1.7 Four Wheel Drive

1.7.1 Advantages and disadvantages

In four-wheel drives, either all the wheels of a passenger car or commercial vehicle are continuously - in other words permanently - driven, or one of the two axles is always linked to the engine and the other can be selected manually or automatically. This is made possible by what is known as the " center differential lock ". If a middle differential is used to distribute the driving torque between the front and rear axles, the torque distribution can be established on  the basis of the axle-load ratios, the design philosophy of the vehicle and the desired handling characteristics. That is why Audi choose a 50%:50% distribution for the V8 Quattro and Mercedes-Benz transmits only 35% of the torque to the front axle and as much as 65% to the rear axle in vehicles belonging to the E class.

1.7.1 Advantages and disadvantages

Advantages :

• better traction on surfaces in all road conditions, especially in wet and wintry weather;
• an increase in the drive-off and climbing capacity regardless of load;
• better acceleration in low gear, especially with high engine performance;
• reduced sensitivity to side wind;
• stability reserves when driving on slush and compacted snow tracks;
• better aquaplaning behavior

Disadvantages :

• acquisition costs;
• not always clear cornering behavior;

To transmit the available engine torque to all four wheels, interaxle differential (such as cone, planet or  Torsen differentials), which are manually or automatically lockable, or clutches (such as sprag, multi-disc or visco clutches) must be installed on the propsshaft between the front and rear alxes. Differentials must be presented on the both drive  axles. However, on roads with differential coefficients of friction on the left and right wheels, known as 'u-split', and with traditional differential, each driven axle can, at most, transmit double the propulsion force of the wheels running on the side with the lower coefficient of friction (u-low).

P.80