The Wanderer
Whilst often credited to U.S. inventor Charles H. Martin of the Martin Rocking Fifth Wheel Co (around 1911), the concept and the term ‘fifth wheel’, have been in use since the 1700s. In 1874 a mechanical dictionary defined it as ‘a wheel or segment above the fore-axle of a carriage...'
Alaska’s Dick Rubbins too, strongly claimed to have invented it. His tombstone is inscribed: “Here lies Dick Rubbins, Inventor of the fifth wheel . . . buried face down, so that Charles Martin can kiss my royal Irish ass.”
Several US manufacturers claim to have built the first fifth wheel caravans in the 1960s, but there were many from the mid 1920s onward. As with caravans vs motorhomes, people have been debating their pros and cons ever since.
As with conventional caravans a fifth wheelers major benefit is that the motorised part is usable independently of the living part.
A fifth wheeler costs more than a good caravan and its tug but the larger rigs cost less than motorhomes with comparable living area. What is less widely realised is that a fifth wheeler is as practical as a conventional caravan, and is far more stable.
An early 1920s Curtiss Aerocar. Far more sophisticated than it looks. The Aerocar had aircraft type construction, initially of timber braced by wire cables, later by light metal tubing and wire bracing. Pic: Glenn Curtis Museum, USA).
On-road dynamics
A fifth wheel caravan’s dynamic behaviour differs from that of a trailer on an overhung hitch. The reason is related to fundamental laws of motion relating to ‘actions and reactions’.
A conventional caravan and its towing vehicle behave dynamically as one pendulum (the caravan) suspended from the bob of another (the tow vehicle).
One pendulum, given a shove, swings from side to side and eventually stops. But suspend one from the bob of another one, when one swings (say) to the right, the other swings to the left.
Once beyond a critical amount of swing, they trigger into non-predictable and increasing interactions that are literally impossible for any driver, no matter how skilled, to correct.
Whilst rare with trailers under 15-16 ft towed by vehicles substantially heavier, it is a major issue with trailers approaching the size and weight of a motor home, and not always towed by appropriately sized vehicles.
Inherent stability
A correctly designed fifth wheeler has no inherent stability issues as long as its hitch is directly over the tow vehicle’s axle/s. If it is, the rig behaves as a single pendulum. It is stable and predictable. Swaying occurs but is mostly self correcting.
Further, as a metre or more of the fifth wheeler is carried by and above the towing vehicle (and there is no drawbar as such) the overall length of a fifth wheeler rig is three or so metres less than of a caravan (of equal length) and car. Or one can have that much more accommodation.
Most are designed such that the tow vehicle carries about 25% of the weight. This is not an issue with lightweight construction and/or fifth wheelers up to 22 ft or so long, as many tow vehicles have a payload of 1000 kg or so. It is however with long ultra-heavy units with slide-out everything, and also some imports primarily intended for permanent or semi-permanent living in US trailer parks.
At a typical five plus tonne, the only way to prevent them flattening the tow vehicle is to position their axles close to the chassis’ centre - resulting in cyclic and unpleasant pitching. Also, because the ends rise and fall in an arc, the resultant (vectored) force constantly pushes the tow vehicle to and fro. This uncomfortable ‘shunting’ is masked (but not corrected) by air sprung hitches.
Australia, and the now many UK makers, mainly produce light to medium sized units that can be towed by locally produced utilities.
How fifth wheelers handle
The ‘feel’ of a fifth wheel combination is not dissimilar to a large rigid vehicle. They are a revelation (for caravan owners) in windy conditions as they are only barely affected by side gusts. Cornering feels vaguely similar to motorhomes excepting that the rear follows a tighter radius. The turning circle however is far tighter. Many can turn such that the tow vehicle is at a right angle to the fifth wheeler. They are also easier to reverse.
In Australia the overall weight of fifth wheeler and towing vehicle is limited to the GCM (Gross Combined Mass) of the tow vehicle. If for example 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.
The weight carried by the towing vehicle must not exceed the legal carrying capacity of that vehicle, and particularly the carrying capacity of that vehicle’s tyres and individual axle loading.
Unlike a big motorhome, a fifth wheeler requires the driver only to hold a current licence appropriate to the GVM of the towing vehicle. As with caravans, a combination weighing well over seven tonnes may be legally driven by holders of a car licence.
The case against
The major argument against fifth wheelers is that a more-or-less dedicated towing vehicle is required.
These rigs are thus unlikely to compete with the traditional small caravan market - where even the best-made rarely exceed $50,000.
A further and possibly minor point is that as such a high proportion of the overall weight is carried by the towing vehicle as payload, this, plus the lack of storage space, substantially reduces that vehicle’s ability to carry heavy odds and ends. The fifth wheel buyer is thus likely to be weight-conscious - but that might well be beneficial!
Imported Units
Intending buyers need to be aware that, unless brought totally into compliance with mandatory Australian electrical standards it is an offense (and in NSW a criminal offense) to sell that vehicle. This is a serious issue with second-hand imports as many are known to have been falsely certified. That such units are still around is known to registration authorities that now inspect them thoroughly if resold.
(Note: Imperial feet have been used instead of metres, as most caravanners and caravan makers still use them.)
The 13 metre long ‘Highway Palace’ was built in the mid-1940s for Harry Parr and his wife Daisy of Pinnaroo, SA, by Grace Brothers of Adelaide (a construction company) that built two standard fifth wheel caravans. The ‘Highway Palace’ was a ‘one off’, costing over £2000, a huge sum then. The tow vehicle is a Dodge Fargo. Both were meticulously restored by Dick Hart and his friends after the caravan was found in a serious state of disrepair. Pic: courtesy of Dick Hart.
Collyn Rivers
For an in-depth coverage of trailer and vehicle stability, refer ‘Caravan Dynamics’, and also ‘Vehicle Dynamics’ in the articles section of www.caravanandmotorhomebooks.com.
Collyn Rivers writes/publishes books on every aspect of camper trailer, caravan and motorhome use. Most are stocked and sold by CMCA NHQ.
Category: Technology
Written: Thu 21 Jun 2018
Printed: February, 2013
Published By: