Electronic stability control, often simply referred to by its acronym 'ESC', is an automatic safety system that is designed to prevent a driver losing control of their vehicle.
It achieves this by individually braking the car's wheels, often in conjunction with reducing engine power, in order to maintain the desired course. Doing so allows the system to combat understeer and oversteer and - in the case of taller vehicles - can help prevent a dangerous rollover.
You may find some systems from certain manufacturers have their own name but, at their core, they function in the same fashion. For example, Mercedes refers to its stability control system as an 'Electronic Stability Program', while BMW calls its set-up 'Dynamic Stability Control'.
How does electronic stability control work?
A modern electronic stability control system uses a car's anti-lock braking and traction control systems, along with additional sensors, to monitor and regulate the behaviour of a vehicle.
ESC systems, besides relying on the car's existing wheel speed sensors that are used for ABS and traction control, also receive data from several other inputs. These typically include a yaw rate sensor, a lateral acceleration sensor and steering wheel, brake and accelerator position sensors.
The system then uses this data to compare the car's responses to the driver's steering input. If the system senses that the car isn't following the driver's steering inputs, indicating instability, it will then intervene in an effort to get the car back on the right track.
This intervention is carried out by using the car's existing ABS hardware, which allows the ESC to brake each individual wheel. Engine output can also be reduced to help restore vehicle control.
What situations prompt an intervention - and what happens?
If a driver enters a corner too quickly, suddenly swerves or is cornering in poor conditions, the front tyres can lose grip and the car will begin to understeer. The ESC will detect that substantial steering input is being applied but the yaw rate and lateral acceleration sensors will show that the car is continuing straight on.
In this instance, the ESC will briefly apply the brake on the rear wheel that's on the inside of the corner. This intervention causes the car to pivot and rotate in the desired direction, helping restore control.
If the car begins to oversteer, however, the ESC will brake a front wheel instead. If the tail of the car swings out to the left, when viewed from behind, then the front-left brake will be applied. This can bring the rear of the car back in line and prevent a spin.
Power may also be reduced in order to more quickly regain control and avoid the situation getting further out of hand. This ability to quickly stabilise a vehicle is what also helps ESC systems reduce the chance of a roll.
More modern set-ups may offer other features, including trailer stabilisation that helps prevent a trailer swerving uncontrollably. In any case, ESC does not guarantee complete safety - as it can only work with the grip that's on offer.
A brief history of ESC
The first production ESC systems arrived on the market in 1995; Mercedes-Benz unveiled a set-up called 'Electronic Stability Program', which had been co-developed with Bosch. Toyota unveiled its 'Vehicle Stability System' and BMW also launched its 'Dynamic Stability Control'.
The benefits of the system quickly became clear; Mercedes-Benz began making ESC standard equipment in 1997 and, by 2003, its cars involvement in accidents had reduced by 42 per cent. By 2014, Bosch's figures suggested that 190,000 accidents had been prevented by ESC and some 6,000 lives had been saved.