Smart Cars Part II - Vehicle Ops -

Smart Cars Part II

Dynamic stability-control systems



JP Molnar | From the February 2007 Issue Wednesday, January 31, 2007

Jan. 26, 1979 the day my future driving career took a wild slide through the back roads of an obscure southern county. The Dukes of Hazzard television show debuted, and millions of pre-license adolescents like me drooled over a show featuring a shiny orange muscle car with its weekly how-to primer on sliding sideways, burnouts and jumping an occasional bridge. To watch Bo and Luke slide that big ol General Lee around the dirt roads of Hazzard County with Boss Hog and the boys after them made it seem that driving was one exercise in controlled chaos after another.

Of course, back then, the producers of the show never mentioned the few hundred Generals that were sacrificed in the name of good TV. So, as I grew up and began to drive, the maneuvers that looked so cool on TV led to real-world realities that usually resulted in replacement fenders, a few trips to traffic school and a bruised ego. The Duke antics did show that an exceptionally talented driver could make a big, heavy, solid rear-axle muscle car handle pretty darn good. Unfortunately, none of those talents were ever taught in local high school driver s ed classes. Since then, driver s education for the masses hasn t really improved much, but thankfully the cars we drive have.

In my first installment of Smart Cars (November/December 2006, p. 20), I discussed how emerging technologies previously reserved for elite automobiles have trickled down into mainstream automotive offerings. That same technology has also worked its way into the vehicles you and I drive every day on patrol. In Part 1, I discussed ABS and traction control. In this installment, I will discuss another radical technology that has worked its way into the modern day patrol car and SUV: stability control.

What It Is

If ABS is the head of the family and traction control the prodigal son, think of stability control as the first cousin. To review, both ABS and traction control use wheel sensors to either detect impending wheel lockup or wheelspin. A computer integrated into the system takes the information from the sensors and reacts accordingly. Both of these systems are designed to deal with vehicle motions that occur generally when the vehicle is traveling in a straight line.

Stability-control systems use these same sensors, plus other components to address lateral (side-to-side) motions of the vehicle associated with chassis behavior. Electronic stability-control systems were first introduced in 1995 by Mercedes Benz in its S-Class sedans. The system, called ESP (Electronic Stability Program), was developed by the Bosch Corporation, and was the first system to address lateral stability issues in a production vehicle.1

Nowadays, stability control comes standard in many mainstream automobiles, including the new Dodge Charger Police Package and the four-wheel-drive (4WD) version of the 2007 Chevy Tahoe Police Package. The system comes under many names, such as ESP (DaimlerChrysler), StabiliTrak (GM), Advance Trac (Ford) and others, but all of the systems essentially operate under the same principles.

Why It's Important

As patrol officers who must successfully negotiate roadways under the worst of conditions, anything that can make our driving responsibilities easier is always welcome. Furthermore, with the hundreds of pounds of equipment we carry in our cars, that extra weight can take a bad-handling condition and make it worse. I noted in Part I how ABS systems haven t led to a marked decrease in accidents, but the implementation of stability-control systems definitely has. A 2004 study by the U. S. Department of Transportation s National Highway Traffic Safety Administration (NHTSA) determined that stability control systems do appear to reduce accidents, especially in SUVs. According to NHTSA, 7.4 percent of the light-vehicle fleet in 2003 was sold with some form of stability-control system. Of the vehicles studied, stability-control systems reduced single-vehicle crashes in passenger cars by 35 percent when compared to the same models sold in prior years without the technology.

The effects on SUVs were even more impressive. The study showed that SUV single-vehicle crashes were reduced by 67 percent in models with stability control. Evaluating fatal crashes only, stability control was associated with a 30-percent reduction for passenger cars, and 63 percent for SUVs. According to NHTSA, This technology appears to provide safety benefits by reducing the number of crashes due to driver error and loss of control because it has the potential to anticipate situations leading up to some crashes before they occur and automatically intervene to assist the driver. 2 So, it appears a stability-control system in your patrol car can help you stay safer behind the wheel.

How It Works

As mentioned above, one component of a stability-control system is the use of wheel sensors also used for ABS and traction control. Stability control adds steering-angle sensors, and a yaw sensor. Yaw is movement of an object as it rotates on a vertical axis. In other words, when your car starts to spin and the lateral forces make the rear end want to touch the front end of your car, blame it on the yaw.

When that begins to happen, the yaw sensor determines that you re about to get in over your head, and works with a lateral-force sensor as well as the steering-wheel sensor to evaluate whether you are about to go from hero to zero on that off ramp. If the answer is yes, the system begins to apply braking force to individual wheels to steer the vehicle back toward its intended path. In some cases, the system can also reduce power and torque to assist in traction and weight transfer.


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JP MolnarJP Molnar, Law Officer's Cruiser Corner columnist, is a former state trooper and has been teaching EVOC since 1991 for numerous agencies.


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