Yet open diffs have no means of limiting the speed variation between the two outputs, so torque is free to follow the path of least resistance. An open differential, a variation of the common planetary gearset found in automatic transmissions, splits a single torque input (the transmission) into two outputs (the front and rear axles) but allows them to rotate at different speeds. The humble open center differential-simple, reliable, cheap-has been driven to near extinction by electromechanical alternatives that offer more control and greater efficiency. What follows is a closer look at the most popular hardware used by today’s all-wheel-drive vehicles to direct power to the ground: Most, however, are on-demand systems based on front-wheel-drive powertrains. Many of today’s all-wheel-drive vehicles rely on the far more common center differential, which is a proven means of controlling torque delivery to both axles. Though their popularity is increasing, only a handful are sold in the U.S. Gas/electric all-wheel-drive systems, which are still in their infancy, range wildly in cost and purpose, and e-axle vehicles are outliers. Appearing at the other end of the performance spectrum is Toyota’s RAV4 hybrid crossover, which powers its rear wheels exclusively with an electric motor. Yet it’s not alone in the world of the electrically driven axle. By powering its front wheels with an electric motor, Porsche’s 918 Spyder recalibrated not only our definition of rapid but also our definition of all-wheel drive.
One of them is to forgo a mechanical connection altogether and power one axle with electricity. If your goals are more ambitious-like turning, for example-there are more-effective ways of dividing torque to the front and rear axles than simple transfer cases. Or our favorite, jumping over cars in beer-doused arenas. Or yanking boats up moss-covered launch ramps. It’s a little elementary, sure, but quite often so are the problems trucks aim to solve. And they do it that way whenever they’re driving all four wheels. By our definition, four-wheel-drive vehicles, mainly trucks, can only lock their front and rear driveshafts so that each axle always turns at the same speed. It’s a distinction that matters in these pages. Notice that we wrote all-wheel drive, not four-wheel drive. How today’s all-wheel-drive vehicles direct torque is varied, but doing it well means distributing the right amount of torque to the right wheels at the right time. Thus, all-wheel drive, which divides the work of moving us not among two wheels but among four. Not that we mind spinning tires, but since an engine’s job is to get us where we want to go, harnessing its torque to accomplish that task is only pragmatic. And in automotive terms, that most frequently means spinning tires. Left undirected, like toddlers or teenagers, it will frustrate, always preferring the path of least resistance. Torque, despite its industrious reputation, is lazy. Herewith, splitting some hairs over how all-wheel-drive systems split torque. But not every all-wheel-drive system is created equal. These systems promise all-weather assurance as well as dry-road dynamic benefits, and many car buyers believe them to be essential to any new-vehicle checklist. All-wheel-drive systems are proliferating through the car market like so many tribbles on Kirk’s Starship Enterprise.
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