Four Wheel Drive
Is four wheel drive needed for motorhomes – if so, here’s why and how to know what can travel where, with and without it. Here’s all you need to know if you decide to build or buy one.
The need varies from country to country. In Australia, some 99% of all regularly used dirt tracks are readily drivable by a rear-wheel-drive motorhome. What all need, however, is truly ample ground clearance. Many such tracks have high and often rocky centre ridges. These tear-off exhaust systems and severely damage the underside of low vehicles. The minimum is about 300 mm.
Coach and truck-based motorhomes are usually fine. So too are Toyota Coasters and Nissan Civilians. But not so most converted delivery vans. This also excludes those with 4WD. That function assists traction on icy roads – but ground clearance is rarely increased. Also unsuitable is front-wheel drive anything. All lack traction on mildly slippery hills – that a 2WD climbs with ease.
Is four wheel drive needed for motorhomes – they can be crude and lack comfort
Today’s car like four wheel drives are quiet, fast, smooth and soft riding. But, for many, off-road performance consequently suffers.
Four wheel drive trucks are very different. Most are as rugged and capable as ever. Their downside is noise and discomfort. There are exceptions: e.g the rare (and costly) Pinzgauer. But even that’s far from car-like and very noisy.
The rare Pinzgauer – at little over four metres it’s not as big as it looks. It weighs 2.4 tonnes and carries up to 1500 kg (3300 lb). Pic Pinztrek.com
When asking if four wheel drive is needed for motorhomes, before settling for a four wheel drive motorhome, ensure it truly is what you need. Such need (as opposed to wanting) does not include driving on almost any dirt road. Few dirt roads need four wheel drive.
Is four wheel drive needed for motorhomes – height, length and weight may limit.
The main need for four wheel drive in Australia is rare. It’s needed for a few remote tracks at the tip of Cape York. It’s (currently) needed to access Jim Jim Falls (Kakadu), and the track into Purnululu. But such tracks are really too narrow for motorhomes anyway.
Its main need is for accessing camping sites away from main tracks. Four wheel drive eases soft going, but is far from reliable. If a big 4WD motorhome gets bogged, its recovery is truly serious.
Plus six tonne OKA needed this to pull it free. Pic: Peter Wright (2008)
Many bush tracks are accessible only by vehicles less than 2 metres wide. Overhanging trees limit height to about 2.6 metres. Most big four wheel drive motorhomes are higher.
The necessarily large suspension movement necessitates ample clearance between the front axle and the engine’s sump. That necessitates an elevated driving cab. The rear is equally high set.
Whilst many have a fixed roof with full standing headroom, this can cause problems on many forest and similar roads. These are typically limited to that allowing access by fire trucks – a typical 2.4 metres. Whilst converting our OKA, we included a custom-made pop-top fibreglass roof. Ventilation too was hugely improved.
A further limitation is a turning circle restricted by front drive-shafts. This can be a problem at creek crossings. Some have tightly curving entries/exits. Compounding this may be a steep drop on one side of the track. Plus an equally steep bank (or close trees) on the other. Worse, there’s often nowhere to turn around. It can result in reversing for kilometres.
Is four wheel drive preferable to six or eight.
There is a trend to ‘off-road motorhomes’ converted from enormous eight-wheel drive vehicles. These seem pointless. They are hugely too large for use on all but the few major dirt tracks in Australia. But four-wheel drive is not needed for those anyway. The same applies to much of Africa and Asia.
Looks impressive – but its 25 or so tonne bulk restricts where it can go. Pic MAN.
Multiple axles on big off-road trucks are used due to maximum permitted on-road axle weights and the availability of tyres. Where no restrictions apply (as with 400-tonne quarry trucks), two axles are commonly used.
Four wheel drive motorhomes – design differences
A four wheel drive motorhome is only usefully so whilst all four wheels are firmly on the ground. There are two different approaches to achieving this. The first is a narrow chassis that flexes diagonally. This works extremely well but its suspension must be firm and have limited travel for safe handling. It also necessitates a flexible three-point mounted body.
The second approach taken by OKA and Unimog is a less flexible chassis and extensive suspension travel. Shock absorbers limit sideways lurching as well as suspension damping. Springing must progressive, not stiff, or it limits suspension travel, and hence traction. Anti-roll bars cannot be used (unless disabled when driving off-road). By definition, they inhibit individual wheel deflection.
Independent front suspension provides greater but variable ground clearance. It does, however, prejudice weight distribution and hence traction.
Beam axles (OKA, Bedford, International, Unimog etc) provide better weight distribution and hence better traction. They must, however, have reliable progressive shock absorbers. If not they develop severe front axle tramping. This is due to gyroscopic precession caused by steerable wheels rising and falling in an arc. (Spinning a bicycle’s front wheel, then lifting it in arc demonstrates the effect). For similar reasons beam-axled wheels need steering dampers.
A 4WD motorhome requires its body to be flexibly mounted on the chassis. All cables, hoses etc need ample room to move. Dust proofing is vital. This can be achieved but requires the slightest hole to be sealed. And the sink plug in place whilst travelling on dirt.
Wheels and tyres for four wheel drive motorhomes
Off road, large single wheels and tyres are far preferable to twin wheels and tyres. With twin wheels, the outer tyre, particularly, can become seriously overloaded. Rocks become trapped between the facing tyre side-walls. They may penetrate one or both tyres. Furthermore, it is difficult to access the valves of the inner tyres (needed to vary the pressure in sand).
Stay with road pattern or all-terrain tyres. The more rugged tyres (e.g. Michelin XZL) are intended for mining and quarry work. They are needed there, but are extremely noisy and ‘lumpy’. Such tyres develop uneven wear patterns impossible to reverse by wheel changing. They thus have a shorter life. On-road truck tyres are far better all-round, particularly so in the sand.
Tyre pressures for four wheel drive motorhomes
Tyres for off-road vehicles are considerably de-rated. Normally recommended tyre pressures for on-road usage at full load are usually far too high. Measure laden axle weights, and consult the tyre manufacturers for recommended pressures.
The off-road payload is invariably lower. A coach bodied OKA can only carry about 1500 kg (3300 lb), an R-type Bedford 3000 kg (6600 lb). It is difficult to keep a laden 4WD motorhome within legal limits. This catches out many DIY builders.
We were well aware of this. Our OKA’s interior was built from aluminium accordingly. When fully laden with 420 litres of diesel and 250 litres of water (a plus 6000 kg [13,225 lb]) and twin spare wheels it weighed 5.2 tonnes. Most are well over 6.0 tonne. One is known to be close to 7.0 tonne.
My own four wheel drive motorhomes
My first was a 1940 Bedford QLR. During 1959 and 1960, I drove it twice the length and breadth of Africa – including two Sahara crossings. No vehicle has ever made that journey (via the centre of Africa) since. See: Driving Across Africa
My Bedford QLR undergoing hill-climbing testing in the UK prior to its African trip. Despite its over 7 tonne and a 3.5-litre petrol engine it could climb a 45-degree slope (slowly) but with ease. Pic: rvbooks.com.au
Engine size and power of four-wheel drive trucks are typically low. On-road, some seem seriously under-powered. This is rarely an issue off-road. All have exceptionally low gearing. This is also invaluable for ‘walking’ them across rough going. The OKA (pic below) had a four-cylinder (4 litres) Perkins diesel. It was not fast but would climb any gradient.
Because the chassis is high built, heavy items (e.g. fuel, water batteries, winches, tools) must be kept low. There must, nevertheless, be ample ground clearance between the axles. High arrival and departure angles are essential.
Our OKA cresting the Big Red sand dune (Simpson desert) on route to Sydney from Broome. Pic. rvbooks.com.au
Apart from size, ground clearance will mostly determine where the vehicle can go.
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. Use wire racks and baskets within strong enclosures. China and glassware will be broken. Stainless steel is light and cheap (but we retain wine glasses for our nightly treat).
Four wheel drive motorhome – driving technique
Off-road for a big four wheel drive motorhome is very different from that with a smaller vehicle. With the latter, four-wheel drive assists you through heavy going. With a big truck, however, four-wheel drive is used only for avoiding it.
If your big motorhome gets thoroughly bogged you’ll need another at least the same size to pull you out. It may need something far bigger.
In soft sand, reduce tyre pressures to about 155-170 kPa (22-24 psi) keeping the speed below 40 km/h. Do not use low range first gear (if that is needed you are very close to being bogged). If necessary drop tyre pressures progressively to 110 kPa (about 16 psi). Don’t drive into sand at these low pressures. Only dropping below 155-170 kPa will get you out of it.
If bogged, do not use low range four wheel drive unless locking front hubs are engaged. Doing so causes the full low range torque (which can exceed 30,000 Nm) to be directed to the rear axle shafts – with a high likelihood of breakage.
If planning serious off-road driving, you must fit differential locks front and rear. Without that, a four wheel drive becomes a no-wheel drive. (One wheel on each axle can spin).
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 lift 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.
A truck-strength snatch strap is worth carrying. It may enable a lighter vehicle to pull you out of sand or mud.
My previously owned OKA was caught out by this freak ultra-soft salty patch on a tidal creek bed. Despite its 5.2 tonnes it was retrieved via a strong snatch strap by a Toyota Land Cruiser half the OKA’s weight. Pic: rvbooks.com.au
Consider completing a 4WD truck driving course. Enrol only in one that is government accredited and experienced with four-wheel-drive trucks. Many teach council staff, bush fire brigade drivers, etc.
The 4WD club courses may have only some value. The instructors are not necessarily aware of the different and more cautious approach needed with big four wheel drive motorhomes.
How strong is a four wheel drive motorhome?
A four wheel drive truck is a very rugged piece of machinery. Nevertheless, do not set out to see how tough they really are. They can break.
If bogged in soft sand the most vulnerable components are the transfer box and drive shafts. The available torque in low-range bottom gear is huge. If the engine even looks like stalling in second gear (low-range if applicable) – reduce tyre pressure before going any further.
Is four wheel drive needed for motorhomes – further information
My Caravan & Motorhome Book includes designing and building motorhomes including 4WDs. It has detailed instructions relating to their recovery if bogged. and RVs. Motorhome Electrics covers all aspects of that topic. Auto-electricians use it too. Solar That Really Works covers solar in cabins and RVs. Solar Success relates to homes and properties.