Four-wheel drive systems - there are a number of 4WD systems, generally split
between full-time and part-time operation. As the names imply, power is sent to
all four wheels either all the time or some of the time. Full-time 4WDs can be
driven on all road surfaces. Some full-time systems are "selectable", running
in two-wheel drive when the driver so chooses. Part-time systems run as
two-wheel drive until the driver selects four-wheel drive for off-road use.
On-demand part-time systems automatically transfer power to the rear wheels on
all surfaces when slip is detected.