dictate what happens in a hard swerve at high speed. The rear slip angles may exceed their limit. The rear tyres then suddenly lose all grip and the rig jack-knifes.
Bye and large if you need to stiffen the suspension, that tow vehicle is being laden beyond that intended. If you really have to – do so equally front and rear. Never at the rear alone. A few suspension shops know this – but sadly far from all.
How and why caravans roll over – tow vehicle tyre pressures
Varying tyre pressure ratio (front/rear) affects their slip angles. The tow vehicle maker specifies pressures that result in the required slip angles.
When you tow a caravan, increase the tow vehicle’s rear tyre pressures as the vehicle maker advises (or the tyre maker advises for that laden weight – including caravan nose weight).
Never increase the tow vehicle’s front tyre pressures. Lowering them by about 35 kPa, (5 psi) whilst towing usefully restores some otherwise lost understeer.
How and why caravans roll over – caravan tyre pressures
Caravan’s tyres take up slip angles but (unless all grip is lost) this does not affect stability nearly as much as with the towing vehicle. Having the rear pair of dual axle tyres about 5 psi (35 kPa) higher than the front usefully assists stability.
That which assists (by reducing slip angles) are tyres with stiffer sidewalls (for both caravan and tow vehicle). LT tyres certainly assist.
How and why caravans roll over – correct nose weight
Until 2013 or so, most caravan makers recommended that caravan nose weight be about 10% of the caravan’s laden weight. Then, as lowering emissions requires lighter vehicle construction, many vehicles suitable for towing had maximum tow ball loadings decreased. Rather than designing accordingly many (local) caravan makers ceased recommending tow ball weight. Many now reveal only that when totally unladen. That maybe as low as under 5%. Unless its weight is mainly central, 10% is still necessary for any caravan over 5 metres long and weighing over 1800 kg.
Excess nose weight – pic original source unknown
How and why caravans roll over – tow hitch overhang
A major factor in caravan stability (affecting why caravans roll over) is the distance from the centre line of the tow vehicle’s rear axle to the tow ball. The average distance is 1.24 metres. This should be minimised. Be careful when selecting a hitch. Some otherwise excellent ones add up to 300 mm. A few as shown below (adapted to carry a bicycle) are absurd.
Fifth wheel caravans have their tow hitch directly above the tow vehicle’s rear axle. There is no overhang. If either trailer or tow vehicle yaws (snakes), it barely affects the other. As a result, these caravans are far more stable. If seeking a long heavy caravan, consider one seriously.
Earliest known (circa 1917) fifth-wheel caravan – the Adams Motor Bungalow. Pic: Glenn H Curtiss Museum (USA).
How and why caravans roll over – the effect of a weight-distributing hitch
The nose weight of a caravan that is suspended on a tow ball some distance behind the tow vehicle’s rear axle increases the down force on the rear tyres of the tow vehicle. It decreases that on its front tyres.
A weight distributing hitch (WDH) forms a semi-flexible beam between the caravan and tow vehicle. By using the tow vehicle’s rear tyres as a pivot, it levers the front of the tow vehicle back down. It also transfers a small part of the tow ball weight to the caravan’s tyres.
In doing so, however, that WDH reduces understeer – to the extent that it limits the rig’s maximum cornering ability. This effect can be serious if it is pitching and/or yawing. Both result in large (cyclically changing) slip angles. This may cause the tow vehicle to heavily oversteer – and possibly jack-knife.
The major USA WDH maker Cequent (Hayman Reese in Australia) now recommends using a WDH to correct only 50% (of the effect of tow ball weight). This will result in the front of the caravan being lower than the rear by about 50 mm. However, as any use of a WDH reduces understeer (as increasing road safety reduces the need for crutches) it is better by far to have a rig for which no WDH is required. That is likely to be one where the laden caravan weighs about 70% to 80% of the laden tow vehicle.
How and why caravans roll over – anti-sway devices
Anti-sway (yaw) devices are of two kinds. One assists to dampen yaw energy (as heat) via friction. It also stiffens the link to the tow vehicle. That it turn, transfers some of the yaw energy to the rear tyres of that vehicle. Sidewall and rear tyres footprint ‘shuffling’ dampens it. However, as that increases rear slip angles it adversely steers the rear of that vehicle.
Dual cam versions virtually lock the moving caravan and tow vehicle in a straight line. Cornering, except tightly at low speed, is by distorting tyre slip angles. The cams release only whilst tight cornering at low speed. Anti-sway units are effective only at low speeds. Use them only to further stabilise already stable rigs.
Reese dual cam anti-sway system. The heavy tyre-lever like spring bars normally lock the caravan and tow vehicle in a straight line. The release only on sharp turns.
How and why caravans roll over – electronic stability control
Electronic stability control systems for caravans work by monitoring the caravan’s yaw. They actuate the caravan brakes if that exceeds predetermined levels. Whilst effective their ability to control major yaw at high speed is unknown. Their makers warn that these systems cannot overcome the laws of physics.
How the AL-KO ESC works. Pic: AL-KO Europe
Yaw is controllable manually via the caravan brakes. It is, however, questionable if a driver can/will do this effectively when a rig is yawing strongly.
Never apply vehicle brakes when yawing. Likewise, do not attempt to control yawing. Hold the steering wheel firmly and keep it centralised.
How and why caravans roll over – tow vehicle/caravan weight ratio
Caravans towed via overhung hitches can never be 100% stable (unless towed by an infinite mass). They are inherently unstable but tamed within reasonable limits if designed and used correctly. This excludes caravans heavier than whatever pulls it.
Towing a long heavy caravan behind a lighter tow vehicle greatly increases the causes of why caravans roll over. The risk is, however, less with short heavy caravans. It is all but unknown with camper trailer as few exceed four metres – and are centre heavy. Whilst it is common to see 2.5 tonne vehicles legally towing 3.5 tonne caravans (in Australia), the UK and EU regulations restrict this to less than 80% (unladen). The Caravan Council of Australia seeks to restrict (the laden) ratio to 70%. So does Caravan & Motorhome Books.
How and why caravans roll over – critical speed
Every caravan/tow vehicle combination has a so-called ‘critical speed’. This is the speed at which a sufficiently strong disturbing side force may build up yaw that is not be correctable. No data exists for local product that enables this speed to be calculated. Known (UK) data suggests that for long end-heavy ‘vans towed by vehicles that are lighter that speed may close to Australian speed limits.
How and why caravans roll over – various
Mass (such as washing machines, tool boxes and/or spare wheels) at the extreme rear
Doing anything that raises the height of the centre of gravity
Inappropriate tyre sizes and/or pressures
Messing around with the suspension.
Anything heavy (e.g. batteries, water tank/s, stored goods, and personal effects) above or as close to the axle/s as possible.
This article summarises that accepted (and proven) in the tow vehicle and caravan area. I particularly acknowledge Richard Klein (for his work on the effects of WDHs etc). Thanks also to Colin, Julian, nswoutback, Sal & John and Scott for appreciated support, assistance and advice.
Caravan & Tow Vehicle Dynamics: Collyn Rivers. http://caravanandmotorhomebooks.com/caravan-and-tow-vehicle-dynamics/ This is an ongoing work in progress. It is updated and expanded frequently.
Understanding the Dynamics of Towing: Simon P Barlow. www.caravanchronicles.com/guides/understanding-the-dynamics-of-towing/ This is a very readable approach primarily relating to UK/EU type caravans.
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