Many years have passed since I stopped believing in the popular myth that four-wheel-drive would unfailingly take me where two-wheel-drive would not. As a rookie on early visits to Africa I used to place blind reliance upon a Land Rover, only to wonder why, when the going became sticky, it used to get stuck along with 2-w-d vehicles. Not only that Land Rover, of course, but other so-called all-terrain vehicles floundered just as readily when taken into mud.
Bush farmers often prefer light pick-up trucks to heavier 4-w-d vehicles. If they get stuck, they are much easier to drag out, and you can always carry your own “push crew” in the back!
Much of the ability of some 4-w-d vehicles comes not from any particular kind of transmission system, but simply from ground clearance, and many a driver has mistakenly thanked providence for his two driven axles whereas his thanks should have been for their height above the ground.
In many of its forms, 4-w-d most certainly does not give the constant all-round traction that some people imagine. Some can be reduced to one-wheel-drive very readily indeed, whilst others can be left with two spinning wheels on one side and no drive at all on the other.
It’s all down to the differentials, which are the units that make or break a 4-w-d system. A normal differential will immobilise a 2-w-d car if one of its wheels starts to spin, for the differential action will prevent any drive getting to the wheel on the opposite side of that axle.
The same will happen to each axle of a 4-w-d vehicle with two standard differentials, and often we have seen such vehicles with two wheels on one side spinning in a ditch, and both other wheels motionless and quite useless.
If there is a third normal differential mounted between the two axles, then that could easily split the drive if any of the four wheels start to spin, resulting in a minimum of just one driven wheel, spinning uselessly.
Of course, if the central differential can be locked, or partially locked, the drive will be maintained front and rear — as it does in a Range Rover—and the driver will be assured of at least two driven wheels, one front and one rear.
The Audi Quattro has lockable differentials rear and centre, and a normal diff at the front, so that when both diff locks are engaged at least three driven wheels will result, two rear and one front. This works extremely well if the car has to be manoeuvred on soft ground, because a locked front differential produces very heavy and difficult steering.
A vehicle which goes all the way and guarantees four driven wheels whenever required is the Mercedes G, a somewhat box-like cross-country vehicle which, like similar ones of other makes, comes in various dimensions and with various engines. There are two differentials, one front and one rear, and both can be locked from the driver’s seat whilst on the move, although drive to the front wheels can be disengaged by use of a secondary gear lever.
With both diffs locked, there is none of the usual differential action anywhere, and all four wheels remain driven no matter whether any of them are spinning on slippery ground. The vehicle is difficult to drive other than in a straight line when so trimmed, but as a means of getting out of, or through, difficult mud, the system is excellent, especially as both differentials can be freed without stopping, once the bad patch has been negotiated, thus restoring the steering to manageable state by allowing the front wheels to revolve at different speeds on curves.
There have been many design copies of the trusty Land Rover, and now just as many of the more luxurious Range Rover which aimed at a more pretentious market than its workhorse forebear. The Mercedes G, which we drove for a week in foul weather last November, retains the appearance of a utility, but has creature comforts which make for tiredness-free long trips.
We don’t intend to compare its appointments with those of its rivals, for many options available such as long / short wheelbase, petrel / diesel engine, two / four doors, transverse / longitudinal rear seating, manual / automatic four-speed gearbox and various others.
Our main concern was its off-road ability, and in this respect we have nothing but praise after using it on roads, steep grassy slopes, a waterlogged moor in Wales and a ploughed field — the latter with consent, incidentally. It is a mite top-heavy, but that’s the price of good ground clearance which all such vehicles have to pay. Even its dry road handling was far better than expected, and the injected 6-cylinder twin-cam version (short wheelbase) which we used was never at risk of not holding its own on twisty hilly roads.
The top ratio of the four-speed automatic gearbox (and presumably of the manual too) could do with being more of an overdrive than one-to-one, thus improving the 14 m.p.g. which we achieved during the week, and we felt that the rear seat could made to fold away a little further than it did, to improve the rear load space.
Inclusive prices between £14,000 and £15,460 are a hole high if all you need is something to impress your friends, but if it’s genuine four-wheel-drive you want, then you should try a G before making a purchase. — G.P.