Fifth wheelers are becoming increasingly popular in the USA, where they rival sales of conventional ‘travel trailers’. There, they are expected to eventually outsell all such trailers - except small ones. There are several reasons for this but the most important has to do with towing stability - Newton’s ‘actions and reactions’.

A conventional trailer towed by a conventional car or 4WD has about 10% of its mass superimposed on a hitch located well over a metre behind the rear axle of the towing vehicle. This overhanging mass causes the front of that vehicle to lift and this reduces the grip of that vehicle’s front tyres on the road. The front end lifting effect is similar to what happens when one person (only) sits on the end of a see-saw. The other end inevitably rises. If the van snakes, the imposed forces cause the overhung rear of that vehicle to be pushed sideways.

To some extent the lifting effect can be counteracted by a ‘weight distributing hitch’ (springs and levers that transfer part of the imposed mass back onto the front wheels of the towing vehicle and, to a lesser extent, the trailer). But this is only partially effective as dynamic forces resulting from pitching far exceed the static weight transfer that these hitches correct. In engineering terms, a conventional trailer and its towing vehicle behave dynamically as coupled pendulums.

The above is not that serious a problem where lightish trailers are towed by relatively heavier vehicles. It is however an issue with trailers approaching the size and weight of a light motorhome - but not always towed by appropriately scaled vehicles.

Fifth Wheel Stability

The US RV industry and its customers appear set on a seemingly inexorable path of bigger and longer trailers with slide-out everything, and the fifth wheel approach was a logical way to avoid increasingly serious and inherent stability defects.

There was also a growing recognition that motorhome users sought independent mobility whilst on site, and that towing a small vehicle was a clumsy and inconvenient solution.

The fifth wheeler solves the towing instability problem by eliminating its major cause: that trailer mass was imposed on an overhanging hitch. By contrast, a fifth wheeler imposes some 25% of the mass directly over or slightly in front of the towing vehicle’s rear axle. By so doing, pitching and snaking are all but eliminated.

Curiously, Australian Design Rules dictate the type of hitch that must be used, but not the location of that hitch. From an engineering standpoint however, the issue is simple: if the hitch is behind the rear axle, the front of the towing vehicle inevitably lifts under pitching conditions; if that hitch is over the rear axle, front axle loading stays the same; if it is in front of the rear axle, front axle loading is increased. There is, due to the triangle of forces involved, a slight front-end lifting effect that increases with speed, but this is only noticeable when accelerating hard from a standstill. This is minimised by keeping the towing attachment (‘fifth wheel’) as low as possible.

The CMCA’s requirement (hitch must be in front of the centre line of the rear axle) is safe, practical and logical. If the hitch is any distance at all behind the towing vehicle’s rear axle, the towed device is dynamically a conventional trailer.

Weight Restrictions

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.

Whilst it is normal practice to have about 25% of the fifth wheeler’s weight carried by the towing vehicle, that weight must not exceed the legal carrying capacity of that vehicle, and particularly must not exceed the carrying capacity of that vehicle’s tyres nor that vehicle’s individual axle loading. As a fifth wheeler’s imposed mass is borne almost entirely by the rear axle, this may well limit the imposed weight below that of the towing vehicle’s payload.

Due to a seemingly odd quirk in driver licence thinking a fifth wheeler thus requires the driver only to have a 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.

Length & Layout

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 two to three metres shorter than that of a caravan (of equal length) and car/4WD. An alternative view is that one can have far larger living accommodation for similar on-road length. The necessity for a front raised section logically results in at least one bedroom to be up front, and the heavier kitchen/bathroom to be placed more or less over the rear wheels. A fifth wheeler and its tug are much shorter than a motorhome of the same accommodation if it is towing a car.

Current Australian and UK practice appears to be concentrating on light medium-sized units that can be towed by locally-produced utilities. American practice however seems increasingly “biggeristic” - with large, heavy units (usually with slide-outs) towed by huge 7-litre turbo-charged Ford diesels, or even full-sized semi-trailer tractors.

As previously mentioned, a further problem resolved by the fifth wheeler is the ongoing traditional motorhomer’s dilemma that transport and home are combined. This is inconvenient at best, and can be quite a problem in the event of serious mechanical breakdown. Instead of the motorhome pulling a towed vehicle, the fifth wheeler towing vehicle pulls a motor-less home but without the substantially increased on-road length and stability issues.

Driving a Fifth Wheeler

My own experience (with semi-trailers) is that the ‘feel’ and ‘utility’ is closer to a large rigid vehicle, than when towing a conventional trailer. Fifth wheelers are a revelation on motorways and in windy conditions as they are only barely affected by side gusts. Cornering feels vaguely similar to motorhomes except that the rear end follows a tighter radius and the turning circle is far tighter than with a conventional trailer. Many such rigs can turn with the towing vehicle at a right angle to the fifth wheeler - but see below. They are also far easier to reverse than a conventional trailer.

Particularly with pop-top examples, the roof line of a fifth wheeler is often lower than that of most motorhomes, and this can be a big advantage if venturing into some National Parks and State Forests.

Until recently it has been necessary to have close to a metre and a half clearance between the rear of the towing vehicle’s cabin and the front of the fifth wheeler and this precluded the use of most dual cab tugs. A recently developed (US) hitch that slides at low speeds on tight turns substantially reduces the clearance required. It is not so far approved for use here but at the time this feature was ready for press, trial units were about to be submitted for ADR approval.

Imported Units

Intending buyers need to be aware that, unless modified, imported fifth wheels are most unlikely to meet obligatory Australian Standards, particularly regarding mains voltage, and also gas installation (etc). US imports in particular may not meet Australia’s electrical insulation standards as the US single phase system operates at half our voltage. Non-compliance could cause serious complications in case of an insurance claim.

The Case Against

The major argument against fifth wheelers is that a more-or-less dedicated towing vehicle is required. Thus these rigs are unlikely ever to compete with the traditional small caravan market - where even the best-made caravans rarely exceed $40,000.

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 more likely to be weight-conscious - but that might well be beneficial!

In Australia at least it seems likely that whilst some people may be tossing up whether to buy a three-tonne trailer and a seriously big (US) 4WD to pull it, most seeking this amount of space will be deciding between a conventional motorhome or a fifth wheeler and appropriate towing vehicle. But that’s only conjecture!

Please note that the copyright of all Collyn’s books and articles is held by Caravan and Motorhome Books, Broome, WA. Full details of these books (available through the CMCA) can be found at www.caravanandmotorhomebooks.com