Four Wheel Drive

Four Wheel Drive (Automobile)

The two main traction problems associated with a two-wheel drive (4 x 2) vehicle are loss of traction during cross-country operation and loss of adhesion during acceleration. To provide a solution to these problems, Harry Ferguson, the inventor of the light weight tractor, was the first person to understand the importance of "all-wheel drive". In 1954 he patented the "Ferguson Formula" (FF), which was used on the Jensen car in the early 1960s. In a four-wheel drive (4 x 4) vehicle (four by four vehicles), the drive is transmitted to all the four wheels. The intended use of the vehicle governs the type of 4 x 4 drive system that is offered by the manufactures.

Some vehicles are designed to work efficiently both on and off the highway. These vehicles use two-wheel drive on 'hard' surfaces and restrict the use of four-wheel drive for cross-country operation where it is likely to encounter muddy surfaces leading to loss of traction from the two driving wheels. High-performance cars built for highway running use four-wheel drive at all times, which gives improved handling and extra safety because the total weight of the car is distributed over all of the driving wheels. The utilization of the total vehicle weight also considerably increases the tractive effort so that a high rate of acceleration is obtained.

Cross-country Operation.

When a conventional two-wheel drive vehicle negotiates rough terrain and muddy road normally found in cross-country travel, the rear wheels lose their adhesion. As a result the traction available is not sufficient for a drive. With a normal differential also if the adhesion of one driving wheel is lost, the drive provided to the other wheel is often too low to propel the vehicle, although a differential locking device reduces the change of this occurrence. A four-wheel drive overcomes this situation. When either front or rear axles lose traction the other axle maintains a drive for the movement of the vehicle.

A typical layout of a four-wheel drive vehicle is illustrated in Fig. 26.60. A transfer gearbox is installed behind the main gearbox to divide the drive between the axles. The transfer box generally uses extra gears to provide a very low ratio and is controlled by two gear levers. One of these levers selects four-wheel drive and the other provides 'high' or 'low' gear. Drive to the front wheels is transmitted through the final drive, differential and drive shafts. Extra universal joints at the wheel end of the drive shafts are incorporated for steering movement of the front wheels. The constant-velocity type of joint is generally fitted to minimize vibration.

The four-wheel drive system presented in Fig. 26.60 is not suitable for a hard-road due to the risk of transmission 'wind-up'. With this arrangement, if a vehicle takes a turn, the mean speed of the front wheels becomes higher than that of the rear wheels, so that the speed difference causes the propeller shafts to deflect. To overcome the wind-up problem and make the four-wheel operation suitable on hard surfaces, a third differential 'between' the driving axles is necessary. This differential is not necessary for cross-country operation, as this can not fulfill the main purpose of four-wheel drive because if one wheel loses adhesion the vehicle is immobilized.

Hard Surface Operation

Tractive and braking efforts are governed by the adhesive force at the road wheels and this force in turn equals to the product of co-efficient of friction and load on wheel. Therefore, the

Fig. 26.60. Four-wheel drive layout for off-highway use.

total weight of the vehicle must be utilized to the maximum possible extent to obtain the maximum adhesive force. In case of only two driving wheels, the weight on the non-driving wheels proportionally reduces the adhesive force, thereby limiting the maximum tractive effort.

Once the tractive effort equals the adhesive force, the driving wheel starts spinning which limits the rate of acceleration, and also can lead to loss of control of the vehicle. The similar situation arises with respect to braking, i.e. skidding occurs beyond the point where the braking force equals the adhesive force. Distribution of the driving and braking forces over all of the wheels, backed up with a device for sensing the approach of wheel spin or slip and then adjusting the effort accordingly, causes maximum utilization of tractive and braking efforts providing greater safety. Cars of this type are not designed for serious off-road use, but for all-weather driving with an added security for tackling icy or wet conditions.

Ferguson Formula

A typical transmission layout similar to the one applied to a Jensen car is shown in Fig. 26.61. A transfer box, containing the sensing mechanism, is placed adjacent to a centre (third) differential, and two propeller shafts transmit the drive to front and rear axles. A Maxaret unit, driven by the transmission, is installed to prevent the possibility of locking of all wheels during

Fig. 26.61. Transmission layout.

braking. It senses the mean deceleration of front and rear road wheels and releases the hydraulic pressure to the brake just before the wheel locks.

The Ferguson control system uses a master differential and two one-way multi-discs clutches. The arrangement divides the torque in the ratio of 37 percent front and 63 percent rear, and also allows for a speed variation between front and rear wheels. The differential is basically a simple epicyclic gear train where the input shaft drives the planet carrier, and the annulus and sun are connected to rear and front wheels respectively. Application of a force to the centre of the planet provides equal force at the annulus and sun. The force at the annulus is converted to a greater torque, as it acts at a larger radius, and hence is connected to the rear wheels. In this layout speed variation takes place as with a normal differential. Due to the difference in speed between the output shaft and the planet carrier, the planet rotates around the slower-moving gear, so that the other members speed up. The control unit governs this speed variation. In this particular design the front wheels can over-run the rear ones by 16.5 percent, whereas the rear wheels can over-run the front ones by 5.5 percent. Once this limit is reached a multi-disc clutch locks the planet carrier (input shaft) to the sun wheel (front output shaft), so that the centre differential becomes ineffective. This action can take place during acceleration as well as during braking.

Viscous Coupling Application

Some form of control is necessary to limit the action of a central differential; otherwise due to the loss of adhesion at any one wheel the vehicle becomes immobile. On the other hand, if a central differential is not used, the driveline components are stressed due to transmission wind-up. A compromise is attained by incorporating a viscous coupling, which can be used in one of two ways. It can serve either as a control unit to limit the speed difference between front and rear wheels, or as a viscous transmission to connect the front drive system to the rear when necessity arises.

26.8.1.

Four-wheel Drive extended from the Rear-wheel Drive

The engine axis being parallel the use of a transfer gearbox becomes necessary, which is positioned behind the main gearbox (Fig. 26.62). The Ford four wheel drive system uses an epicyclic gear acting as a central differential and a viscous coupling to control the speed difference between the front and rear wheels (Fig. 26.63). The input shaft of the differential is joined to the planet carrier and the front- and rear-wheel output shafts are connected to the sun wheel and annulus respectively. This provides a torque distribution of 34:66 between the front and rear axles.

A standard differential is installed at the front to the right-hand side of the engine. A drive shaft transmits the power through CV joints to the right-hand front wheel. The drive train to

Fig. 26.62. 4x4 layout-longitudinal engine (viewed from underside).

Fig. 26.63. 4 x 4 transfer box (Ford)

the left-hand front wheel requires an intermediate shaft which, due to the location of the engine, passes through the engine sump to connect the left-hand drive shaft. The rear axle is driven in the normal manner using a two piece propeller shaft and a visco-differential. Compared to a rear-wheel drive, the four-wheel drive arrangements require modification of the front axle steering geometry to meet different driving conditions.

On this layout a silent chain powers the offset output shaft, which connects with the front axle shaft. The viscous coupling is a shaft-to-shaft design. When connected between the axles in this manner, the coupling senses the speed differences between the axles and provides a torque difference twice the value of the torque transmitted by the coupling. Control of speed through viscous coupling depends on the operating conditions. When the mean speed of front and rear wheels is nearly equal, the coupling becomes ineffective. As the speed difference increases, the coupling resistance and hence the locking characteristic rise progressively.

The coupling experiences a large slip when complete loss of adhesion at any wheel occurs. As a result the coupling exerts a high locking torque on the gears to maintain a drive to the wheels, which are tending to spin. Frequent use of coupling under maximum slip conditions causes it to overheat, and the coupling is designed to go into the hump mode to protect it against this problem.

Four-Wheel Drive extended from the Front-wheel Drive

This layout is comparatively easy to adapt because the complicated part of the assembly, which is the front drive layout and steering geometry, is already available. Since most front-

wheel drive vehicles use a transaxle, the drive is taken from this unit through a central differential to the rear wheels (Fig. 26.64).

Fig. 26.64. 4x4 layout transverse engine.

The distribution of torque between front and rear depends on the weight distribution and the desired characteristics. The front/rear proportion generally varies from about 56:44 to 35:65. Most central differentials use an epicyclic gear train, which is controlled by a viscous coupling. This coupling is either mounted between shaft-to-shaft and take-off to front and rear axles, or between input drive and take-off to either the front axle or rear axle.

26.8.3.

Variable Four-wheel Drive

This arrangement provides the benefits of four-wheel drive, without the drawbacks of a permanently connected system. The viscous transmission layout illustrated in Fig. 26.65 uses a propeller shaft and viscous coupling to interconnect the two axles. The system does not require a central differential, but a free wheel is used in the rear axle final drive gear.

Fig. 26.65. 4x4 layout with viscous transmission (VW).

During normal operation, when the axle speeds are nearly equal, the coupling transmits a very limited drive to the rear wheels so that the vehicle practically works on normal front wheel drive system. When the front wheels lose their adhesion, the difference in speed causes the coupling to use the rear wheel drive system. The free wheel ensures that no drive is transmitted from the rear axle to the engine during overturn, there by making the arrangement suitable for use with anti-lock brake systems. An electro-pneumatic free wheel locking device makes four-wheel drive available for reversing, when reverse gear is selected.