How Electronic Stability Control Keeps a Car on Course
Electronic stability control compares where you steer with where the car goes and brakes single wheels to correct a skid. Here is how it works, its history and limits.
In this article
Electronic stability control (ESC) is a safety system that notices when a car is not going where the driver is steering, and then brakes individual wheels to bring it back in line. Transport Canada describes it as a system that automatically brakes one or more wheels for short periods, reduces engine power, or both. The main point is that it helps a driver keep control in a skid, but it cannot create grip that the tyres and road do not have.
Comparing intention with reality
ESC works by comparing two things. The first is what the driver wants, which the system reads mainly from the steering wheel angle. The second is what the car is actually doing, which comes from sensors for yaw (how fast the car rotates), sideways acceleration and the speed of each wheel.
When the two disagree, the system decides that control is slipping. Wikipedia's article on the technology explains that it can then brake individual wheels and, in some systems, cut engine power too. Transport Canada says only that it uses sensors and a computer, without naming the sensors, so the list above follows the more detailed Wikipedia description.
Understeer and oversteer
There are two classic ways a car loses its line in a corner. In understeer, the front tyres slide and the car runs wide of the curve. In oversteer, the rear tyres slide and the tail swings out.
ESC treats them differently. According to Wikipedia, it brakes the outer front wheel to counter oversteer and the inner rear wheel to counter understeer. Braking one wheel creates a turning force on the car that nudges the nose or tail back toward the path the driver chose, without the driver having to do anything special.
It builds on anti-lock brakes
ESC reuses hardware already fitted for anti-lock brakes (ABS): wheel-speed sensors, a hydraulic unit that can apply pressure to each wheel separately, and a control computer. The addition is the extra sensing of steering angle and rotation, plus the software that decides which wheel to brake and by how much.
A short history
Wikipedia reports that Mercedes-Benz and Bosch developed their Electronic Stability Program between 1987 and 1992. Mercedes-Benz was the first to put it on a production car, in 1995 on the S 600 Coupe. Toyota's Vehicle Stability Control appeared the same year on the Crown Majesta.
The same article describes a well-known test in October 1997, in which a Mercedes A-Class rolled over in a swerving test at 78 km/h. Mercedes then recalled and retrofitted about 130,000 cars with ESC and firmer suspension. The episode is often cited as a turning point for the technology becoming common.
Where it is required
- Canada: required on new passenger vehicles from 1 September 2011, and Transport Canada says mandatory on cars, SUVs, pickups, minivans, heavy trucks and buses from the 2012 model year
- United States: required for 2012 model-year passenger vehicles under FMVSS 126, phased in from 2009 models
- European Union: from 1 November 2011 for new type approvals, and from 1 November 2014 for all newly registered cars
The dates for Canada's passenger cars and for the US and EU come from Wikipedia; Transport Canada's own page confirms the 2012 model-year requirement.
What studies found
Wikipedia summarises two older studies. A 2004 field study by the US National Highway Traffic Safety Administration found ESC reduced crashes by 35%. In June 2006, the Insurance Institute for Highway Safety estimated it cut the likelihood of fatal crashes by 43%, fatal single-vehicle crashes by 56% and fatal single-vehicle rollovers by 77 to 80%. These figures are from the mid-2000s, and the article itself notes that parts of it need updating, so current agency data may differ.
What ESC cannot do
Wikipedia makes a related point: ESC does not improve a car's cornering ability. It helps the driver keep control on slippery surfaces. Transport Canada adds that worn, under-inflated or overloaded tyres and poor brakes reduce its effectiveness.
Summary
ESC compares the driver's steering with the car's actual motion, and when they diverge it brakes single wheels and may trim engine power. It grew out of ABS, reached production in 1995, and is now required on new cars in Canada, the United States and the European Union. It is a safety net, not a way to drive faster, and its benefit depends on tyres, brakes and sensible speed.
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