by Collyn Rivers
Making stable caravans
Making stable caravans 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 their 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 stable caravans is always desirable. For Australian (and USA) caravans it is essential. Such trailers are increasingly heavier and (worse) longer, 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. It did, however, overlook that most were long twin-axle units.
An only too typical caravan roll-over. Pic: original source unknown.
Making stable caravans – weight
Many caravans built since 2015 are over 6.5 meters. They weigh well over 2000 kg (4400 lb). Some are 7-9 meters and weigh over 3500 kg (7700 lb). A few are over 4000 kg (8800 lb). Many are towed by vehicles far lighter than the trailer. And at 100-110 km/h (plus 60 mph). Worse, an Australian caravan magazine stated that many caravans reviewed were heavier than their makers claimed.
Before finalizing payment for any caravan, weigh it on a certified weighbridge. Never assume the 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.
Whilst excess weight is undesirable the major factor determining a caravan‘s stability is its length. In particular the distance from its tow hitch to its axle/s. Known as the ‘radius of gyration’, the greater 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. By and large, however, it is excess caravan length and where weight is distributed along that length that is now the major issue,
Weight distribution truly matters
That not generally understood is that towing stability is substantially related to a caravan‘s length. In particular, where weight is distributed along that length.
A dangerous assumption (still found on RVs forums) is that as long as recommended nose mass is retained, a caravan can be loaded as wished. An extreme example, seen at a camp-site, had a motorcycle on its rear. The motorcycle was ‘balanced’ by 200 kg (440 lb) of barbell weights on the trailer’s A-frame. Another was a widely spread article suggesting that tow ball mass be adjusted via sandbags at the trailer’s front end – a seriously bad way of adjusting tow ball mass.
The ideal caravan has its axle/s set way back. Then laden with everything heavy as close to the axle/s as possible.
Never locate anything heavy (particularly high-slung spare wheels) at a trailer’s extreme rear. Locating a tool-box on a rear bumper is an absolute no-no. At the front, for stability, the longer the A-frame the better.
Tow ball mass
For a trailer towed via an overhung hitch, to be stable it must be nose heavy. There is a known relationship between its nose weight and speed. The lower that nose weight the lower the safe speed.
Australian trailer makers initially recommended a nose mass of about 10% of the laden weight. Ongoing emission legislation, however, causes vehicle makers to reduce their products’ weight. As a result, trailer makers reduce their nose mass recommendations. Or, and increasingly, do not advise it. While that 10% is still really required, that often now exceeds current tow vehicles’ ability to support it.
The average maker-recommended tow ball mass of typical 2020 Australian-built caravans is now 5%-7%. One is a mere 4.0%. Only a few remain at 9% to 10%. Some makers suggest towing with the water tanks empty. That seemingly negates having them.
European caravans are about 40% lighter (per metre) than most local products. There, 7% has long been seen adequate (and still is).
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.
Another major factor in trailer stability is the length from the tow vehicle’s rear axle to the overhung 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.
When making stable caravans, the tow hitch too should have the minimum possible overhang. Reducing that alone assists stability.
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 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 the 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 pic – source unknown.
A heavy caravan‘s nose weight imposed on lighter tow vehicles is often more than its rear tires 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 tires cornering power.
Major WDH maker Cequent (parent of Hayman Reese) advises restoring no more than 50% of the ‘lost’ front axle load. This usually results in the laden trailer’s nose being down by about five centimeters. Better by far, however, is to have a rig that has no need for a WDH. Doing so has long been routine in Europe.
Sway control systems
Any trailer towed via an overhung hitch has a natural tendency to sway. With well-designed and laden trailers towed by a suitable vehicle, such swaying 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 the square of the speed. At 100 km/h a friction hitch is close to useless. Its damping is down to 1% or so.
So-called dual cam systems ‘lock’ the caravan and tow vehicle together in a straight line. Normal cornering is enabled by tire 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 trailer 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.
Electronic stability control
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 one to three seconds at 75% of full braking. This reduces the sway, and particularly, reduces speed below the critical level. They can only be fitted to trailers that use the maker’s respective brakes.
How the AL-KO system works
The US products (from ALKO/Dexter) 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 trailers with any form of braking. Either system is worth fitting. They do not, however, substitute for making stable caravans. When these systems are triggered they do not initially 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.
Manufacturers stress that such products cannot overcome the laws of physics. With that presumably in mind, most test them at under 100 km/h (about 60 mph).
The tow vehicle
For truly making stable caravans the tow vehicle must be heavier than the trailer. The more so the better.
Recommendations about trailer/tow vehicle weight stable stem from the 1930s. Caravans back then had an interior length of 4-5 metres. They weighed about 1000 kg (2200 lb). Most were towed by cars heavier than that 1000 kg (2200 lb). Few exceeded 80 km/h (about 50 mph).
Most EU/UK caravan bodies now recommend laden trailer weight should not exceed 85% of the tow vehicle’s unladen weight. They suggest that experienced owners may go up to 100% of the car’s unladen weight. Germany legislates that such trailers 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. A 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 caravan-owner and industry rage. See Caravan Council of Australia. Few Australian caravan tow vehicle combinations meet these independent recommendations.
It is also becoming necessary to stress that limiting caravan length is even more important than weight.
Whilst environmentally unsound, when making stable caravans, for any over 2200 kg (4850 lb) laden, or over 6 or so metres, buy the heaviest tow vehicle you can find. See below for the minimal hitch overhang.
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 trailer 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 trailers. To reduce risk, keep the speed below 70 km/h. Never brake (the tow vehicle) hard when descending a hill. It may cause the trailer 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.
The effect of speed
Any given combination of the 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 traveling vehicles. Towing a trailer 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 stable caravans – various aids
There is no stability benefit in having dual axles unless caravan weight demands. If anything the opposite is so – they add weight and (worse) length.
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.
When designing a caravan, set the axle/s as far back as you can – yet maintaining a tow ball weight that the tow vehicle can realistically handle.
This well-balanced 1998 Phoenix’s set-back axles resulted in almost legendary stability. Pic: Caboolture Caravan Repairs.
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 a side-wind gust and by long trucks passing, or passed, at speed. The 1990s Phoenix shown above has diagonally sloping walls at its front. This ensured the set-back axles did 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. If the trailer sways, that automatically causes the tow vehicle to sway in the opposite manner. And vice versa. Nose mass and correct weight distribution assist 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 trailers often have sway damping as standard. But makers first ensure sway is addressed as described above.
Long end-heavy trailers 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 trailer begins to yaw. Then, that very mass that normally keeps it so stable will overcome the tow vehicle’s ability to control it. The trailer 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 until then’.
Making trailers 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 Roll Over – 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.