A ot a dry road there is seldom a lack of traction. The driving force of the vehicle is transmitted to the road via the wheels without any problems. This essentially depends on two factors: the weight that is placed on a driven wheel and the coefficient of friction, which describes the current state of the two friction partners road and tire.
This is on a good, dry asphalt road Value around 1, on black ice it can drop below 0.1. This means that the tire can only transmit a fraction of its power on ice, so it spins much earlier. Four driven wheels can theoretically bring twice as much power to the road as only two, so they have a clear advantage, especially in low-friction conditions.
But what happens now with the different drive variants on slippery roads? The rear-wheel drive car with a front engine performs worst. Under load, it oversteers when cornering because the driven wheels have little weight, so they cannot exert any great force on the road. At the same time, the drive wheels spin without stopping. The electronic helpers ESP and ASR try to prevent this.
The rare rear motor and drive variant is much better. The VW Beetle is a classic example of this. With its good-natured, high-torque boxer engine, which sits above the driven axle, it brings good traction. Although the heating is a mean disaster in winter, the Beetle is simply fun on a slippery road surface.
The most common type of drive is the combination of front-engine and front-wheel drive, which is quite suitable for winter because of the heavy one Motor loads the drive axle and thus mostly ensures good propulsion. If you accelerate too much in a snowy bend, the car gently pushes the front wheels outwards, causing understeer. Here, too, the ESP intervenes, bringing the car back on track, provided the physical limit has not yet been exceeded.
Advantages of all-wheel drive
If the advantages of the two types of drive are now combined, we end up with all-wheel drive. But he also needs more helpers to guarantee progress under difficult conditions. If, for example, the right wheels are in the icy snow of the roadside and the other two are on dry tar, the desperate driver will still get stuck because the differentials only accept the power according to the motto 'We take the path of least resistance'pass the wheels spinning in the snow. This can easily be remedied by locking the differentials, so the power is transmitted evenly to all wheels.
The easiest way is a 4WD over the A second drive axle that can be mechanically activated via a dog clutch A solution that is dying out in the age of the electronification of the car. Modern systems, controlled by sensors and computers, can, as soon as slip is detected on the drive axle, quickly involve the second axle via Visco or Haldex coupling in the drive of the vehicle.
In what ratio the drive force to the is transferred to both axes is defined by the vehicle manufacturer. The axle, which can transmit more propulsion in the respective situation, also receives more torque. Ideally, the distribution is so flexible that each wheel is supplied with just as much power as it can currently bring on the road.
All-wheel drive is not all-wheel drive
However, it can the inexpensive 4 x 4 cannot, they only involve the second axis in the drive if necessary. This has advantages over a conventional drive, but only becomes a usable all-wheel drive when all four wheels can be braked independently of each other, which is technically not a problem in the age of ESP and ASR. With the trick of the simulated differential lock, the function of a conventional differential can be outwitted, and the power is actually allocated to the wheels, which can also transmit it. This solution is at the expense of the brakes, which wear out faster with frequent all-wheel drive. Nevertheless, the gain in driving safety is enormous. And not only on snowy terrain, but also on wet roads.
If an all-wheel drive vehicle drives too briskly through a snowy curve, the wheel sensors register any slip. Torque is immediately withdrawn from the wheel looking for grip and allocated to the second axle via the electromechanical multi-plate clutch. This enables the affected wheel to transfer side forces again. However, this line is thin. If you drive too fast into a curve, you will fly off despite all 4WD aids, because the physics cannot be outwitted and the number of driven axles has no influence on the maximum transferable lateral forces.
Next step to perfect all-wheel drive are three independently lockable differentials between the wheels and axles. In extreme cases, all three differentials are locked and each wheel receives 25 percent of the available power. Such a rustic system with transfer case and switchable reduction makes the Mercedes G-model an extremely potent climber. The configuration is ideal for difficult terrain, but not for everyday use.
Here, driving stability and dynamics are more important. And modern all-wheel drive vehicles like VW Amarok or Audi Q7 can do that much better with their complex all-wheel drive. They have a self-locking center differential that allows variable torque distribution between the axles. In normal operation, the front /rear distribution is 40 to 60 percent; if an axle slips, it can vary between 20 and 80 percent. Depending on the situation, each wheel can be supplied with a different torque. This takes place via the respective brake intervention or, much better, via two further actively regulating differentials on the axles. These can vary the torque allocation to the wheels of an axle between zero and 100 percent. With this solution, the ideal of completely free power distribution to all wheels is possible.
All-wheel drive increases consumption
In addition to the advantages on low-friction routes and non-routes, all-wheel drive also has system-related disadvantages. It is expensive and the additional parts add weight and increased friction. This is noticeable at the gas station. Even a modern SUV that only engages its 4 x 4 when needed needs at least ten percent more fuel than the variant without all-wheel drive. The high technical effort for the all-wheel drive has two main goals: better traction and greater driving stability. The braking distance is by no means shorter, no matter how smooth the surface is. After all, every car with four wheels brakes.
In order to actually be able to transfer the better traction of the 4 x 4, very good winter tires are essential. The fine profiles interlock with the subsurface and thus increase the coefficient of friction, so more power can be transmitted. This of course also applies to front and rear-wheel drive cars. Equipped with the best winter tires, they are on a par with all-wheel drive riders until the ice becomes too smooth, the snow too deep or the mountain too steep. It is gratifying that the sensible 4 x 4 drive is now available in almost every vehicle class.