The transfer case is the device that
splits the power between the front and rear axles on a
four-wheel-drive car.
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A typical part time four-wheel
drive transfer case. The planetary gear reduction
can be engaged to provide the low-range gearing.
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Back to our corner-turning example:
While the differentials handle the speed difference
between the inside and outside wheels, the transfer case
in an all-wheel-drive system contains a device that
allows for a speed difference between the front and rear
wheels. This could be a viscous coupling, center
differential or other type of gearset. These devices
allow an all-wheel-drive system to function properly on
any surface.
The transfer case on a
part-time four-wheel-drive system locks the front-axle
driveshaft to the rear-axle driveshaft, so the wheels
are forced to spin at the same speed. This requires that
the tires slip when the car goes around a turn.
Part-time systems like this should only be used in low
-traction situations in which it is relatively easy for
the tires to slip. On dry concrete, it is not easy for
the tires to slip, so the four-wheel drive should be
disengaged in order to avoid jerky turns and extra wear
on the tires and drivetrain.
Some transfer cases, more
commonly those in part-time systems, also contain an
additional set of gears that give the vehicle a low
range. This extra gear ratio gives the vehicle extra
torque and a super-slow output speed. In first gear in
low range, the vehicle might have a top speed of about 5
mph (8 kph), but incredible torque is produced at the
wheels. This allows drivers to slowly and smoothly creep
up very steep hills.
The
type of part-time system typically found on
four-wheel-drive pickups and older SUVs works like this:
The vehicle is usually rear-wheel drive. The
transmission hooks up directly to a transfer case. From
there, one driveshaft turns the front axle, and another
turns the rear axle.
