How Permanent 4 system works
On vehicles with permanent four-wheel
drive new technical solutions have been
adopted characterized by a series of
devices able to guarantee optimal traction
even under critical road holding
conditions. These results have been
obtained thanks to the adoption of a
central “viscous coupling” able to
transfer part of the deflecting torque from
one drive shaft to another whenever there
is wheel slip due to bad road conditions.
Under good road holding conditions the
vehicle behaves liked a front wheel drive
vehicle but has the advantage that the rear axle permanently contributes to the thrust with a small torque (about
5%) transferred by the viscous coupling which, like a differential device, allows small differences in the speed
of the front and rear wheels. Drive is transmitted to the viscous coupling (2) from the electromagnetic coupling
(1) which is controlled by an electronic control unit which automatically engages/disengages it in accordance
with evaluation parameters set by the control unit itself and measures by the relevant sensors and components.
The rotating mass (3) located at the end of the drive shaft enables the vibrations generated by the imbalance of
the system to be absorbed.
1 - Electromagnet coupling
2 - Viscous coupling
3 - Rotating mass
4. Body
5. Shaft
A. Integral disc with coupling body 4
B. Integral disc wit shaft 5
Electromagnetic coupling (1)
The electromagnetic coupling forms part of a sophisticated system of control managed by a specific control unit
which, from the-processing of the data measured by a series of sensors, manages the traction on the basis of the
best road holding conditions obtained trough the disengagement of the four-wheel drive which is the normal
operational state.
The-functions of the system are basically as follows:
- Engagement/disengagement of the coupling
- Anti-disengaging function
- Self diagnose.
Viscous coupling (2)
As shown in the following page the connection between the two drive shafts (front and rear) is formed by a
special silicone fluid in which discs “A” and “B” are immersed. These discs are integral with body 4 and with
the shaft 5 (through a grooved coupling). Under normal conditions, as already described, with a minimum
difference in the speed of the two discs, a minimum torque is transferred as the traction is almost entirely on the
front axle. When one of the drive shafts is slipping resulting in a great difference in speed between the two
disks, (e.g. front wheels on ice and rear wheels on dry road) there is an increase in temperature resulting in a
compression of the disks and relative blocking of the coupling itself.
