Updated 19 June 2016
Making caravans more stable is readily possible by design, loading, and tow vehicle use and choice. This article by Collyn Rivers explains how. It also provides practical guidelines for buying a caravan and tow vehicle, their loading and on-road usage. For a full technical explanation of why rigs can be unstable please see my Caravan and Tow Vehicle Dynamics.
Making caravans more stable has always been desirable. For Australian (and US) caravans it is often essential. Such caravans are increasingly heavier, yet new tow vehicles are increasingly lighter. In 2013, one Australian insurer (CIL Insurance) alone reports 135 caravan rollovers.* Of these, 88% related to ‘loss of control’, and 4% ‘fell off the towball’. According to CIL ‘Lost control accounts for 91% of the rollovers . . . in all cases, the van began to fishtail and the driver wasn’t able to bring it back under control.’ The ‘loss of control’ points to incorrect loading, inadequate tow vehicle weight, excess caravan weight, excess hitch overhang, driver error, and particularly speed.
An only too typical caravan roll-over. Pic: original source unknown.
The heavier the tow vehicle relative to the caravan the better
Recommendations about caravan/tow vehicle weight and making caravans more stable stem from the 1930s. Caravans back then had an inside length of 4-5 metres. They weighed about 1000 kg. Most were towed by cars heavier than 1000 kg. They rarely exceeded 80 km/h. Associated legislation also stems from this era and relates only to maximums. Most recommendations however noted that the laden caravan weight should not exceed the unladen weight of the tow vehicle. Many such recommendations are now under review.
Most Australian caravans are now about 6.5 metres. They weigh about 2200 kg. Some are 7-8 metres and weigh over 3500 kg. One recently released is 4000 kg. Many are towed by vehicles far lighter than the caravan – at 100-110 km/h. Worse, a recent issue of a major Australian Caravan magazine reports that a substantial percentage of heavy caravans submitted for review were even heavier than their makers claimed.
Before finalising payment for any caravan (new or used) have it it weighed in your presence on a certified weighbridge. Never assume the Tare Mass – legally required to be shown on a plate affixed to the chassis – reflects reality. That weight is as the unit left the factory. It does not include the 1kg/litre of all water carried. Nor (in many cases) any of the optional extras even if included in the original order. It is not unknown for a caravan to be so heavy that there is no allowance for anything to be legally carried within it.
Tow ball mass
An essential element for making caravans more stable is that the caravan must be nose heavy. From the 1930s until 2013 or so virtually every caravan maker in Australia recommended nose mass of about 10% of the caravan’s loaded weight. One now retired technical writer (Tom Olthoff) built a decades-long career on this virtual mantra. It worked well enough for caravans of 4-5 metres (14-16 ft) that were of 1100-1400 kg, and tow vehicles that could handle that and more.
It does not work however for caravans that are longer and heavier because it is contrary to basic laws of physics. It overlooks the effect of Moments along a pivoted beam. This precludes basing tow ball mass on a percentage of overall weight alone. That which matters – for making caravans more stable, is the trailer’s length and where and how much weight is located along it. Because of this, many a tow vehicle seen as having ‘inadequate’ tow ball loading may be totally capable of towing a short light central kitchen caravan. Such practice is common in Europe. There, tow ball weight is typically 5%. (Extensive research however indicates that 6%-7% is preferable of EU type caravans.)
Tow ball mass too low?
Were many new locally-built caravans to adhere to the 10% (that the industry long maintained was required for making caravans more stable) the tow ball mass required would be way above that now permitted by many tow vehicle makers. Yet (often with with no apparent changes to the caravan’s design) many local caravan makers simply reduced their tow ball mass recommendations accordingly. The average of thirty typical 2015 locally built caravans is (according to their own specifications) 7.1%. One is a mere 4.2%. Some however remain at 9%-10%.
It is often argued this is unladen mass but, as the units are intended to be laden evenly for and aft of the axles, full loading will barely affect tow ball mass. Some makers suggest towing with one or more water tanks empty: substantially negating the very point of having them.
Legal reasons preclude my suggesting anything other than to ‘follow what the caravan maker recommends’ re tow ball mass. They do not require me to endorse such recommendations. I don’t.
It is not so much weight but weight distribution that matters
In making caravans more stable overall weight is less of an issue than where weight is distributed. You can increase stability (when loading) by having anything heavy above and close to the axle/s. Never locate anything heavy (particularly high-slung spare wheels) at the extreme rear of a caravan – either inside or outside. Locating a tool box on a rear bumper is an absolute no no.
Many owners believe that as long as a caravan’s recommended nose mass is attained it is fine to load up the front and rear to achieve this balance. This is a dangerous misunderstanding.
A short centre heavy caravan is far less a concern than an end-heavy caravan of the same length. The same applies to loading the caravan. (Some camper trailers weigh 1500 kg or more, but as most are under 4 metres and with minimal rear overhang, tow comfortably with well under 5% tow ball load).
If buying a caravan my advice is that its fully laden on road weight should not exceed about 70% of the laden weight of the tow vehicle. Further that its maximum body length should not exceed about 5.5 metres unless towed by a seriously heavy 4WD.
The tow vehicle
Making caravans more stable requires the tow vehicle to be heavier than the caravan. The greater the ratio the better.
The Camping and Caravanning Club of the UK, and other UK bodies, now recommend the weight of the laden caravan should not exceed 85% of the car’s unladen weight. There is a suggestion that experienced caravanners may go up to 100 per cent of the car’s unladen weight.
Germany legislates that caravans with brakes and hydraulic shock absorbers may not (by legislation) exceed 0.8 times the unladen weight of the tow vehicle.
The Caravan Council of Australia suggests that ‘for added safety and peace of mind, the laden tow-vehicle should weigh 30% more than the laden caravan/trailer’. This implies that that the caravan should not exceed about 77% of the laden weight of the tow vehicle. Its recommended 77% is less stringent than that of the UK and Germany (where towing speed limits are 20 km/h lower). See Caravan Council of Australia.
Few Australian caravan/tow vehicle combinations meet these independent recommendations. For actual details see http://caravanbuyersguide.com.au/tow-vehicle-caravan-weightWhilst environmentally unsound, for making caravans more stable, for any over 2200 kg laden, buy the heaviest tow vehicle you can find. But see below re minimal hitch overhang.
Another major factor in making caravans more stable is the distance from the tow vehicle’s rear axle (centre line) to the tow ball. The shorter that rear overhang, the less a caravan’s tendency to pitch and snake. The average hitch overhang of typical Australian tow vehicles is 1.26 metres. The longest are extended chassis utes. It is unlikely to be a coincidence that so many caravan roll-overs now involve these vehicles.
For making caravans more stable, the tow hitch too should have the minimum possible extra overhang. Reducing that alone assists stability.
Making caravans more stable. Excess hitch shank length this should be avoided. Some hitches have adjustable shanks. If yours is like this fit one that is shorter – or have an engineer shorten the shank and drill a new hole.
Speed is a critical factor
Any given combination of tow vehicle and caravan has a unique critical speed. Above that speed any sufficiently strong disturbing force may trigger it into non-recoverable jack-knifing. This typically results in the rig overturning. Despite Europe’s general 80 km/h tow speed limit and inherently more stable caravans, AL-KO (Europe) rates speed as the major factor in caravan roll-overs. Tests in the UK showed that, optimally locating typical personal effects, food etc alone, can affect the critical speed by as much as 25 km/h. One source is Dynamics of Towed Vehicles (Bath University).
An issue in Australia is that (except in WA) caravans can legally be towed at up to 110 km/h. This is a speed limit however, not a ‘recommended’ speed. Many drivers resent being held up by slow travelling caravans but to tow a caravan that is heavier than the tow vehicle at speed is nevertheless risky. An evasive swerve, or strong side gust, can trigger the caravan into an irreversible snaking cycle. Most owners never experience such an event. Some do.
Braking and accelerating
Driving fast down a hill that has bends or changes in road camber whilst towing is a major risk. When doing so, keep in a low gear at no more than 70 km/h. Never brake hard because doing so can cause the caravan to sway. When descending a hill, and particularly if braking, gravitational forces acting on the rear of a swaying caravan will increase that sway. This is also an issue on long winding motorway downgrades. This mainly affects end-heavy caravans. Except in extreme cases, light caravans, up to five or so metres, are less at risk if loaded sanely.
Advice is often given on forums to the effect that accelerating (by pulling the rig straight) effectively corrects sway. It is better (where the caravan braking system permits) to gently brake the caravan alone). Accelerating whilst swaying at higher speeds may cause the rig to exceed the critical speed at which the sway suddenly escalates. In that event jack-knifing is likely.
Sway control systems
Any trailer towed via an overhung hitch has a natural tendency to sway. With well designed and laden caravans towed behind a suitable vehicle, such sway normally dies out of its own accord within two/three cycles. It is mildly annoying but harmless.
After-market sway control systems usefully and effectively control such swaying. Most introduce friction that dissipates sway energy as heat. So-called dual cam systems ‘lock’ the caravan and tow vehicle together in a straight line. Normal cornering is effected by tyre distortion. The cams release for tight cornering, but also when sway forces are excessive.
Whilst both are effective to a point, if fitted to a caravan that does not self-correct swaying within those two/three cycles, these devices mask a dangerous underlying condition. They are like take pain killers for a serious condition, instead of medical treatment. Sway control is routinely included with some UK/EU caravans – but only as an aid to comfort.
Electronic stability control
Electronic stability systems have been used in some cars and 4WDs for many years. Apart for the BMW X3, none relates to caravan stability. Those for caravan stability were pioneered in Europe but there are now USA products.
Europe’s IDC, and AL-KO ESC activate when caravan sway that exceeds an acceleration level of about 0.4 g (an uncomfortable level) and automatically apply caravan braking – in the case of the Al-KO for 1-3 seconds at 75% of full braking. This reduces the sway, but particularly reduces the speed below the critical level. They can only be fitted to caravans that use the maker’s respective brakes.
Making caravans more stable. How the AL-KO system works
The US products (from Dexter, and Tuson) operate at lower levels of sway acceleration (about 02 g) and brake each caravan wheel at whatever level is deemed optimum for each. The makers’ claim they can be fitted to caravans with any form of braking. Whilst both systems are worth fitting they are not a substitute for making caravans stable.
When these systems are triggered they not only attempt to reduce the sway but also to brake the caravan’s speed. If using any such system never engage cruise control whilst towing. They will attempt to accelerate the rig to its preset (and previous) speed.
Various aids to making caravans more stable
There is no stability benefit in having dual axles unless caravan weight demands. If anything the opposite is so – they add weight.
Follow European practice by having nothing mounted on the A-frame. Locate gas bottles (if used) in a ventilated centrally located locker – or as close to the axles as possible.
For making caravans more stable, set the axle/s as far back as you can – yet maintaining a tow ball weight that the tow vehicle can realistically handle.
The side wall area ahead of the axle/s needs to be marginally greater than behind the axle. This reduces sway caused by side wind gust and by long trucks passing, or passed, at speed. The 1990s Phoenix shown above has diagonally sloping walls at its front to ensure the set-back axles do not result in excess side (frontal) area. It is still respected for its excellent stability.
How can I tell if my rig is unstable
No trailer pulled via an overhung hitch can ever be 100% stable. The technical explanation is that if the caravan sways that automatically causes the tow vehicle to sway in the opposite manner. And vice versa. Nose mass and correct weight distribution assists to limit this – as does a tow vehicle much heavier than the trailer.
Bye and large a rig is likely to be acceptably safe if minor sway automatically dies out (without driver correction) after two or three such cycles It is mildly uncomfortable but not necessarily dangerous. EU designed caravans often have such sway damping as standard but first ensure the caravan sway is as described above.
Long end heavy caravans with substantial nose mass typically feel ultra-stable on tow. That high nose mass reduces the effect of sway forces. Problem occur however if that caravan does begin to sway – as that very mass that normally keeps it so stable will overcome the tow vehicle’s ability to control. The caravan will begin to fishtail and may cause the rig to jack-knife.
This has been stated time and again but the most common statement made by the driver following such an incident is:
‘It always felt so stable up till then’.
* A breakdown of the CIL Insurance data can be viewed at: http://caravanbuyersguide.com.au/caravan-accidents/
This topic is far too big to cover fully in shortish article form. For more details see my Caravan and Tow Vehicle Dynamics – http://caravanandmotorhomebooks.com/caravan-and-tow-vehicle-dynamics/
The UK article Understanding the Dynamics of Towing: by Simon P Barlow. www.caravanchronicles.com/guides/understanding-the-dynamics-of-towing/ is a generally similar and very down to earth approach. It is eminently readable but less relevant (in parts) to typically heavier Australian/American caravans.
If you liked this article you will like my books! All are written in a similar practical down to earth plain English manner. For comprehensive information on every aspect of camper trailer, caravan and motor home usage (and also solar and RV electrics) please check out my globally selling books. The all-new Caravan & Motorhome Book covers stability issues in depth. My other books include the The Camper Trailer Book, Caravan & Motorhome Electrics, and Solar That Really Works! (for cabins and all RVs). Solar Success relates to large home and property solar. For information about the author please Click on Bio.
This article is copyright (2016) Caravan and Motorhome Books, PO Box 356, Church Point NSW 2105 Australia.