What causes it, what you can do about it but more importantly how to avoid it in the first place – advice that might just save your life!
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The dashcam video shows a horrific sequence of a fast overtaking caravan and tow vehicle whose minor sway escalates until the combination jack-knifes seconds after.
That the dashcam captures the incident is not necessarily chance. That overtaking manoeuvre is likely to have caused it. So too can a fast-moving truck travelling toward a fast-moving rig on a two-lane road. This was suspected as far back as 1983: an NRMA booklet (Towing in Australia – Pain or Pleasure) warned that . . . in really serious cases this can cause the trailer combination to go completely out of control and jack-knife.
The causal mechanism was described in detail by traffic engineer Rob Caldwell in his (2014) paper Caravans and Trucks Sharing Roads in Australia. Caldwell stated that ‘…these situations can lead to loss of control, a collision with the truck or jack-knifing, possibly ending in vehicle rollover…’
Here’s what happens – and the consequences.
A big truck, particularly if flat-nosed, generates a diagonal bow wave of considerable force. It also creates a vortex (a sort of air-like whirlpool) along its immediate side. This vortex causes an overtaking rig to be initially drawn toward the truck when it is some metres behind the truck’s cab. But, just as the tow vehicle nears the front of that truck, the very strong bow wave pushes the front of the tow vehicle strongly away from it. Meanwhile, the vortex sucks the front of the caravan towards the truck – a movement enabled by the overhung hitch causing it to sway strongly.
If in such a situation, hold the steering wheel firmly but use pressure only to prevent the sway building up. Do NOT attempt to correct the sway by using opposite lock. Do NOT brake the tow vehicle but, if feasible, use the override on the caravan brake controller to apply the caravan brakes.
An AL-KO or Tuson electronics sway-control system may usefully actuate in this situation.
Such sway, from any cause, is fundamental to all rigs with an overhung hitch. When the tow vehicle swings one way, that overhang causes the caravan to sway the opposite way. Likewise, a caravan’s sway causes the tow vehicle to sway. This sway is damped by intended friction between the tow ball and its receiver (but not if greased!), and a friction like shuffling effect inherent in the footprint of the tow vehicle’s rear tyres on the road.
The greater that overhang, the worse the effect. It is no coincidence that so many such accidents involve dual-cab utes with extended chassis.
If that overhang is not excessive, and the caravan is a sane percentage of the tow vehicle weight (and correctly loaded, and with adequate ball mass) sway should die out within two to three seconds. If otherwise, sway may escalate.
This instability is fundamental. It is not simply allowed by that overhung hitch: it is created by it. If it dies of its own accord (at all speeds) it is uncomfortable and annoying – but mostly harmless. Friction mechanism and locking cams increase that damping but do cope with potential jack-knifing forces. When overwhelmed, they suddenly release pent-up energy into the system when least needed. Further, and as with electronic mechanisms that act at low levels of sway, they mask fundamental instability.
At low-ish speeds a strong side force as with that truck-affected rig may just cause the rig to sway around a bit and then settle down. The effect, however, is very much is speed related. If you are attempting to overtake a truck travelling at (say) 100 km/h, your rig is likely to will be travelling at 110-120 km/h. What may happen next is explained (by what physicists call) chaos theory.
Given a sufficient nudge (here, the truck’s bow wave) and an adequate energy source (momentum) a fast towed caravan may exceed a certain critical speed (unique to each rig). If that is so the rig is triggered into a sequence the behaviour of which is not random, but not realistically predictable either. It is now totally out of driver control. The violently swaying caravan generates such side forces on the tow vehicle’s rear tyres that they cause that vehicle to fish-tail violently. That overhung hitch, in turn, causes causing the caravan to do likewise – but in the opposite manner. After one or two more increasingly violent swings the rig jack-knifes.
At speed, any strong enough side force can do this: cornering too fast, sudden changes in camber, inner wheels run onto a soft surface. Its possibility can be minimised but never totally eliminated in any rig with that overhung hitch. A fifth-wheeler (that has its hitch over the tow vehicle’s rear axle) will, in this scenario sway a few times. But that’s all.
The caravan weight as such is not the major issue.
Vital, however, is where that caravan’s weight is located. The ideal is a compact mass just to the front of a well set-back axle. That’s impracticable, but the closer to that ideal the better. Only too many caravans are virtually the opposite. Long and heavy, with 100 kg or more of batteries, gas bottles and spare fuel up front. Heavy washing machines and spare wheels at the extreme rear: and even tool boxes and laden cycle racks. Virtually recipes for jack-knifing because the effect of weight at the end of a beam (mass) is far greater than that weight centralised. Two 40 kg spare wheels at the extreme rear of a long rig have an effective total mass of more than 300 kg! This issue is worsened if the tow vehicle is not substantially heavier (many are much lighter).
As with horse-drawn carriages, a towed caravan ultimately relies on the mass and stability of whatever pulls it. This is basic physics. With the Olympics on as I write, the ‘throwing the hammer’ event shows that even the heaviest athlete can barely control that rapidly spun 16 pound (7.28 kg) ‘hammer’ just prior to release. When released it goes hard and fast and in a straight line.
The current and (in my opinion) inevitably escalating caravan rollover rate is reversible only by manufacturers and owners accepting that caravans are not exempt from the basic laws of physics relating to moving mass. This rules out anything long and end-heavy towed via an overhung hitch.
For those with a deeper interest in such issues please read my ongoing work in progress (http://caravanandmotorhomebooks.com/caravan-and-tow-vehicle-dynamics).
The author and RV Daily acknowledge input from Rob Caldwell MITE (Life) MAITPM in the preparation of this article.
5 Rules you must Obey
- Do not even think of exceeding 110 km/h when towing (ideally at 100 km/h)
- When loading, place everything heavy close to the axle/s – never at the rear.
- When using a WDH with a heavy caravan correct to only 50%. Ideally, the caravan should then be about 5 cm low at the front.
- When towing, add at least 5 psi (35 kPa) to the tow vehicle’s rear tyre pressures. Reduce front tyre pressures by the same amount.
- If the rig’s left side wheels run onto soft going never attempt to correct by steering. Do not brake. Allow the rig to move as is – letting it gently roll to a stop. Only then, restart and bring it back gently onto the road.