172 Fourwheel drive vehicles with overdrive

In four-wheel drive vehicles with overdrive the middle differential is not used. The engine torque is distributed to all four wheels by means of a clutch on the propshaft, as required. The clutch can be engaged manually, or automatically in response to slip. With the use of sprag clutches, which are usually engaged manually, the torque is transmitted in a fixed ratio between the front and rear axles; multi-disc or visco clutches permit variable torque distribution. As these systems have essential similarities with permanent four-wheel drive varieties, they are discussed in Section 1.7.4.

With sprag-clutch engaged transmissions, the design complexity, and therefore the costs, are lower than on permanent drive. Usually there is no rear axle differential lock, which is important on extremely slippery roads; while this results in price and weight advantages, it does lead to disadvantages in the traction.

Front-wheel drive is suitable as a basic version and the longitudinal engine has advantages here (Fig. 1.48). With the transverse engine, the force from the manual gearbox is transmitted via a bevel gear and a divided propshaft, to the rear axle with a differential (Fig. 1.68). There is relatively little additional

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Fig. 1.68 The Fiat Panda Treking 4 x 4, a passenger car based on front-wheel drive with transverse engine. The vehicle has McPherson struts at the front and a rigid axle with longitudinal leaf springs at the back. The propshaft leading to it is divided into three to be able to take the rotational movements of the rigid axle around the transverse (y) axis during drive-off and braking and to absorb movements of the drive unit. The Fiat Panda is an estate car with the ratio

Fig. 1.68 The Fiat Panda Treking 4 x 4, a passenger car based on front-wheel drive with transverse engine. The vehicle has McPherson struts at the front and a rigid axle with longitudinal leaf springs at the back. The propshaft leading to it is divided into three to be able to take the rotational movements of the rigid axle around the transverse (y) axis during drive-off and braking and to absorb movements of the drive unit. The Fiat Panda is an estate car with the ratio

3689 mm complexity compared with the front-wheel drive design, even if, on the Fiat Panda (Trecking 4 X 4), there is a weight increase of about 11% (90 kg), not least because of the heavy, driven, rigid axle. It is possible to select rear-wheel drive during a journey using a shift lever that is attached to the prop-shaft tunnel.

Manual selection on the Subaru Justy operates pneumatically at the touch of a button (even while travelling). This vehicle has independent rear-wheel suspension and weighs only 6% more than the basic vehicle with front-wheel drive. Traction is always improved considerably if the driver recognizes the need in time and switches the engine force onto all four wheels. In critical situations, this usually happens too late, and the abrupt change in drive behaviour becomes an additional disadvantage.

Conversely, if the driver forgets to switch to single axle drive on a dry road, tensions occur in the power train during cornering, as the front wheels travel larger arcs than the back ones (Figs 1.69 and 3.91). The tighter the bend, the greater the stress on the power train and the greater the tendency to unwanted tyre slip.

A further problem is the braking stability of these vehicles. If the front axle locks on a wet or wintry road during braking, the rear one is taken with it due to the rigid power train. All four wheels lock simultaneously and the car goes into an uncontrollable skid.

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Fig. 1.69 The front wheel on the outside of the bend draws the largest arc during slow cornering, the track circle diameter Ds, while the inner wheel draws the considerably smaller arc Df,i. This is the reason for the differential in the driven front axle of the front-wheel drive. The bend diameters Ds,r and Drj to the rear are even smaller, so the rolling distance of the two wheels of this axle decreases further and there can be tensions in the drive train if both axles are rigidly connected, a bend is being negotiated and when a dry road surface makes wheel slip more difficult because of high coefficients of friction.

Fig. 1.69 The front wheel on the outside of the bend draws the largest arc during slow cornering, the track circle diameter Ds, while the inner wheel draws the considerably smaller arc Df,i. This is the reason for the differential in the driven front axle of the front-wheel drive. The bend diameters Ds,r and Drj to the rear are even smaller, so the rolling distance of the two wheels of this axle decreases further and there can be tensions in the drive train if both axles are rigidly connected, a bend is being negotiated and when a dry road surface makes wheel slip more difficult because of high coefficients of friction.

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Fig. 1.70 Complex power distribution on the Fiat Campagnolo, a four-wheel drive, all-purpose passenger car. The drive moment is transferred from the manual gearbox via a centrally located two-gear power take-off gear to the differentials of the front and rear axles. Efficiency is not likely to be especially good.

Fig. 1.70 Complex power distribution on the Fiat Campagnolo, a four-wheel drive, all-purpose passenger car. The drive moment is transferred from the manual gearbox via a centrally located two-gear power take-off gear to the differentials of the front and rear axles. Efficiency is not likely to be especially good.

1.7.3 Manual selection four-wheel drive on commercial and all-terrain vehicles

The basis for this type of vehicle is the standard design which, because of the larger ground clearance necessary in off-road vehicles (Fig. 1.67), has more space available between the engine and front axle differential and between the cargo area and the rear axle. Figure 1.70 shows the design details:

• a central power take-off gear with manual selection for the front axle, plus a larger ratio off-road gear, which can be engaged if desired;

• three propshafts;

• complex accommodation of the drive joints if there is a rigid front axle (Fig. 1.3).

1.7.4 Permanent four-wheel drive; basic passenger car with front-wheel drive

All four wheels are constantly driven; this can be achieved between the front and rear axle with different design principles:

• a bevel centre differential with or without manual lock selection;

• a Torsen centre differential with moment distribution, based on the traction requirement (Fig. 1.71);

• a planet gear central differential with fixed moment distribution and additional visco clutch, which automatically takes over the locking function when a difference in the number of revolutions occurs or a magnetic clutch (which is electronically controlled, Fig. 1.79);

• electronically controlled multi-disc clutches (Haldex clutch, Fig. 1.73);

Fig. 1.71 Torsen central differential fitted in Quattro models (apart from the TT) by Audi. It consists of two worm gears, which are joined by spur gears and, depending on the traction requirement, can distribute the driving torque up to 75% to the front or rear axle. Under normal driving conditions 50% goes to each axle.

Fig. 1.71 Torsen central differential fitted in Quattro models (apart from the TT) by Audi. It consists of two worm gears, which are joined by spur gears and, depending on the traction requirement, can distribute the driving torque up to 75% to the front or rear axle. Under normal driving conditions 50% goes to each axle.

Worm Drive Axle

Fig. 1.72 Four-wheel drive Golf 4motion (1998). In the four-wheel drive vehicle, Volkswagen uses a multi-link suspension consisting of one longitudinal and two transverse links mounted on a subframe. The driving torque is transmitted to the rear axle via a wet multi-disc clutch by the Swedish company Haldex which is flange-mounted on the rear axle drive and runs in oil. This electronically controlled clutch can build up a coupling torque of up to 3200 Nm even at small cardan-shaft rotation angles of 45° and can be combined to good effect with brake-power control systems. The drive train of the Audi TT Quattro (1998) built on the same platform is built to almost the same design.

Fig. 1.72 Four-wheel drive Golf 4motion (1998). In the four-wheel drive vehicle, Volkswagen uses a multi-link suspension consisting of one longitudinal and two transverse links mounted on a subframe. The driving torque is transmitted to the rear axle via a wet multi-disc clutch by the Swedish company Haldex which is flange-mounted on the rear axle drive and runs in oil. This electronically controlled clutch can build up a coupling torque of up to 3200 Nm even at small cardan-shaft rotation angles of 45° and can be combined to good effect with brake-power control systems. The drive train of the Audi TT Quattro (1998) built on the same platform is built to almost the same design.

• a visco clutch in the propshaft power train, which selects the initially undriven axle depending on the tyre slip (Figs 1.72, 1.74 and 1.75).

Here too, the front-wheel drive passenger car is suitable as a basic vehicle. In 1979, Audi was the first company to bring out a car with permanent four-wheel drive, the Quattro, and today vehicles with this type of drive are available throughout the entire Audi range. On a longitudinally mounted engine, a Torsen centre differential distributes the moment according to the traction requirement (Fig. 1.71). The four-wheel drive increases the weight by around 100 kg.

Four Wheel Clutch

Fig. 1.73 Multi-disc clutch of the Swedish company Haldex, used in the Golf 4motion (1998) and Audi TT Quattro (1998). When there is a difference in speed between the front and rear axles, the disc cam 6 on the output shaft activates the working elements of the axial-piston pump 12 by means of the rollers 7. Via the control valve 14, the pressure produced activates the working piston which moves the discs. The torque transmitted is adjusted continuously by the control unit up to the maximum value, depending on the driving situation described by the wheel sensors, the signals from the slip and brake-power control systems, the position of the accelerator pedal, the engine speed etc. The clutch is disengaged when the ABS function is used. 1 electronic control unit, 2 connector vehicle (voltage, CAN, K leads), 3 oil filter, 4 shaft bevel wheel exit (rear axle gearing), 5 lamella, 6 cam plate, 7 coil, 8 relief valve, 9 pressure regulating valve, 10 accumulator, 11 input shaft, 12 axial piston pump, 13 pre-load pump, 14 control valve, 15 intermittent or step motor.

Fig. 1.73 Multi-disc clutch of the Swedish company Haldex, used in the Golf 4motion (1998) and Audi TT Quattro (1998). When there is a difference in speed between the front and rear axles, the disc cam 6 on the output shaft activates the working elements of the axial-piston pump 12 by means of the rollers 7. Via the control valve 14, the pressure produced activates the working piston which moves the discs. The torque transmitted is adjusted continuously by the control unit up to the maximum value, depending on the driving situation described by the wheel sensors, the signals from the slip and brake-power control systems, the position of the accelerator pedal, the engine speed etc. The clutch is disengaged when the ABS function is used. 1 electronic control unit, 2 connector vehicle (voltage, CAN, K leads), 3 oil filter, 4 shaft bevel wheel exit (rear axle gearing), 5 lamella, 6 cam plate, 7 coil, 8 relief valve, 9 pressure regulating valve, 10 accumulator, 11 input shaft, 12 axial piston pump, 13 pre-load pump, 14 control valve, 15 intermittent or step motor.

Fig. 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.

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Fig. 1.75 Driven front axle of the Porsche 911 Carrera 4 (1996, 1998). The visco clutch is flange mounted directly on the front axle to achieve a better distribution of axle load. With corresponding slip of the rear wheels, up to 40% of the driving torque is transmitted to the front axles. Particular attention was paid during the adjustment of the four-wheel drive to predictable self-steering properties independent of drive distribution and to controllability of the handling characteristics even at the stability limit. Instead of differential locks, specific wheel brake engagements are made in order to retard spinning wheels. Four-wheel drive is integrated into the Porsche Stability Management (PSM), a system for controlling the dynamics of vehicle movement with brake actuation.

Fig. 1.75 Driven front axle of the Porsche 911 Carrera 4 (1996, 1998). The visco clutch is flange mounted directly on the front axle to achieve a better distribution of axle load. With corresponding slip of the rear wheels, up to 40% of the driving torque is transmitted to the front axles. Particular attention was paid during the adjustment of the four-wheel drive to predictable self-steering properties independent of drive distribution and to controllability of the handling characteristics even at the stability limit. Instead of differential locks, specific wheel brake engagements are made in order to retard spinning wheels. Four-wheel drive is integrated into the Porsche Stability Management (PSM), a system for controlling the dynamics of vehicle movement with brake actuation.

Fig. 1.76 Double wishbone rear axle on the Audi A4 Quattro. The suspension subframe 1 is fixed to the body with four widely spaced rubber mountings (items 2 and 3) and houses the differential casing 8 and transverse control arms (items 4 and 5). The springs and shock absorbers are mounted next to the fixings for the upper control arms 7. The location 6 of the wheel hub carrier 5 was raised (long base c, Fig. 1.4) and drawn outwards. The lower transverse control arm 4 is fixed to part 1 with widely spaced mountings. These measures ensure a wide boot and low forces, making it easier to attain the desired kinematic characteristics.

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VW used a visco clutch in the power train (without centre differential) for the first time on the Transporter (Fig. 1.74) and then subsequently used it in the Golf syncro. The clutch has the advantage of the engine moment distribution being dependent on the tyre slip. If the slip on the front wheels, which are otherwise driven at the higher moment, increases on a wet or frozen surface or off-road, more drive is applied to the rear wheels. No action on the part of the driver is either necessary or possible. The transverse engine makes a bevel gear in front of the split propshaft necessary. The visco clutch sits in the rear differential casing and there is also an overrunning clutch, which ensures that the rear wheels are automatically disengaged from the drive, on overrun, to guarantee proper braking behaviour. This type of drive is fully ABS compatible. When reverse is engaged, a sliding sleeve is moved, which bridges the overrunning clutch to make it possible to drive backwards.

When selecting their rear axle design, manufacturers choose different paths. Audi fits a double wishbone suspension in the A4 and A6 Quattro (Fig. 1.76), Honda uses the requisite centre differential on the double wishbone standard suspension in the Civic Shuttle 4WD (Figs 1.77 and 1.62).

Fig. 1-77 Double wishbone rear axle of the Honda Civic Shuttle 4 WD. The visco clutch sits (held by two shaft bearings) in the centre of the divided propshaft. The rear axle differential has been moved forwards and is mounted to the rear on the body via a cross-member. Apart from the different type of wheel bearings and the lower transverse control arm positioned somewhat further back (to make it possible to bring the drive shafts through in front of the spring dampers), the axle corresponds to Fig. 1.62 and resembles the suspension shown in Fig. 1.1.

Visco clutch

Visco clutch

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Fig. 1-77 Double wishbone rear axle of the Honda Civic Shuttle 4 WD. The visco clutch sits (held by two shaft bearings) in the centre of the divided propshaft. The rear axle differential has been moved forwards and is mounted to the rear on the body via a cross-member. Apart from the different type of wheel bearings and the lower transverse control arm positioned somewhat further back (to make it possible to bring the drive shafts through in front of the spring dampers), the axle corresponds to Fig. 1.62 and resembles the suspension shown in Fig. 1.1.

Four Wheel Axle Picture

Fig. 1.78 Drive train of the four-wheel drive Mercedes-Benz E class 4MATIC (from 1997). In order to be able to control the drive shafts to the front wheels, an integrated spring-and-shock absorber strut in the shape of a fork on the lower transverse link is used. In the almost identical suspension design of other than off-road varieties, the springs and shock absorbers are separate.

Fig. 1.78 Drive train of the four-wheel drive Mercedes-Benz E class 4MATIC (from 1997). In order to be able to control the drive shafts to the front wheels, an integrated spring-and-shock absorber strut in the shape of a fork on the lower transverse link is used. In the almost identical suspension design of other than off-road varieties, the springs and shock absorbers are separate.

1.7.5 Permanent four-wheel drive, basic standard design passenger car

Giving a standard design car four-wheel drive requires larger modifications, greater design complexity and makes the drive less efficient (Fig. 1.78). A power take-off gear is required, from which a short propshaft transmits the engine moment to the front differential. The lateral offset must be bridged, for example, with a toothed chain (Fig. 1.79). The ground clearance must not be affected and so changes in the engine oil pan are indispensable if the axle drive is to be accommodated (Fig. 1.80).

The power take-off gear (Fig. 1.79) contains a planet gear centre differential which facilitates a variable force distribution (based on the internal ratio); 36% of the drive moment normally goes to the front and 64% to the rear axle. A multi-disc clutch can also be installed that can lock the differential electromagnetically up to 100%, depending on the torque requirement (front to rear axle). Moreover, there is a further electrohydraulically controlled lock differential in the rear axle which is also up to 100% effective.

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Fig. 1.79 The torque coming from the engine is apportioned by the Planet Wheel-Centric Differential 1 in such one, to the rear cardan shaft 2 (64%) and to the front one 3 (36%). The offset to this shaft is bridged-over by the inserted tooth type chain 4. The adaptation of the distribution of driving power is taken over the the multiple-disk clutch 5, which is driven (controlled) by the electromagnet.

Power Divider A110 of the Fa. ZF. (Zahnradfabrik Friedrichshafen)

Fig. 1.79 The torque coming from the engine is apportioned by the Planet Wheel-Centric Differential 1 in such one, to the rear cardan shaft 2 (64%) and to the front one 3 (36%). The offset to this shaft is bridged-over by the inserted tooth type chain 4. The adaptation of the distribution of driving power is taken over the the multiple-disk clutch 5, which is driven (controlled) by the electromagnet.

Power Divider A110 of the Fa. ZF. (Zahnradfabrik Friedrichshafen)

Power Take Off Cross Section

Fig. 1.80 Front cross-section view of the engine; and drive axle of a standard four-wheel drive vehicle (BMW assembly diagram). The basic vehicle has rear-wheel drive and, in order to also be able to drive the front wheels, the front axle power take-off 4 had to be moved into the space of the oil pan. The intermediate shaft 1 bridges the distance to the right inner CV joint and thus ensures drive shafts of equal length to both wheels (items 2 and 3 and Fig. 1.51). Part 1 is mounted on one side in the non-lockable differential 4 and on the other side in the outrigger 5. This, and the casing 6, are screwed to the oil pan.

Fig. 1.80 Front cross-section view of the engine; and drive axle of a standard four-wheel drive vehicle (BMW assembly diagram). The basic vehicle has rear-wheel drive and, in order to also be able to drive the front wheels, the front axle power take-off 4 had to be moved into the space of the oil pan. The intermediate shaft 1 bridges the distance to the right inner CV joint and thus ensures drive shafts of equal length to both wheels (items 2 and 3 and Fig. 1.51). Part 1 is mounted on one side in the non-lockable differential 4 and on the other side in the outrigger 5. This, and the casing 6, are screwed to the oil pan.

The two differentials with variable degrees of lock offer decisive advantages:

• to reach optimal driving stability, they distribute the engine moments during overrun and traction according to the wheel slip on the drive axles;

• they allow maximum traction without loss of driving stability (Fig. 1.66).

The locks are open during normal driving. By including the front axle differential, they make it possible to equalize the number of revolutions between all wheels, so tight bends can be negotiated without stress in the power train and parking presents no problems. If the car is moved with locked differentials and the driver is forced to apply the brakes, the locks are released in a fraction of a second. The system is therefore fully ABS compatible.

In its four-wheel drive vehicles of the E class (Fig. 1.78), Mercedes-Benz uses

Fig. 1.81 Front suspension and drive axle of the Mercedes-Benz off-road vehicle of the M series. In off-road vehicles, rigid axles are mostly used. Instead of these, Mercedes-Benz installs double wishbone suspensions at the front and rear. In this way, the proportion of unsprung masses can be reduced by approximately 66%; driving safety and riding comfort are increased. For space reasons, torsion-bar springs are used for the suspension of the front axle.

1 lower transverse link in the form of a forged steel component because of the introduction of torque by the torsion bars (2) and notch insensitivity off road conditions; 2 torsion bars (spring rate of 50 Nm/degree); 3 vertically adjustable torque support which can be placed in any position in a transverse direction; 4 integral bearers (subframe) attached to the box-type frame by 4 bolts; 5 upper transverse link in the form of a forged aluminium component; 6 rack and pinion power steering, 7 twin-tube shock absorber with integrated rubber bump stop, 8 transverse link mounting points; 9 stabilizer application of force to lower transverse link.

Fig. 1.81 Front suspension and drive axle of the Mercedes-Benz off-road vehicle of the M series. In off-road vehicles, rigid axles are mostly used. Instead of these, Mercedes-Benz installs double wishbone suspensions at the front and rear. In this way, the proportion of unsprung masses can be reduced by approximately 66%; driving safety and riding comfort are increased. For space reasons, torsion-bar springs are used for the suspension of the front axle.

1 lower transverse link in the form of a forged steel component because of the introduction of torque by the torsion bars (2) and notch insensitivity off road conditions; 2 torsion bars (spring rate of 50 Nm/degree); 3 vertically adjustable torque support which can be placed in any position in a transverse direction; 4 integral bearers (subframe) attached to the box-type frame by 4 bolts; 5 upper transverse link in the form of a forged aluminium component; 6 rack and pinion power steering, 7 twin-tube shock absorber with integrated rubber bump stop, 8 transverse link mounting points; 9 stabilizer application of force to lower transverse link.

a transfer gear with central differential situated on the gearbox outlet and a front axle gear integrated into the engine-oil pan. The (fixed) driving torque distribution is 35%:65%. Instead of traditional differential locks, the wheel brakes are activated on the spinning wheels as in off-road vehicles of the M class. This system permits maximum flexibility, its effect not only corresponds to differential locks on front and rear axles as well as on the central differential, but also makes it possible for other functions such as ABS and electronic yaw control (ESP) to be integrated without any problem. Design complexity - and thus cost - is considerable.

1.7.6 Summary of different kinds of four-wheel drive

The list in Fig. 1.83 shows the increasing use of slip-controlled clutches (visco and Haldex clutches) for the transmission of torque instead of an interaxle

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Fig. 1.82 Rear axle of the Mercedes-Benz off-road vehicle of the M series. Suspension and damping are ensured by the spring strut (1) whose spring is tapered for reasons of construction space (spring rate gradually increasing from 70 to 140 N/mm), 2 brake disc with integrated drum parking brake, 3 upper transverse link (forged aluminium component), 4 lower transverse link (forged aluminium component), 5 tie rod (forged steel component), 6 integral bearer (subframe), 7 stabilizer, 8 transverse link mounting points.

Common characteristics of front and rear axles: camber and castor are adjusted by positioning the transverse link mounting points (8) in long holes during assembly. Technical data: spring travel ±100 mm, kingpin offset -5 mm, disturbing force moment arm 56.7 mm, kingpin inclination 10.5°, camber angle -0.5°, castor for front axle/rear axle 7/-8.5°, castor trail for front axle/rear axle 37/-55 mm, wheel castor trail for front axle/rear axle 5/-4.5 mm, instantaneous centre height for front axle/rear axle 80/119 mm, braking-torque compensation for front axle/rear axle 38/21%, starting-torque compensation for front axle/rear axle -7/-3%. The axle concept was designed and developed by Mercedes-Benz. Mass production and assembly is undertaken by Zahnradfabrik Friedrichshafen AG who, via Lemforder Fahrwerktechnik AG, supply the complete subassemblies to the assembly line as required.

differential and the importance of electronic brake application systems which are used instead of lockable differential gears. Modern four-wheel varieties operate without functional restrictions with antilocking, slip and driving stabilization systems.

Fig. 1.83 Different kinds of four-wheel drive.

Motor

Reduction

Drive

Four-wheel drive

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