Page 245 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Types of suspension and drive CHAPTER 8.1

Fig. 8.1-74 Visco clutch with slip-dependent drive moment distribution. Two different packages sit in the closed drum-shaped
housing: radially slit steel discs, which are moved by the serrated profile of the hollow shaft, and perforated discs which grip (as can
be seen below) into housing keys. The shaft is joined with the differential and the casing with the propshaft going to the rear axle.
The discs are arranged in the casing so that a perforated disc alternates with a slit one. The individual parts have no definite spacing
but can be slid against one another axially. The whole assembly is filled with viscous silicone fluid and the torque behaviour (therefore
the locking effect) can be adjusted via the filling level.
If slip occurs between the front and rear axle, the sets of discs in the clutch rotate relative to one another and shearing forces
are transferred via the silicone fluid. These increase with increasing slip and ensure a torque increase in the rear axle. The
power consumed in the visco clutch leads to warming and thus to growing inner pressure. This causes an increase in the
transferable torque which, under conditions of extreme torque requirement, ultimately leads to an almost slip-free torque
transfer (rigid drive). With ABS braking, a free-wheeling device disengages the clutch; the latter must be engaged again when
reversing.

to all four wheels by means of a clutch on the propshaft, (Fig. 8.1-68). There is relatively little additional com-
as required. The clutch can be engaged manually, or au- plexity compared with the front-wheel-drive design,
tomatically in response to slip. With the use of sprag even if, on the Fiat Panda (Trecking 4 4), there is
clutches, which are usually engaged manually, the torque a weight increase of about 11% (90 kg), not least because
is transmitted in a fixed ratio between the front and rear of the heavy, driven, rigid axle. It is possible to select
axles; multi-disc or visco clutches permit variable torque rear-wheel drive during a journey using a shift lever that
distribution. As these systems have essential similarities is attached to the prop-shaft tunnel.
with permanent four-wheel drive varieties, they are Manual selection on the Subaru Justy operates
discussed in Section 8.1.7.4. pneumatically at the touch of a button (even while
With sprag-clutch engaged transmissions, the design travelling). This vehicle has independent rear-wheel
complexity, and therefore the costs, are lower than on suspension and weighs only 6% more than the basic
permanent drive. Usually there is no rear axle differential vehicle with front-wheel drive. Traction is always im-
lock, which is important on extremely slippery roads; proved considerably if the driver recognizes the need in
while this results in price and weight advantages, it does time and switches the engine force onto all four wheels.
lead to disadvantages in the traction. In critical situations, this usually happens too late, and
Front-wheel drive is suitable as a basic version and the the abrupt change in drive behaviour becomes an ad-
longitudinal engine has advantages here (Fig. 8.1-48). ditional disadvantage.
With the transverse engine, the force from the Conversely, if the driver forgets to switch to single
manual gearbox is transmitted via a bevel gear and a di- axle drive on a dry road, tensions occur in the power train
vided propshaft, to the rear axle with a differential during cornering, as the front wheels travel larger arcs

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