4MATIC control strategies for wintry road conditions: Best possible traction, stable and safe handling

Muamer Hodzic December 8, 2008

As is the case with 4MATIC’s fine-tuning on a dry or wet road, directional stability and therefore the active safety of the GLK models is always to the fore when wintry conditions prevail, too. The basic 4MATIC mechanism with a 45:55 percent torque split between the front and rear axles along with the multiple-disc limited-slip centre differential with a basic locking torque of 50 Nm adds up to an ideal solution. This basic design delivers high traction since, on the one hand, the dynamic shift in axle load toward the rear axle that occurs during acceleration is used to deliver more drive torque to the rear. Meanwhile, the multiple-disc limited-slip centre differential can also variably shift the drive torque from 30 to 70 or 70 to 30 percent between the front and rear axle, whenever the road conditions so dictate. As such, the electronic control systems ESP®, 4ETS or ASR can intervene as late as possible; the bulk of the torque is converted into traction on slippery roads.

All interventions go virtually unnoticed, but the drivers are still informed immediately if they are driving on the limit. In this case a yellow warning light flashes in the instrument panel – a clear signal to adjust their driving style to the road conditions.

The arrangement featuring a permanent design mechanism has key advantages over other systems that first need to diagnose a lack of traction before activating their 4×4 all-wheel drive. The 4MATIC on the GLK will have already made use of this valuable time to transmit drive torque via the wheels to the road.

Kamm circle: the laws of physics sets limits

Just like any all-wheel-drive system, 4MATIC in the GLK has to abide by the laws of physics, a fact clearly illustrated in what is known as the “Kamm circle”. The fundamental rule is that a tyre is only able to transfer a certain level of overall force to the road surface. If a large amount of force is required in a longitudinal direction, for example when accelerating or braking, the lateral forces available are reduced. When cornering the reverse applies. In this case a particularly high degree of lateral force is needed for directional stability, whilst the longitudinal force potential is restricted. When designing the drive mechanism and fine-tuning the control systems, engineers’ skills are called upon to use this correlation in such a way that the very best possible handling is ensured in any conditions. The frictional coefficient µ describes the physical grip between the tyres and the road surface. On a dry road this figure is high (µ=0,9), and on a snow-covered road it is low (µ=0,3).

Despite the engineers’ best efforts, it is ultimately the drivers themselves who determine how safe they are on the road. They should always adapt their driving style in accordance with wintry conditions and ensure that their vehicle is suitably equipped; winter tyres are absolutely essential.

Balance of forces in the “Kamm’s Circle”

Like every other all-wheel-drive system, the 4MATIC on the GLK-Class obeys the laws of driving physics. These interrelationships are described in the “Kamm’s Circle”. A basic law of physics states that a tyre can only transfer a certain total force onto the road. If a large amount of longitudinal force is required during acceleration or braking, the available lateral force is reduced (left-hand graphic).A large amount of lateral force is required during cornering to maintain directional stability; the longitudinal force potential is thus limited (right-hand graphic). The engineers’art in designing the drive mechanism and fine-tuning the control systems is to exploit these interrelationships to ensure optimum handling characteristics whatever the conditions.

The employees at the Mercedes-Benz Technology Centre have a wealth of experience at their fingertips in this respect. The first all-wheel-drive system was created over 100 years ago, and the engineers also occupy a leading position where the fine-tuning of all-wheel-drive control systems is concerned. 4ETS celebrated its world premiere in 1997 in the first generation of the M-Class, development work having begun as far back as 1993. Mercedes-Benz thus enjoys the most extensive experience of all car manufacturers with these systems.

4ETS: integrated “automatic start-off aid” for ice and snow

When pulling away in wintry conditions the engineers use the off-road algorithms on the GLK 4MATIC to develop optimum traction. Certain road conditions are automatically detected and the interventions of the 4ETS electronic traction control system adjusted so that as much acceleration as possible can be achieved with minimal wheel slip, providing optimum directional stability in the process. This strategy also allows the vehicle to pull away under adverse conditions, such as when one side of the vehicle is on an icy slope (the so-called µ split) or both wheels on the front or rear axle have limited grip (µ-jump).

So when starting off on µ-split one side of the GLK is on snow or ice and the other on asphalt. 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 lowest coefficient of friction limits the maximum transferable driving power. If the driving power rises above the maximally transferable power, the wheels on snow or ice start to spin, and the GLK would not be able to start off. This situation is immediately registered by 4ETS and the spinning wheels are baked by precisely the right amount of pressure build-up in the wheel brakes. As the wheel with the higher coefficient of friction is now supported via the brake force of the wheel with the lower coefficient of friction, the GLK starts to move. Following the start-off process, the wheel characteristics are observed very closely and the brake pressure controlled 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 simulates, as it were, a higher coefficient of friction on the side with ice or snow, which, in an ideal case, corresponds to the coefficient of friction on the asphalt side. This produces an optimal transverse locking effect on the axle differentials, resulting in the maximum possible acceleration on µ-split.

Pulling away easily on different road surfaces (µ-split)

The adhesion between the tyres and road is considerably higher on the asphalt side than on the side of the road covered with ice. 4ETS detects this situation and prevents the wheels from spinning on the icy road by applying the brakes on specific wheels.The torque is thus balanced at the axle differentials because the same adhesion is simulated on the ice-covered side as on the asphalt side. The GLK manages to pull away.

Starting off on “µ-jump”, where one of the GLK’s axles is entirely on ice or snow and the other axle is on asphalt, is made more difficult by the large differences in coefficient of friction between the front and rear axle. On vehicles with an open longitudinal differential the axle with the lower coefficient of friction determines the transferable driving power. Whilst the multiple-disc limited-slip centre differential with its basic locking torque of 50 Newton metres does have a balancing effect, it is unable to even out these extreme differences in coefficient of friction. In these conditions, too, both wheels on the axle with the lower coefficient of friction start to spin if the driving power exceeds the maximum transferable driving power at this coefficient of friction. 4ETS helps here by immediately recognising and braking the spinning wheels. This enables the axle with the higher coefficient of friction to be supported by the braked axle with the lower coefficient of friction – and the GLK starts to move.

Pulling away easily on different road surfaces (µ-jump)

The adhesion between the tyres and road is considerably higher on the asphalt than on the road covered with ice. This situation is offset partially by the multiple-disc limited-slip centre differential, while at the same time the 4ETS detects the rear wheels spinning on the icy road and eliminates wheel spin by applying the brakes on specific wheels. The torque is balanced between the front and rear axle in the centre differential because the same adhesion is simulated on the icy road as on the asphalt. The GLK manages to pull away.

Stable conditions on wintry roads

On winding roads covered with snow and ice, vehicle stability is primarily controlled by means of engine torque regulation in the acceleration skid control system ASR. Taking into consideration the longitudinal and lateral dynamics of the vehicle continuously determined by the ESP® sensor system, the ASR control thresholds are adjusted according to the driving situation, as illustrated by the aforementioned Kamm circle. In order to prevent the vehicle from tail-skidding, on winding roads the longitudinal force must be controlled via engine torque regulation in such a way that sufficient lateral force is always available. In order to comply with this law of physics, when driving on winding roads at low frictional coefficient, vehicle stability is mainly controlled by means of intervention from ESP® and the acceleration skid control system ASR so that a sufficient reserve of lateral force is always available. Initially, only sufficient engine torque is transferred to the outer wheels to ensure the tyres can develop sufficient lateral force. If this control function cannot generate directional stability, then ESP® applies the brakes to stabilise the vehicle.

Stable handling characteristics while cornering

A tyre can basically only transfer a certain total force onto the road.If a large amount of lateral force is required to maintain directional stability on winding roads, the available longitudinal force is reduced. Conversely, limited lateral force is required when driving in a straight line. On winding roads covered with snow and ice, vehicle stability is mainly controlled by means of the ASR acceleration skid control system so that a sufficient reserve of lateral force is always available. If necessary, the ASR acceleration skid control system reduces the engine torque and, in turn, the longitudinal force to the point where the outer wheels can produce sufficient lateral force. If the ASR control function cannot generate sufficient directional stability, then ESP® applies the brakes to stabilise the vehicle.

In contrast to the control mechanisms operating when driving on winding roads, considerably more longitudinal force can be provided when accelerating in a straight line, as the tyres hardly have to transfer any lateral force. Here it is important that the tyres can work in the optimum µ-slip curve range. In order to achieve this, in this driving situation the control thresholds for the engine torque regulation are raised. The multiple-disc limited-slip centre differential simultaneously works on the transfer case, locking the powertrain longitudinally with 50 Nm. This locking factor substantially increases traction, without compromising the control systems. The final drive ratios therefore allow the 150 Nm to be transferred to the wheels.

In certain wintry conditions it may be necessary to deactivate the control systems via the “ESP® OFF”-switch. This is the case where high slip values are required at the wheels – for example to get out of deep snow with or without snow chains. When braking drivers still have full support at their disposal, even in ESP OFF mode. Once back on a road surface with a normal covering of snow, drivers should then activate the control systems again.