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Drivetrains Explained

How each system works—and suits your driving style

Just about every vehicular drive system imaginable is currently in production—each touted by its manufacturer as the obviously superior choice for you. With conflicting claims and confusing terminology, it's hard to know what's what, and what's best for you. The optimum drive-system choice for a given vehicle depends on many factors. If you're realistic about the type of driving you do, you can narrow down the possibilities—and probably save a lot of money in the process.

Front Drive

For modestly powered passenger cars, front-wheel drive (FWD) is the best compromise of performance, cost and efficiency. By putting the weight of the engine over the driving wheels, traction off the line is excellent. Plus, overall advantages include reduced vehicle weight and interior roominess thanks to not having a long driveshaft/raised tunnel down the center of the car.

The downside of FWD is mainly in aggressively sporting driving, where the front tires—burdened with steering, most of the braking and all the power delivery—get overloaded in turns. The result is something called understeer, when the front tires begin to slide much more than the rear tires do, and the car won't answer the steering faithfully. This is almost never an issue for normal drivers, since they don't maneuver very aggressively, even in a panic situation.

Another downside to front drive is torque steer, which is feedback through the steering caused by heavy applications of engine power. Refinements to the design of the front suspension and the driveshaft layout have made torque steer something of a non-issue in current vehicles, at least in cars with less than about 250 horsepower or so.

Rear Drive

Rear-wheel drive (RWD) is favored in powerful sports cars, due to its superior performance in high-energy driving and racing. With power being sent to the rear wheels, the front tires have less work to do and the vehicle has better handling balance as it reaches its absolute cornering limits. Understeer is reduced, and a skilled and experienced performance driver can enjoy a superior feeling of control and connection with the car.

However, if you don't drive hard, the difference in handling between front and rear drive is often indistinguishable. This is particularly the case in vehicles of average power output. Rear drive can perform poorly when leaving the line (or attempting to) in snow or ice, when the lightly loaded rear wheels are prone to spin. Rear drive cars are also typically less stable in violent transient maneuvers like sudden lane changes or when the driver runs out of talent unexpectedly. A spin is the typical result in these admittedly atypical situations. New electronic vehicle stability-assist/traction-control systems address this issue and can make even rear-drive cars highly forgiving at the limits of control.

Traction Strategies

Traction-control systems are designed to make the most of the traction available. By using electronic circuitry that senses when the drive wheels are beginning to spin uselessly, these systems automatically modulate the individual wheel brakes or throttle to minimize wheelspin. So one way or another, traction control reduces the engine power applied to the wheels until slip is under control. But it can't fix everything: Even with the help of traction control, rear drive typically performs worse than front drive on snowy or icy surfaces.

Limited-slip differentials are another traction strategy found for the most part in performance-oriented vehicles or 4x4s. When one of the drive wheels begins to slip, the limited-slip differential automatically sends a greater proportion of the engine's power to the wheel that is NOT slipping. Limited-slip doesn't reduce the power applied to the drive wheels, but instead redistributes it.

Four-Wheel Drive

Most people can get by just fine with front or rear drive. Still, in some climates, driving conditions and testosterone levels are better handled with the help of four-wheel drive. Though manufacturers use a variety of terms to describe their systems, the types of four-wheel drive fall into two main categories: all-wheel drive and four-wheel drive.

All-wheel-drive (AWD) systems are a diverse bunch of technologies that share one key feature: They either drive all the wheels all the time, or automatically drive all four wheels at the moment that wheel slip is detected. No driver intervention is required. There are significant differences among systems. Some are designed almost exclusively for road use, and have just a single range of ratios (no low range). These systems are ideal for people who drive on paved or dirt roads in all kinds of weather, but aren't interested in doing hard-core off-road stuff like plowing through deep mud, slogging through natural stream crossings or crawling over rocks.

Getting a full-time all-wheel-drive system that also offers true off-road capability will cost you more money at the time of purchase, and in some cases can hurt fuel mileage and on-road handling. But if you truly need full off-road capability, it's the only choice. Select vehicles offer dual-range all-wheel-drive systems that have a high range for normal-speed use but where additional traction is needed, plus a low range for slow-going off-road. These systems combine the excellent all-weather on-road traction of road-oriented all-wheel-drive with serious off-road capability. If you want to be ready for anything, this is the best type of system to have.

Part-time four-wheel drive (4WD) is designed to operate as a rear-drive system on the highway, then be shifted into four-wheel-drive when the vehicle is driven off-road or in particularly slick highway driving. Since these systems are designed for serious off-road use, they let you choose between high- and low-range ratios, depending on whether you're cruising a smooth dirt road at 50 mph (high range), or creeping through a boulder-strewn stream bed (low range). Part-time four-wheel drive systems offer a lot of capability, but require a high level of driver knowledge and experience to extract their maximum performance. And since they can't be run in four-wheel drive on the highway continuously, they're no better than rear drive in most road driving.