In a 4WD vehicle, as mentioned earlier,
the transfer case delivers power to both the front and rear assemblies.
Two drive shafts normally operate from the transfer case, one to each
The transfer case itself is constructed
similar to a standard transmission. It uses shift forks to select the
operating mode, plus spines, gears, shims, bearings, and other
components found in manual and automatic transmissions. The outer case
of the unit is made of cast iron or aluminium. It is filled with
lubricant (oil) that cuts friction on all moving parts. Seals hold the
lubricant in the case and prevent leakage around shafts and yokes. Shims
set up the proper clearance between the internal components and the
It must be kept in mind that vehicles with
two driving axles have different gear ratios between the front and rear
driving axles, resulting in a pull-push action. The result of having the
two axle ratios is a phenomenon called driveline windup. Driveline
windup can be explained by associating the driveline to a torsion bar.
The driveline twists up when both driving axles are rotating at
different speeds, pushing and pulling the vehicle on hard, dry pavement.
Also remember that neither the front nor rear axle has any compensating
factor for speed and gear ratio differences between the front and rear
Driveline windup can cause handling
problems, particularly when rounding turns on dry pavement. This is
because the front axle wheels must travel farther than the rear axle
wheels when rounding a curve. On wet or slippery roads, the front and
rear wheels slide enough to prevent damage to the driveline components.
However, this may not be the case on dry surfaces. This is why many
older 4WD systems that do not include components to dissipate driveline
windup can only be safely driven on wet or slippery surfaces.
Interaxle Differentials The
most common method of dissipating driveline windup is to include a third
or transaxle differential in the transfer case gearing.
The front and rear drivelines are
connected to the interaxle differential inside the transfer case. Just
as a drive axle differential allows for different left and right drive
axle shaft speeds, the interaxle differential allows for different front
and rear driveline shaft speeds. The driveline windup, developed as a
result of different front and rear axle gear ratios, is dissipated by
the interaxle differential.
While the interaxle differential solves
the problem of driveline windup during turns, it also lowers performance
in poor traction conditions. This is because the interaxle differential
will tend to deliver more power to the wheels with the least traction.
The result is increased slippage, the exact opposite of what is desired.
To counteract this problem, some interaxle
differentials are designed much like a limited slip differentials are
designed much like a limited slip differential. They use a multiple-disc
clutch pack to maintain a predetermined amount of torque transfer before
the differential action begins to take effect. Other systems, use a cone
braking system rather than a clutch pack. However, the end result is the
same. Power is supplied to both axles regardless of the traction
Most systems also give the driver the
option of locking the interaxle differential in certain operating modes.
This eliminates the differential action altogether. However, the
interaxle differential should only be locked while driving in slippery