Four-wheel drive systems

the front and rear wheels. Figure 10.5 shows the
layout of the system used on the Ford Maverick.
Note the extra components compared to a normal
two-wheel drive system:
●
transfer gearbox
to provide an extra drive
output;
●
differential on each axle
to allow cornering
speed variations;
●
centre differential
to prevent wind up between
the front and rear axles;
●
extra drive shafts
to supply drive to the extra
axle.
The transfer gearbox on some vehicles may also
contain extra reduction gears for low ratio drive.
One problem to overcome, however, with
4 WD is that if three differentials are used then the
chance of one wheel slipping actually increases.
This is because the drive will always be trans-
ferred to the wheel with least traction – like run-
ning a 2 WD car with one driving wheel jacked up.
To overcome this problem and take advantage of
the extra traction available a viscous coupling is
combined with an epicyclic gear train to form the
centre differential.
The drive can now be distributed proportion-
ally. A typical value is about 35% to the front and
65% to the rear wheels. However, the viscous
clutch coupling acts so that if a wheel starts to
slip, the greater difference in speed across the
coupling will cause more friction and hence more
drive will pass through the coupling. This tends to
act so that the drive is automatically distributed 
to the most effective driving axle. A ‘Hyvo’ or
silent chain drive is often used to drive from the
transfer box.

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