How it Works: Four Wheel Drive

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How it Works: Four Wheel Drive


The need to produce a light and reliable vehicle capable of making progress across country in all conditions was highlighted by the outbreak of World War 2 and it was the pioneering four-wheel drive Bantam Jeep which set the pattern for later developments by other manufacturers. Although originally designed for utility vehicles, the advantages of 4WD were such that its appeal widened and many 4WD vehicles were bought by the general public, a trend which the manufacturers were not slow to exploit. The main and undisputed advantage of AWD was that it provides extra traction, especially in off-road conditions. Thus, in conditions where a vehicle with two-wheel drive (2WD) may be unable to make further progress, a rival with AWD can often continue. It has also been argued - perhaps best by Subaru - that 4WD confers handling benefits as long as the drive system was arranged in a particular way. In general, however, it was the superior traction, coupled with the robustness of the mechanical systems, that persuades most punters to buy the wide range of 4WD vehicles on the market.

To simply say that 4WD improves traction was to ignore several other factors, because it was possible to improve the traction and the general cross-country ability of vehicles with 2WD, an example being the Y66 produced by DAF which used 2WD and the Variomatic transmission. The simplest method, and the one that was most generally seen, was to fit tyres with a knobbly, self-cleaning tread pattern. This achieved two important things. First, the extra grip lessened the chance of a single driving wheel spinning. Secondly, the total grip area was increased so that the vehicle stood a better chance of crossing poor surfaces, such as deep mud. If the mud was really deep, ground clearance also became an important factor: almost any normal car would perform much better across country if its ground clearance was increased by half again.

Even with the extra grip of cross-country tyres, the standard 2WD vehicle will stop as soon as one of its driven wheels lose grip, because the wheels will spin helplessly due to the action of the differential. A considerable increase in cross-country ability can, therefore, be achieved by fitting a device to lock the differential, so that the vehicle will continue to make progress as long as at least one of its driven wheels can gain a grip of some description. Finally, if the grip was good enough but the resistance to motion was too great - if the gradient was too steep or the mud too deep, for example - the engine could be prevented from stalling by lowering the overall gearing. It is not generally appreciated how much can be achieved with a fairly ordinary 2WD vehicle by taking these measures. One of the most familiar off-road vehicles, the Land-Rover which was introduced in 1948, has much more ground clearance than any ordinary car and its transmission not only gives the option of 4WD but also of selecting an entire range of much lower gears, as, in fact, did the original military Jeep.

It should also be pointed out that wheel-spin was potentially just as much of a hindrance to a 4WD vehicle as it was to a 2WD car unless the transmission is equipped specifically to overcome it. Even if all four wheels are driven, a single spinning wheel will cause all drive to be lost unless limited-slip differentials or differential locks are fitted. Furthermore, at least two differential locks are essential, while a third will enable the best possible traction to be maintained in all conditions. Any form of 4WD is not without its disadvantages, however. It is much more complicated than 2WD since it must, at the very least, have one more "live" axle, one more prop-shaft and possibly two more differentials. The three main drawbacks are the cost of production, the weight of the system and the extra servicing that is required. Another disadvantage is that there are more components that can malfunction. In addition, problems of heavy steering and front tyre wear have to be considered also.

A 4WD vehicle, therefore, has to incorporate all the refinements necessary in a Front-Wheel-Drive design, such as constant-velocity (CV) joints in the front drive-shafts to ensure that the drive was transmitted smoothly as the wheels are steered. However, most older four-wheel-drives, notably some Land-Rover models, can be driven in rear-wheel drive (RWD) only when 4WD is unnecessary, which saves front tyre wear on hard surfaces, lightens the steering and improves fuel economy.

Series 1 Land Rover

Range Rover Series 1

Daihatsu F20

International Scout

Daihatsu Rocky

Subaru Brat

Land Rover Transfer Gearbox The transfer gearbox allows the driver to choose two separate gear ratios, one of which is extremely low to allow progress in difficult conditions. The above is a Land-Rover unit, with gearbox set in high ratio with drive directed to the rear wheels only.

Jeep 4WD Layout On the early Jeeps the centre differential split the drive between the two propeller shafts. The drive was directed to all four wheels by further differentials, mounted on each live axle. A transfer gearbox allowed low ratio gear selection.

The Late Arrival Of The 4WD System

The engineering problems of Front-Wheel-Drive design were sufficiently severe to ensure that it was only in the 1930's that the first mass-produced FWD car, the Citroen Traction Avant, was introduced. It was hardly surprising, therefore, that the 4WD vehicle had to wait until just before WW2 to be accepted as practical and would probably have waited another 10 years if the war had not accelerated its development for military purposes. It was the Americans who foresaw the need for a vehicle that was able to operate in extremely poor conditions. Clearly it was vital that the vehicle should keep going as long as any sort of progress was possible, so 4WD formed part of the specification. The successful prototypes were actually made by an American Company called Bantam, which had started life building Austin Sevens under licence for the American market.

Before the advent of the original Jeep, which was developed for the U.S. Army, some trucks had used 4WD without a great deal of success. Where more traction was needed, as in the trucks built by the French for the run across the Sahara for example, it was more easily found, together with better load capacity, by having four or more driven wheels situated at the rear of the vehicle. This was not a satisfactory arrangement for the Jeep, which needed to be small and light, to be air-transportable, to be manhandled if, in spite of its 4WD, it still became stuck and, not least in view of its original military role, to present as small a target as possible.

In fact, the Jeep was so successful that it set the pattern for virtually all of the utility vehicles that followed, except that most of them have placed more emphasis on load-carrying capacity and less on sheer performance and manoeuvrability. It had a live axle at each end with a propeller shaft running from each of the axle-mounted differentials to a third differential that served to split the drive between the front and rear wheels. Apart from the fact that independent front suspension was by no means universal before WWII there were good reasons for employing live axles. They were stronger, less susceptible to damage and, above all, gave constant ground clearance. This latter feature being most important because if the nose of a vehicle with independent front suspension crashes into a dip, the chances of grounding the sump are that much greater.

The Transfer Gearbox

The Jeep also introduced the idea of the transfer gearbox, effectively a kind of under-drive giving two complete gear ranges, one for normal progress and the other a much lower set for use on rough surfaces in poor conditions. Certainly the Jeep's excellent cross-country performance was due in large measure to its 4 WD but it should not be forgotten that it also had very good ground clearance and an excellent power to weight ratio with 60 BHP and 14.6 kg/m (1051b/ft) at 2,000rpm from the 2.2-litre engine in a comparatively light 950 kgs (2,1001b) body. Neither the British nor the Germans produced a true Jeep equivalent during the war. The Volkswagen-based Kubelwagen was driven by its rear wheels like the original saloon, although an amphibious development did have 4WD. During WW2 the British adopted the Jeep where their own staff cars were not adequate and it was not until 1948 that the Land-Rover appeared.

Apart from being bigger, the Land-Rover had a good deal in common with the Jeep as far as its general layout was concerned. The central differential, the live axles and the transfer gearbox were features of both. Four and six-cylinder Land-Rovers, however, had provision for freeing the drive to the front wheels so that they could proceed on rear wheel drive alone, saving fuel, noise and tyre wear, although the more recent, V-8 model was a permanent 4WD concept. This apart, the most notable feature of the British vehicle was, perhaps, its use of body panels made of aluminium alloy which were, therefore, rust-free. The general chassis layout has proved to be so sound that it has continued in production in virtually the same form since it was introduced and has proved itself adaptable to many changes, not least the forward control layout of certain military versions in which the driver's cab was positioned ahead of the front wheels.

Land-Rover Developments

Although there was much of interest in the way the Land-Rover was developed into a wider range of special versions than any other vehicle, most of the technical interest centres on the transmission. While its standard form has hardly changed in its production life (except that synchromesh, originally fitted only on third and top gears, was later fitted to. all four forward gears), there are a multitude of factory-fitted options which help either cross-country ability or fuel economy. Differential locks were not part of the standard specification, although in their absence the ability to keep going in the worst conditions was inevitably more limited. A wide range of wheels and tyres was, therefore, offered as optional equipment. On the economy side, a popular fitting was a free-wheel mechanism on the wheel hubs, and later an over-drive was made available. This overdrive unit made possible much quieter and more economical road cruising without sacrificing the low overall gearing needed to provide the torque necessary in extreme conditions.

Without greatly changing its form, the Land-Rover was adapted from a general utility vehicle into a fairly luxurious wagon, though for these applications it had the drawback of a hard and somewhat uncomfortable ride across country, which set a definite limit to the speed which could be attained without discomfort. To some extent this was a matter of engineering convenience. Softer springs would have meant longer suspension travel, which in turn would have meant longer multi-leaf springs and these would have been difficult to install. Longer damper travel would have resulted in greater problems of damper fade. Above all, however, the harsh ride was a protection for the vehicle itself ensuring that the driver eased off before they reached the point where damage could be caused to the vehicle. This was Rover's deliberate policy, both at the design stage and during development, and it was one of the most important factors in ensuring the Land-Rover's world-wide reputation for reliability.

This aspect of the Land-Rover was accepted by most early customers who bought it as a genuine utility vehicle. Later on, however, it became popular with a different class of buyer which valued its strength and more particularly its towing ability but also wanted to use it as an alternative to a family sedan. The lack of ride comfort was a drawback in this case and was one of the considerations which led Rover to develop the Range Rover. The Range Rover was in almost every sense a completely different vehicle from the Land-Rover but the biggest differences were in the body design, the suspension and the transmission. The body was designed to present a far more car-like aspect. The suspension used coil rather than multi-leaf springs, had a much longer travel and provided much greater ride comfort. The transmission was, to some extent, simplified by having 4WD permanently engaged. The Range Rover's suspension was of interest in that, despite the greater emphasis on ride comfort, it still used live axles rather than an independent arrangement. This was an indication of the supreme importance of maintaining ground clearance beneath the differential casing and, additionally, the vehicle as a whole.

As far as the transmission of the Range Rover was concerned, it should be noted that the use of permanent 4WD was not altogether dictated by a desire for simplicity. As in the Land-Rover, a transfer gearbox was fitted to give a much lower set of alternative gear ratios, the bottom gear of the lower set giving a ratio of around 9:1. This meant that the transmission as a whole had to cope with the torque of the 3.5-litre V8 engine multiplied by this ratio; the use of permanent 4WD meant that each axle differential normally had only to withstand half of the torques. Otherwise the rear axle differential, at the least, would have had to be much stronger and therefore bigger and heavier and, since the differential was unsprung weight, that would have made the ride considerably less comfortable.

An Expanding Market

The Range Rover may have set a new benchmark, but it was soon to be joined by some pretty formidable competitors. In America, the original Jeep was developed into a series of 4WD models built by American Motors. Some of them were luxuriously-equipped and used automatic transmission and power-steering to ease the driver's considerable task. Such was the appeal of the Jeep series that they, too, soon had several rival American makes, including one from GM and even one from non-car specialists such as the International Harvester Corporation. The two major Japanese companies, Nissan-Datsun and Toyota, each produce one model of similar size to the Range Rover but, technically, they are much less advanced and are more like the Land-Rover in their mechanical details. These two vehicles, the Nissan Patrol and the Toyota Land Cruiser were exported to many territories where their extra power and load capacity gave them an advantage over the Land-Rover. Alongside these more or less traditional vehicles, Japan also produced designs that looked very much like miniature Jeeps.

Daihatsu produced these smaller vehicles, such as the Daihatsu F20 and Daihatsu Rocky, which had 1.6-litre petrol engines or 2.5-litre diesel engines, with the usual 4WD layout. They were apparently intended as a return to the original Jeep formula, with the most important consideration being cross-country ability with a couple of occupants and a small load aboard, rather than the more substantial load-carrying capability of the Toyota Land Cruiser or the British Leyland Land-Rover. Extending their range even further, the Japanese then began to offer vehicles which were, in effect, conversions of normal 2WD designs. The first was the Subaru Leone 4X4 Station Wagon, in which the normal Front-Wheel-Drive design was modified to also have a drive to the rear wheels. At the same time, the suspension was revised to give better ground clearance. Toyota also produced a AWD version of the company's light pick-up truck which in some ways complemented the better-known Land Cruiser. In this case the basic vehicle had Rear-Wheel-Drive, so that conversion took the form of adding a live front axle with a prop-shaft from a centre transfer gearbox.

Other countries also produced their own 4WD vehicles and some designs originally thought of as purely for military application were successfully adapted for the civil market. In many cases they remained too heavy and expensive to be widely sold, as was the case with the Volvo Laplander vehicles and the Fiat Campangnola. The Mercedes Gelaendewagen, on the other hand, was specifically designed from the outset for civilian use and its relatively high price was more a reflection of Mercedes' high standards of engineering. It was interesting that here again, though Mercedes started with "a clean sheet of paper" when they designed the Gelaendewagen, the result looked similar to many other vehicles. An even more interesting vehicle in many ways was the Russian Lada Niva, which was not unlike a small Range Rover in concept. Because of its compact dimensions, it achieved reasonable performance with a 1.6-litre engine. Its high ground clearance and live axles revealed that it was designed from the outset as a 4WD vehicle.

4WD Conversions

As mentioned earlier, some of the first Japanese 4WD designs were effectively conversions of ordinary production 2WD vehicles, though the conversions were skilfully and thoroughly carried out. Elsewhere, smaller firms began designing conversions to fill gaps on the market. This was most noticeable in France where there had never been a popular equivalent of the Land-Rover or Jeep and many cross-country operators were forced to prove just how much could be achieved with, for instance, the Citroen 2CV and the Renault 4. Citroen did produce at one stage a military version of the 2CV known as the Sahara in which a second engine drove the rear wheels.

One French company that started up around this time was Sinpar, which began converting the Front-Wheel-Drive Renault 6 and Renault 12 to AWD. In some ways a FWD car was easier to convert than a RWD model since a second drive could be taken to a rear axle without too much trouble and a live axle installed on the pick-up points of the original rear suspension. In Sinpar's case, it was clear that the Renault 12, with its engine at the very front and its gearbox behind the front axle line, was easier to convert than the Renault 6 which used the older Renault mechanical layout with the gearbox situated right at the front of the car. In both cases, however, the use of a FWD basis meant that they could not avoid the inherent weakness in the independent front suspension of these models. Even with extra ground clearance, a strong under-shield was still needed to make sure the vulnerable mechanical components were not damaged in very rough conditions.

Another 4WD system often overlooked these days was the Ferguson design, which was fitted to the Jensen FF and first put into production in 1964. The system represented a great advance in 4WD design because it incorporated sensors which controlled clutches that adjusted the available torque between the front and rear of the car in the event of wheel-spin developing at any of the wheels. Unless all four wheels lost their grip simultaneously such as when hitting black ice, wheel-spin was completely eliminated - a major advance in road safety.

Permanent 4WD

At face value, permanent 4WD appears much simpler than a system giving a choice of 2WD or 4WD but in practice there was not much of a saving as the standard method of engaging the second drive line was to use a simple "dog" clutch assembly rather than a friction device, so the extra expense of enabling the driver to select 2WD was not so great as might be expected. The employment of a dog clutch system did mean that 4WD could only be engaged or disengaged when the vehicle was at a standstill or travelling at very low speeds but it was preferable to a friction clutch which would not only be heavier and bulkier but also subject to a considerably higher rate of wear.

The main reason for having the choice of 2WD was to overcome certain problems that can arise on a 4WD car with a live front axle when it was used at higher speeds on hard surfaces. If the front wheels remain driven, there will be three principal undesirable effects. The steering will tend to be heavier and there may be some "feedback" of power and traction changes through the steering wheel. Secondly, front tyre wear will be increased and, thirdly, road holding was bound to be affected, with a tendency to understeer which will make the steering seem even heavier, since more lock will have to be applied to take the same corner and will have to act against the torque being transmitted to the wheels.

Hookes-Type Universal-Joints

Apart from these three immediate effects, it may be possible to dispense with constant-velocity (CV) joints, which were expensive to produce, in the front axle-shafts if 4WD was only used in the slower, more slippery conditions of cross-country driving and not on the road. Under these conditions cheaper Hookes-type universal-joints could be used and this is, in fact, the case with the four and six-cylinder model Land-Rovers. Other important factors are that, by using 2 WD, power losses in the transmission are reduced, performance and fuel consumption are improved and there was also-likely to be less noise while the vehicle was in motion. It is worth noting that many older 4WD vehicles in which it was possible to drive the rear wheels only were fitted with free-wheeling hubs on the front wheels. The hubs free the wheels from the drag of turning the front drive-shafts and front differential and thus improve performance and economy still further. Although free-wheeling hubs have always been an option rather than standard equipment on the Land-Rover, they were a very popular fitting.

The main point of having permanent 4WD, as in the case of the Range Rover, was that a powerful 3.5-litre engine could be used without having to stress the entire transmission to take the maximum torque output multiplied by the lowest gear - which, given the gearing commonly found in this kind of vehicle, could result in a massive figure which would require proportionately-sized components, particularly the axle-shafts. Cost apart, such bulk was undesirable since it was generally added to the unsprung weight and could, therefore, be detrimental to the ride and handling qualities. If permanent 4WD was used, however, a centre differential became essential. If it was possible to disengage the drive to one pair of wheels, the centre unit was not essential, providing, however, that the driver did not engage 4WD unless it was absolutely necessary.

The Transfer Gearbox

A four or five-speed gearbox provides a sufficient spread of gears for a normal car under the wide range of normal road-going conditions. However, a cross-country vehicle may encounter many conditions in which it needs a gear lower than the standard-first gear. It may, for instance, have to tackle very steep gradients or tow extremely heavy loads through very soft ground. If the final drive ratio was lowered to make the overall first gear ratio low enough for this kind of work, a very under-geared vehicle was the likely result and this would be too slow and uneconomical for normal road use. If, conversely, first gear only was lowered and the other gears are spread out above it, the gaps between the various gears could make driving the vehicle very awkward indeed.

The usual solution was to adopt a transfer gearbox, which was simply an extra two-speed gearbox that was added to the output shaft of the standard gearbox. The transfer gearbox differed from an overdrive unit in two respects. It operated on all of the forward gearbox ratios, not just top or third and top gears as was usual with an overdrive. It also provided a much higher step-down ratio than the usual step-up ratio of an overdrive. Early designed transfer gearboxes rarely employed synchromesh and it was usually necessary to change from the high to the low range when the vehicle was at a standstill. With a transfer gearbox fitted, its higher gear ratio was often known as "high" and the lower ratio as the "low range". These two ratios were chosen so that the overall gear ratios did not greatly overlap; thus low range top gear was actually lower than high range second gear in some cases. The result was a range of eight gear ratios with an extremely wide spread, to cater for a variety of conditions and loads.

The use of a transfer gearbox with suitable ratios enabled a relatively under-powered vehicle to have reasonable cross-country performance, though it must be remembered that in all cases the transfer gearbox must be stressed to take the torque output of the main gearbox's first gear. This means that the transfer gears themselves must be relatively large and robust. Extra power was absorbed in the transfer gearbox when it was in use but this was unlikely to be noticeable at the slower speeds involved. If the power-to-weight ratio of the vehicle was high enough it may have been possible to do without a transfer box at all. This was the case in some of the car conversions to 4WD such as the Subaru and the automatic transmission Jeeps which had acceptable performance with standard gearing.

The Austin Champ, formerly used by the British Army, had yet another arrangement; it used a five-speed gearbox, plus a "transfer" gearbox which selected forward or reverse so that, in theory, it could travel as fast backwards as it could forwards. The removal of reverse from the main gearbox was a worthwhile simplification and the use of a 2838cc engine in a relatively small vehicle meant that good performance was available with the five forward gears. In some ways, it was tempting for a designer to use a transfer gearbox whether or not the vehicle really needs it, because it was so easily incorporated in the central drive assembly. There has to be some means of splitting the single drive output from the main gearbox into two drives, one to each end axle of the vehicle, and the gearing needed to do this can very easily form a neat package with a transfer box.

The Centre Differential

A standard differential balances the rotation of two wheels so if a vehicle has 4WD it seems logical to fit a differential between the two driven axles so that they are balanced as well and earlier 4WD vehicles, such as the Jeep and the Land-Rover, were equipped in this way. The centre differential can be considered as a FWD final drive unit turned sideways: there was a spur gear drive from the transfer gearbox to the crown wheel, within which the differential unit itself acts to split the torque equally between the two-shafts or, in this case, between the front and rear axles.

At this point, several other factors have to be taken into account. First, if there is an arrangement to enable the front axle to be freed so that the vehicle can be RWD only, then there must be provision for locking the centre differential if one is fitted, so that the drive may continue to be fed to the rear axle. In practice this was not difficult, because the same dog clutch movement which disconnected the front drive line could also serve to lock the differential. A second consideration was that if a centre differential was fitted, the spinning of a single wheel at either end of the vehicle will bring it to a halt unless there was provision for locking the differential. Conversely, if there was no centre differential fitted, drive would continue to be available as long as some type of grip can be found at either end of the vehicle.

When permanent 4WD was used, a centre differential was essential. This was because there must be some mechanism for absorbing slight differences in the speed of the front and rear wheels, which might, for instance, be due to wheelspin or to small differences in tyre size. Without a centre differential any such variations would cause stress at the point where the drive was split front and rear, with the possibility of eventual damage. On soft ground this was no problem, because any build-up of torque would lose itself in wheel slip but on hard surfaces where tyre grip was good this was another matter. This was why 4WD vehicles with 2WD only facility and no centre differential, such as the Subaru Leone 4X4, the Toyota Hi-Lux and the Mercedes-Benz Gelaendewagen, were produced with a handbook which firmly instructed the driver to use only 2WD on metalled or tarmac road surfaces.

The Differential Locks

If a centre differential was fitted, there will be a total of three differential units on the vehicle. Between them they form a mechanism through which the spinning of a single wheel can cause the loss of all drive. The question therefore arises as to the relative values of fitting one, two or three differential locks or limited-slip units. The idea of creating a vehicle which can proceed as long as one of its four wheels can find a sufficient grip was highly attractive to the designer but it was not without certain problems. Whenever a differential was locked, its torque-splitting ability was lost and it was possible for all of the available torque to be fed in one direction by locking all of the differentials; therefore, a situation could arise whereby the maximum engine torque, probably multiplied by the low range bottom gear ratio, was wholly applied to only one wheel through one drive-shaft. In order for each drive-shaft to be capable of transmitting such torque without damage being caused they would each have to be as strong and thick as the propeller shaft.

While this would be possible it was hardly desirable because of the increase in weight and manufacturing cost. In practice, therefore, locks were not fitted to the axle differentials of all 4WD vehicles and, where they were fitted, to vehicles such as the Jeep and the Mercedes-Benz Gelaendewagen, for example, great care had to be taken by the driver not to over-stress the drive-shafts. Even when only the centre differential was locked, there may be some danger of over-stressing the drive line if the engine was sufficiently powerful. In the Range Rover the centre unit was actually a limited-slip design to delay the onset of spin and the necessity to positively lock the differential. If it was locked, the handbook warned the driver to be cautious in their use of power and to try to edge the vehicle clear of any obstruction without using full throttle.

It is worth remembering that, especially where the front axle was concerned, locking the differential was likely to lead to severe handling problems on anything but soft surfaces. Where the tyres can find good traction, a 4WD vehicle with a locked front axle would tend to understeer excessively. This was yet another reason why axle differential locks were not always part of a 4WD vehicle's specification and why even centre differential locks called for caution to be exercised on the part of the driver.

Transmission Control Layouts

In most older 4WD vehicles, there was a virtually standardized transmission control layout. Among other things, this was designed to prevent the use of low range gearing with 2WD where this was available, since this again would mean that very high drive line torque would all be directed to one end of the vehicle. It was therefore normal, in addition to the gear selector lever of the main gearbox, to have a single lever controlling the transfer box and the engagement of 4WD. This selector lever had three positions in sequence, generally labelled 2H (2WD, high range), 4H and 4L. A neutral position - neutral in the transfer box, that was - was sometimes also marked. Such a control arrangement was simpler and neater than having two separate levers, as in the four and six-cylinder Land-Rovers, for low range and 4WD engagements which would call for a mechanical interlock to prevent 2WD and low range being selected at the same time.

Live Axles v Independent Suspension

In the past, most manufacturers of 4WD vehicles favoured rigid arles, mainly because they gave constant and (with large diameter wheels) generous ground clearance. The Range Rover and the Mercedes-Benz Gelaendewagen showed that even in vehicles where comfort was a more important consideration than overall price, live axles still had their place. The preference was not, however, universal. The Austin Champ had all-independent suspension as did the Fiat Campagnola, which used MacPherson strut suspension all round, with torsion bar springing. Given suitable control of the spring rates and initial ground clearance, Fiat's choice suggested that the advantages of live axles were not overwhelming.

The choice did not greatly matter to the transmission designer except to place different emphasis on the problems he must consider. With live axles, these are mainly centred round the propeller shafts linking the centre drive unit to axle differentials, since the axles themselves will be free to move up and down through considerable distances. The propeller shafts themselves will be short, inevitably less than half the wheelbase of the vehicle, and the change of length and angle are correspondingly more severe than for the full-length prop-shaft of a normal car. Some care must be taken, especially in vehicles with a short wheelbase, to place the centre drive unit at the midpoint and avoid making one propeller shaft so short that it needs expensive CV joints to run satisfactorily. For this reason, many older AWD designs have yet another short propeller shaft linking the output from the main gearbox to the input flange of the transfer gearbox assembly. The Range Rover was, in fact, one of the few vehicles to have a solid layout with the two gearboxes mounted in union.

With live axles, once the drive had been delivered to the differentials, there were few further problems other than to decide whether CV joints would be required at the front. Several vehicles in which AWD was only used off the road, for example, the four and six-cylinder Land-Rovers, made do with plain UJ's. Independent suspension moved the problems into a different area. The differentials at one or both ends of a car with all round independent suspension were attached to the chassis and the design of propeller shafts to carry the drive from the centre unit was relatively simple. Instead, the problems arose in taking the drive from the differentials to the wheels. These problems and their solutions became much easier as 4WD vehicles adopted all round independent suspension.

Consequently, it might be said that there are fewer technical problems to be encountered in designing the transmission for a 4WD vehicle fitted with independent suspension, than there were for a vehicle with live axles. In either case the designer had to choose between symmetry and simplicity. The easiest way to install the centre drive unit was to have the transfer box behind the main gearbox and the drive-split gearing (with or without a differential) alongside it. This means that the drive line to the axles was off-centre and that, in turn, the differentials themselves are off-centre unless the propeller shafts are angled across the chassis. Even off-centre differentials present no problems now, as the designers of several FWD cars have proved that unequal length drive-shafts can perform their duties in a totally satisfactory manner.

The Ferguson System

While few people doubt the advantages of 4WD in providing extra traction for utility vehicles, there have been far fewer attempts to drive all four wheels of an ordinary road-going car which has no cross-country pretensions. In fact, as long as drive can be maintained to all four wheels and the layout of the components was properly balanced in the first place, there was every reason to expect a AWD road car to handle extremely well. The major problem was that a plain centre differential arrangement was unsatisfactory because it will allow wheelspin, developed at one end of the car, to upset its whole handling behaviour.

The Ferguson system, first seen on the F1 Grand Prix car in 1961, overcame this problem and was the only such system to have been sold on a high-performance car, the Jensen FF. The system overcomes the limitations of the differential by using two one-way sprag clutches, each bridging the two sides of the differential but acting in opposite directions. The gears connecting the clutches with the differential are chosen so that as long as all of the wheels are being driven normally the two clutches idle round in their "free" direction. If, however, a wheel does lose its grip and starts to spin, the action of the differential locks the appropriate clutch, limiting the amount of spin and transmitting the excess torque to the other end of the car.

The system does allow the shafts driving the front and the rear wheels to revolve at different speeds (an essential factor because the front and rear wheels take a different line round a corner) but it prevents this difference from exceeding a pre-determined percentage of the average speed. This percentage was determined by the diameters of the sun and planet gears and these gears will therefore have to be changed if a different percentage figure was required for some reason.

A second advantage of this arrangement was that the control system also works during braking so that if any wheel locks, the braking torque through the drive-shafts was transferred to the opposite end of the car. When the Jensen FF was introduced this was considered a major advance in road safety and in recognition of this the car was awarded the Don Safety Award in 1965. From the driver's point of view, the Ferguson system means that wheelspin was eliminated, unless all four wheels lose their grip simultaneously, and that the chances of the wheels locking under braking are greatly reduced. As applied to the Jensen FF the system proved itself beyond all doubt but its expense and complication continue to weigh against its more widespread adoption.

Also see: The History of the Four Wheel Drive | The History of the Four Wheel Drive Pictorial
Free Wheeling Hubs
Free-wheeling hubs were fitted to a number of 4WD vehicles which could also be driven in 2WD mode. When disengaged the wheel was free to rotate without also turning the drive-shafts and differential. The hub cover was turned clockwise to lock the hubs, compressing the two springs and engaging the clutch ring which bears on the drive shaft.
Range Rover Centre Differential
The centre differential of the Range Rover split the torque between the front and rear axles and also accomodated any variations in speed of the front and rear wheels. The high or low range gear was driven by a spur gear from the main gearbox and the drive was transferred by the planet gears which were capable of rotating on their axes to absorb any variations.
Mercedes-Benz Gelaendewagen Live Axle Setup
The original Mercedes-Benz Gelaendewagen was fitted with rigid axles. Despite the fact that independent suspension gave a more comfortable ride, live axles were preferred because they gave a constant and generous ground clearance.
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