Vehicle Stability Control Systems
by Jim Kerr

First, there was anti-lock braking. Then came traction control systems. Stability control is the third step in electronic systems that help us control our vehicles. Almost all automobile manufacturers offer a stability control system on select models. Most of them are on cars, although trucks are quickly adding stability control to the option list.

While all systems are similar in concept, one system has important market significance. AdvanceTrac, the stability control option on the 2001 Ford Focus is the first system offered on an economy car, and it brings vehicle stability controls within the financial grasp of the majority of the buying public. AdvanceTrac is available on two of the Focus models, the economical and sporty ZX3 three-door, and the top of the line ZTS four-door sedan.

AdvanceTrac came from Ford's involvement with Formula One racing. Although active vehicle controls like AdvanceTrac are no longer used in F1 racing, their benefits are very real in everyday driving. Ford first offered AdvanceTrac in the Lincoln LS. This system, on the Focus, brings a new level of safety to small cars.

Stability control systems are especially beneficial on wet, snowy, or icy conditions, although they do offer handling benefits during emergency maneuvers on dry pavement. The design intent of stability control is to keep the vehicle going in the direction the driver is steering the car. To do this, the brakes are applied on one wheel to help steer the car in the correct direction. For example, if poor traction causes the front end of the car to slip sideways when you are going around a corner, the computer will apply the wheel brake on the inside of the corner causing the car to turn and slow down. If the back end of the car slips sideways, the brake on the outside of the corner is applied to bring the car back into line. The system works when the car starts to slide on a straight road the same as it does when turning corners.

Not all stability control systems feel the same. The difference is mainly in the computer programming. I find systems on Mercedes and Lexus cars react aggressively by reducing engine power and vehicle speed at the first instance of vehicle instability. Their emphasis is on always keeping the car in total control. Other systems, such as the Subaru Outback Vehicle Dynamic Control system, allow the vehicle to move around a bit for a more sporty feel, but as soon as the movement is too much, control is smoothly engaged. The Subaru system operation is my personal favourite. Most systems, including AdvanceTrac, fall somewhere between these two. Control is smooth and seamless.

Several inputs are required for stability control operation. The wheel speed sensors for the anti-lock brakes are used to detect wheel spin. A steering wheel position sensor is used to determine where the driver is wishing the car to go. A yaw rate sensor measures the speed at which the car is rotating around its centre, while a lateral acceleration sensor measures sideways force on the car. The AdvanceTrac computer monitors these sensors 150 times per second and can react within milliseconds to briefly apply individual brakes, retard ignition timing, and cut back on fuel delivery to keep the vehicle under control.

Stability control systems are a fantastic safety system. According to Johnny Unser, race car driver and performance driving consultant, "the yaw control is phenomenal in avoiding an accident by keeping the car pointing in the right direction, instead of getting out of control."

There is one caution about stability control. It can't overcome the laws of physics. Trying to corner at excessive speeds or driving beyond the limits of tire traction can cause the car to lose control. The stability system will help, but there is only so much it can do.

I have enjoyed driving many cars in the $35,000 and up price range that were equipped with stability control. The systems work even better than they sound. Now that stability control is available on a Ford Focus at under $22,000, I look forward to the safety benefits on even more cars.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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