Four wins - all-wheel drive

Four driven wheels provide more traction, guaranteeing progress on slippery surfaces or increasing safety on dry roads. How it works

Front-wheel or rear-wheel drive - for many car buyers that is secondary and at most a financial question. For some, however, the drive concept is almost a philosophy and plays a decisive role: Just remember the screams of the rear-wheel drive-spoiled BMW disciples when the Munich-based carmaker announced a few years ago that the smaller models around the 1 Series will be the future Drive front wheels. And who can't decide? It is best to use all-wheel drive right away.

More security and stability

The advantage of all-wheel or 4 × 4 drive can be described very briefly: More traction. This helps on the one hand in the field, if it gets slippery under one or two wheels, the other two can push the car even further and often literally pull the cart out of the dirt. On the other hand, the all-wheel drive ensures more grip even on dry roads and, especially in curves, more safety and stability. In order to understand this, one has to deal a little with physics - all lexicon enthusiasts should be given the keyword "Kammscher Kreis": A tire must transmit both driving forces forwards and cornering forces. Both are not infinitely possible, the sum of the two forces is limited. In other words: If two wheels take over the complete drive, they can withstand less lateral forces in the curve at the same time. By distributing the drive torque to four wheels, each tire can take care of cornering a little more. This reduces understeer in front-wheel drive models and oversteer in rear-wheel drive vehicles. By the way: Even if you have more grip on snow and ice with all-wheel drive, the technology only helps when driving, but not when braking.

But how does the power get to all four wheels? With combustion engines there are traditionally two ways: On the one hand, the power can be sent from the engine to a so-called differential, which then divides the drive power between both axles. In this case, one speaks of permanent all-wheel drive because the power is always sent to all four wheels. However, the division between the front and rear axles can be varied depending on the driving situation.

Only when it is necessary

As an alternative to this, the power can only be delivered to one of the two axes as standard. The second is only switched on via a clutch when required. This can either be controlled by the driver - at the push of a button or via an extra gear lever and with the help of a dog clutch - or it can be controlled automatically by the car's control systems. As soon as the technology detects slip on the actual drive axle, power is sent to the other. In the latter case, multi-plate clutches are often used, in which it is also possible to regulate how much power is redistributed mechanically or electronically.

Interesting: The so-called switchable all-wheel drive is often marketed by the manufacturers as "permanent", but the term "permanently available" would be more correct. Because on a dry road, these cars generally transfer 100 percent of their power to one axle. This is intended to eliminate a disadvantage of all-wheel drive: the higher fuel consumption.

Another possibility of all-wheel drive is of course to use two sources of power. However, two combustion engines are rarely found, so far they have only been found in studies and one-offs. It looks different with electric cars. E-motors are relatively simple and inexpensive, so that many manufacturers now use four-wheel drive for their electric vehicles with their own drive per axle. And petrol or diesel can also be easily upgraded to 4 × 4 models: the combustion engine drives one axle and the electric motor pushes the other.

 

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