There was a time - not so long ago - when the idea of all-wheel drive (AWD) or four-wheel drive (4WD) was a largely confusing concept reserved for land-crushing 4x4s, small-volume European cars, and the people who loved either or both.

But over the past decade or so, car manufacturers and consumers have come to appreciate the benefits of having power distributed to four wheels instead of just two, temporarily if not permanently, particularly when the going gets tough.

Of course, with so many different systems on the market - and with every manufacturer promoting their own approach to the technology - things can still get confusing.

In honor of the wintry weather that has struck or is about to strike many parts of North America, here is a two-part overview of the pros and cons of current AWD and 4WD systems and where this technology is headed down the road, so to speak. In part two, we’ll look at AWD systems; but first, a delve into four-wheelin’.

Full-time vs. part-time 4WD

Full-time 4WD vehicles are those permanently fixed in 4-wheel-drive; power is sent to all four wheels at all times.

Originally known as 4x4s ("four by fours"), these vehicles have both a regular set of "street" gears plus a range of low gears meant for tackling serious off-road trails at low speed - high speed being traded in exchange for lots of power. They also employ what is called a "center differential" allowing 4WDs to travel on regular paved roads without much issue, though that hasn't always been the case.

A differential, of course, is the precision assemblage of gears (usually) found in every car. In 2-wheel-drives, it allows a single source of power (as torque) to be distributed left and right - basically a variable-speed Y-splitter.

After all, when a vehicle rounds a curve or turns a corner, the inside wheels have less distance to travel than the outside wheels. While the left and right wheel are receiving power from the same source, they need to rotate at different rates, the inside wheel rotating less (wheels not under power simply "free wheel" along for the ride). Without a differential, the inside wheel would be pushing the vehicle contrary to the intended direction of the turn, burn little spats of rubber and spin as it tries to rotate at the same rate while covering less ground, or, if it has more traction, drag the outside wheel which is trying to cover the greater distance at the same rate of rotation. On pavement, this would be bad, resulting in a constant war on physics, handling characteristics no better than a chain-driven go-cart and quite a bit of nasty tire wear.

4WD doubles the conundrum as all four wheels take power and all spin a different rates when turning or cornering. For these vehicles, a center differential allows both of the outside wheels, front and back, both under power, to rotate at higher speeds when cornering. Even with a center differential, older or lesser quality 4WDs tend behave awkwardly on dry pavement as each tire still travels a bit more or a bit less than the others through twists, tweaks and turns, which is why their tires are known to "chirp," "hop" or "bind." However, without a differential, the effect would flagrant an unwieldy - not to mention damaging to the 4WD system components.

These days, fortunately, center differentials are so technically proficient, more so in the upper class, the ride in 4WD is pretty smooth.


Off road, on the other hand, where traction is varied and comparatively slippery, binding and hopping is not an issue, because a little dragging or pushing with without firm traction is no big deal. In fact, a center differential can also be "locked," allowing the vehicle to act as if it has no differential at all. Thus, whatever tire has the most traction at any given moment will also be getting all power available to it; the tire(s) with less traction will be dragged along or spun inconsequentially considering the aforementioned lack of traction.

While full-time 4WD vehicles are definitely beneficial when the going gets rough, there are some drawbacks. Typically, these vehicles suffer in terms of fuel efficiency because all four wheels are always under power. They also tend to be more expensive because they’re equipped with extra features and more components for going off-road.

Of the current vehicles on the market today, the Land Rover range is the most representative of a full-time 4WD offering. Although the manufacturer’s entire line boasts a massive amount of luxury, you can also practically drive them up a redwood tree if you so desire.



Part-time 4WD vehicles have most the capabilities of their full-time 4WD counterparts, but with the added benefit of being able to run in 2WD mode with the flick of a switch (these days).

The thing is, convenience wasn’t necessarily the name of the game when full-time 4WD first appeared; the driver would have to stop the vehicle and yank a lever or two and often step outside to manually lock the wheel hubs before being able to engage the system. These days, part-time 4WD vehicles come with "shift-on-the-fly" capability; the driver simply presses a button on the console and the magic unfolds.

As with full-time 4WD vehicles, these part-timers have an optionally selectable low range providing extra torque for slow-speed off-roading. However, they do not have a center differential (but one differential for the front axle and another for the back). This makes for a lighter and less expensive solution but it's not as effective as a center differential. As a result, the 4WD mode of a "part-time 4WD" vehicle should only be used under muddy, snowy or wet conditions.

Many trucks and SUVs intended for serious off-road use feature part-time 4WD; these include the Chevrolet Silverado, Ford F-Series, Jeep Grand Cherokee, Nissan Xterra andToyota FJ Cruiser.

Next up, a look at AWD, or "all-wheel drive" vehicles which, despite referring to all four wheels on a vehicle, isn't quite the same thing as four-wheel drive.
 
 

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