As with the 4MATIC system's set-up on a dry or wet road, the directional stability and, therefore, the active safety of the 4MATIC models are paramount at all times when wintry conditions prevail, too. The mechanical principle of the 4MATIC system featuring a 45:55 torque split between the front and rear axles and the multi-disc limited-slip centre differential with a basic locking torque of 50 Newton metres offer all the right ingredients.
This basic design enables high levels of traction, firstly because the dynamic shift in axle load toward the rear axle that occurs during acceleration is harnessed to deliver more drive torque to the rear wheels. Secondly, the multi-disc differential lock is also able to shift the drive torque between the front and rear axles, varying the split as the road conditions dictate. Consequently, intervention by the ESP®, 4ETS or ASR electronic control systems can be delayed for as long as possible and the bulk of the drive torque converted into tractive power, even on slippery roads. All control system interventions go virtually unnoticed, yet drivers still know straight away if they are driving on the limit, as a yellow warning symbol will flash in the instrument cluster. This serves as a prompt to adapt the driving style to the road conditions.
The drive mechanism's permanently engaged design offers key advantages over other systems that first need to diagnose a lack of grip before activating the all-wheel drive. The 4MATIC will have already made use of this valuable time to start transmitting drive torque via the wheels to the road.
All-wheel drive is not a substitute for winter tyres
Like any other all-wheel drive system, the 4MATIC has to abide by the laws of driving physics. The fundamental rule is that a tyre is only able to transfer a certain level of overall force to the road surface. If a particularly high degree of longitudinal force is required during acceleration or braking, for example, the available lateral force is reduced. When cornering, the reverse applies: now a large amount of lateral force is required to keep the vehicle on course, while the longitudinal force potential is restricted. The engineers' skill in designing the drive mechanism and fine-tuning the control systems lies in their ability to exploit these correlations in such a way as to ensure the best possible handling characteristics under all conditions. The frictional coefficient µ describes the physical grip between the tyres and the ground. This figure is high on a dry road (µ = 0.9) and low on a snow-covered road (µ = 0.3).
Despite all the engineers' efforts, it is ultimately the drivers themselves who determine how safe they are out on the road. They should always adapt their driving style in accordance with the wintry conditions and ensure that their vehicle is suitably equipped; winter tyres are absolutely essential here.
Pulling away with ease on ice and snow
When starting off in wintry conditions, certain road surface conditions are automatically detected and the interventions of the 4ETS electronic traction control system adjusted so as to achieve the greatest possible acceleration while minimising wheel slip, ensuring optimum directional stability in the process. This strategy also allows the vehicle to pull away under the most adverse conditions, such as when one side of the vehicle is on an icy slope (µ-split) or both wheels on either the front or rear axle have limited grip (µ-jump).
When starting off on µ-split, one side of the vehicle is on snow or ice and the other on asphalt. This means there are major differences in the coefficient of friction between the left and right-hand side of the vehicle. On all vehicles with open axle differentials, the wheel with the smallest friction coefficient limits the maximum transferable drive power. If the drive power exceeds the maximum transferable power, the wheels on snow or ice will start to spin and the vehicle would be unable to start off.
4ETS instantly detects this situation and prevents the wheels from spinning by building up exactly the right amount of pressure in the wheel brakes. As the wheel with the higher friction coefficient is now braced by the brake force at the wheel with the smaller friction coefficient, the 4MATIC model starts to move. After pulling away, the wheel characteristics are closely observed and the brake pressure regulated in such a way that, as far as possible, there is no difference in speed between the individual wheels. The braking force applied by 4ETS effectively simulates a higher coefficient of friction on the side of the road covered with ice or snow; ideally, this will match the coefficient of friction on the asphalt side. This produces an optimal inter-wheel locking effect at the axle differentials, resulting in the maximum possible acceleration on µ-split surfaces.
Starting off on "µ-jump" surfaces – where one of the vehicle's axles is entirely on ice or snow and the other on asphalt – is hampered by the large differences in friction coefficient between the front and rear wheels. On vehicles with an open inter-axle differential, the axle with the lower coefficient of friction determines the transferable drive power. Whilst the multi-disc differential lock with its basic locking torque of 50 Newton metres does have a balancing effect, it is unable to compensate for these extreme differences in friction coefficient. Here again, both wheels on the axle with the lower coefficient of friction start to spin if the drive power exceeds the maximum transferable drive power at this friction coefficient. The 4ETS resolves this problem by instantly detecting spinning wheels and braking them. In this way, the braked axle with the smaller friction coefficient braces the axle with the higher friction coefficient – and the 4MATIC model starts to move.
Assured handling on wintry roads
On twisting roads covered with snow and ice, vehicle stability is primarily controlled by the 4ETS system's acceleration skid control function regulating engine torque. The 4ETS control thresholds are adjusted according to the driving situation based on the vehicle's longitudinal and lateral dynamic readings as continuously measured by the ESP® sensor system. To prevent the vehicle from tail-skidding, the longitudinal force when cornering must be controlled by means of engine torque regulation in such a way that there are sufficient reserves of lateral force at all times. In order to comply with this physical correlation, when cornering on road surfaces with a low friction coefficient vehicle stability is mainly controlled by the ESP® and ASR acceleration skid control systems intervening to ensure that there is always sufficient lateral force in reserve.
Initially, only enough engine torque to allow the tyres to develop sufficient lateral force is transmitted to the wheels on the outside of the bend. If the ASR control function cannot restore directional stability, the ESP® system will intervene by applying the brakes to stabilise the vehicle.
In contrast to the control mechanisms triggered when cornering, considerably more longitudinal force can be provided when accelerating in a straight line, as the tyres hardly have to transfer any lateral force. What is important here is that the tyres can work in the optimum µ-slip curve range. In order to achieve this, the control thresholds for engine torque regulation are raised in this driving situation. When driving on snow-covered or icy roads with low friction coefficients, the multi-disc differential lock in the transfer case takes effect, producing an inter-axle locking torque of 50 Newton metres in the powertrain. This locking effect boosts traction considerably without having any negative impact on the control systems. When the final-drive ratios are factored in, this allows 150 Newton metres to be delivered to the wheels.
In certain wintry conditions it may become necessary to deactivate the control systems using the "ESP® OFF" switch. This is the case where high slip values are required at the wheels – when powering out of deep snow, for instance, either with or without snow chains. Even in "ESP OFF" mode, drivers can still count on the full support of the control systems when braking. Once back on a road surface with a normal covering of snow, the control systems should be reactivated again.