Quote:
Originally Posted by walile
It is not entirely correct to call the xDrive a "reactive" system. While the xDrive system reacts and redistributes power according to the information from the sensors and driver inputs, it is technically an active system.
This is how it is defined. With an active AWD system or any active differential, torque delivery can be varied at will usually via a computer controlled clutch. So, Haldex based VW/Audi will be considered as an active system. The new active rear diff in the F10 M5 and C7 Corvette are other examples.
The Torsen based VW/Audi system or other viscous coupling based systems on the other hand are considered as passive. Because there are no means to actively control torque distribution via computer programming. Power distribution is varied when wheel slippage occurs.
Of course with Audi you can also have a passive Torsen center diff coupled with an active rear diff.
While you go look for mentions of torque vectoring on the f30, check out this video on xDrive as well. Both the blonde and brunette cute. 
http://www.bmwusa.com/Standard/Conte...ng/xDrive.aspx |
I suppose your definition of active is probably better, whereas mine is more in regards to how the torque split is optimized. In the case of xdrive you have an electronic control system, which filters sensor measurements in conjunction with driver inputs and uses a plant model to calculate actuator inputs, which in this case goes to the clutch pack. Thus, it is a combination of a feed-forward and feedback control system. The actuator input commands are limited with respect to dynamic response to reject noise from the sensors. You can generally sense this bandwidth limitation with xdrive/haldex, where the car powertrain seems to "change" once you start to hit a different surface.
On the other hand, a torque sensing differential, ie conventional limited slip differential, uses a mechanical gear set to optimize torque appropriation. There is no limitation on the actuator bandwidth other than the physics of the coupling mechanism (viscosity for example). These systems will provide optimum torque distribution in terms of maximum traction. On the other hand, one cannot enforce constraints such as: force all torque to the rear wheels, which might reduce understeer in certain instances.
Side - I don't think either the M5 nor C7 have active rear differentials. Only Audi, Lancer Evo, Acura SH-AWD, Nissan Nismo stuff and BMW x5m-x6m-x6 as far as I can tell