Stability Control: Get Your Yaw-Yaws Out!

You're driving on a rain-slickened, single-lane highway with your one-year-old toddler securely buckled in her car seat behind you. It's been raining all day, and the roads are slick, so of course you have your stability control system engaged. You drive into a small valley, unaware that you've picked up a little too much downhill momentum into the curve ahead. As you near the curve, your daughter cries out for her bottle. You take your eyes off the road for a second to fish through your bag. When you look up, you're startled to see a pickup truck darting out of a blind driveway. You swerve to avoid the pickup, and the rear end of your car begins to "fishtail." Just then, with no effort on your part, the stability control system kicks in. Braking force is applied to the appropriate wheels, and the car slows until the fishtailing ceases. You accelerate up the other side of the hill and proceed on your way. Disaster averted. Stability control may have just saved your life — and the life of your child.

*It seems every time we turn around, a new technology enters the car. Of course, this has been going on since the horseless carriage days.

Imagine how shocked the witnesses to the first motorcar must have been. A quick look at the front of the carriage — "Hey, where the hell's the horse?" then a glance behind — "Hey, where's that stuff the horse usually leaves on the road?" You can even imagine onlookers saying, "What's the funny sound coming from that pipe in back?"

Years later, at the appearance of the first rear-engine, air-cooled VW, how confused onlookers must have been. Opening the hood, they'd say, "Hey, where's the engine?" Then, finding the engine in the rear, they'd ask, "OK, but where's the radiator?"

If there's one thing we're guaranteed in the automotive business, it's change — but not always for the better. Was the Edsel an improvement? Don't think so. What about the Pacer? Hardly. Or take the 2001 Pontiac Aztek — please.

OK, OK, so that one was just too easy.

The point is, the automobile has evolved — or, in the case of the above examples, devolved — in fits and starts. Ideas come and go. Some technologies stick, and others become historical footnotes.

In the safety arena we've seen major advances in the last few decades. These developments — from three-point seatbelts to airbags to ABS — have saved thousands of drivers' lives. And recently, it's gotten even better. Accelerated by the advent of the microchip, in the last ten years we've witnessed an incredible array of safety devices entering the vehicle. One of the newest and most intriguing of these "high-tech" advancements is yaw-sensing technology, more commonly referred to as stability control. Let's take a look at what it does and how it benefits consumers.

Before we do, though, let's step back a few years and get a historical perspective.

Stability control is actually the latest in a series of braking-related developments that began appearing in production vehicles in the 1980s. As with many high-tech advances, the original designs debuted in German vehicles, typically Mercedes S-Class and BMW 7 Series sedans.

There's a good reason for the German connection. Robert Bosch GmbH, headquartered in Stuttgart, Germany, is one of the largest original equipment manufacturers (OEMs) in the world. The company, a world leader in safety development, works extensively with German automakers.

Laying the groundwork for stability control, in the mid-80s Bosch brought the antilock braking system (ABS) to market through Mercedes and BMW. As most consumers probably know by now, ABS has become a standard feature on many new cars. It works by sensing and preventing wheel lock-up, thereby improving the vehicle's traction and enhancing steerability during hard braking.

According to Eric Kosmider, spokesman for Robert Bosch Corp., the American subsidiary of Robert Bosch GmbH, "ABS was the first 'building block' in the chain of components that eventually led to stability control."

After Bosch perfected ABS, the company moved on to the second "building block" — traction control. Sometimes referred to as ASR traction control (the ASR stands for "Acceleration Slip Regulation" and typically referees to systems appearing in German cars), the technology works in a similar fashion to ABS, but at the opposite end of the performance spectrum. Whereas ABS focuses on eliminating lock-up in braking situations, traction control regulates wheelspin during acceleration. In other words, when speeding from a standing stop or while in motion, a driver may give too much accelerator input, causing the wheels to spin freely. In such cases, traction control monitors wheel speed, cuts engine power or even applies the brakes to optimize contact between the tires and the road surface.

This brings us to our present topic: stability control. The third "building block" in modern braking systems, stability control incorporates everything ABS and traction control do plus a yaw-sensing feature that works to increase traction during potential side-skidding situations. In other words, whereas both ABS and traction control work on the longitudinal (front-to-back) axis of the vehicle, stability control operates on the lateral (side-to-side) axis. Bosch's Electronic Stability Program (ESP), the first such system on the market, began appearing in 1995 Mercedes-Benz S-Class sedans. It has since become a popular feature on many upscale vehicles.

According to Bosch's Kosmider, "The platform for ABS, traction control and stability control is essentially the same. We simply add sensors to get the desired effect."

In addition to discrete electrical components, ceramic sensors and solenoid valves, stability control systems typically utilize wheel-speed sensors, steering-angle sensors and a hydraulic modulator. The key component is, however, something called a rotational speed sensor (also known as a yaw-rate sensor). Yaw can be described as "the movement of an object turning on its vertical axis."

The yaw-rate sensor determines how far off-axis a car is "tilting" in a turn. This information is then fed into a microcomputer that correlates the data with wheel speed, steering angle and accelerator position, and, if the system senses too much yaw, the appropriate braking force is applied.

As you can imagine, stability control systems are particularly effective in inclement driving conditions, where the roadway may be covered with rain, ice or snow and the normal friction between the tires and the road is reduced.

There are a couple of things you may want to know about stability control. First, the system will do most of the "thinking" for you. Depending on the particular driving situation, the system may activate an individual wheel brake or any combination of the four, as well as control the throttle, until the vehicle is once again stable.

Second, the system is fully independent of the driver's actions. Even if the car is free-rolling (no acceleration or braking input from the driver), the stability control system will kick in and perform its duty. All you need to do is steer.

Remember, though, that stability control, like any technology, is not fool-proof. All vehicles must ultimately obey the laws of physics. Members of our road test staff report spinning sedans with activated stability control systems on dry pavement. Be sure to make your driving decisions based on the appropriate criteria — visibility, road conditions, speed, the condition of the tires and brakes — and not to rely on any technology to correct for unsafe driving.

Still, for most drivers, the technology offers the promise of increased safety and control. For this reason, stability control is becoming a popular — and in some cases, inexpensive — option on many new vehicles. For instance, the system in the Dodge Caliber R/T has an MSRP of just $250.

In addition to Bosch, three other braking manufacturers build stability control systems for passenger cars. They are TRW, Delphi and Continental-Tevis.

We've supplied a partial list below of automakers that offer stability control. Be aware that the technology is marketed under a number of different trade names, so check's vehicle detail pages to confirm that the option is offered on your chosen vehicle.

Audi: Electronic Stability Program (ESP).

BMW: Dynamic Stability Control (DSC).

DaimlerChrysler: Electronic Stability Program (ESP).

Ford Motor Company: Advance Trac.

General Motors: Active Handling System (Corvette), Stabilitrak (Pontiac, Buick, Cadillac).

Jaguar: Dynamic Stability Control (DSM).

Lexus: Vehicle Skid Control (VSC)

Porsche: Porsche Stability Management (PSM).

Subaru: Vehicle Dynamics Control (VDC).

Volkswagen: Electronic Stability Program (ESP).

Volvo: Dynamic Stability Traction Control (DTSC).

Stability control represents one more example of the significant impact advanced electronics are having on the automobile. In this case, the results can be life-saving.

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