How Electronic Stability Control Works

Image Gallery: Car Safety A man dressed as a crash test dummy promotes Electronic Stability Control (ESC) at the booth of auto supplier Continental in Frankfurt, Germany, on Sept. 12, 2007. See more car safety pictures.
AP Photo/Thomas Kienzle

Driving safety took a big step forward in the mid-1990s when electronic stability control was introduced. The German auto supplier Bosch developed the first system, and the Mercedes-Benz S-Class and BMW 7-series were the first cars to use the new safety and regulatory devices.

It's been more than a decade since electronic stability control was first introduced, and it goes by many names, depending on the company. Audi calls it the Electronic Stability Program, or ESP; at Ford, it goes by Advance Trac. GM has Stabilitrak, and Porsche puts its stamp on the system by dubbing it Porsche Stability Management.


But all of these systems, no matter their names, use high-tech sensors, the car's central computer and mechanical actions to assist in driving safely. We often read about high-performance cars having a tendency to understeer or oversteer; but the truth is, any car can veer off course, especially if the road is slippery.

Understeer happens when the front wheels don't have enough traction and the car continues moving forward rather than turning. Oversteer is just the opposite: the car turns farther than the driver intended causing the rear wheels to slide and the car to spin. ESC, as electronic stability control is often known, can help correct both of these situations.

Now that you know the basics, read on to find out how this advanced safety system operates during an emergency.


Electronic Stability Control Explained

A test vehicle with its electronic stability control turned off slides over cones during a test in Auburn Hills, Mich., on July 16, 2003.
AP Photo/Carlos Osorio

The electronic stability control system doesn't work all alone -- it uses the car's other safety and regulatory devices, like anti-lock braking and traction control, to correct problems before they become accidents.

The center of the ESC system is also the center of the car: the yaw control sensor. It's almost always located as close to the very center of the car as possible. If you were sitting in the driver's seat, the yaw control sensor would be under your right elbow, somewhere between you and the passenger.


But what the heck is "yaw?" If it sounds like something a pirate would say, that's because they probably would. Ships and cars both experience yaw, which is a movement around a vertical, or z, axis. It's as if the car were pinned like a butterfly in a display case, where the pin is the z axis. The yaw sensor is right at the center of the pin. If the ESC system detects that the car is swinging too far (or not far enough) around that up-and-down axis, it springs into action to assist.

Using all the modern electronic systems at its disposal, the ESC can activate one or more individual brakes, depending on which wheel can increase driving safety the most, and control the throttle to lessen the speed at which the car is traveling. The sensor is looking for differences between the direction of the steering wheel and the direction the car is headed; the car's computer then makes the necessary corrections to bring the vehicle's direction of travel in line with what the driver wanted.

To find out which components electronic stability control has at its disposal, keep reading.


Electronic Stability Control Components

An Electronic Stability Program (ESP) system on a Mercedes-Benz S550.
AP Photo/Chris Greenberg

There are lots of safety and regulatory devices in cars these days, and they all work together to keep the wheels on the road and the passengers safe. Electronic stability control, in particular, takes advantage of two other systems, ABS and traction control, plus a few special sensors, to do its job

Before the 1990s, drivers were taught to pump the brake pedal to keep the brakes from locking up and causing a slide. With the invention of anti-lock brakes, driving safely became much easier. ABS electronically pumped the brakes faster than the driver could, which kept them from locking and causing understeer or oversteer. ESC uses this system to correct the problem almost before it can start by activating the ABS for as many wheels as needed, from one individual wheel to all four. The nature of ABS keeps the over- or understeer from getting worse while slowing the car to a controllable speed.


ESC also uses traction control for driving safety. If ESC is in charge of monitoring side-to-side motion around a vertical axis, traction control is in charge of front-to-back motion. If the traction control system is detecting wheel slippage, the electronic stability control sensor will pick up on the direction of the slide. If there's a difference between the angle of the steering wheel and the direction the car is sliding, the ESC will work with the traction control system to engage the ABS at the proper wheel (or wheels) and control the throttle to reduce the speed of the vehicle, too.

ESC information is fed into the car's central computer via three types of sensor:

  • Wheel-speed sensors: One wheel-speed sensor at each wheel measures the speed of the wheel which the computer can then compare to the speed of the engine.
  • Steering-angle sensors:This sensor, in the steering column, measures the direction the driver intends to aim the car. If it's different than the direction the car is actually traveling, the ESC system will kick in.
  • Rotational-speed sensor:This is also known as the yaw sensor. It's the one in the middle of the car that measures the side-to-side motion of the vehicle.

If you want to know what ESC can do for driving safely in the real world, head to the next page.


Benefits of Electronic Stability Control

Transport minister Anthony Albanese experiences the Electronic Stability Control system during a test drive at the driver training centre in Queanbeyan, NSW, on June 23, 2009.
AP Image/Alan Porritt

The most important role ESC plays in driving safety is reducing the number and severity of crashes. Almost everyone ends up in nasty, slippery driving conditions at some point, whether it's a rainstorm, a sudden patch of ice or a snowy road. Electronic stability control, along with the other safety and regulatory devices on-board today's vehicles, can help drivers maintain control on the road.

Electronic stability control will not engage in the event of a fender-bender -- the kind of accident that typically happens in stop-and-go traffic. However, some cars have other systems to help with this, including sensors in the front of the car that measure the distance between your bumper and the bumper of the car in front of you, but electronic stability control doesn't really come into play at that point. It's more helpful when slippery conditions mean a loss of control, regardless of whether there's anyone else on the road or not.


ESC has made driving easier and less likely to end in a serious accident. In fact, the Insurance Institute for Highway Safety (IIHS) said that ESC could prevent as many as 9,000 fatal crashes per year, and the National Highway Traffic Safety Administration (NHTSA) found that ESC reduces single-vehicle car crashes by 26 percent, and single-vehicle SUV crashes by 48 percent [source: IIHS].

These kinds of numbers have prompted the U.S. government to require electronic stability control on all passenger vehicles by 2012. Consumer Reports found that by 2009, 73 percent of all cars and a whopping 99 percent of SUVs already had standard ESC. Another 11 percent offered it as optional equipment on cars [source:]. As for Mercedes-Benz, which first used ESC in 1995, all the cars under its corporate umbrella (including tiny smart cars and luxurious Maybachs) have electronic stability control as standard equipment.

For more information about electronic stability control as well as other safety and regulatory devices, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • "Electronic stability control to be standard by 2012." September 2006. (Nov. 10, 2009)
  • Dang, Jennifer N. "Preliminary Results Analyzing the Effectiveness of Electronic Stability Control (ESC) Systems." National Highway Traffic Safety Administration. September 2004. (Nov. 10, 2009)
  • Insurance Institute for Highway Safety. "ESC and how it helps drivers maintain control." (Nov. 10, 2009)
  • Insurance Institute for Highway Safety. "Q&As: Electronic Stability Control." October 2009. (Nov. 10, 2009)
  • Memmer, Scott. "Stability Control: Get Your Yaw-Yaws Out!" (Nov. 10, 2009)
  • National Highway Traffic Safety Administration. "Federal Motor Vehicle Safety Standards; Electronic Stability Control Systems; Controls and Displays." April 5, 2007. (Nov. 10, 2009)
  • Rosenberg, Jeremy. "Electronic Stability Control." June 18, 2009. (Nov. 10, 2009)