Four-wheel drive systems
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There are many 4WD systems, the most common ones are full-time 4WD and part-time 4WD. Obviously in Full Time 4WD the power is sent to all four wheels all the time, in more modern Full Time 4WDs, this setting can be changed by the driver. However in some Four Wheel Drive cars, such as those by Subaru (AWD) the power is going to all four wheels and is unchangeable. While in part time 4WD the car can work out when it is not needed (or the driver can tell the car) nevertheless in some systems, given the right conditions part-time systems automatically transfer power to the rear wheels on all surfaces.