The torque an engine produces only becomes movement at the moment the tire manages to transfer it to the ground. Each tire, however, has a limited amount of grip available, which has to be shared between longitudinal thrust, braking and the lateral force needed in a corner: when the torque demanded exceeds the grip available, the wheel spins and the energy is wasted. It is from this observation that all-wheel drive is born: by distributing drive torque across four wheels instead of two, proportionally less is asked of each tire, the result being a wider grip margin under acceleration, on wet, snow-covered or broken surfaces, and a more neutral behaviour on corner exit. One clarification is in order: all-wheel drive acts on the transmission of torque, not on braking or on lateral hold, which remain the responsibility of the tires and of the braking system. The benefit also has to be paid for in weight, constructional complexity and internal friction, with on average higher fuel consumption than the corresponding two-wheel drive version. The designer’s task is therefore to decide how and when the torque should reach the second axle. The solutions adopted today can be grouped as follows:
- Permanent all-wheel drive with mechanical torque split
- Electronically engaged (on demand) all-wheel drive
- Lateral torque distribution (torque vectoring)
Permanent all-wheel drive with mechanical torque split
In this architecture both axles are permanently connected to the output of the gearbox. Between the two sits a centre differential, which is essential because in a corner the front and rear axles follow paths of different length: without it, the transmission would build up torsional stresses, triggering vibration and wear. An open differential, however, would send torque to the axle offering the least resistance, which is precisely the one that is slipping. This is why the centre differential is of the self-locking type: as soon as a difference in speed appears between the two axles, the very geometry of its gears generates axial forces which, by way of packs of friction plates, produce a locking effect and divert most of the drive towards the axle that still has grip. All of this happens mechanically, without waiting for an electronic command. The basic split is not necessarily even: it is chosen at the design stage to define the dynamic character of the car, often with a slight bias towards the rear axle. The price to be paid is the constant presence of rotating components, with the friction that goes with them, and a higher overall weight.
Electronically engaged (on demand) all-wheel drive
Here the centre differential is replaced by a multi-plate clutch, operated electro-hydraulically or electromechanically and managed by a control unit. One axle, the primary one, always receives drive; the second comes into play only when the clutch is closed. The degree of closure can be modulated continuously, so the torque split is no longer a fixed value but a variable one, ranging between two-wheel drive and an almost complete transfer to the other axle. The control unit does not merely read wheel slip through the wheel speed sensors: it talks to the stability control, to the steering angle and to the driver’s torque request, and is therefore able to anticipate a loss of grip rather than simply correct it. This is the lightest solution and the least penalising in terms of fuel consumption, because at a steady cruise the components of the second axle can remain unloaded. It does, however, involve a delay in intervention, however small in the most recent designs, and it exposes the clutch pack to thermal stress if used at length in demanding conditions.
Lateral torque distribution (torque vectoring)
The two previous groups act longitudinally, that is between front and rear axle. Torque vectoring, by contrast, works between the two wheels of the same axle. In a corner the outer wheel, more heavily loaded by the transfer of mass, can exploit more grip than the inner one: sending it a greater share of torque generates a yaw moment that helps the car tighten its line, countering understeer. This can be achieved in two ways. In the first, cheaper approach, the system uses the braking system, applying a light brake to the inner wheel and obtaining a similar effect by difference. In the second, an active differential is used, fitted with clutches or gear sets that genuinely transfer torque from one wheel to the other. It should be made clear that torque vectoring is not in itself an all-wheel drive system: it is also found on two-wheel drive cars, but it is in combination with all-wheel drive that it expresses its potential best. On electric cars with more than one motor the function is intrinsic to the architecture, since the torque of each motor can be regulated independently.
The main applications
Audi quattro
Introduced in 1980 on the coupé that bears its name, it is the historical benchmark for permanent all-wheel drive on a road car. On longitudinal-engine models the torque is managed by a mechanical self-locking centre differential – of the Torsen type in the classic generations, of the crown-gear type in the more recent ones – with a basic split in the order of 40% to the front axle and 60% to the rear, able to shift markedly towards the axle that has grip. Since 2016 Audi has also offered the quattro ultra variant, which belongs instead to the on demand family: two clutches, one at the gearbox output and one in the rear differential, decouple the propeller shaft when all-wheel drive is not needed. On transverse-engine models, finally, the quattro name identifies an electronically managed multi-plate clutch system.
Mercedes-Benz 4MATIC
Introduced in the second half of the Eighties, the 4MATIC badge today covers solutions that are technically quite different from one another. On longitudinal-engine platforms an epicyclic centre differential is used, accompanied by a plate pack that provides a basic locking effect, with a split close to even and slightly biased towards the rear axle. On compact transverse-engine cars the principle is instead that of torque on demand: a power take-off integrated into the dual-clutch gearbox takes the drive to the rear axle, where a hydraulically operated multi-plate clutch decides how much torque to let through, right down to cutting it off altogether and giving back a front-wheel drive car.
BMW xDrive
Presented in 2003 on the X3 and X5, it is the electronic interpretation of all-wheel drive applied to a marque with a rear-drive tradition. The heart of the system is a transfer case at the gearbox output, inside which an electronically controlled multi-plate clutch regulates the share diverted to the front axle; the rear axle remains directly connected. With the clutch open the car behaves like a rear-wheel drive, while in normal conditions the split stays biased rearwards, roughly 40% front and 60% rear. The transfer case control unit is networked with the stability control and can therefore act on the split to counter understeer and oversteer in their earliest stages.
Volkswagen 4MOTION
Here too the commercial name gathers together two distinct approaches. On transverse-engine cars, from the 1998 Golf onwards, all-wheel drive is entrusted to the Haldex coupling, an electro-hydraulic multi-plate clutch mounted ahead of the rear differential: in normal driving the torque stays almost entirely at the front axle and is diverted to the rear when a difference in speed between the axles appears or when a large amount of torque is requested, while the more recent generations keep the pack already pressurised in order to anticipate the intervention. On longitudinal-engine models such as the Touareg, a permanent system with a mechanical centre differential and an asymmetric split is used instead, in keeping with the off-road vocation of these cars as well.
What emerges is that there is no single best system. A permanent mechanical split offers promptness and predictability, at the cost of greater weight and friction; electronic engagement favours efficiency and light weight, accepting a response that is not instantaneous; torque vectoring does not increase the grip available, but exploits it better where it is needed. The choice depends on the architecture of the powertrain, on the mission of the car and on the compromise the manufacturer intends to strike between performance, fuel consumption and cost.