Updated May 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 at some distance behind the tow vehicle’s rear axle. That overhang, however, does not just permit sway. It causes it. When a tow vehicle sways (yaws) anti-clockwise (e.g due to camber change, or wind gust) it thus 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 thus causes the 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. If sway dies out within two to three cycles it is annoying but mostly harmless. As long as it does, it is safe to add an anti-sway device.
If (without an anti-sway device) swaying continues undamped – its cause must be found and fixed before adding that device.
Reducing caravan sway – identifying the causes
Any/all of the below generally assist to reduce caravan sway.
* Minimising tow hitch overhang. Of tow vehicles used in Australia, distance from the rear axle to the tow ball is typically 1.24 metres. Be wary of any utes with extended chassis – as that extends this.
* Ensuring adequate tow ball mass. That recommended by for typical Australian caravans is 8%-10%. That for EU design caravans is 6%-7%.
* Having a (laden) tow vehicle heavier than the laden caravan (ideally 30% heavier), particularly with with long end-heavy caravans.
* 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). Also 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. Also, but to a lesser extent, the caravan’s. It is enhanced by stiffer tyre side-walls. It generally 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 caravaneers (dangerously) increase the tow vehicle’s front tyre pressure as well as (correctly) the rear. Doing so seriously reduces straight line stability. It assists to reduce front tyre pressure by about 35 kPa (5 psi).
By inherently reducing desirable tow vehicle understeer, any WDH reduces tow vehicle stability. Stability is further prejudiced by overly adjusting a WDH. USA maker now advise to correct only 50% of the tow vehicle’s front end lift.
Reducing caravan sway – design
A (2017) Australian-built caravan is typically 6-6.5 metres. It weighs (empty) 2100-2200 kg. Laden mass is typically 2500-2600 kg. But some are 4000 kg plus.
Until recently, most had a recommended (and desirable) tow ball mass about 10% of 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. It relates to what can pull uphill, also its 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. 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 usually 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 thus 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 ‘lock-up the tow ball’. Tow vehicle and caravan are thus literally held in a straight line. Normal cornering is enabled by tyre side wall and footprint distortion. Tight radius turns exert forces that cause the cams to release. The device is thus ‘all or none’. Further, when emergency swerve forces exceed the sprung cam’s ability to remain closed, pent-up energy is released when/where 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 speed, however, may result in forces that far 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. Moreover, the rig has literally become a chaotic system. It’s ongoing actions cannot consequently be realistically determined.)
Recent products sense sway level and brake the caravan’s wheels (only). This assists straighten the caravan (and tow vehicle). More importantly, it drops the speed below that critical. There are two main approaches:
Tuson Dexter’s operates at minor levels of sway. If exceeded it brakes the caravan wheel/s opposite to the sway direction. Whilst effectively eliminating minor sway, it masks inherent stability. The maker warns it cannot overcome the laws of physics – but this is often ignored.
The IDC, and AL-KO ESC, are emergency systems. AL_KO’s 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 braking to (braked) caravan wheels in 1 to 3 second bursts. It is limited to caravans under 2500 kg.
The AL-KO ESC system works like this Pic: AL-KO Europe.
These products are, however being 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 at (say) 100 km/h are huge. These devices rely on the caravan tyres’ small rubber footprints to brake the forces. No such system is effective on dirt roads (where many roll-overs occur). Caravan and Motorhome Books consequently regards these units as last-resort ‘parachutes’ for use on hard surfaced roads. Not substitutes 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 thus by far the most preferable for long heavy caravans.
See: Fifth-wheel Caravans are Safer/ . For an (ongoingly updated) technical explanation of caravan and tow vehicle behaviour see: Caravan & Tow Vehicle Dynamics . See also Caravan Weight Safe to Tow, and Why Caravans Roll Over. There are also other articles on all aspects of caravan on-road behaviour on this website. All are routinely updated.
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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