stability.gif (2423 bytes)
Taking Safety to the Next Level
by Kim Wolfkill
A new generation of stability control systems helps drivers maintain command of their vehicles even before they realize they've lost it.
understeer.gif (13301 bytes)

In an understeer situation ("plowing"), the stability system gently brakes the inside rear wheel to help the front end follow the path of the curve.

In an oversteer situation ("fishtailing"), the stability system gently brakes the outside front wheel to bring the rear end back in line with the driver’s intended path.

Halfway through a long, rain-soaked curve, a deer pops out of the woods, leaving you very few options and little time to react. Without thinking you jerk the wheel to the left, then right, hoping for the best. In most cases, even in the hands of an experienced driver, the car is off the road and into the woods before you realize what has happened. You've put your life at risk and the deer lopes away.

It's a scenario on which automakers have focused a lot of attention over the past few years, and the resulting solutions—built on and integrating key features of anti-lock-brake systems (ABS) and traction control technologies—now have made it to market.

Introducing Stability Control
"Think of stability control as an invisible copilot who intuitively knows when you've lost control of your car, and then comes to your aid," says Fred Heiler of Mercedes-Benz when talking about his company's Electronic Stability Program (ESP). Available as standard or optional equipment on many new high-end cars, stability control systems with names like StabiliTrak, Dynamic Stability Control, Stability Management, and Vehicle Skid Control are the latest advancement in vehicle safety.

So how does stability control differ from the traction control systems currently on the market? Essentially, traction control acts on a vehicle's drive wheels to prevent unwanted wheel spin under acceleration. While this helps in low-traction situations such as snow or rain, traction control's ability to assist in more extreme emergency situations is limited. A stability control system, on the other hand, goes one step further by actually detecting when a driver has lost some degree of control. It then automatically stabilizes the vehicle to help the driver regain control.

The "it" of this scenario is a complex system of sensors and microprocessors that continually monitors the vehicle for any signs of instability. Once detected (usually in the form of a slide or skid), the system automatically applies selective braking to specific wheels thereby stabilizing the vehicle. This split-second intervention often happens so quickly it's over before drivers even realize they were in danger of losing control.

Cadillac's manager of vehicle performance, Scott Badnoch, told Microsoft CarPoint, "These systems are designed to deliver transparent intervention the moment the situation becomes unstable. By gently stabilizing the car at the critical moment, control is returned to the driver with minimal fuss and alarm."

What Makes it Tick?
All the enhanced stability systems currently being marketed by manufacturers such as BMW, Cadillac, Chevrolet, Lexus, and Mercedes-Benz approach vehicle stability in similar ways.

The heart of all these systems is a central processor that takes information from a number of sensors, and then determines whether the car is in a stable or unstable state. By combining the data from ABS sensors (for wheel speed), steering angle sensors, yaw sensors (measuring the amount a car fishtails, or rotates around its vertical center axis), and lateral force sensors (measuring the amount of sideways g-force generated by the car), the central processing unit can actually detect when a vehicle is behaving in a way contrary to how the driver intends.

If the processor does detect instability such as a slide produced by a sudden swerve, it automatically applies light brake pressure to a select wheel (or wheels) to maintain or restore control. On some systems such as the BMW Dynamic Stability Control III and the Porsche Stability Management system, engine torque is also automatically decreased to aid in the stabilizing process.

The Stability System at Work
The most common types of slides are referred to as understeer and oversteer. In an understeer situation, the front of the car plows toward the outside of a turn without following the curve of the turn. When the stability control system detects understeer, it applies light brake pressure to the inside rear wheel (see first illustration, above). This helps "tug" the front of the car back onto the intended line.

In an oversteer situation, the rear of the car fishtails toward the outside of a turn, increasing the chance of a spin. To counteract such a situation, the stability system applies braking to the outside front wheel, bringing the rear end back in line (see second illustration, above).

On most vehicles, a warning light on the dashboard illuminates when the system is operating. In the case of the Lexus Vehicle Skid Control system, a subtle audio signal also sounds .

When Does it Help?
Like the safety systems that preceded it, stability control is designed to step in when human input is incapable of effectively controlling the vehicle. In most cases, critical situations are the result of human error in the first place—driving too quickly for conditions, inattention, misjudgment or simply panicking in an emergency situation.

In these situations, everyone can benefit from a safety system that occasionally helps regain vehicle stability, while never taking full control out of the driver's hands.

As Scott Badnoch of Cadillac is quick to point out, "The StabiliTrak system on the Cadillac is designed to improve a driver's ability to avoid accidents, but not intrude on the overall driving experience."

Not since the introduction of ABS has a safety advancement added such a high level of driving security. When used with ABS and traction control, stability control systems significantly increase a driver's chances of recovering from potentially dangerous situations.

But no matter how advanced the safety aid, never forget that the ultimate fate of a vehicle and its occupants remains in the hands of the driver. No safety system should ever be expected to protect unconditionally. So while the latest generation of stability control systems offer drivers increased protection from both themselves and the unexpected, they can never overcome poor judgement or the laws of physics.


Home Dealer Documents Accomplishments Links ABS Brakes Association Profile Associate Members Why Buy From a Dealer Buy with confidence Buying Privately Car Seat Info Code of Ethics Food For Thought Hot Wheels How We Began Ready For Winter? Take Advantage Stability