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From BMWUSA.com -
Unlike other all-wheel drive systems, we've built xDrive, BMW's intelligent all-wheel drive system with rear-wheel bias for surefooted grip on slick or uneven roads, without sacrificing the feel of rear-wheel drive. It smartly sends power to the wheel or wheels with the surest footing-and it does so in milliseconds. That means under normal conditions, drive forces are spread out with a rear-wheel bias. But the moment the system senses excessive wheel slip, it subtly shifts torque to meet the changing conditions, offering maximum available traction and superior handling on all surfaces-despite the weather conditions.