THE 4 WHEEL DRIVE MOTORHOME

The four wheel drive was originally a large, rugged working vehicle intended for crossing equally rugged terrain. The concept is over a century old - Federal Motors (USA) offered four wheel drive trucks as early as 1901. They were in common use in World War I.

Historically, 4WD's were practical, tough and uncomfortable, but as less than 20% of (non truck) 4WD's ever go off road, the car like versions progressively became quieter, faster, smoother and softer riding, at the expense of off road performance. Four wheel drive trucks have not changed this way. They are as rugged and capable as they ever were. But none is a ball of fun in a city!

Why a 4WD?

Before settling on a 4WD motorhome, ensure it really is what you need Such needs (as opposed to wants) do not include dirt roads. Outside the wet, virtually every major inland road in Australia, and most minor ones, are readily passable by a mechanically sound 2WD with good ground clearance.

Minimum Size Essential

The most common usage for a 4WD motorhome is in the less accessible National Parks, State Forests, remote tracks at the tip of Cape York, to Jim Jim Falls in Kakadu, and for the more adventurous - a Simpson Desert crossing.

Many bush tracks are accessible only by vehicles no wider than 2 metres and no higher than 2.8 metres (2.4 metres is better). Even then, one may need to bring the rear view mirrors flush with the body from time to time.

A further limitation is a turning circle restricted by front drive shaft angles. This limitation can be a problem at creek crossings. These often have tightly curving entries/exists with a steep drop on one side of the track, and an equally steep bank (or close trees) on the other (There is a particularly nasty one on the "back track" out of Lichfield Park). In these situations there is often nowhere to "back and fill".

Such usage is not for the vehicle proud - bodywork inevitably becomes scratched. At the Narrabeen Rally, one CMCA member commented that our OKA looked a "bit rough". I refrained from telling him it was just back from Broome, via the Tanami, Simpson and Oonadatta tracks.

Design Differences

A 4WD is only usefully as such whilst all wheels are firmly on the ground. There are two different approaches to achieving this. The first, typified by the R type Bedford and the International ACCO range, is a narrow chassis that flexes diagonally. This works extremely well off road but suspension must be both firm and have limited travel for safe on road usage.

The second approach taken by OKA, Unimog and the Hummer, is for a massively rigid chassis and extremely long travel springs that provide extensive suspension travel. Shock absorbers must inhibit sideways lurching as well as performing their normal functions. They need to be progressive rather than stiff or they will limit suspension travel (and hence traction). Anti roll bars cannot be used (unless disabled when driving off road) as, by definition, they too inhibit suspension travel. Independent front suspension (Hummer, Unimog) provides greater ground clearance, but prejudices weight distribution and hence traction. Beam axles (OKA, Bedford, International, etc) provide better weight distribution and hence better traction (but less ground clearance).

All 4WD trucks require the body to be flexibly mounted on the chassis, with all cables, hoses etc having ample room to move. Dust proofing is vital, but readily achieved as long as one tracks down even the slightest hole (and one remembers to put the sink plug in place whilst travelling).

Four Wheel Drive Preferable to Six/Eight

Three or four drive axles on bigger off road trucks are forced on manufacturers by legally permitted on road axle weights and the availability of suitable tyres, rather than improved cross country performance. Where these restrictions do not apply (as with the huge 100-400 tonne quarry trucks), only two axles are commonly used.

Wheels and Tyres

Off road, large single wheels and tyres are preferable to twin wheels and tyres. Over rocks, the outer tyre, particularly, can become seriously overloaded, rocks become trapped between the facing tyre side walls, and it can be difficult to access the valves of the inner tyres to reduce pressure in sand.

The more rugged 4WD truck tyres (e.g. Michelin XZL) are intended for mining and quarry work - where they are extremely effective. But tyres like these are pigs on the road - they are extremely noisey and "lumpy", develop uneven wear patterns impossible to reverse by wheel changing, and have a short life. On road truck tyres are better all round, particularly in sand.

Tyre Pressures

Tyres for off road trucks are considerably de-rated. Normally recommended tyre pressures are likely to be too high. Measure axle weights, and consult the tyre manufacturers for recommended pressures.

Height Inevitable

The necessarily large suspension movement necessitates ample clearance between the front axle and the engine sump, in turn necessitating a high driving cab. The rear is equally high set, making a pop top virtually obligatory.

Off road payload is lower than for a 2WD of generally similar size. A coach bodies OKA, can carry about 1500 kg, an R-type Bedford 3000 kg. It is difficult to keep a laden 4WD motorhome within legal limits - we had our OKA's interior built from aluminium to stay within the limits.

Low Power is Common

Engine size and power is typically low. On hard roads, most 4WD trucks seem hopelessly underpowered. Off road, this is never a problem - all have exceptionally low gearing - invaluable for walking them across rough going (and enabling them to climb up the side of a house).

Because they are high built, heavy items (e.g. fuel, water batteries, winches, tools) must be kept low down whilst retaining good ground clearance between the axles, and good arrival and departure angles (see pix). Apart from size, ground clearance will mostly determine where the vehicle can go.

A fully locking differential is worth fitting, particularly at the rear. If used also (but never only) at the front it must be driver selectable, e.g. the excellent ARB Air Locker.

Body Roll

Despite adequate shock absorbers, body roll and pitching can be violent. Normal cupboards and shelves are useless unless everything is tightly packed. All doors and hatch lids need strong latches. It is hard to go past wire racks and baskets housed in a strong enclosure. China and glassware is likely to be broken. Stainless steel is light and cheap, but we still prefer wine glasses for our nightly half bottle.

Driving Technique

Off road driving technique is different from that with a smaller 4WD. With the latter four wheel drive assists you through heavy going - with a big truck it's better to reserve four wheel drive for getting you out of it. (If your ACCO or R-type gets thoroughly bogged you'll need another 4WD truck of at least the same size to pull you out!) In soft sand, reduce tyre pressures to about 23-25 psi - and again have diff locks disengaged. If you get stuck (because one wheel is spinning), engage the diff lock/s. If that fails, drop tyre pressures progressively to 15 psi. Don't drive into sand at these low pressures - only drop below 23-25 psi to get you out of it. Never use low range 4WD unless locking front hubs are engaged because the whole low range torque (which can exceed 30,000 Nm) is directed to the rear axle shafts - with every likelihood of breakage.

De-ditching equipment is worth carrying, particularly a long handled shovel. Also heaps of thick wooden blocks, at least two jacks, plus a high lift jack strong enough to life one side of the vehicle. A big winch is comforting, but one large enough to shift over five tonnes really needs to be mechanically or hydraulically powered.

Consider a 4WD driving course. Only enrol in one that is both government accredited and experienced with 4WD trucks (many teach council staff, bush fire brigade drivers, etc).

How Strong

A 4WD truck is a very rugged piece of machinery - but don't set out to see how tough they really area - they can be broken. The most vulnerable components are usually the transfer box and drive shafts if bogged in soft sand. The available torque in low range bottom gear is huge. If the engine stalls in this situation - let the tyres down and dig the truck out before trying again.

Collyn Rivers W8054
CMCA magazine - The Wanderer May 2002