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A part-time four-wheel drive system powers only two wheels when sufficient grip is available. When those two wheels (either the front or the rear) start to lose their grip, then the system either automatically distributes torque to the other set of wheels, or the driver has to push a lever or button to engage the other two wheels. On some vehicles, a low range can be selected to further increase the torque being transmitted to the wheels. The CR-V is one example of a automatically-engaging part-time system. The “Real-Time 4WD” system usually powers only two wheels—the front. When the fronts start to lose grip, say on mud or sand, the system’s computer can detect the speed difference between the front and rear tires and automatically engage the transfer case. A pump activates and hydraulically engages a multiplate clutch, sending torque to the rear wheels as well. Some driving tests have shown that the Real-Time system does help in slippery conditions, but is ultimately not meant for off-roading. Deep sand can easily bog down the system, when the fronts are already deep in the loose sand before the rear tires can help out. The Mercedes Benz 4-matic is another example of an automatic part-type system but with all torque being sent to the rear, and the fronts engaging only when needed. 4Matic also uses a hydraulically-activated multiplate clutch. There are several advantages with part time systems. Since they propel only two wheels most of the time, there is less transmission loss in the system due to friction. This can lead to better fuel economy versus a full-time system. Audi, however, claims that full-time 4wd is actually more fuel efficient, as there is less traction loss due to wheelspin. Also, the car can retain the handling characteristics of a front or rear-wheel drive car. Drivers who enjoy controlling the car direction using the throttle as well as the steering can still have the “pure” feel of a rear-wheel drive car, with the fronts ready to help out in slippery conditions. |
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