Traction Control and Stability Control

How these automotive systems help protect you and your passengers.
By Brian Alexander
Vehicle safety systems have become increasingly advanced over the past few decades. Where the seatbelt and airbag used to be our primary defenses against injury, cars are now equipped with active safety systems to prevent us from losing control of the vehicle in the first place.

The proliferation of onboard electronics in the eighties enabled carmakers to fit vehicles with increasingly advanced safety systems. The idea behind these new "proactive" safety systems was to correct potentially dangerous driver inputs. The first such system developed was antilock braking, which keeps drivers from locking up tires during hard braking by rapidly pulsating the brake calipers, thus safely minimizing braking distance.

Traction control is a logical evolution of the ABS system's underlying principles, only reversed. While ABS works to maximize traction during braking, the purpose of traction control is to mitigate loss of grip during acceleration.

When a vehicle accelerates from a stop or accelerates to pass another car, it does so by transferring power through friction created between the tires and the road. There are limits to how much friction a tire can take however, and once traction is lost, the tire stops accelerating and begins to spin freely. Think of a car doing a burnout - the tires are spinning beyond the limits of traction.

"Traction control systems work by electronically detecting tire slip and overriding the driver's throttle input, which limits torque to the driven wheels," says Alex Cardinali, Product Safety Engineer at Nissan of America.

When wheel spin is detected, the traction control system instantly compensates by correcting the driver inputs to the engine management system and applies less torque. Advanced traction control systems are even capable of applying light braking to slow the tires down to a speed at which they can regain traction. In older systems where the gas pedal is directly connected to the engine injection system via a throttle cable, traction control retards the engine by keeping cylinders from firing, limiting power to the driven wheels.

The next step on the safety ladder then is to gain control of a car's handling, also known technically as its yaw movement. While traction control focuses on the longitudinal (front-to-back) forces the car experiences, stability control focuses on the lateral (side-to-side) forces a car is subjected to and attempts to keep the front and back ends of the car in line through use of yaw sensors.

This means controlling "oversteer," a condition in which the vehicle's rear tires lose traction, as well as "understeer," a condition in which the vehicle's front tires lose traction.

"A stability control system such as Nissan's VDC (Vehicle Dynamic Control) works by controlling the rotational speed of each individual wheel. It does so by applying braking force to wheels individually. The result is a yaw movement in the opposite direction of the perceived loss of control," explains Cardinali.

For example, if you are driving through a left hand turn and the right rear tire hits a puddle and breaks loose, the system will compensate by braking only the right rear wheel until it has decelerated to a speed where it achieves traction again. The system can also input throttle to regain control of the car if need be, and will always override the driver's inputs once activated.

"It's important to remember that our VDC system isn't driving for the driver, it is merely correcting dangerous inputs. People tend to have the misconception that the car will drive itself, but the driver still holds primary responsibility for their own safety," comments Cardinali. "These systems work well, but they still have to obey the laws of physics."

The National Highway Traffic Safety Administration has recognized the safety stability management systems provide and, as of September 2008, has made stability control mandatory in all passenger vehicles sold within the United States. The program, known as FMVSS 126, will be phased in over a four-year period, concluding in 2011.

Much like the airbag and seatbelt before them, traction control and stability control have become safety standards in the modern automobile. While they should not be relied upon to avoid an accident, they certainly help drivers prevent making mistakes.

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