How ESC uses single-wheel braking and engine torque to pull a skid straight, how it differs from traction control, and why regulators made it compulsory.
Electronic stability control is a computerised system that compares where the driver is steering with where the car is actually going. If they differ, it brakes individual wheels and may cut engine power to stop a skid. Unlike traction control, it is designed to aid steering, and it is mandatory in new cars across Canada, the US and the EU.
Electronic stability control has saved an estimated 15,000 lives in Europe as of 2020, and it does that without adding a gram of grip. All it does is brake the right wheel at the right moment.
What is electronic stability control?
Electronic stability control, or ESC, is a computerised system that keeps a car stable by spotting the start of a skid and shrinking it. When it senses that the car has stopped following the steering, it applies the brakes automatically to point the nose back along the intended path. Some systems also trim engine power.
The same idea carries several labels. ESP, short for electronic stability program, and DSC, for dynamic stability control, mean the same thing. ESP began as a Mercedes-Benz and Bosch trademark, while makers have since attached their own names: StabiliTrak at General Motors in the US, AdvanceTrac at Ford, Vehicle Stability Control at Toyota, and Dynamic Stability and Traction Control at Volvo. GM uses plain Electronic Stability Control on its Opel, Holden and Saab cars.
Stability control cannot add grip; it can only decide which wheel to brake so the car goes where the driver is steering.
How does ESC catch a skid before you do?
ESC watches the steering wheel angle to learn where the driver wants to go, then compares it with where the car is actually heading, using lateral acceleration, vehicle rotation and individual wheel speeds. If the two disagree, it brakes single wheels until they match again, often before the driver has noticed.
Think of a shopping trolley that pulls to one side. You do not fight it by pushing harder on the handle; you drag a finger on the one wheel that is leading it astray. ESC does something similar with the brakes. It works out which way the car is sliding, then brakes individual wheels unevenly, which creates a twisting force about the car's vertical axis that opposes the slide.
Which wheel it picks depends on the problem. To counter oversteer, where the tail swings wide, it brakes the outer front wheel. To counter understeer, where the nose runs wide, it brakes the inner rear. The system may also cut engine power or work the transmission to slow the car.
It stays quiet until steering control looks likely to slip away. Typical triggers are an emergency swerve, understeer or oversteer through a badly judged corner on a slippery road, or hydroplaning. The system works on any surface, from dry tarmac to frozen lakes, and reacts to a skid much faster than a typical driver can.
What sensors and hardware does ESC use?
ESC is an evolution of the anti-lock braking system. A minimum of two sensors are added to the ABS hardware: one measures the steering wheel angle and one is gyroscopic. When the gyroscope shows that the car's direction does not match what the steering sensor reports, software brakes whichever wheels are needed.
The foundation is ABS, which typically has a central control unit, a speed sensor at each of four wheels and a minimum of two hydraulic valves. The unit watches each wheel's speed, and if one turns much more slowly than the car is moving, it cuts the brake pressure at that wheel and then reapplies it. Some anti-lock systems repeat this 15 times per second, which is why a driver feels the brake pedal pulsate.
Most sophisticated ESC systems can brake up to three wheels at once. The steering sensor also feeds Cornering Brake Control, which decides how much harder the inner pair of wheels in a bend should brake than the outer pair.
Cheaper electronics explain much of why the technology spread. ABS is offered or standard on most road vehicles, and stability control has grown rapidly in popularity as the price of vehicle electronics has fallen.
What is the difference between traction control and stability control?
Traction control prevents wheelspin from the driven wheels under acceleration, and it is typically a secondary function of ESC. Stability control goes further. It compares steering input with the car's actual path and corrects skids in any direction, while traction control, unlike ESC, is not designed to aid in steering.
Traction control acts when the throttle, the engine's power and the torque reaching the road are out of step with the surface. It can brake one or more wheels, suppress the spark to one or more cylinders, reduce the fuel supply, close a drive-by-wire throttle, or, in a turbocharged car, use a solenoid to reduce boost.
The two share hardware. Traction control typically uses the same electrohydraulic brake actuator and wheel-speed sensors as ABS. When a driven wheel spins significantly faster than another, the system brakes it, and the differential passes power across to the wheel with traction.
The roots go back to the limited-slip differential, a purely mechanical device. Buick's MaxTrac of 1971 used an early computer to spot rear wheel spin and modulate power, and Cadillac followed in 1979 with the Traction Monitoring System on the Eldorado. Mercedes-Benz, BMW and Toyota all launched their first traction control systems in 1987.
Traction control also has a place in cornering. If a driver applies too much throttle mid-corner, the driven wheels lose grip and slide, which shows up as understeer in front-wheel-drive cars and oversteer in rear-wheel-drive cars. Limiting power to the overdriven wheels can reduce the slide, though it cannot raise the grip available.
There are times when you want it off. Allowing one wheel to spin can rock a car out of snow or mud, whereas two wheels each driven gently will not manage the same trick. Many cars have a shut-off switch for this reason.
How do ABS, traction control and ESC compare?
| System | What it prevents | How it acts |
|---|---|---|
| ABS | Wheels locking under braking | Valves release and reapply brake pressure at the wheel that slows too fast |
| Traction control | Driven wheels spinning under acceleration | Brakes the spinning wheel; may cut spark, fuel, throttle or boost |
| ESC | Loss of steering control in a skid | Brakes individual wheels, up to three on advanced systems, and may cut engine power |
Why is ESC mandatory in most markets?
Because the evidence of lives saved was too strong to ignore. In 2004 and 2006, the US regulator NHTSA and the insurers' research body IIHS concluded that a third of fatal crashes could be avoided with the technology. Canada then required it in new cars from 2011, the US from 2012 and the European Union from 2014.
The EU resolved in 2009 to require it. From 1 November 2011, a model without ESC could no longer win EU type approval, and from 1 November 2014 every newly registered car had to have it. In the US, all new passenger vehicles needed it from the 2012 model year, and the NHTSA estimated that it would prevent 5,300 to 9,600 fatalities a year.
Sweden offers a neat case study. In 2003, only 15% of new cars were bought with ESC. After Sweden's road safety administration pushed a firm recommendation, the purchase rate reached 58% by September 2004, 16 months later. A stronger recommendation followed, taking it to 69% that December and 96% by 2008. Worldwide, 82 percent of all new passenger cars now carry the system.
A single test helped push the industry along. In October 1997, the Swedish journalist Robert Collin of Teknikens Varld flipped an A-Class that lacked ESC, at 78 km/h, during a moose test. Mercedes, with a reputation for safety to protect, called back 130,000 A-Class cars to fit stiffer suspension and sportier tyres, and made ESC standard on every new one. Crashes fell significantly, and because ESC was now available in a small car, it became available across the market, as standard or as an option.
Where did stability control come from?
The earliest steps were brake-based skid systems. In 1983 Toyota introduced a four-wheel electronic Anti-Skid Control on the Crown. In 1990 Mitsubishi released the Diamante in Japan with a system that tracked steering angle, throttle position and the speed of each wheel, and regulated engine output and braking through a corner. It was not a modern stability control system, because it had no yaw input.
BMW, together with Bosch and Continental, built a system that trimmed engine torque to stop control being lost, and fitted it to most of its range for 1992, excluding the E30 and E36. Between 1987 and 1992 Mercedes-Benz and Bosch jointly developed the Elektronisches Stabilitaetsprogramm, trademarked as ESP.
Then came 1995, when three manufacturers introduced ESC systems. Mercedes-Benz was first to put ESP on a car, the S 600 Coupe, with Bosch as supplier, and Toyota's Vehicle Stability Control appeared on the Crown Majesta. General Motors teamed up with Delphi on StabiliTrak, which reached selected Cadillacs in 1996 as a 1997 model-year feature.
Audi was first to put ESP into series production on all-wheel-drive cars, with the A8 and A6 quattro in 1997, and Volvo offered Dynamic Stability and Traction Control on the new S80 in 1998. Ford's AdvanceTrac followed in 2000, with Roll Stability Control added first on the Volvo XC90 in 2003.
Adoption took a while to reach the whole range. Ford and Toyota each pledged ESC as standard across their North American ranges by the end of 2009, though as of November 2010 Ford still sold models there without it. GM's StabiliTrak was standard on all its US and Canadian SUVs and vans by 2007, except certain commercial and fleet vehicles.
What does the ESC light on the dashboard mean?
It tells you the system is working. Because ESC can fix a slide faster than a driver notices, makers want you to know that the car has reached its handling limits, so most systems activate a dashboard indicator light, an alert tone, or both whenever they intervene.
Some go further and let the corrected course deviate very slightly from the direction the driver commanded, even when a closer match is possible. The slight difference is deliberate, a reminder that the system has stepped in.
A separate warning concerns the ABS. Should any part of it fail, a warning light usually appears on the instrument panel and the ABS stays disabled until the fault is rectified.
What can ESC not do?
It cannot add grip. ESC adds no traction, so it will not let a car corner faster, although it can make cornering better controlled. It works within the limits of the available traction between tyres and road, and a reckless manoeuvre can still exceed them. Makers stress that it is a safety technology rather than a performance aid.
Hydroplaning illustrates the point. The wheels it relies on for the correction can lose contact with the surface, which blunts its effectiveness. There is also a concern that drivers may become overconfident, which is part of the reason systems alert them when they intervene.
On a track or a loose surface, ESC can get in the way. During high-performance driving, steering input may not show the intended direction of travel, as in controlled drifting, so the system can intervene when it is unwanted. Many vehicles therefore have an override control: in simple systems one button switches everything off, while more complicated set-ups have a multi-position switch or are never fully disengaged.
Does ESC help off-road?
Often it does. With an open differential, power takes the path of least resistance, which means it flows to the wheel that has lost grip. ESC brakes wheels spinning at a rate sharply different from the opposite wheel on the axle, and off-road use typically needs consistent or even increased power while the brakes work on the slipping wheel.
Some vehicles switch between modes automatically. Mitsubishi's Super-Select 4WD, found in the Pajero, Triton and Pajero Sport, behaves as on-road in 2WD, and in high-range 4WD with the centre differential free. Lock that differential, in high or low range, and off-road traction control comes on while ABS braking is switched off.
The brake-based approach has advantages over mechanical limited-slip and locking differentials. Steering is easier, so the system can stay switched on all the time, and there is less stress on powertrain components and fewer moving parts to fail. Range Rover adopted the approach in 1993.
Where this goes next
Stability control is also being joined by related functions. Cadillac's Integrated Chassis Control System, on the Eldorado, was an early attempt to link engine, traction control, stability control, steering and suspension in one computer, intended to improve responsiveness to driver input, performance and safety.
