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The 4x4 Equation

What Makes Four-Wheel and All-Wheel Drive Vehicles So Popular?
Joshua Shapiro
From the Print Edition:
Linda Evangelista, Autumn 95

(continued from page 1)

Although SUVs, with their rigid, heavy truck bodies, are perceived to be more crashworthy than cars, in some accidents they are more dangerous. Unlike passenger cars, SUVs are not usually designed with energy-absorbing crumple zones. As a result, a Volvo 940 protects a driver from fatal injuries in a frontal crash four times better than, for example, a Land Rover Discovery or a Mitsubishi Montero.

Actually, the biggest danger in driving an SUV is false confidence. Many SUV accidents are caused by a driver's inability to stop suddenly--better traction doesn't mean better stopping. No matter how expensive and sophisticated the vehicle, control, traction, steering and stopping ultimately depend on four little patches of tire, where the rubber meets the road.

Four-wheel drive divides the engine's power among all four wheels instead of two, the result being superior lateral stability and handling. After all, no vehicle today uses only two wheels to brake, and some sophisticated sports cars even steer with all four wheels. But 4WD systems differ greatly in performance, function and sophistication.

The heart of any 4WD system is a gearbox next to the transmission called a transfer case. It divides the engine torque and sends it to the front and rear axles. Most 4WDs also have a separate set of low gears for engine breaking, hauling a trailer or crawling over rough terrain. What sets apart 4WD systems are the number and type of grapefruit-sized gear arrangements called differentials. A 2WD vehicle only has a single differential, while part-time 4WDs have two, and full-time 4WD or AWD vehicles have three. When a car goes through a turn, each wheel is turning at a different speed. Differentials allow each wheel to be driven at a different speed.

Part-time 4WD is the least expensive, most common and least convenient type of 4WD. The Isuzu Rodeo, Suzuki Samurai and SUVs with similar part-time systems require that a driver come to a complete stop before shifting between 2WD and 4WD. The transfer cases of some part-time 4WD systems--such as those standard in the Rodeo and the Jeep Wrangler, Cherokee, Grand Cherokee Laredo and SE--lock the front and rear driveshafts together when entering 4WD. SUVs in part-time 4WD mode should not be driven on dry or damp hard-surfaced roads, because it forces all four wheels to rotate at fixed speeds, causing the tires to skip. They also require driving slowly in strong crosswinds and around curves.

Higher in cost and convenience are the various "shift-on-the-fly" part-time systems standard in the Ford Bronco, the Nissan Pathfinder and the Toyota 4Runner. With shift-on-the-fly, the transfer case allows shifting in and out of 4WD mode while the vehicle is in gear. Depending on the model, mode shifting is possible at speeds as fast as 55 mph.

Part-time and shift-on-the-fly systems have either manual or automatically locking front wheel hubs that must engage the front axle for a driver to shift into 4WD mode. When returning to 2WD, a driver must either stop, for manual-locking hubs, and get out of the SUV to twist the lock on the hubs back to the "free" position, or he must back the vehicle up as much as 15 feet to disengage automatic-locking hubs. Were this unlocking not done, driving in 2WD mode would result in unnecessary and damaging driveline wear and much lower fuel economy.

The most functional and expensive type of 4WD is full-time 4WD. The A.M. General Hummer, all Land Rover models, the Toyota Land Cruiser, the Mitsubishi Montero, the 1995 Ford Explorer and the Chrysler Jeep Grand Cherokee Limited are equipped with full-time, all-surface 4WD. These models require special preparation to be towed and must only be transported on a flatbed tow truck.

Full-time systems are either permanent or set-and-forget type 4WD systems. Their transfer cases have either a "limited slip" differential or an "open" differential and a "viscous coupling" that variably distributes torque to the front and rear axles as needed, while allowing the axles to turn at different rates. Set-and-forget 4WD systems also have an automatic front axle system that eliminates the need for locking front wheel hubs.

All-wheel drive (AWD) is a form of permanent, full-time 4WD without a low gear range. Manufacturers have chosen not to publicize AWD or educate the public about its value; instead, they focus attention on their more profitable, but less effective, traction control systems. As a result, even though AWD improves handling on wet or dry roads, it sells primarily in northern snow states. When Ford recently started to advertise the AWD option for its Aerostar minivan in the Canadian market, orders jumped by a factor of four.


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