The Different Differential

Pig, pumpkin, rear end, differential – call it what you like, but even some serious gearheads don’t know how the things work, especially when you throw in qualifiers like “limited-slip,” “posi-traction,” and “locker.”
For a good primer on the traditional and the computer-assisted what’s to come, check out the NYT story on how BMW made the differential a steering booster in its new x-drive all-wheel drive system. Clue: it’s got to do with “yaw torque.”
The traditional differential:

“Its array of gears and shafts makes it possible for the left and right wheels to spin at different speeds even though they are geared together on the same axle.”

The new BMW:

“Dynamic Performance Control pitches in once the hardest-working tires have reached their traction limits. This added yaw torque — essentially an extra nudge to help the vehicle complete the cornering maneuver — is generated at the rear. By forcing the outside rear tire to push forward while the inside-rear tire pulls back a bit, a helpful turning force is generated about the imaginary vertical axis.”

That can’t be cheap.