Updated April 2017
Reducing caravan sway (yaw) necessitates minimising its causes – and only then adding devices promoted as reducing it. This article shows why and how.
Minor caravan sway is inevitable with any caravan towed via a hitch that is some distance behind the tow vehicle’s rear axle. That overhang does not just permit that sway – it causes it. As shown below, if a tow vehicle sways (yaws) anti-clockwise (due say to a minor camber change, or wind gust) it causes the tow vehicle to sway clockwise. If the caravan sways it likewise causes the tow vehicle to sway.
If the caravan yaws (sways) in one direction, that overhung hitch causes tow vehicle to yaw in the other direction. Pic: caravanandmotorhomebooks.com
The sway energy is dampened by tow ball friction, tyre footprint tread distortion and wind resistance. As long as the sway dies out within two to three cycles it is annoying but mostly harmless. As long as it does so, it is safe to add one of the many anti-sway devices available.
If however (without an anti-sway device) swaying continues undamped – the cause of that must be found and fixed before adding any type of anti-sway device.
Reducing caravan sway – identifying the causes
Any/all of the below assists to reduce caravan sway.
* Minimising tow hitch overhang. Of tow vehicles typically used in Australia the average distance from the centre of the rear axle to the tow ball is 1.26 metres. (Be wary of any utes with extended chassis.)
* Ensuring adequate tow ball mass. That recommended by for typical Australian caravans is 8%-12%. That for EU design caravans is 6%-7%.
* Having a (laden) tow vehicle heavier than the laden caravan (ideally 30% heavier) and particularly with with end-heavy caravans longer than about 5.5 metres.
* Reducing (for towing only) the tow vehicle’s front tyre pressure by 3-5 psi (20-35 kPa).
* Loading all heavy items as close as possible to the axle/s (or in the tow vehicle) and loading light bulky items (such as clothing, bedding etc.) equally front and rear.
Reducing caravan sway – tyres
Caravan sway is primarily reduced by the action and grip of the rear footprints of the tow vehicle tyres, and to a lesser extent the caravans’. It is enhanced by stiffer tyre side-walls. It assists to use Light Truck tyres of the same size. Use them also on the caravan. (Light Truck tyres are also known as ‘C’ rated.)
Many caravanneers (dangerously) increase the tow vehicle’s front tyre pressure as well as (correctly) the rear. Doing so seriously reduces straight line stability – it assists a lot to reduce them by about 5 kPa (35 psi).
By inherently reducing desirable tow vehicle understeer, any WDH reduces the tow vehicle’s straight line stability typical. Stability is further prejudiced by overly adjusting weight distribution via a WDH. Their makers (in the USA) now advise to correct only 50% of the tow vehicle’s front end lift.
Reducing caravan sway – design
A (2017) Australian-built caravan is 6-6.5 metres. It weighs (empty) 2100-2200 kg. The laden mass is typically 2500-2600 kg. Some are 4000 kg plus.
Until recently, most had a recommended (and desirable) tow ball mass of about 10% of their laden weight. Some still do. Many tow vehicle makers, however, are reducing their permitted tow ball loading. It is now typically 250 kg or less. Many local caravan makers then reduced unladen tow ball mass – to as low as 4.2%.
Low tow ball mass increases probability of non self-correcting sway. Caravan & Motorhome Books can (legally) only suggest owners follow the makers’ recommendations. It does not endorse such recommendations.
A vehicle’s ‘towing capacity’ does not relate to its ability to support a trailer but what it can pull up a hill, ability to restart on a hill, etc. It’s about engine torque, gearing, strength of drive shafts etc. In effect it is what that vehicle can pull where – on the end of a rope.
Reducing caravan sway – loading
* Avoid locating heavy stuff behind the axle/s.
* Avoid travelling with filled water tanks – if other than close to centrally mounted.
* If feasible re-house rear mounted spare wheel/s under the chassis – ideally ahead of the axle/s.
* Never have a tool box or cycle rack at the rear.
* Re-house A-frame mounted gas bottles in ventilated side lockers, Re-house batteries close to or over the axle/s.
Reducing caravan sway – basic sway control
Consider adding ‘sway control’ devices only after everything possible has been done as outlined above. If you do so, reducing caravan sway is usually possible such that it is minor and self corrects. If this is done an anti-sway device will virtually eliminate sway discomfort.
The AL-KO tow ball housing has pressure loaded friction pads. It is simple and effective. But, as sway force energy increases at speed yet pad friction remains constant, it is less effective at high speed. It is best limited to short, light caravans. There are also other friction devices. These are not necessarily better or worse. Some have clearly been designed to avoid infringing prior patents.
The AL-KO sway reducing friction tow hitch. Depressing the handle forces friction pads against the side of the tow ball. Pic: AL-KO Europe.
There are also sprung loaded cam mechanisms. These, in effect, ‘lock-up the tow ball’ such that tow vehicle and caravan are literally held in a straight line. Normal cornering is enabled by tyre side wall and footprint distortion. Low speed tight radius turns exert forces that overcome the spring mechanism and enable the cams to release. The device is thus ‘all or none’. Whilst effective, when sway forces (in say an emergency swerve) exceed the sprung cam’s ability to remain closed, pent-up sway energy is suddenly released when least needed.
The Hayman Reese dual cam sway control system. Pic: Reese USA.
Reducing caravan sway – electronic stability control
Sway (with well designed and correctly loaded) caravans is normally controllable. An emergency swerve at high speed, however, may result in forces that massively exceed your rig’s ability to self-correct. If that happens above a critical speed (specific to each rig and its loading), recovery is virtually impossible. (The rig has literally become a chaotic system in that ongoing actions cannot be realistically be determined.)
That caravans can be become seriously unstable is shown by a huge world market for products aimed at correcting it.
Recent products sense the sway level and brake the caravan’s wheels (only). This assists to straighten the caravan (and tow vehicle). More importantly, it drops the speed below that critical. There are two main approaches:
That of Tuson Dexter operates at relatively minor levels of sway. If exceeded it brakes the caravan wheel/s opposite to the direction of sway. Whilst effectively eliminating minor sway, by do so it masks inherent stability failings. The maker warns that it cannot overcome the laws of physics but, as many a caravan forum shows, this is often ignored.
The IDC, and generally similar AL-KO ESC, are emergency systems. The AL_KO unit operates only at a high level of ‘sway force’ (about 0.4 g), or four repeated at 0.2 g). If exceeded it applies 75% of full caravan braking to all (braked) caravan wheels in short 1-3 second bursts. Currently (April 2017) it is suitable only for caravans under 2500 kg.
The AL-KO ESC system works like this Pic: AL-KO Europe.
These products are being routinely fitted to long, high and end-heavy caravans (often) towed by much lighter vehicles.
The forces associated with 5-7 tonnes of caravan and tow vehicle swaying strongly at (say) 100 km/h is huge. These devices ultimately rely on the caravan tyres’ small rubber footprints’ grip to brake such forces.
No such system is likely to be effective on dirt roads (where many roll-overs occur). It is the view of Caravan and Motorhome Books that these units be seen only as last-resort ‘parachutes’ for use on hard surfaced roads. Not a substitute for safe design.
Reducing caravan sway – the very best solution
Early goods carrying trailers towed via overhung hitches swayed badly. Many overturned. By 1920 (US) Fruehauf trailer maker realised how and why. Moving that hitch to directly over the tow vehicle axle solved the problem. The resultant fifth-wheel trailers rarely sway at all. This format is by far the most preferable for long heavy caravans.
See: http://caravanandmotorhomebooks.com/fifth-wheel-caravans-safer/ For an (ongoingly updated) technical explanation of caravan and tow vehicle behaviour see: http://caravanandmotorhomebooks.com/caravan-and-tow-vehicle-dynamics/ There are also other articles on all aspects of caravan on-road behaviour on this website.
See also the video-enhanced feature re this at the October 2016 edition of Rvdaily.com
If you found this article of value my books will prove even more so. To quote Caravan World magazine: ‘Collyn Rivers has put his encyclopedic knowledge into print . . . there is virtually no issue he hasn’t covered.’
Collyn’s all-new Caravan & Motorhome Book covers caravan towing in depth. His other books are the Camper Trailer Book, Caravan & Motorhome Electrics, Solar That Really Works (for RVs) and Solar Success (for home and property systems). For information about the author: Click on Bio.
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