Un rezumat al sistemelor 4x4, by Mercedes

BMW X3 - x-Drive


The BMW X3 certainly also has a lengthways built-in drive train, but does without a permanent all- wheel drive: the drive to the front wheels is only engaged via an electrically actuated multi-disc clutch with a continuous adjustment from 0% front/100% rear to 50% front/50% rear as required. The develop- ers had the high driving dynamics especially in mind. This is matched by the very stiff springing that already limits the traction over light terrain and so necessitates braking action from the all-wheel drive system.
> The xDrive can certainly simulate a central differential lock for a short period of time but a mid-differential brake is lacking.
> The transition from rear to all-wheel drive causes unaccus- tomed under/oversteer. When manoeuvring, the X3 only uses rear-wheel drive in order to avoid stressing the transmission.
> The friction clutch is subject to a certain wear due to its de- sign - in contrast to the differential in the GLK-Class.

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Land Rover Freelander, Volvo XC60 and Volkswagen Tiguan require an (uneconomical) torque redirec- tion to the rear axle because of their transversely installed engines. This is why they nearly always drive using the front wheels. An active, electronically controlled multi-disc clutch from the Swedish supplier, Haldex, connects the rear axle to the front drive train only if required. "Fast computers, such as those in the Tiguan, can certainly simulate a ... four-wheel drive, but cannot react quickly enough when un- der pressure" (Off Road/D, 02/08).

> Only 10 % of the torque gets to the rear. The multi-disc clutch responds to slip on the front axle and then involves the rear axle up to 50 % in the propulsion. When manoeuvring, only the front wheels are driven in order to avoid stressing the transmission.
> As the Haldex clutch normally distributes 90 % of the torque to the front, the typical disadvantages of front-wheel drive are retained: steering effects and the tendency to understeer.

sa inteleg ca in curbe, unde ai avea mai mare nevoie de tractiune pe toate patru roti, te lasa doar in doua?