Tải bản đầy đủ

Automotive mechanics (volume II)(Part 4, chapter25) four wheel drive and all wheel drive

507-532_May 2chap 25

13/9/06

4:16 PM

Page 507

507

Chapter 25

Four-wheel drive and
all-wheel drive

Drive-line arrangements
Transmission with gear transfer
Transfer case with chain drive
Centre differentials
Transaxle centre differentials
Viscous couplings

Transfer unit with chain and viscous coupling
Suspension arrangements
Four-wheel-drive service
All-wheel drive
Types of all-wheel-drive systems
Technical terms
Review questions


507-532_May 2chap 25

13/9/06

4:16 PM

Page 508

508 part four automatic transmissions and drive
Four-wheel-drive vehicles have drive lines to both
front and rear axles so that all four wheels of the
vehicle can be driven. Some vehicles are designed for
optional off-road use, where four-wheel drive is
selected for rough, soft or sandy conditions. Other
vehicles, mainly passenger cars, have all-wheel drive
which can be used full time for better on-road
performance.

Drive-line arrangements
In general, there are three different arrangements for
four-wheel drive for passenger and light commercial
vehicles:
1. Larger passenger and light commercial vehicles.
These have a normal drive line to the rear wheels
and an additional drive line to the front wheels.

2. Smaller passenger vehicles and those with
transaxles. These have a transaxle with a normal
front-wheel-drive arrangement. They also have a
drive line to the rear wheels.
3. Vehicles with rear engines. These have their normal

rear-wheel drive and an additional drive line to the
front wheels of the vehicle.
While some vehicles have four-wheel drive as an
option, there are others which are made with only fourwheel drive.
Larger passenger and light commercial vehicles
Figure 25.1 shows the basic arrangement for a fourwheel-drive vehicle with its engine mounted
longitudinally. This has a four-speed or five-speed
transmission of conventional design behind the engine,
with a transfer case beside it (Figure 25.2). The
transfer case is actually a smaller transmission with
two speeds that provide a high and a low range. It has
output shafts at both front and rear. The front and rear
axles are both rigid axles.
Propeller shafts are connected to the output shafts
to carry the drive from the transfer case to the front
and rear axles. The axles then transfer the drive
through the final-drive gears, the differential and the
axle shafts to drive the wheels.
Figure 25.3 is another arrangement. The main
transmission has an output shaft for the rear propeller
shaft, and the transfer case has an output shaft for the
front propeller shaft.

figure 25.1

Basic arrangement of a larger four-wheeldrive vehicle ROVER AUSTRALIA

figure 25.2

One arrangement of a transmission and transfer case for four-wheel drive

ROVER AUSTRALIA


507-532_May 2chap 25

13/9/06

4:17 PM

Page 509

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.3

Arrangement of the drive line of a four-wheel-drive vehicle

Smaller passenger vehicles
and those with transaxles
The four-wheel-drive arrangement for a smaller
vehicle with a transverse engine is shown in Figure
25.4. This has a transaxle and a normal front-wheel
drive with open drive shafts. As well as this, it has a
transfer case attached to the transaxle. The transfer
case has an output shaft to the rear of the vehicle.
A two-piece propeller shaft with a centre bearing is
used to carry the drive from the output shaft of the
transfer case to the rear axle. The rear axle has a final
drive and differential assembly mounted to the body of
the vehicle, and swing axles connect it to the rearwheel hubs.
Vehicle with rear engine
Figure 25.5 shows the arrangement of a vehicle with a
rear engine. The transaxle is at the front of the engine
and open drive shafts are used to drive the rear wheels.
It is a normal type of manual transmission, except that
it has extra gearing and an output shaft at the front.
A propeller shaft connects the output shaft to the final
drive in the front axle.
The front final drive includes a viscous coupling,
final-drive gears and a differential. Open drive shafts
carry the drive to the front wheels.

509

TOYOTA

Transmission with gear transfer
A transmission and transfer case are shown in Figure
25.6 with the parts identified. The transmission has
four speeds that can be selected with the gear lever.
The transfer gearing has two speeds that are selected
with a separate lever. This provides a high range and a
low range. With the four speeds of the main transmission and the two speeds of the transfer, eight
different forward gear ratios are available to the driver.
The main transmission is similar to a transmission
for a two-wheel-drive vehicle, except that it has an
extended mainshaft with some of the transfer gears.
Transfer gearing
The arrangement of the gearing is shown in Figure
25.7. This has to do three things:
1. provide drive for high range
2. provide a gear ratio for low range
3. transfer power to the front drive.
The transfer gearing is shown in neutral in the
diagram. To obtain drive, the sliding sleeves are
moved along their splines to engage with the dog teeth
on one of the gears. The sliding sleeves and the dog
teeth on the adjacent gears form a dog clutch. With the
teeth engaged, the sleeve locks the gear to the shaft.


507-532_May 2chap 25

13/9/06

4:17 PM

Page 510

510 part four automatic transmissions and drive

figure 25.4

Arrangement of four-wheel drive for a vehicle with a transaxle and transverse engine

High range
For high range, the front sleeve is moved forwards to
engage with the dog teeth on the input gear. This
connects the input gear with the rear output shaft, and
drive is carried straight through the transfer case.
There is no gear reduction.
Low range
For low range, the front sleeve is moved to the rear.
This engages the dog teeth on the rear drive gear and
locks it to the rear output shaft.
Drive from the input gear is now through the
countergears to the rear drive gear and the rear output
shaft. A gear reduction is provided by the countergears.
Transfer
For front drive, the rear sliding sleeve is moved
forwards. This engages the dog teeth of the transfer
drive gear and locks it to the rear output shaft. Rotation
of the rear output shaft is now transferred by the
transfer drive gear, via the idler gear, to the front drive
gear and the front output shaft.

FORD

Front drive can be selected to give four-wheel drive
whether the transfer is in high range or low range.
Transfer case with two output shafts
The transfer case in Figure 25.8 has a different
arrangement. It has two output shafts, one for the front
propeller shaft and one for the rear propeller shaft.
Gearshift lever positions
In manually selected transmissions with four-wheel
drive, there are two gear levers, one for the normal
transmission and one for the transfer case (Figure 25.9).
The main transmission can have four or five gears.
The transfer case has a high range and a low range for
four-wheel drive (H4 and L4), a high range for twowheel drive (H2), and also a neutral position (N).
Neutral is used if auxiliary equipment is being driven
from the transfer case.
The main gearshift lever is shown with common
selector positions, and the transfer shift is shown with
one arrangement. However, there are variations to


507-532_May 2chap 25

13/9/06

4:17 PM

Page 511

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.5

Arrangement of a rear-engine vehicle with four-wheel drive

VOLKSWAGEN

511


507-532_May 2chap 25

13/9/06

4:17 PM

Page 512

512 part four automatic transmissions and drive

figure 25.6

Transmission and transfer case assembly

TOYOTA

those shown. The transfer positions will be different
for a vehicle with full-time four-wheel drive.

Transfer case with chain drive
Figure 25.10 shows a transfer assembly that has a
chain drive. The gearing in the transfer case provides
a gear reduction of approximately 2.5:1 when low
range is selected. The chain has no effect on the gear
ratio. Its purpose is to transfer drive from a sprocket
on the rear output shaft to a sprocket on the front
output shaft.
The arrangement of the chain can be seen in Figure
25.11. The chain is a silent type, which has a number
of links across its width that form teeth in the chain.
The links are of normal tooth shape when the chain is
flat, but are shaped so that the teeth spread as the chain

passes around the sprockets. The spread teeth fill the
space between the sprocket teeth so that there is no
clearance between the chain teeth and the sprocket
teeth – this reduces chain noise.
■ Because of the design of the teeth, the chain is
much quieter than other chains and that is why it
is called a silent chain.
Power flow with chain drive
The diagrams in Figure 25.12 show the power flow
through the transfer gearing and chain. There are three
positions:
1. two-wheel drive, high range
2. four-wheel drive, high range
3. four-wheel drive, low range.


507-532_May 2chap 25

13/9/06

4:17 PM

Page 513

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.7

Arrangement of the gearing in a transfer case
H high range, L low range, F front drive

figure 25.8

Manual transmission with a transfer case

TOYOTA

513


507-532_May 2chap 25

13/9/06

4:17 PM

Page 514

514 part four automatic transmissions and drive
There are two selector forks connected to the transfer shift lever. These move a sleeve which engages or
disengages the dog clutches to alter the power flow
through the transfer gearing.
Two-wheel drive, high range
When the transfer selector lever is moved to the
2H position, the front sleeve is moved forwards. This
connects the transmission main output shaft to the
transfer rear output shaft. In the transfer case, this is a
straight-through drive.
Drive from the transfer case is transmitted through
the propeller shaft to the rear wheels only. There is no
gear reduction in the transfer case and no power to the
front output shaft.
Four-wheel drive, high range
When 4H is selected, the rear sleeve is moved
forwards. This connects the chain drive sprocket to the
rear output shaft. Drive is taken through the transfer
chain to the sprocket on the front output shaft.

figure 25.9

Transmission and transfer selector positions
– there are also other arrangements TOYOTA

figure 25.10

Transfer case in which a chain is used to transfer the drive

FORD


507-532_May 2chap 25

13/9/06

4:17 PM

Page 515

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.11

Drive chain linkage

figure 25.12

Power flow through a transfer case with a drive chain

515

DAIHATSU

The front propeller shaft carries the drive from the
front output shaft to the front axle, and the rear
propeller shaft carries the drive from the rear output
shaft to the rear axle.

DAIHATSU

Four-wheel drive, low range
When 4L is selected, the front sleeve is moved rearwards. This introduces the transfer countergear into the
gear train. Instead of the drive going directly from the


507-532_May 2chap 25

13/9/06

4:17 PM

Page 516

516 part four automatic transmissions and drive
input shaft to the transfer rear output shaft, the countergear provides a reduction.
The countergear is driven by a constant-mesh gear
on the input shaft, and it also drives a constant-mesh
gear on the transfer rear output shaft. When the sleeve
is moved, it connects the constant mesh gear to the
output shaft. Drive now passes through the countergear
so that the rear output shaft is driven at reduced speed.
The chain also drives the front output shaft at this
reduced speed.

Centre differentials
Some transmissions are provided with a third
differential. This is located in the transmission. Its
purpose is to equalise the drive between the front and
rear axles. It acts in the same manner as a normal
differential.
With a differential in the drive line, four-wheel
drive can remain engaged, and the centre differential
will prevent transmission wind up. This is a condition
that can occur if a vehicle without a centre differential
is driven in four-wheel drive over hard-surfaced roads
or through tight turns. Tyre scuffing and transmission
damage could result.
When a vehicle without a centre differential is
operated off-road, the difference in wheel speeds is
taken care of by the loose or softer surfaces, and windup does not become a problem.
■ A centre differential compensates for variations in
speeds between the front and rear wheels.
Centre-differential lock
A lock is provided on some centre differentials. This
allows the differential to be locked when driving in
conditions where there would be wheel spin. Without a
differential lock, one wheel in loose conditions could
spin and immobilise the vehicle. With the differential
locked, the rear wheels continue to drive, even if one
front wheel has no traction.

Transaxle centre differentials
The arrangement of a lockable centre differential for a
transaxle is shown in Figure 25.13.
The transaxle centre differential and the front
differential are combined into a single compact unit
which allows full-time four-wheel drive. The centre
differential is always in the drive line, but can be
locked when necessary for bad driving conditions.

The front differential is the normal transaxle differential with side gears and pinions. The side gears drive
the front-wheel drive shafts in the usual way.
The centre differential is of a different design. It
has a simple planetary gear system which consists of
internal teeth inside the final-drive ring gear, a planet
carrier with pinions, and a sun gear. The internal teeth
are part of the final-drive ring gear, and the sun gear is
connected to an intermediate gear. With this arrangement, the final-drive ring gear is connected to the
intermediate gear by the planetary gearing.
If the action of the planetary gearing of the centre
differential is ignored for the time being, the drive
through the unit is:
1. from the output pinion of the transaxle to the finaldrive ring gear
2. then through the planetary gearing to the intermediate gear
3. from the intermediate gear to the idler gear
4. then through the crown wheel and pinion to the
pinion’s shaft.
The pinion shaft is connected by the propeller shaft to
the rear axle, which also has a differential, and so the
drive goes to the rear wheels.
The side gears of the front differential drive the
front-wheel drive shafts (not shown) in the usual way
for a transaxle.
Planetary action
The planetary gearing is located between internal teeth of
the ring gear and a sun gear attached to the intermediate
gear. With equal load on the front and rear wheels of the
vehicle, there is no planetary action. The gearing merely
transfers drive from the final-drive ring gear to the
intermediate gear without any change in speed.
Whenever there is a difference in speed between
the front and rear wheels, planetary action occurs. This
allows the ring gear and the intermediate gear to rotate
at different speeds.
While the gears are arranged differently, the action
is similar to that of the gears in a normal differential –
the planetary gearing carries drive between the finaldrive ring gear and the intermediate gear while still
allowing movement between the two gears.

Viscous couplings
A viscous coupling can be used with a centre
differential, with a rear differential, or with a front


507-532_May 2chap 25

13/9/06

4:17 PM

Page 517

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.13

Transaxle centre differential combined with the front differential – the centre differential is lockable

differential. Its purpose is to limit the difference in
speed between the front and rear drives. It acts like a
limited-slip clutch for the centre differential. It allows
a small difference in speed between the front and rear
drives, but acts automatically to prevent any great
difference in speeds.
Figure 25.14 shows the construction of a viscous
coupling. It consists of a number of plates, with one set
of plates splined to the coupling casing and the other
set splined to its hub. Spacer rings locate the plates in
the casing. The coupling contains silicone oil and is
sealed with X-shaped seals. These retain the oil under
operating pressure.
The coupling operates whenever there is a difference in rotational speed between the hub and the
casing. Slots in the plates cause shearing of the silicone
oil, and this creates a fluid-coupling effect.
■ The plates of a silicon coupling do not come into
contact with each other; they are separated by the
silicon oil.

517

FORD

Centre viscous coupling
Figure 25.15 shows a viscous coupling fitted to the
centre-differential assembly of a transaxle. This is the
same type of transaxle as in Figure 25.13, but with a
viscous coupling added.
Transaxle with crown wheel and
pinion and viscous coupling
The transaxle in Figure 25.16 is used with a longitudinally mounted engine. It has a crown wheel and
pinion, a centre differential and a viscous coupling.
The front drive shafts are connected to the differential assembly at the front of the transmission in the
normal way. The propeller shaft to the differential
at the rear of the vehicle is connected to splines on
the output shaft that extends from the rear of the
transaxle.
The viscous coupling and the differential are,
in effect, an extension of the countershaft of the
transmission.


507-532_May 2chap 25

13/9/06

4:17 PM

Page 518

518 part four automatic transmissions and drive

figure 25.14

Construction of a viscous coupling

SUBARU

Rear differential with viscous coupling
Figure 25.17 illustrates a rear differential with a
viscous coupling. This is a conventional design of
differential assembly and final drive, but it has a
viscous coupling between the two output shafts.
Viscous couplings can be located anywhere in the
drive line between the front and rear drives. They are
generally fitted wherever it is most convenient for the
particular design of transmission and transfer unit.

Front final drive with viscous coupling
The front-axle final-drive unit in Figure 25.18 has a
viscous coupling. This automatically distributes the
driving forces to the front and rear wheels. It is
mounted on the pinion shaft so that the pinion splines
engage with splines in the hub of the coupling. The
coupling casing is connected to the drive flange by
the coupling shaft.
Power is transmitted from the flange through the
coupling to the pinion, then to the crown wheel and
the differential.

Transfer unit with chain
and viscous coupling
The transfer unit in Figure 25.19 has planetary reduction gearing, a transfer chain, a centre differential and
a viscous coupling. There are therefore three actions to
consider: gearing, differential and coupling.
Gearing
The planetary gearing provides a high range, a low
range and neutral. The ring gear with its internal teeth
is fixed to the casing. Input to the gearing is from the
transmission output shaft to the sun gear of the
planetary gearing. The gearing functions as follows.
Neutral
The sun gear drives the planet pinions, and these walk
around inside the ring gear. No drive is transmitted.
High range
The coupling sleeve is moved to the right so that the
sun gear is connected to the chain drive sprocket.


507-532_May 2chap 25

13/9/06

4:17 PM

Page 519

chapter twenty-five four-wheel drive and all-wheel drive

519

figure 25.15

Centre differential with a viscous coupling and a differential lock

figure 25.16

Full-time four-wheel-drive transaxle has a centre differential and viscous coupling – it also has a crown
wheel and pinion for the front-wheel drive SUBARU

FORD


507-532_May 2chap 25

13/9/06

4:17 PM

Page 520

520 part four automatic transmissions and drive
Low range
The coupling sleeve is moved to the left so that the
carrier is connected to the chain drive sprocket. There
is now a reduction in speed in the planetary gearing.
This occurs between the sun gear and the planet
carrier. This will reduce the speed of the chain
sprocket, the chain and the differential assembly.
Differential
The differential has the chain driven sprocket attached
to its case. Ignoring the viscous coupling, the differential side gears are connected to the front and rear
output shafts, and this provides normal differential
action.
Viscous coupling

figure 25.17

Rear differential with a viscous coupling
FORD

Drive is transmitted by the chain to the driven sprocket
on the differential assembly.
■ The sprockets are the same size, so there is no
reduction.

figure 25.18

Front-axle final drive with a viscous coupling

The viscous coupling is connected between the two
output shafts. One side gear has internal splines for the
rear output shaft, which also extends into the hub of
the coupling. The other side gear has external splines
which carry the viscous-coupling casing. The casing is
joined to the front output shaft.
With the vehicle being driven on a dry, sealed
road surface, there will be little movement between
the plates in the coupling. The differential will act
normally to compensate for slightly different speeds
between the front and rear input shafts.
When there is a difference in speeds between the
output shafts, the silicon coupling will automatically
hold to prevent wheel spin.

VOLKSWAGEN


507-532_May 2chap 25

13/9/06

4:17 PM

Page 521

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.19

521

Transfer case with planetary gearing, a transfer chain and a viscous coupling (low range is selected)
ROVER AUSTRALIA

Suspension arrangements
Rugged suspension and steering systems are required
for larger four-wheel-drive vehicles that are intended
for off-road use. Leaf springs, coil springs and torsion
bars are all used for larger vehicles. Passenger vehicles
that are normally used on surfaced roads usually have
strut-type suspensions.
Four-wheel-drive vehicles need special arrangements for the front axle so that the vehicle can be
steered while the front wheels are being driven. This is
accomplished by using drive shafts with constantvelocity joints. Front-wheel-drive vehicles with
transaxles already have these. Other vehicles that
normally are rear-wheel drive are fitted with a front
axle on which the wheel hubs can pivot.

The suspension assembly includes a sway bar,
shock absorbers, leaf springs and a torque rod. The
steering system has a hydraulic damper that reduces
the road shocks which would otherwise be transmitted
to the steering wheel when driving in rough conditions.
The axle has a normal final-drive unit, with a crown
wheel and pinion and a differential. The axle shafts,
which carry the drive to the wheels, have constantvelocity joints (CV joints).
Figure 25.21 shows a section through the end of a
rigid-axle and wheel assembly. The steering knuckle is
pivoted on tapered roller bearings on the ball end of
the axle housing. The drive shaft is fitted with a
Birfield constant-velocity joint.
■ Birfield-type joints are one of the types of CV joints
used with front-wheel-drive vehicles.

Rigid front axle with leaf springs
A rigid front-axle assembly with leaf spring suspension
is shown in Figure 25.20. The axle consists of a tubular
axle housing, similar to a rear-axle housing, but with a
large ball at each end. These carry the steering knuckles
and wheel spindles, which pivot on the balls for steering.

Components
The parts of a dismantled steering-knuckle and axle
shaft assembly are identified in Figure 25.22.
The main parts shown are the end of the
axle housing, the axle shaft, the steering knuckle, axle


507-532_May 2chap 25

13/9/06

4:17 PM

Page 522

522 part four automatic transmissions and drive

figure 25.20

Components of a leaf spring front suspension for a four-wheel-drive vehicle

spindle, hub and disc assembly and the disc brake
caliper.
Freewheel hubs
Special freewheel hubs can be fitted to the front
wheels (Figure 25.23). This enables the drive to be disconnected at the wheels when front-wheel drive is not
being used. The front part of the drive train is not
needed and so remains stationary. Without freewheel
hubs, parts of the drive train would rotate, but not carry
load, even though front-wheel drive is not engaged.
Most freewheel hubs are locked and unlocked
manually by turning the centre of the hub to the free
position as shown in Figure 25.24. Some hubs have an
automatic lock.
■ Both hubs must be in either the lock or free positions.
Front suspension with coil springs
The front suspension in Figure 25.25 has a rigid-axle
(or beam-axle) housing. In this design, the coil springs
are arranged so that there is a large vertical wheel
movement. Long telescopic shock absorbers are

TOYOTA

mounted inside the springs and in towers attached to
the frame.
Forged-steel radius arms pivot on rubber bushes
and prevent fore and aft movement of the axle.
A Panhard rod locates the axle laterally.
Rear suspension with coil springs
Figure 25.26 shows a rear-axle assembly. This has a
rigid-axle housing which is located against backwards
and forwards movement by two trailing links. Lateral
movement of the axle is prevented by a central
A-frame which is attached to the centre of the axle and
to the vehicle frame.
A ride-level unit is fitted to the suspension. It has a
self-energised oil-damped piston that operates in a
sealed cylinder which is gas-filled. When the vehicle is
loaded and stationary, it will be lower at the rear.
However, once the vehicle is in motion, irregularities
in the road surface will energise the levelling unit. This
will raise the rear of the vehicle and bring it back level.
The self-levelling device will support the load on
the rear axle, and the springs will act normally to
provide a smooth ride.


507-532_May 2chap 25

13/9/06

4:17 PM

Page 523

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.21

Steering-knuckle, hub and wheel assembly for a four-wheel-drive front axle

Four-wheel-drive service
Four-wheel-drive vehicles have the same service
requirements as two-wheel-drive vehicles, except that
extra cleaning and attention is needed for off-road
vehicle use. Some relevant points are as follows:
1. Protection is needed against corrosion where the
vehicle is used on beaches.

523

TOYOTA

4. Off-road vehicles and light commercial vehicles
usually have grease nipples, or plugs for grease
nipples, on steering and suspension ball joints.
These need regular lubrication.
5. Universal joints are usually fitted with grease
nipples so that they can be easily lubricated
(Figure 25.27).

2. Air cleaners will require more frequent service
where a vehicle is operated in dusty conditions.

Four-wheel-drive operation

3. The engine oil and the oil filter will need changing
more often for off-road operation.

Following are lists of general do’s and don’ts that
relate to operating a four-wheel-drive vehicle. Some of


507-532_May 2chap 25

13/9/06

4:18 PM

Page 524

524 part four automatic transmissions and drive

figure 25.22

Parts of a dismantled steering-knuckle assembly

figure 25.23

Parts of a freewheel hub for the front wheel of a four-wheel-drive vehicle

TOYOTA

TOYOTA


507-532_May 2chap 25

13/9/06

4:18 PM

Page 525

chapter twenty-five four-wheel drive and all-wheel drive

figure 25.24

525

Setting the position of the freewheel hub to
‘free’ or ‘lock’ TOYOTA

figure 25.27

Lubrication points on universal joints

TOYOTA

these apply generally to all four-wheel drives; others
will vary according to the particular vehicle –
especially full-time drives.
Do’s
These are as follows:
figure 25.25

Front suspension with a rigid axle and coil
springs ROVER AUSTRALIA

1. Do lock freewheel hubs, where fitted, before
engaging four-wheel drive.
2. Do turn freewheel hubs to ‘free’ when four-wheel
drive is not to be used.
3. Do select low range for extreme off-road conditions
or wherever low-speed manoeuvring is needed.
4. Do use engine braking for sharp descents. Select a
low gear so that the vehicle moves slowly.
5. Do reduce the tyre pressures if driving on marshy
ground or sand. This will improve traction by
increasing tyre flotation.
6. Do lock the centre differential when negotiating
rough tracks where wheel spin could occur.
7. Do dry out brakes after driving through water. This
can be done by driving a short distance with the
brakes applied.
Don’ts
These are as follows:

figure 25.26

Rear suspension system with coil springs
and a rigid axle ROVER AUSTRALIA

1. Don’t engage four-wheel drive with the rear wheels
spinning.


507-532_May 2chap 25

13/9/06

4:18 PM

Page 526

526 part four automatic transmissions and drive
weight shifted
to rear

2. Don’t engage the differential lock unless the wheels
are stopped.
3. Don’t lock the differential on hard-surfaced roads.
4. Don’t engage four-wheel drive on hard-surfaced
roads.
5. Don’t allow the engine to labour before changing to
a lower gear.

traction lost
(wheel could spin)
(a)

6. Don’t change to low ratio with the vehicle moving.
7. Don’t use the clutch as a footrest.

weight shifted
to front

All-wheel drive
All-wheel drive is a system that provides drive to all
four wheels of a passenger car. Because the drive is
permanently connected to both front and rear wheels,
vehicles of this type are often referred to as having
full-time four-wheel drive.
The main difference between all-wheel drive for
passenger cars and four-wheel drive in other vehicles
is the arrangement of the gearing. Vehicles that are
classed as four-wheel drives have some form of
transfer case or gearing. This provides low gear ratios
and also allows the four-wheel drive to be disconnected, so that the vehicle can be operated in either
two-wheel drive or four-wheel drive.
All-wheel drive vehicles have standard gear ratios
and do not have a transfer case. Many of these vehicles
have permanent all-wheel drive. The all-wheel drive
cannot be disconnected, but the ratio of drive can be
split between front and rear drive in various ways.
■ The way that the drive is split is a function of the
system and is not directly controlled by the driver.
Advantages of all-wheel drive
Traditionally, passenger cars have either front-wheel
drive or rear-wheel drive. Both drive systems have
advantages and disadvantages. For example, an
attempt to drive a front-wheel vehicle up a steep slope
could encounter difficulties, due to weight transfer
from the front to the rear wheels as shown in Figure
25.28(a). A rear-wheel drive vehicle attempting to
reverse up the slope, as shown in Figure 25.28(b),
could encounter a similar problem – again, due to the
weight transfer away from the driving wheels.
In both instances, if all four wheels were driving,
the vehicle would be able to negotiate the slope more
successfully.
All-wheel drive also improves traction and stability

traction lost
(wheel could spin)
(b)

figure 25.28

Vehicle being driven on a slope
(a) front-wheel drive vehicle being driven up
the slope (b) rear-wheel drive vehicle reversing up the slope

for normal driving. With two-wheel drive passenger
cars, it could be difficult to prevent wheel spin during
acceleration, particularly on wet and loose surfaces.
This creates a dangerous situation for drivers with
inexperience or low driving skills. With all the wheels
driving wheel spin is avoided, except in unusual
circumstances.
Another way to prevent wheel-spin is by the use of
traction control (TC). Traction control systems use the
antilock braking system (ABS) of the vehicle to apply
the brake to prevent a wheel from spinning. Traction
control, with the aid of the engine management system,
also reduces the engine torque.
With both systems operating, all-wheel drive gives
improved traction during normal driving, and traction
control generally assists to prevent wheel spin under
particular operating conditions.
■ All-wheel drive and traction control are both used
on some vehicles.

Types of all-wheel-drive systems
A variety of all-wheel-drive systems are fitted to
passenger vehicles. The components of these are
similar to those already covered for four-wheel drive,
but there are some different mechanical arrangements.


507-532_May 2chap 25

13/9/06

4:18 PM

Page 527

chapter twenty-five four-wheel drive and all-wheel drive

The other (and main) difference is the method of transferring the drive between the front and rear wheels and
controlling how it is split.
For convenience, the systems for all-wheel drive
can be referred to by the type of transfer clutch or
coupling and the controls that are used. Some of these
are:
1. multiplate clutch, electronically controlled
2. multiplate clutch, electro-hydraulic
3. viscous coupling drive
4. viscous coupling and centre differential.
Multiplate clutch, electronically controlled
Figure 25.29 shows a sectional view of an automatic
transaxle with a multiplate transfer clutch. The clutch
is applied by fluid pressure from the automatic
transmission’s hydraulic system and it is controlled by
a solenoid operated by an electronic control unit
(ECU).
For the rear wheels, drive from the torque converter
is transmitted through the transmission’s epicyclic
gearing to the rear output shaft via the transfer clutch.
The drive is then carried from the output shaft by the

propeller shaft to the differential assembly at the rear
axle.
For the front wheels, drive is transmitted inside the
transmission by the transfer gears and transfer shaft to
the differential assembly at the front of the transaxle.
From there it is carried by the front drive shafts to the
front wheels.
The front wheels are in constant drive, but the rear
wheels are not. The ECU varies the pressure to the
transfer clutch in line with expected drive demands.
This applies and releases the clutch in a controlled
manner to provide more drive or less drive, as
required, to the rear axle assembly.
■ This system is sometimes referred to as an active
torque split system.
Drive split
The system uses the ECU to distribute the torque in a
ratio of approximately 60% front wheels and 40%
rear wheels. This ratio applies when the vehicle is
evenly loaded. The control system is capable of distributing drive from the 60–40% ratio of front to rear,
to 95% front and 5% rear as required by operating
conditions.
torque converter
input shaft

transfer clutch

rear output
shaft

transfer
gears

transfer
shaft

selector
lever
valve body
assembly

crown wheel

front
drive shaft
pinion
differential
assembly

figure 25.29

527

Automatic transaxle with both front-wheel and rear-wheel drive

SUBARU


507-532_May 2chap 25

13/9/06

4:18 PM

Page 528

528 part four automatic transmissions and drive
At light throttle, driving on the highway in good
conditions and at constant speed, the vehicle will have
95% front-wheel drive and 5% rear-wheel drive. This
helps to produce good fuel economy.
However, when full power is applied the ECU
reacts instantly without driver involvement and applies
increased drive through the transfer clutch to the rear
axle. This improves the traction that is distributed to all
the wheels.

clutch plates
transfer clutch

transfer duty
solenoid

transfer control
valve

Clutch control
The system operates by the ECU receiving input
signals from many sources. These input signals are
processed by the ECU and compared with preprogrammed tables. From these tables the ECU selects
and sends an output signal to the pressure control
telling it to apply more or less pressure to the transfer
clutch piston. In this way the driving ratio between the
front and rear axle assemblies is monitored and
controlled to suit the driving conditions.
A diagram of a basic control system is shown in
Figure 25.30. The parts of this are the transfer clutch, a
transfer control valve and a transfer duty-solenoid
valve.
The ECU operates the solenoid which, in turn,
operates the control valve. The pressure valve directs
fluid pressure to apply the transfer clutch, or it relieves
pressure to release the transfer clutch.
The duty cycle of the transfer solenoid valve (its
rate of pulsing) is dictated by the ECU.

figure 25.31

fluid flow
out

figure 25.30
system

line pressure in

Diagram of an electronically-controlled
transfer clutch as used in an all-wheel drive

SUBARU

Multiplate clutch, electro-hydraulic
The basic drive setup for this system is similar to that
of the transfer clutch with electronic control. That is, it
is a front-wheel-drive system with rear-wheel drive
provided via the transfer clutch. This system uses a
clutch that has its own pump to generate clutch
clamping pressure (Figure 25.31).
■ This system is often used with a manual transmission.

Schematic diagram of an electro-hydraulic transfer clutch

AUDI


507-532_May 2chap 25

13/9/06

4:18 PM

Page 529

chapter twenty-five four-wheel drive and all-wheel drive

■ The drive through the multiplate clutch will be
varied according to the actuating pressure on the
clutch piston.

The main feature of this system is the multiplate
clutch which transfers drive from the transmission to
the rear axle. The plates are clamped by pressure
generated by the annular piston pump.
The annular piston pump is attached to the output
shaft and the axial cam-disc is attached to the
input shaft of the clutch assembly. If the speed of
the input shaft and output shaft are the same, the piston
and cam-disc will both rotate together. Thus no
clamping pressure will be produced and effectively
no drive will be transferred to the rear axle.
When traction is lost and spin occurs with the front
wheels, there will be a difference in rotational speed
between the input and the output shaft of the clutch
assembly. When this occurs the cam-disc drives the
pump thus generating pressure and clamping the clutch
plates together.
The oil pressure, and as a result the clamping force,
is controlled electronically via the control valve. The
vehicle’s electronics receive and process signals from
many sources. The ECU then outputs a signal to the
control valve which adjusts the pressure to the clutch
actuating piston. This clamps the plates and completes
the drive to the rear axle.

This system usually includes limited slip functions
in both front and rear differentials and can also be
supplemented by individual wheel braking to temporarily retard a slipping wheel.
Viscous coupling drive
This system has the same basic arrangements as the
two previous systems. That is, direct front-wheel drive
with controlled rear-wheel drive. The difference with
this system is that it uses a viscous coupling to
complete the drive from the transmission to the rear
differential. It also has a one-way clutch and a centrifugal clutch in the drive line.
Figure 25.32 shows a diagram of the viscous
coupling and its location in the drive line. Under ideal
conditions this system is predominantly front-wheel
drive with very little drive to the rear wheels. However, if the front wheels lose drive traction, the input
shaft of the viscous coupling will rotate faster than the

transaxle

viscous coupling
assembly

front drive
shafts

rear drive
shafts

(a)

viscous coupling

centrifugal
lock-up clutch

drive input
drive output

(b)

figure 25.32

529

All-wheel drive with a silicon transfer coupling in the drive line to the rear axle
(a) arrangement of drive (b) coupling assembly VOLVO


507-532_May 2chap 25

13/9/06

4:18 PM

Page 530

530 part four automatic transmissions and drive
output shaft. This will cause the viscous coupling to
slip and generate heat within the silicon oil.
The heat causes the silicon oil to become more
viscous (thicker) and the coupling will commence to
drive the rear wheels.
This system is simple and is neither electronically
nor hydraulically controlled. Control occurs automatically by the increasing viscosity of the silicon oil.
Because it relies on the generation of heat before it can
respond, there is usually a small time delay before
drive is connected to the rear wheels.
■ This system has a limited slip function in both front
and rear differentials.
One-way and centrifugal lock-up clutches
As well as a viscous coupling, the drive line has a
combined one-way clutch and centrifugal lock-up
clutch as shown in Figure 25.33. When the vehicle is
being driven forwards, the one-way clutch automatically locks to carry the drive through to the rear
axle.
In reverse, the one-way clutch freewheels. This also
prevents the rear axle from driving the front axle
during severe braking.
Figure 25.34 shows the one-way clutch locked
while driving in a forward direction and freewheeling
when in reverse. It consists of an outer member and an
inner member with sprags in between. The clutch locks
automatically whenever the outer member rotates
faster than the inner member.
Centrifugal lock-up clutch
The centrifugal lock-up mechanism consists of two
sections, the lock-up and the centrifugal release

(Figure 25.33). The lock up has an inner hub with
grooves that carry a number of steel balls, and an outer
ring with cutouts.
At lower forward speeds and in reverse, the springloaded cage in the centre of the assembly holds the
balls into the cutouts of the outer ring. This locks the
clutch and it transmits drive. With the centrifugal
clutch locked up, the one-way clutch is isolated and the
drive for low forward speeds and reverse is transmitted
through the centrifugal clutch.
At speeds above 50 km/h the centrifugal release
comes into operation. The centrifugal balls are moved
outwards against their retainers by centrifugal force.
This overcomes the spring that holds the spring-loaded
cage. The lock-up balls are released and the clutch is
unlocked. This allows the one-way clutch to come into
operation.
Figure 25.34 shows the actions of both the centrifugal clutch and the centrifugal lock-up clutch in both
forward and reverse.
Viscous coupling and centre differential
Figure 25.35 shows a typical arrangement for an allwheel-drive system with a manual transmission. This
has a centre differential that is controlled by a viscous
coupling. Its function is to split the drive between the
front and rear wheels. In effect, the left side gear of the
differential drives the front wheels and the right side
drives the rear wheels.
During highway driving in good conditions, the
silicon coupling is not effective and the split of driving
forces is approximately 50% to the front axle and 50%
to the rear axle.
The centre differential, with the help of the viscous
coupling, will distribute the drive in accordance with

one-way clutch
lock up ball
viscous coupling

centrifugal ball

ball in cutout

figure 25.33

spring loaded
cage

Drive line with a viscous coupling, a one-way clutch and a centrifugal lock-up clutch (shown in section)

VOLVO


507-532_May 2chap 25

13/9/06

4:18 PM

Page 531

chapter twenty-five four-wheel drive and all-wheel drive
one-way clutch
(locked)

one-way clutch
(unlocked)

centrifugal
clutch (unlocked)

centrifugal
clutch (locked)

(a) Driving

figure 25.34

531

(a) Reversing

Operation of a one-way clutch and a centrifugal lock-up clutch
(a) one-way clutch locked, centrifugal clutch unlocked (b) one-way clutch freewheeling, centrifugal clutch
locked VOLVO

engine

clutch

manual
transaxle

transfer

viscous
coupling

centre
differential

drive shaft
front
differential

rear differential

figure 25.35

Drive train arrangement for an all-wheel drive vehicle, with a centre differential controlled by a viscous
coupling


Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay

×