A swaying caravan – we’ve all been there. It’s a scary feeling, isn’t it? It’s not hard either, these days, to find a video of some unfortunate – unprepared and possibly ill-equipped – driver losing the lot while overtaking a semi-trailer at freeway speeds. That semi is blasting air sideways as it pushes forward and as soon as you enter that turbulence your combination starts to snake – or yaw, to use the proper term. Trailer yaw is caused by the overhang that results from the hitch on a regular van set-up being behind the rear axle of the tow vehicle. As soon as a side force, such as that windblast, hits either the tow vehicle or the van then the snaking begins. What happens next is crucial.
Towball and other friction devices in effect partially lock caravan and tow vehicle together such that the yaw energy is dissipated via shuffling of the caravan’s and tow vehicle’s tyre footprints. Snaking is also related to tow ball mass. Within reason the greater that mass, and the heavier the tow vehicle, the less the tendency to snake. The UK and EU approach achieves realistic stability using a tow ball mass of 5%-6% of the laden trailer’s weight. (UK research indicates 6%-7% is desirable for such caravans). Some Australian mega vans now, however, have only 5%. A few have even less and (sane) loading does not change that substantially. This is way below the previously recommended 10%. The reduction seems, primarily, to reflect many local tow vehicles’ reduced permitted tow ball mass.
As that tow ball mass becomes limited, additional friction, such as AL-KO’s AKS3004 stabiliser friction 50mm ball coupling, is required. To a varying extent these devices partially ‘lock’ trailer and tow vehicle in a straight line – with mild cornering enabled via tyre footprint deflection. The devices release for tight turns. These mechanisms dampen low level snaking, but may thereby mask inherent instability. Such instability is often seen as that which does not die of its own accord within two-to-three cycles.
Practical testing in the late 1970s resulted in rigs jack-knifing if the yaw angle exceeded about 15 degrees at speeds as low as 70km/h, and/or the caravan’s yaw acceleration exceeding 0.3 or so g. Automatically braking the trailer wheels was found to help the driver regain control. It was also found (the hard way) that if side forces exceeded a certain level whilst above a critical speed (unique to the specific tow vehicle, trailer and loading) that rig was instantly triggered into what physicists call ‘chaotic’ behaviour – resulting in almost instant jack-knifing and possible roll-over.
Recovery in a chaotic situation is beyond driver control. This is because whilst not random, chaotic behaviour is random-like. It is not possible to predict what it is about to do next by observing what it is doing now.
That onset of chaotic behaviour is called the rig’s critical speed. If that onset is detected (by yaw that exceeds a certain level) instant braking (of the caravan alone) can bring the rig below that critical speed. This enables a reasonably competent driver to regain control.
Electronic Stability Control
In the early 2000s, BPW Fahrzeugtechnik (in Germany) and AL-KO Europe developed devices that addressed this issue. Both monitor snaking (yaw) and apply the caravan brakes if yaw exceeds preset levels. The two systems do this in different ways but are, to some extent, comparable.
The AL-KO system works only with AL-KO brakes, but as these are used on most locally-made caravans this is a minor issue. It is powered from the tow vehicle and actuates when yaw exceeds 0.4 g (or four consecutive levels of 0.2 g). It operates the caravan brakes in bursts of two to three seconds at 75% of full braking, repeating this cycle if required. Each system is tailored for the size and weight of the caravan. The upper limit is 2500kg for single axle and 3000kg for twin-axle caravans.
The BPW Fahrzeugtechnik IDC unit works in much the same way. It is made in various sizes for caravans up to 2800 kg laden weight. The company warns that whilst increasing safety IDC cannot increase the safe driving speed of the rig concerned – and that physical limitations are not overridden by its use. Both systems wipe-off speed, enabling a reasonably competent driver to regain control. In effect, they are parachutes that actuate to prevent otherwise probable (or inevitable) jack-knifing.
DYNAMIC STABILITY CONTROL
The Dexter DSC system, manufactured by Tuson RV Brakes (USA), is powered via the caravan battery. It detects vertical as well as lateral movement – disabling itself during severe off-road going. This enables it to avoid unwanted braking due to sudden low-speed side forces.
DSC is triggered at lower levels of snaking. It activates proportionally to the magnitude of yaw angle and rate of change and at lower levels of yaw with smaller amounts of braking. It increases braking intensity as yaw increases. (Whilst the system does not monitor g level as such it is generally seen as operating at about half that of the IDC/AL-KO systems.)
DSC differs from the BPW IDC and AL-KO systems in that it brakes the caravan wheels differentially. Speed is reduced and snaking forces directly reacted. Dexter DSC is not weight sensitive: it self-adjusts. It is claimed to work with virtually any type of brakes – enabling it to be retrofitted.
The Dexter system assists to reduce snaking at all speeds, but in doing so may mask an inherent tendency to snake that is better addressed at source. This is, however, compensated by the units’ ability to operate at low levels of snaking that straightens the rig, and reduce its speed at lower yaw levels and speed than the BPW (IDC) and AL-KO units.
When I raised this issue with Hugh Evans of local Dexter agent, Melbourne Trailer & Caravan Supplies, he emphasised that DSC is an accepted way of enhancing the safety of already optimally stable caravans. The company does not see it as a way of compensating for poor design, mismatched systems, or incorrect loading. It is refreshing to note that Dexter’s claims are thus less than that of some of its users!
Regarding Dexter’s differential braking, AL-KO’s Brad Hooper advised RV Daily that AL-KO believes the best option in an emergency situation is to brake all wheels (rather than individual sides) as this wipes off speed and stopping distance in the shortest possible time period. For their intended purpose, these systems (generically known as Electronic Stability Control – or ESC) assist rigs to recover from many situations that no driver – no matter how skilled – can possibly hope to do. It is for this reason that some insurers now offer a discount if any of the systems mentioned in this article are correctly fitted.
The major difference between IDC, AL-KO and Dexter ESC systems is that the former two give precedence to reducing the rig’s speed as rapidly as possible (to bring it below that critical speed where recovery is otherwise impossible). They operate only in situations deemed to be potentially dangerous. They are not intended to, nor will, mask inherent instability.
While all these systems are effective, none is idiot-proof. As we raised at the outset, ESC is not a failsafe. They cannot overcome the basic laws of physics – and none of their makers pretends they can. If a rig is driven too fast to recover while cornering it will go off the road regardless of any ESC device.
Whilst Dexter specifically warns against this, there is seemingly a risk of makers or owners of caravans of sub-optimal stability – or towed by an inappropriate vehicle relying on any of these systems, rather than addressing stability issues at the source. They can lull owners into a sense of misplaced security. But for situations that truly reflect their intended purpose and installed in an appropriate caravan towed by an adequate tow vehicle (in my opinion) these systems are of considerable value.
Words Collyn Rivers.
Footnote: The Dexter Tow Assist system shown in the last video above is a result of a partnership between Dexter and AL-KO and Bosch, a leader in automotive safety technology to bring proven intelligence and reliable functionality to the trailer towing market.
