Taken straight from intenret page, so no credit claimed
Does the concept of increasing driver control by removing driver control seem a bit flawed? How about the idea of letting a vehicle, rather than its operator, decide just how far it will turn in a given corner? Sure they sound crazy, but it's these two philosophies that are behind the growing number of yaw control systems appearing on today's high-end cars. Systems like the BMW Dynamic Stability Control, the Lexus Vehicle Skid Control, and the Cadillac StabiliTrak.
If you're thinking that "yaw control" sounds more like an aviation term than an automotive stability system, you are correct. Since the beginning of man's flying days (which actually started in 1903 at a place called Kill Devil Hills, North Carolina not Kitty Hawk), the term yaw has referred to the movement of an airplane rotating from left to right or right to left. An extreme amount of yaw in one direction is called a flat spin and is not a good thing in either aviation or automobile terms (it's what led to the crash that killed poor "Goose" in the movie Top Gun).
While the factors used to control an airplane's yaw rate are highly sophisticated and involve a number of mechanical, computer, and hydraulic systems, the yaw control system on cars like the Chevrolet Corvette or Mercedes S600 sedan are fairly straightforward. By using sensors to determine the angle of a vehicle's front wheels and referencing that to the angle the vehicle is traveling in, the system decides whether or not to intervene. If the difference is too great, meaning that the car is no longer following the front wheels, it redirects the vehicle's angle of travel by applying braking force to one or more of the wheels.
The idea behind all this mechanical wizardry is the same one that brought us antilock brakes. Just as using too much braking force can actually increase a vehicle's stopping distance, turning the front wheels too far while rounding a corner can actually cause a car to slide further to the outside of said corner. If this corner happens to be on a mountain road with no guardrail, yaw control could potentially keep a car that's entered the corner "too hot" from sliding over the edge. And, as with antilock brakes, the effectiveness of yaw control goes up as the road's coefficient of friction goes down (i.e. it's raining or snowing).
It sounds great on paper (or computer monitor), but does it work in the real world? Well, these systems, used by various manufacturers, definitely operate properly. By applying corrective braking force when a vehicle begins to skid, they make sliding a yaw-controlled car nearly impossible. Does it actually reduce the chances of getting into an accident? Again, antilock brakes provide the best comparison so the answer, unfortunately, is maybe. If you don't regularly drive "too fast for conditions" than neither ABS nor yaw control should ever come into play.
But, as we all know, unexpected events involving deer, pedestrians, and the occasional falling rock can thrust even the most prudent drivers into a precarious situation. It's these times when a little technology, used properly, can go a long way. Since many of the problems with ABS have been tracked to improper use by uneducated drivers, it's still too early to say whether or not antilock brakes are effective. Yaw control is still in its infancy with just BMW, General Motors, Lexus, and Mercedes offering it on their high-end models. Only time and insurance studies can decide how effective it is at reducing accidents.
