Types of Four Wheel Drive Systems


To the ignorant, selecting a four wheel drive (4WD) system is like shopping for basketballs. Don’t they all look the same? But beneath the orange veneers lie a diverse set of balls, some bouncy and some not. Similarly, beneath the jovial jargon of an automotive salesperson lie distinctly different 4WD systems, some meant to tackle earth, water and stone, and others designed for the winding road meandering into the sunset. So take your pick of rugged terrain or reflective travel, and here are the 4WD systems to match.

Part-Time (aka “Selectable”) 4WD

Don’t let the name fool you – part-time doesn’t mean half-hearted. A long-standing favorite with off-roading enthusiasts and the beating heart of the Jeep Wrangler, Nissan Xterra and Toyota Tundra, part-time 4WD is designed to tackle steep gradients and craggy territory.

A vehicle equipped with part-time 4WD usually travels in 2WD, more specifically, RWD. When traveling on slippery, wet or rugged surfaces, the driver can manually switch to 4WD, either through pushing a button or pulling a lever. Older part-time 4WD systems demanded drivers to manually lock the hubcaps on the tires and stop the vehicle before switching modes. Today, neither of these annoyances are prominent practices.

When in part-time 4WD, power is shared between the front and rear axles through a transfer case, which normally also contains a powerful low gear for low-speed, high-torque needs. However, there are no center differentials, which effectively lock the speeds of the front and rear axles together. This improves straight line traction and adds maneuverability in slippery, low-traction surfaces, but wreaks havoc in dry, flat surfaces. For this reason, part-time 4WD should never be used on roads, because the wheels will bind and cause premature tire wear and busted bearings.

Full Time (aka “Permanent”) 4WD

The more expensive and talented cousin to part-time 4WD, full-time 4WD never sleeps nor rests. It is active at all times, pavement or dirt. However, it contains a center differential, allowing the front and rear axles to rotate at different speeds so it can drive on dry, flat surfaces. Some off-road-oriented vehicles contain a locking center differential or limited slip differential, which improves traction. Many trucks are equipped with full-time 4WD to allow them to easily switch between off-road hauling and on-road commuting. In short, for some extra mulah, a buyer can get the off-road benefits of a part-time 4WD system with the on-road comfort of full-time 4WD.

Full-time AWD

Full-time AWD is the more intelligent brother to full-time 4WD. Usually an option on an otherwise-2WD vehicles, full-time AWD uses a complex array of sensors to decide which tire gets the lion’s share of power. It adds some type of viscous coupling or multi-plate clutch to the standard batch of drivetrain equipment. However, as with most electrical devices, its mathematical brain can be maddeningly rationale, and has been known to cause problems in the great outdoors. Therefore, it is normally used as traction control for luxury sport cars.

Automatic AWD

Once again, don’t let the name fool you – automatic AWD doesn’t mean off-road capability. Automatic AWD is designed for on-road use and has no 2WD option or ultra-low gearing for tough off-road requirements. It permanently powers both front and real axles, and therefore needs no transfer case. Originally designed for luxury and sport cars, automatic AWD is making headway into the entry-level market as more and more manufacturers are turning to the superior safety and handling provided by automatic AWD. Subaru, in fact, designs all its vehicles with automatic AWD, and is well known for its all-weather stability and control.

Drivability. That’s what 4WD systems are all about. Whether hunting in the outback or roaring down the drag strip, 4WD can help cut corners or hop over hills. The fact is, they aren’t all the same. Some thrive outdoors, some slump. So select a 4WD system and drive away – just not in part-time 4WD.

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