Updated March 2016
Fifth wheel caravans are safer than conventional caravans. They are fundamentally more stable on road and far less likely to overturn. Further, because up to two metres more is usable space they provide that much more living area. They increasingly challenge caravan and motor home sales.
That fifth wheel caravans are safer and inherently more stable was realised in early 1920s following ongoing accidents with centre axled ‘pig’ trailers. This caused the transport industry (led by Fruehof) virtually worldwide to switch to the fifth wheel configuration. Fifth wheel caravan buyers however often choose one without realising they are safer.
Why fifth wheel caravans are safer
A fifth wheel caravan’s on-road behaviour is totally different from that of a conventional caravan. That fifth wheel caravans are safer is fundamental.
The overhung mass (up to 350 kg) of the nose of a conventional caravan causes the front of its tow vehicle to lift. This reduces the grip of its front tyres on the road. This lifting effect can be counteracted by a ‘weight distributing hitch (WDH)’. This, in effect, ‘levers up’ the rear of the laden tow vehicle.The WDH thus restores part (or all) of the undesirable front end lift. It does however introduce major stability issues when the caravan pitches fore/aft. The WDH then causes weight on the tow vehicles’ front/rear tyres to vary cyclically. This introduces (to put it mildly) undesirable dynamic issues.
There is a further and serious problem with an overhung hitch. If the caravan snakes in one direction that overhang causes the tow vehicle to snake in the opposite direction. It is a fundamentally unstable configuration. The effect can be reasonably tamed with short centre-heavy ‘vans, but less so with long and/or end-heavy ‘vans.
The physics of towing (simplified)
A conventional trailer and tow vehicle behave as two pendulums. The lower one (the caravan) swings from the bob (overhung hitch) of the upper pendulum (the tow vehicle).
If a caravan is coupled to an overhung hitch, if either or both are subject to side force (e.g. change in road camber, wind gusts etc), they swing in opposite directions. The rig ‘snakes’. At minor levels, this may dampen out after two or three swings. If however side force exceeds a critical amount above a critical speed, the double pendulum acts in a chaotic manner that is impossible to correct. (http://www.myphysicslab.com/dbl_pendulum.html)
The fifth wheeler is inherently stable. Pic: caravanandmotorhomebooks.com
The ongoing actions of a conventional caravan and tow vehicle in this situation cannot realistically be predicted. It is literally impossible for the driver to correct. If/when that happens the rig almost always jack-knifes.
That this is a serious issue is shown by a major, still growing, industry whose main purpose is to limit the tendency to sway. More recently it has developed systems aimed at correct the consequences. (see Caravan Dynamics).
In essence the overhung hitch of a tow vehicle that pulls a conventional caravan introduces a mechanical ‘phase change.’ that introduces in inherent instability. The trailer acts as a single pendulum. It is inherently more stable and far more predictable. This is the main reason why fifth wheel caravans are safer.
A well designed fifth wheel caravan has its hitch directly over the tow vehicle’s axle/s. It not be to the rear of the axle/s. If it is, the rig is akin to a conventional caravan with massive nose weight.
For optimal stability, a fifth wheel caravan needs its axle/s as far to the rear as possible. This results in a lot of its weight being carried by the tow vehicle. The amount is not critical but typically 15%-25%. As many suitable tow vehicles can carry 750-1000 kg, tow ball weight is not an issue with lightweight construction. It becomes one however when customers and manufacturers seek biggerism at low cost.
This is rarely an issue with local product but some (mainly US) fifth wheelers are massively heavy. The only way to prevent them flattening the typically used big Ford, Dodge or Chevy tow vehicle is to locate the trailers’ axles close to centre. This causes an ongoing ‘rocking horse’ pitching. The resultant forces push the tow vehicle to and fro. This uncomfortable effect can be masked (but not 100% corrected) by using air sprung hitches.
A different and saner approach is exemplified (in Australia) by Glenn Porch’s long, ultra-light units. The one shown above, of 11.3 metres, has twin axles set right at the rear. It weighs 3200 kg and has a legal payload of 1300 kg. It can be pulled by a light tug. See: http://caravanandmotorhomebooks.com/ultra-light-caravans/
How fifth wheelers ‘feel’
The on-road ‘feel’ of a towed fifth wheeler is much as a motor home. Cornering is similar except that the trailer’s rear follows a tighter radius. The turning circle is much smaller. Many turn such that the tow vehicle is at a right angle to the fifth wheeler. They are also easier to reverse. They are barely affected by strong side wind gusts and only rarely snake. If/when they do they usually self-correct. Again, another reason why fifth wheel caravans are safer.
In Australia the overall weight of fifth wheeler and towing vehicle is limited to the GCM (Gross Combined Mass) of the tow vehicle. If the GCM is 4.5 tonnes and the tow vehicle weighs 2.0 tonnes, then the maximum weight of the fully laden trailer must not exceed 2.5 tonne.
Whilst it is common to have 20%-25% of a fifth wheeler’s weight carried by the towing vehicle, that weight must not exceed the legal carrying capacity of that vehicle. In particular it must not exceed the carrying capacity of that vehicle’s tyres, nor that its individual axle loading. If necessary, the nose weight may be reduced but 15% is the (recommended) minimum.
A curious quirk in Australian legislation requires the driver to hold appropriate only to the Gross Vehicle Mass of the tow vehicle. As with caravans, a combination weighing seven tonnes may be legally driven by holders of a car licence. The equivalent limit for a truck is 4.5 tonne.
Fifth wheeler compliance
Unless extensively modified, many fifth wheels imported prior to 2010 are unlikely to meet obligatory Australian Standards. This is particularly regarding mains voltage. Whilst most have been modified by adding a 230/110 volt transformer this allows the original buyer only to use it in that form. A non 100% compliant RV cannot be legally resold unless brought into full compliance. This is not least because the sale of 110 volt domestic appliances is illegal in Australia. Obtaining 100% compliance can costly. If, as some are, the vehicle over-width. is may be impossible. Non-compliance also invalidates insurance. This is a serious issue with second-hand US imports of which many have been wrongly (and some illegally) certified. This is known to registration authorities.They inspect them thoroughly if re-registered. This issue is covered in depth in Imported-rv-electrics/ See also Imported RVs.I
The case against fifth wheelers
Whilst fifth wheel caravans are safer, they need a dedicated tow vehicle. Their high nose weight (and lack of storage space) reduces that vehicle’s ability to carry heavy odds and ends. If the fifth wheeler is light enough however the dual cab Ivecos etc pull them with ease. The fifth wheel buyer does need to be weight-conscious. But that can be beneficial.
Intending buyers need to be wary of ultra-large and heavy (low-price) US units. Many are intended for permanent trailer park living and have only rudimentary suspension.
Further information on caravans and fifth wheelers
Caravan/tow vehicle stability is complex. For in-depth and constantly updated coverage refer to my major article Caravan Dynamics. Also the more technical Caravan and Tow Vehicle Dynamics. Every aspect of fifth wheeler design and usage etc is included in my new Caravan & Motorhome Book. My other books include 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 please Click on Bio.