Selecting the 4WD system for your needs

As 4WD SUVs and Double Cabs grow in popularity, more first-time buyers are faced with selecting a vehicle in these segments with a drivetrain that best meets their requirements. We explain the key differences between the all-wheel drivetrains available.

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Technically, any system where drive is provided to all wheels is categorised as AWD. However, in SA we typically refer to three types – part-time 4WD, full-time or permanent 4WD, and AWD, with the latter usually describing a vehicle which is primarily 2WD but automatically engages 4WD when the system detects that it’s required. Complementing these different systems are features such as diff locks, viscous couplings, and brake traction control; all of which play their own role in the systems’ function and performance.

It is important that prospective owners carefully weigh the capabilities of the different drivetrain types against their requirements and expectations prior to making a purchase decision. Before explaining the workings of the different systems, including their benefits, and shortcomings, it’s important that the functions of the differential, a key system component found in all drivetrains – within a driven axle and sometimes between front and rear driven axles – is clearly understood.

Driven axle and centre differentials

As vehicles turn, the outer wheels follow a larger arc or radius than the inner wheels and are therefore required to rotate more quickly. The axle differential allows these different wheel rotational speeds, while maintaining equal torque to both wheels to maintain the vehicle’s forward motion. However, this only works provided both wheels have traction. As soon as one wheel loses traction, the torque it can transmit is also reduced and the wheel speeds up; in fact, if that wheel is lifted into the air, its only resistance, and hence torque transmitted, will be to overcome drivetrain losses. The same (minimal) amount of torque will also be transferred to the wheel with traction, and the vehicle loses drive. Simply put, in an open differential, the power is always transferred to the wheel with the least resistance. In addition to the outer wheels on each axle following a larger radius, the front axle also travels further as it runs wider than the rear axle which slightly cuts across the corner. Consequently, when cornering, each of the four wheels follows a different path, covering different distances and therefore turning at different speeds. A 2WD vehicle caters for this with an open differential on the drive axle, but a permanent 4WD vehicle requires differentials in both drive axles as well as a differential between the front and rear axles – the so-called centre differential. The shortcoming of the three open differentials is that if any single wheel lifts or otherwise loses traction, drive will be lost – not what a 4×4 owner expects.

Various solutions exist, aimed at ensuring that the wheels with the most traction continue to receive sufficient torque to maintain drive, and these are covered in the descriptions of the different drivetrains.

Part-Time 4WD

Part-time 4WD systems are common fitment to most 4x4s on the SA market. The drive output from the gearbox is routed through a transfer box mounted behind it, which has two outputs: permanent drive to the rear axle and a selectable drive to the front axle. In 2WD mode, all the power is directed to the rear wheels, while selecting 4WD sees power routed to the front wheels, too. The transfer box also features two transfer ratios, high range, and low range, but does not include a central differential, meaning front/rear torque split is fixed at 50:50. With no speed differential possible between the front and rear axles, the vehicle should only be driven in 4WD on loose or slippery surfaces where some wheel slip is possible, otherwise drivetrain binding and damage can occur.

The disadvantage of a part-time system is that the vehicle’s 4WD capability is only accessible following a conscious decision and action from the driver, and then it is limited to low traction surfaces. For many years, Mitsubishi was an exception, their Super Select II system featuring a centre differential which can also be locked if required. This allows the driver to engage 4WD (4H), for example, on wet tarmac and benefit from improved stability and safety. Only when encountering a loose or muddy surface, does the driver lock the centre differential (4HLc) and the system will then perform like a regular part-time system with 4H engaged. Ford’s Advanced 4WD system provides similar benefits using a multi-plate clutch in the drive to the front wheels, instead of the centre differential. The vehicle can be driven on any surface in 4A (auto) mode with the system constantly monitoring traction conditions and automatically apportioning torque between the front and rear wheels to provide optimum grip.

While part-time 4WD systems ensure drive is provided to each axle, open differentials on each axle mean that in an off-road situation where one wheel at each axle loses traction, drive will be lost as the power at each axle is routed to the spinning wheel. The most common solution is fitment of a rear axle diff lock, a mechanical locking device within the differential which locks it into a 50:50 drive split. It is activated by the driver if loss of traction is anticipated but needs to be disengaged when back on a firm surface. Limited Slip Differentials are an alternative, featuring a multi-plate clutch which progressively engages to limit the speed difference between the wheels. Always operational, they are not as effective as diff locks in extreme conditions as complete lock up cannot be achieved but have the advantage of limiting wheelspin in any driving conditions.

The fitment of Brake Traction Control is also becoming more common in this application. An extension of the ASC / ABS system, this utilises wheel speed inputs from the ABS sensors to determine that a wheel has lost traction and applies the brake on that wheel, thus increasing drive to the wheel with more grip.

Full Time or Permanent 4×4

Vehicles fitted with these systems require a centre differential and are typically the more capable off-roaders. Driver-operated rear diff locks are typically fitted, while some even include a diff lock on the front axle, although this is generally used to extract the vehicle from more extreme situations due to its effect on steering. Centre differentials are sometimes a conventional open type, lockable by the driver, fitted with a viscous coupling which progressively varies the front and rear torque distribution according to axle speed differential. The torque-sensing Torsen type provides a mechanically variable torque split within a set range and can also be locked by the driver. Many of these vehicles also feature driver selectable modes to assist driving in sand, mud, snow and other conditions, each featuring pre-programmed settings for throttle response, gearbox, centre diff locking and other systems, to optimise off-road performance.

AWD – All Wheel Drive

AWD systems are typically fitted to mid-size family SUVs, designed primarily for everyday on-road use but capable of providing 4WD traction when required on loose or slippery surfaces. In such systems there is a permanently driven primary axle and a secondary axle which only receives power when a loss of traction is experienced (or anticipated) by the primary axle. Most vehicles using this system are built on FWD, transverse engine platforms shared with passenger cars. They are adapted to 4WD by the addition of a power transfer unit (PTU) which turns the power through 90° and directs it, via a permanently driven propshaft, to the rear axle. Built into the front end of the rear differential housing is a clutch unit that transmits drive to the rear axle when required.

Earlier versions were reactive, the clutch only engaging when a speed differential between the front and rear axles – wheelspin – had already occurred. Current versions are more effective, processing inputs from the ASC/ABS sensors to pre-tension the clutch, and engage 4WD before traction is lost. Brake traction control is also commonly used to limit wheelspin. More sophisticated vehicles in this category feature torque vectoring units that use multi-plate clutches to vary torque between front and rear axles, as well as between the rear wheels.

In comparison with its front-wheel drive equivalent, an AWD SUV will provide superior grip and safety on wet tarmac, improved stability on loose-surfaced dirt roads, and offer improved traction on low-grip sandy or muddy surfaces. However, it’s not generally designed for tackling sand dunes or off-road trails.

A new form of AWD, dubbed AWD-e, features a combustion-driven two-wheel drive setup complemented by a small electric motor independently driving the other axle. Increasingly used in some hybrid applications, the electric motor can boost performance when needed, as well as providing improved traction in low-grip conditions.

An understanding of these basic features and characteristics of these different types of 4WD systems should make it easier to navigate through some of the proprietary names used by manufacturers and ensure that you select a vehicle with a drivetrain that matches your needs and expectations.

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