Making Caravans More Stable
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/. See also Why Caravans Roll Over/
Making caravans more stable is always desirable. For Australian (and US) caravans it is essential. Such caravans are increasingly heavier, yet tow vehicles are increasingly lighter. Accidents have escalated since 2014.
According to one major insurer loss of control accounts for over 90% of all rollovers. ‘ In all cases’, stated that insurer ‘the caravan began to fishtail and the driver was unable to bring it back under control.’ Such ‘loss of control’ appeared due to various causes. These include 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.
Making caravans more stable – caravan weight
Many Australian caravans built since 2015 are over 6.5 metres. They weigh well over 2000 kg. Some are 7-9 metres and weigh over 3500 kg. A few are over 4000 kg. Many are towed by vehicles far lighter than the caravan. And at 100-110 km/h. In 2016, an Australian caravan magazine stated ‘many caravans reviewed were heavier than their makers claimed’.
Before finalising payment for any caravan, weigh it certified weighbridge. Never assume claimed Tare Mass is correct. Few are. That weight is as the unit left the factory. It does not include water. Nor (usually) optional extras originally ordered. Any weight over its actual Tare Mass reduces that for personal effects pro-rata.
Making caravans more stable – overall length
Whilst excess weight is undesirable the major factors determining a caravan’s stability is the distance from its tow hitch to its axle/s. Known as ‘radius of gyration’, the longer that is the better. Also assisting stability is the tow vehicle’s wheelbase (i.e. the distance between its front and rear axle). Here, the longer the better.
Making caravans more stable – It is not so much weight but weight distribution that matters
That not generally understood is that towing stability is related to a caravan’s length. In particular, where weight is distributed along that length.
A dangerous assumption (still found on caravaneers fora) is that as long as recommended nose mass is retained, they may load the caravan however they wish. An extreme example, seen at a camp-site, was a caravan that had a motorcycle on its rear. The motorcycle was ‘balanced’ by 200 kg of barbell weights on its A-frame.
The ideal caravan has its axle/s set way back. Then locate everything heavy as close to the axle/s as possible. Never locate anything heavy (particularly high-slung spare wheels) at the extreme rear of a caravan. Locating a tool-box on a rear bumper is an absolute no-no. For stability, the longer the A-frame the better.
Making caravans more stable – tow ball mass
For a caravan to be stable it must be nose heavy. There is a known relationship between its nose weight and speed. The lower that nose weight to lower the safe speed.
Australian caravan makers initially recommended nose mass of about 10% of the laden weight. Ongoing emission legislation, however, causes vehicle makers to reduce their products’ weight. As a result caravan makers either reduce their recommendations. Or, and increasingly do not advise one. While that 10% is still really required that often now exceeds current tow vehicles’ ability.
European caravans are about 40% lighter (per metre) than of most local products. There, 7% has long been seen adequate (and still is).
For making caravans more stable, the most limiting factor is its length. Overall weight is not that important, that which is where weight is located along it.
The average maker-recommended tow ball mass of typical 2020 locally built caravans is about 7%. One is a mere 4.2%. Only a few remain at 9% to 10%. Some makers suggest towing with the water tanks empty. That seemingly negates having them.
Legal reasons preclude suggesting anything other than: ‘follow what the caravan maker recommends’ re tow ball weight. RV Books does not, however, endorse such recommendations.
Making caravans more stable – hitch overhang
Another major factor in caravan stability is the length from the tow vehicle’s rear axle to the tow ball. The less that overhang, the less a caravan’s tendency to pitch and yaw. The average overhang of Australian tow vehicles is 1.24 metres. The longest (well over 2 metres) are mostly extended chassis dual-cab utes. It is not a coincidence that many roll-overs involve such vehicles.
For making caravans more stable, the tow hitch too should have the minimum possible overhang. Reducing that alone assists stability.
Making caravans more stable. Excess hitch shank length like 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.
Weight Distributing Hitches
The heavy tow ball weight imposed on an overhung hitch pushes down the rear of the tow vehicle. It acts as a lever. As with a heavy person at one end of a see-saw, it levers up the front wheels of the tow vehicle. A weight distributing hitch is simply a springy lever. Its effect is to force those wheels back down.
This effect is often misunderstood. A WDH cannot reduce the side forces resulting from cornering or yaw. Adding a WDH always reduces required understeer. Such understeer ensures the tow vehicle automatically increases turning radius if cornering too fast. See pics below. Why caravans roll over
Understeer and oversteer. Original pix – source unknown.
A heavy caravan’s nose weight imposed on (often lighter) tow vehicles, is often more than its rear tyres can withstand. This necessitates the use of a weight distributing hitch (WDH). These hitches are semi-flexible springy beams that, by using the tow vehicle’s rear axle as a pivot shift part of that imposed weight to the front wheels. In doing so however, removing that weight reduces the rear tyre’s cornering power.
Major WDH maker Cequent (parent of Hayman Reese) advises to restore no more than 50% of the ‘lost’ front axle load. This usually results in the laden caravans’ nose being down by about 5 centimetres. Better by far, however, is to have a rig that has no need for a WDH. Doing so has long been virtually routine in Europe.
Making caravans more stable – sway control systems
Any trailer towed via an overhung hitch has a natural tendency to sway. With well designed and laden caravans towed by a suitable vehicle, such sway normally dies out within two/three cycles. It is mildly annoying but harmless.
After-market sway control systems usefully and effectively control low speed swaying. They introduce friction that dissipates sway energy as heat. That seemingly overlooked is a basic law of physics. That frictional force is a constant – but sway force increases with square of the speed. At 100 km/h a friction is close to useless. Its damping is a mere 1% or so.
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 and that control is most needed.
Both are effective at low speed. But, if fitted to a caravan that is otherwise unstable, these devices mask a dangerous underlying condition. They are akin to pain killers instead of medical treatment. Sway control is routinely included with some UK/EU caravans. But only as an aid to low speed comfort.
Making caravans more stable – 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 USA products.
Europe’s IDC, and AL-KO ESC activate when caravan sway exceeds about 0.4 g (an uncomfortable level). Or four repeated at about 0.2 g. They then automatically apply caravan braking. The Al-KO does so for 1-3 seconds at 75% of full braking. This reduces the sway, and particularly reduces 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/Tuson) operate at lower levels of sway acceleration (about 0.2 g). They brake each caravan wheel at whatever level is deemed optimum. The makers claim they can be fitted to caravans with any form of braking. Either system is worth fitting. They do not, however, substitute for making caravans stable.
When these systems are triggered they not reduce sway. They reduce speed.
If using such systems never engage cruise control. That attempts to accelerate the rig to its preset (and previous) speed.
Making caravans more stable – the tow vehicle
For truly making caravans more stable the tow vehicle must be heavier than the caravan. The more so the better.
Recommendations about caravan/tow vehicle weight and making caravans more stable stem from the 1930s. Caravans back then had an interior length of 4-5 metres. They weighed about 1000 kg. Most were towed by cars heavier than that 1000 kg. Few exceeded 80 km/h.
Most EU/UK caravan bodies now recommend laden caravan weight should not exceed 85% of the tow vehicle’s unladen weight. They suggest that experienced caravanners may go up to 100 per cent of the car’s unladen weight. Germany legislates that caravans may not exceed 0.8 times the tow vehicle’s unladen weight.
Whilst the UK’s and European towing legislation has been updated, Australia’s has not. It relates only to maximums. Review is long overdue. The Caravan Council of Australia suggests that ‘for added safety and peace of mind’, the laden caravan should not exceed about 77% of the laden weight of the tow vehicle. That recommended 77% is less stringent than that of the UK and Germany – where towing speed limits are 20 km/h lower. Despite that, this recommendation was greeted by owner and industry rage. 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-weight
Whilst environmentally unsound, for making caravans more stable, for any over 2200 kg laden, buy the heaviest tow vehicle you can find. See below re minimal hitch overhang.
Making caravans more stable – braking and accelerating
A major towing risk is driving fast down a hill that has bends or changes in road camber. Particularly if braking, gravitational forces at the rear of a swaying caravan increase that sway. This is a particular risk on long winding motorway downgrades. And even more so in strong side winds. This mainly affects end-heavy caravans. To reduce risk, keep speed below 70 km/h. Never brake (the tow vehicle) hard downhill. It may cause the caravan to sway.
Advice on forums is that accelerating corrects sway. Whilst true – it is safe only at low speed. Accelerating whilst swaying at higher speeds may cause the rig to exceed the critical speed at which sway suddenly escalates. In that event jack-knifing is likely. If your system permits, a safer way is by gently braking the caravan alone.
Making caravans more stable – the effect of speed
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.
That critical speed is determined by a number of factors. Tests in the UK show that, optimally locating typical personal effects alone, can affect it by as much as 25 km/h. See Caravan and Tow Vehicle Dynamics for a full technical explanation.
In Australia is that (excepting WA’s limit of 100 km/h) caravans under 4.5 tonne can legally be towed at up to 110 km/h. This is a speed limit however. It is not a ‘recommended’ speed. Many drivers resent being held up by slow travelling caravans. Towing a caravan that is heavier than the tow vehicle at speed, however, is risky. An emergency swerve, or strong side gust, can trigger irreversible snaking. Most owners never experience this. But some do.
Making caravans more stable – electronic stability control
Electronic stability has 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). Or four repeated at about 0.2 g. They then automatically apply caravan braking. The AL-KO does for 1-3 seconds at 75% of full braking. This reduces sway, and attempts to reduce 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 product (from Dexter/Tuson) operates at lower levels of sway acceleration (about 0.2 g). It brakes 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. These systems are worth fitting but 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.
Making caravans more stable – various aids
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 not carrying anything heavy 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 to the rear of the axle/s needs to be marginally greater than in front of 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.
Making caravans more stable – how can I tell if my rig is unstable
No trailer pulled via an overhung hitch can ever be 100% stable. 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. So does a tow vehicle much heavier than the trailer.
A rig is likely to be acceptably safe if minor sway automatically dies out (without driver correction) after two or three cycles. It is mildly uncomfortable but not necessarily dangerous. EU designed caravans often have sway damping as standard. But makers first ensure 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. Problems occur however if that caravan begins to yaw. Then, that very mass that normally keeps it so stable will overcome the tow vehicle’s ability to control it. The caravan will begin to fishtail, and this may cause the rig to jack-knife.
The most common statement made by the driver following such an incident is: ‘It always felt so stable up till then’.
Making caravans more stable – further information
This topic is far too big to cover fully in article form. For full details see Caravan and Tow Vehicle Dynamics
The UK article www.caravanchronicles.com/guides/understanding-the-dynamics-of-towing/ by Simon P Barlow. is a generally similar and very down to earth approach. It is accurate and eminently readable but relates mainly to the much lighter UK and EU caravans.
If you liked this article you will like my books! All are written in a similar manner. Why Caravans Rollover – and how to prevent it covers stability issues in depth. So too does the Caravan & Motorhome Book. My other books are The Camper Trailer Book, Caravan & Motorhome Electrics, and Solar That Really Works! (for cabins and RVs). Solar Success relates to home and property solar. For information about the author please Click on Bio.
This updated article is copyright (2020) RV Books, NSW 2105 Australia.