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All-wheel drive: eight 4x4 systems in a large comparison test

Achim Hartmann
Large 4x4 system comparison
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It's actually unbelievable. The Jeep Wrangler, the archetype of the overwhelming all-rounder, is stuck in the deep snow into which its overly optimistic driver has steered it. What should also happen there? After all, as the top Rubicon model, the Jeep has the most uncompromising of all four-wheel drive systems: through-drive to all four wheels based solely on gearwheels creates a firm connection between the front and rear axles, and there are 100 percent lockable locks in both differentials. The effect is visible: all wheels turn. But none of them find enough grip to move the jeep even an inch. Tow rope please.

The reason for failure in a seemingly harmless situation is the tires. The Wrangler has stubby universal tires, useful for the road, good for the terrain. But despite the M&S marking, they are as suitable for snow as disco slippers. Which already answers the first question about the best all-wheel drive: All the genius of development, which manifests itself in the sensitive allocation of torque to the wheels that can be carried out in milliseconds, is wasted effort if the tires cannot do anything with the drive force they are given.

Locking principle

The best all-wheel drive is the one with the best tires. In winter there are finely laminated winter tires with the snowflake symbol. Basta. Only when there is agreement can the various all-wheel drive systems show their advantages. The technical diversity is great. There is the simple viscous coupling, the silicone oil filling of which becomes viscous when there are speed differences between the front and rear axles and thus creates the flow of power. Porsche used to have something like this, but this technology was more of a driving stabilizing element than a real traction aid. In this comparison, the Fiat Panda uses the advantage of the viscous coupling of being light and relatively inexpensive. Simply rigidly coupled all-wheel drives, which can only be used on loose ground because of the tension that occurs on firm ground, remain the brutal solution for real off-road vehicles such as the jeep. From a traction point of view, they leave nothing to be desired, but they are questionable for improving driving safety.

The classic all-wheel drive, once brought into large-scale production by Audi with the Quattro concept, uses a differential between the front and rear axles to reduce the speed differencesCan compensate for cornering. The basic force distribution depends on the design, but is varied depending on the slip conditions. If one axis cannot transmit any torque at all, nothing will get through to the other. Unless the center differential can be locked. So-called Torsen differentials, as in Audi and Lexus, do this automatically; Subaru uses a viscous lock that is based on the same principle as a viscous coupling. Mercedes uses a multi-disc lock.

No advantage due to complex drives

More recent developments dispense with the differential and offer a multi-disc clutch that is more or less closed depending on the driving situation. BMW relies on this design, which is electronically networked with the other driving safety systems and promises an individual, lightning-fast reaction to different driving conditions. The Land Rover uses similar technology, but the driver can choose between different driving programs. What do you get out of it when you are out and about in the Alps in wintry conditions?

Little when it comes to traction-determined progress. The complex drives have no advantages - wherever the Mercedes with its expensive 4matic goes, the little panda goes too. He often struggles with spinning wheels, but he does not get stuck. With BMW and Land Rover, the driver does not notice that the clutches are working, the power on all wheels is always there - just like with the systems with a central differential or the rigid Jeep through drive. When it comes to driving safety in bends, differences become apparent, which are primarily explained by the design of the car.

The Jeep comes through

The Audi and the BMW, although technically different, have a similar character: They emphasize the sportiness and allow the rear to move freely until it is slowed down by the ESP. Mercedes remains, that is the company philosophy, on the safe side. The aim is to offer the driver consistent driving behavior under all circumstances. That succeeded: the C-Class remained completely unspectacular in the driving tests. It is not for fans of the drift angle, but ultimately not slower. The Land Rover shows a comparable reliability of driving behavior, but as a long-legged off-road vehicle naturally cannot keep up with the speed of the sedans.

The Lexus follows a similar line of clearly defined driving characteristics, but is designed to be more understeer. Together with its size and weight, this results in limited handiness. Exactly the opposite is true for the panda. Sometimes he pushes over the front wheels, sometimes his rear breaks out. But since it is very handy due to its size, such excursions to the limit are easy to manage. The driver can control the Subaru with its even power distribution viaBring gas and steering to understeer or oversteer This is all-wheel drive according to the old fathers custom, enjoyable for clever driving dynamics, on the other hand a rich field of activity for the ESP. And the jeep? In terms of driving dynamics it is a failure, but it gets through everywhere. If he has the right tires on it. Actually unbelievable.

Mercedes remains on the safe side

A lamella lock is used for starting behavior, the traction control is provided by the electronically controlled braking intervention on individual wheels.

The result is convincing. The Mercedes shows no traction weaknesses and offers a clearly defined, slightly understeering self-steering behavior when cornering.

CONCLUSION : One of the best all-wheel drive systems, the presence of which is only communicated by the good
traction . In extreme situations, the usual driving behavior remains.

High weight

What comes out under snow conditions, however, is not entirely convincing, what is primarily about the stately Weight of the Lexus is.

In the cornering area, it has a strong tendency to push over the front wheels and is dependent on emphatic interventions by the ESP that hinder the dynamics.

CONCLUSION : The suspension tuning of the Lexus shows there is a need for improvement.
Despite its all-wheel drive, it is comparatively unwieldy on snow.

Sophisticated control electronics

The electronics oversee a multi-plate clutch that Torque distributed between the front and rear axles.

In addition, in the ingenious network of electronics, there is also the locking effect in the transverse direction through targeted brake intervention. The result is convincing.

The Freelander persistently digs through deep snow and, thanks to the well-coordinated ESP, remains easily manageable even in critical situations.

CONCLUSION : Although the Freelander has all-terrain vehicle Genes become noticeable, it is manageable even on snow-covered roads and, with its neutral design, is very safe to drive.

The core is still a central differential that works according to the Torsen principle with a self-locking function.

But the force distribution is no longer even. In the basic configuration, the differential sends 60 percent of the torque to the rear axle.

The reduced front axle load on the A4 also results in a very agile, sporty driving behavior with, in extreme cases, easily controllable oversteer.

The reduced front axle load on the A4 also results in a very agile, sporty drive Driving behavior with oversteer that is easily controllable in extreme cases.

CONCLUSION: The rear-biased 40:60 basic configuration of the all-wheel drive is a challenge for the stability control, but it is mastered without any problems.

As a traction aidit works well, but the driving behavior clearly reveals the changing power distribution.

The understeer typical of a front-wheel drive vehicle can turn into sudden oversteer.

Handy and manageable

Since the panda is small and light, controllability in critical situations is not a problem.

CONCLUSION : The Fiat does well on snow, also thanks to good handiness and precise steering. In extreme cases, oversteer remains easily manageable.

That is why there is no center differential with the so-called x-drive, but a controlled multi-plate clutch, which, however, transfers part of the torque to the front axle even when driving straight ahead.

In general, the design is rear-biased in order to guarantee BMW-adequate handling.

The X3 is nimble and precise in steering, but not easy to assess at the limit, because the self-steering behavior depends on the amount of force used Can be understeer and sometimes oversteer.

CONCLUSION : The X3 shows the well-known agile BMW driving behavior on snow. The electronic x-Drive doesn’t drive any better than a conventional system.

Very good traction

The 50: 50 distribution is classic technology: entire generations of Audi models distribute the power evenly between the front and rear axles.

However, the locking effect with a visco unit is Subaru-specific. This achieves excellent traction on snow, and the power can be optimized with a shorter gear ratio that can be activated.

Less good: the insufficiently defined self-steering behavior.

CONCLUSION : The Subaru shows changeable driving behavior. Depending on the steering angle and the use of power, it pushes over the front wheels or swings out with the rear.

The use of four-wheel drive is therefore limited to loose ground, because it is on firm road due to the different speeds of the front and the rear axle leads to tension that puts a lot of stress on the material.

In the snow, this is compensated for by the wheel slip. The handling is to be enjoyed with caution with such a system.

CONCLUSION: Rigid through drive and two-stage transfer case are the best off-road ingredients. Switching between understeer and oversteer is difficult to calculate on the road.

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