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      01-05-2013, 10:03 PM   #58
Private First Class

Drives: 2013 328i xDrive Luxury
Join Date: Nov 2012
Location: near Calgary

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Originally Posted by Elk View Post
Not really.

The marketing sources you cite are describing what the car does when the tires are actually slipping on the pavement from too much power. This has nothing to do with the fundamental handling balance of the car; any car will exhibit understeer or oversteer if the limits if adhesion are exceeded on that end of the car.
That's what understeer is - the tires slipping on the pavement. If they're not slipping, it's not understeering; it's just steering. When traction is lost, the system detects the difference in the rotational speed of the wheels and redirects power accordingly.

Originally Posted by Elk View Post
Front wheel drive cars exhibit tremendous understeer as a fundamental handling characteristic, as well as torque steer upon throttle application. This does not mean the front wheels are spinning. AWD cars exhibit precisely the same characteristics of a FWD car to the extent the front wheels are powering the car.
By your logic, an AWD vehicle would thus exhibit a percentage of understeer based on the proportion of their front wheel drive power, and a percentage of understeer resistance based on their rear-wheel drive power? And the net effect of this is what? A hole in the space-time continuum?

AWD cars still have the rear wheels pushing the back of the car, which is fundamentally different from a FWD vehicle. So no, an AWD car does not understeer in proportion to the power delivered to the front wheels as your argument suggests. This is fundamentally and physically false.

Originally Posted by Elk View Post
Also, keep in mind the front wheel slipping under power understeer being addressed by xDrive is directly caused by the xDrive system itself! This form of understeer will never occur with RWD so there is nothing to "fix.". This is yet another example of xDrive understeer.
The "form" of understeer is irrelevant. If the RWD vehicles when pushed exhibit an understeer of say 1x, and the AWD vehicles under the same conditions exhibit an understeer of say 1.2x, the "form" of understeer is of no consequence. Understeer is understeer, and the RWD vehicles are certainly not immune. The AWD, under certain conditions (and when pressed very hard), can have slightly more understeer. That's it.

That's why I stated that the xDrive system could produce results that were "quite similar", not identical. I clearly stated that under a limited number of circumstances, the AWD had a higher propensity towards understeer. However, the BMW system does a very good job of managing this phenomenon when the vehicle is pushed hard, and compensates well.

And when the vehicle is not being driven past 99%, it's entirely a non-issue.

So what's the end result? Maybe a tick higher lap times with the RWD, but in every-day usage, you'd be picking nits.