Under the Hood: What is Stability Control?

Under the Hood: What is Stability Control?

AutoGuide’s last installment of “Under the Hood” investigated traction control; today it’s time to look at another safety feature. This one has nothing to do with gripping the road and everything to do with keeping your vehicle shiny-side up.

Stability-Control-01.jpgStability control is designed to make a vehicle go where the driver wants, and to prevent it from tipping, flipping, spinning or rolling, all a great ways to ruin an otherwise good day.

To understand this advanced, computer-controlled technology we contacted the experts. Thankfully Chris Harrison, manager of vehicle development and testing for electronic brake systems at Continental North America lent his expertise.

“I think the big basis is we have to rely on what the driver is telling us on where they want the vehicle to go,” Harrison said, noting they should be in command at all times. Stability control is only there to help, not take over, because chances are drivers won’t deliberately be aiming for oncoming traffic or cement walls. They generally know best.

Stability-Control-03.jpgHarrison said, “we do that by applying a brake force. We can control braking at every wheel – making decisions every 10 milliseconds.” To use a well-worn cliché, that’s faster than the blink of an eye – anywhere from 10 to 40 times faster in fact.

Differential braking is at the heart of stability control, which goes by countless different names. Every automaker brands it something different. For instance, General Motors calls it StabiliTrak, while Audi’s system is known as Electronic Stabilization Program, ESP for short.

No matter the name, they’re all trying to do the same thing, that is control oversteer and understeer. Oversteer is a condition where the rear tires of a vehicle lose traction and slide sideways, this causes the vehicle to steer more than the driver intends. Alternately, understeer is a situation where the front wheels lose grip and a vehicle plows wide through a turn, steering less than the driver commands.

Vehicles equipped with stability control feature a battery of electronic sensors. These systems are constantly measuring things like lateral and longitudinal acceleration, wheel speed and steering angle. “Some vehicles have a roll-rate sensor in them that we can use” Harrison said.

“We can [also] use traction control coupled with all-wheel drive,” he said. Even torque-vectoring systems like Super Handling All-Wheel Drive found on the Acura TL can be commandeered by a stability-control system. This technological collaboration is all in the name of safety.

SEE ALSO: What is Traction Control?

But the driver is still in control, and he or she can oftentimes turn these systems off, or so they think. Depending on the automaker and vehicle, stability control can either be completely defeated or remain on, but with a higher threshold of intervention.

Additionally, Harrison said a lot of times there’s something called an “angel mode,” where stability control will turn itself back on when the vehicle reaches a certain speed. Also, “if you ever turn your car off and then back on it defaults to on,” Harrison said. This is the sneaky side of safety.

But what fun is all that? Sometimes wheel spin and oversteer are desirable things. How boring would a Camaro ZL1 be if you couldn’t hang the back-end out on a race track with just a tap of your tootsies?

Stability-Control-09.jpg“Sports cars do have ‘off’ modes for track driving,” Harrison said, as they should, but overpowered two-doors aren’t the only exceptions to the stability-control rule. “In the [Ford] Explorer for instance they have the terrain switch, which changes how we react,” he said.

In certain off-road situations wheel slippage and body movements that would normally be prevented by the stability control are important, such as when slogging through mud, where maintaining momentum means the difference between getting stuck and getting home.

As such, Harrison said, “people should know not to just turn the system off when they get in the car.” It’s there for a reason. He also said, “it’s really gratifying when somebody comes back and says stability control saved my life,” adding “[it’s] a big motivational factor for the job… [it] makes the long hours worth it.”

Stability-Control-11.jpgSo how effective is stability control? The National Highway Traffic Safety Administration (NHTSA) estimates that if the nation’s entire vehicle fleet were equipped with the technology up to 8,200 lives could be spared every year. But given how many older cars are on the road the adoption rate will never hit 100 percent. Additionally, NHTSA estimates that ESC alone saved more than 2,200 lives between 2008 and 2010.

NHTSA mandated ESC on all new passenger vehicles sold after September 1, 2011, but most manufacturers beat that deadline by years.

GALLERY: Stability Control


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