by Collyn Rivers – Updated 2020
Reducing Caravan Sway
Reducing caravan sway (yaw) necessitates minimising its causes – and only then adding devices promoted as reducing it. This article shows why and how.
A caravan‘s sway is caused by its tow hitch being located well behind the tow vehicle’s rear axle. When the tow vehicle yaws in one direction that overhung hitch causes (not just allows) the tow vehicle to sway in the other direction. Likewise, caravan sway causes tow vehicle sway.
When a caravan yaws in one direction that overhung hitch causes (not just allows) the tow vehicle to yaw in the other direction. Pic: rvbooks.com.au
Sway energy is dampened by tow ball friction, tow vehicle rear-tyre tread distortion and wind resistance. Swaying that ceases within two to three cycles is annoying. Such sway, however, is mostly harmless. As long as it ceases within those two to three cycles friction sway damping device will assist.
If, however, swaying (without an anti-sway device) does not cease within two to three cycles, its cause must be found and fixed before adding any sway damping device.
In this connection, a survey (on caravanners forum) shows that just over half of all caravanners grease their tow balls. This may save long-term tow ball wear – but substantially reduces what is otherwise useful damping. A far better approach is to use AL-KO’s friction-added tow ball.
The AL-KO friction damper assists to reduce caravan sway.
Friction anti-sway devices have a fundamental limitation.
Friction is a constant force. Caravan sway forces, however, increase by the square of the speed. Thus, while effective to about 60 km/h (37.5 mph) at 100 km/h (62.5 mph) most have a frictional and practical effect of less than 1%. They are thus effective at low speed but close to useless at high speed.
UK/EU caravan makers know and accept this. Their approach is to use friction damping for low/medium speed damping. This is reinforced by AL-KO/Dexter electronic stability control system at higher speeds (towing speed limits in the UK, and most of the EU, however, is only 80 km/h). USA drivers tow at higher speeds but usually with far heavier and longer tow vehicles.
The friction damping plus AL-KO Dexter approach will work well with most Australian-made caravans. All such devices, however, ultimately depend on the maintained grip of hand-sized patches of rubber (i.e. your tow vehicle and caravan‘s tyre footprints) to control 2000-3500 kg (4400-7715 lb) of a possibly violently-swaying caravan.
The sway forces concerned increase with the square of the speed. For example, the forces at 100 km/h are 16 times higher, than at 25 km/h. The makers test them at 60 mph (96 km/h). If towing at 110 km/h, as many Australians do, the forces are thus about 31% greater (not 14%).
Reducing caravan sway – practical ways that work
Average tow hitch overhang (distance from the rear axle to the tow ball) of Australian vehicles is 1.24 metres. If your tow hitch has excess overhang, have a machine shop drill a new hole accordingly. Even a centimetre is worth saving. Utes with extended chassis and/or tow hitches are thus at major risk.
Ensure adequate tow ball mass. That recommended for typical Australian caravans is 8%-10%. That for the typical 40% less weight (per metre) UK/EU caravans is 6%-7%. As of 2020, many local caravan makers recommend as low 4%. This inevitably reduces the speed at which a caravan is likely to sway. And also reduces the chance of being able to recover from it.
It helps to have a (laden) tow vehicle heavier than the laden caravan. Be aware, however, that with caravans, it is their length, and where weight is distributed along that length that primarily affects the tendency to sway. Furthermore, the longer the caravan, the lower its safe towing speed.
With long caravans (i.e.) over about six metres (approx 20 ft) have all heavy items as close as possible to the axles. Never load anything heavy at their extreme rear. It is not clever to have spare wheels on a caravan‘s rear wall. Their actual weight there maybe 40 kg (88 lb) or so. Their effective weight, however, is plus 120 kg (265 lb). If your caravan has them there, relocate them in a slide-out cradle underneath the caravan‘s floor.
Caravan underfloor wheel carrier. Pic: AL-KO (UK).
Increase (for towing) the tow vehicle’s rear tyres pressure by 5-7 psi (30-50 kPa). Some caravanners (dangerously) increase the tow vehicle’s front tyre pressure as well as the rear. Never do this as doing so seriously reduces stability. Use the (front) tyre pressure that the vehicle maker recommends for normal driving. Never higher.
Load all heavy items as close as possible to the caravan‘s axle/s. Never at the far ends. Or (and preferably) in the tow vehicle. Always load that tow vehicle to its legal maximum – the heavier it is the better.
Reducing caravan sway – tyres
Caravan sway is primarily reduced by the frictional grip and part molecular grip of the tow vehicle’s rear tyres. Also, but to a lesser extent, that of the caravan‘s tyres. That grip is enhanced by stiffer tyre side-walls. It assists to use Light Truck tyres (also known as ‘C’ rated) for both tow vehicle and caravan. These tyres will cause a slightly harsher ride, but that’s a small price to pay for safety.
The effect of a weight distributing hitch (WDH)
A weight distributing hitch is an (optional) semi-flexible springy beam between caravan and tow vehicle. Its purpose is to restore some part of the imposed tow weight on the tow vehicle’s rear tyres – to its front tyres.
While necessary where the caravan is heavier than the tow vehicle any WDH inherently reduces the tow vehicle’s ‘cornering power’. The amount by which it does so is related to its extent of adjustment. USA makers now advise correcting only 50% of the tow vehicle’s front end lift. That recommendation is also in the SAE Standard J2807. In practice, this usually corresponds to the front end of the caravan becoming about 50 mm (approx. two inches) lower. Ignore caravan forum ‘advice’ that the rig should be levelled.
Reducing caravan sway – design
A typical Australian-built caravan is now typically 6-6.5 metres. It weighs (empty) 2100-2200 kg (4625-4850 lb). Laden mass is typically 2500-3500 kg (5500-7715 lb). Some, however, are 4000 kg (8800 lb) plus. While excess length is even more of a (stability) issue, Excess weight is now a major issue for the newer generation of tow vehicles.
Until 2016 or so, most locally-made caravans had a recommended (and desirable) tow ball mass of about 10% of the laden caravan‘s weight. Some still have. Many tow vehicle makers, however, have reduced their permitted tow ball loading. It is now typically 250 kg (550 lb) or less. Then, inexplicably without making any apparent physical changes, some local caravan makers then reduced recommended tow ball mass to as low as 4.2%.
Low tow ball mass increases the probability of non-self-correcting sway. RV Books can (legally) only suggest owners follow the makers’ recommendations. It certainly does not, however, endorse such recommendations.
A vehicle’s ‘towing capacity’ does not relate to its ability to support a trailer. It relates to its ability to restart on a hill, etc. It’s about engine torque, gearing, its ability to support the tow ball weight, the 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 unless they are close to centrally mounted. Or unless the caravan builder advises otherwise.
If feasible, re-house rear-mounted spare wheel/s under the chassis – ideally ahead of the axle/s. This is not an opinion. It is an issue of basic physics. Likewise, never locate a tool-box or cycle-rack at the rear of a caravan. If feasible have them in or on the tow vehicle.
Re-house A-frame mounted LP gas bottles in ventilated side lockers.
Re-house batteries close to or over the axle/s.
Do not attempt to tow a caravan by an extended-chassis ute. Ideally, nor by a dual-cab ute. This is not least because of the known risk of imposed tow ball weight bending them. Many, such as that shown below, end up like this after attempting to use the Old Telegraph Track near the top of Cape York.
Dual-cab ute’s bent chassis due to excess tow ball weight. Pic: rvbooks.com.au
Reducing caravan sway – basic sway control
Consider adding ‘sway control’ devices only after everything possible has been done by following the weight distribution (loading) advice above. If you do so, reducing caravan sway is usually possible such that it is minor and self corrects after a couple of such sways. If this is done an anti-sway device will then usually all but eliminate sway.
The AL-KO tow ball housing has pressure loaded friction pads. It is simple and effective. But, as noted above, 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 mechanisms ‘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 cornering exerts forces that cause the cams to release. The device is thus ‘all or none’. Further, when emergency-swerves 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 – the (US) Hensely hitch
While hardly known to most non-USA caravanners, the Hensely hitch, in effect, cancels out the effect of caravan sway. It does so by a trapezoid hitch linkage that projects the pivot point of the trailer forward such that it is effectively close the tow vehicle’s rear axle. By not allowing the caravan to pivot side-to-side, trailer sway is virtually eliminated. The unit’s downside is its weight (of over 40 kg (about 100 lbs). This not an issue in the USA as many owners tow with what in Australia are mega-trucks.
The unit was designed and patented in the USA by Norman Gallatin but was used in a substantially modified form by Hensley. A lighter version is made. As far as is known there is currently no Hensely dealer in Australia.
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. Its ongoing actions cannot consequently be realistically determined.
Recent products sense sway level and brake the caravan’s wheels (only). These assists straightening the caravan (and tow vehicle). More importantly, they reduce the speed below that critical. There are two main approaches:
Tucson-Dexter’s operates at minor levels of sway. If exceeded, it brakes the caravan wheel/s opposite to the sway direction. While effectively eliminating minor sway, this unit masks inherent stability. That should be first addressed and corrected. The maker responsibly warns that the unit 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 (5500 lb).
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). RV 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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