Mini All-Wheel-Drive Systems
By mark Williams
Each of our four little 'utes is without an extra low-range gear, traditionally used for heavier towing or rougher 4x4 driving. The advantages of all-wheel drive usually manifest themselves in difficult weather conditions or loose gravelly roads, clearly more in line with our intended use. Here's a brief description of how each all-weather system works, along with our impressions.
Called Control Trac II, Ford's system biases drive to the front wheels on dry pavement and sends power to the rear wheels when a difference in driveshaft speed is sensed in the transfer-case's viscous coupling (a closed, liquid-filled box full of speed-differentiating friction plates). Feel is subtle, practically invisible, but slow to react.
Creatively called Real Time Four-Wheel Drive, the Honda system doesn't use a center transfer case, but rather a series of pumps and friction plates that activate when they sense slip at the front wheels. The system is housed in the front transaxle, adds only 15 lb, and is not as invisible as the other systems, especially when driven energetically.
Land Rover Freelander
Without a dedicated name, Land Rover's system is full-time all-wheel drive and uses traction control, combined with clever technology like Hill Descent Control and Steptronic shifting, to allow for significant driver control in many different situations. A separate transfer case uses a viscous coupling that generally keeps a 50/50 split of power. Versatile and transparent.
Also called Full-Time Four-Wheel Drive, Toyota's system uses a separate transfer case with a viscous coupling to send power to the rear wheels when excessive amounts of shear are detected between the front and rear driveshafts. In high-traction situations, the RAV4 is front drive only. Probably the standard for the class.