Traction control is a system that reduces wheel slip. In its simplest form, traction control is achieved mechanically using a limited slip differential. However, modern traction control systems work together with anti-lock brake systems and usually involve the engine and automatic transmission. Stability control is an improvement to the traction control system, which helps maintain control of the vehicle in a sideways skid. The U.S. Government mandated that all vehicles sold after 2012 must be equipped with stability control. Let’s take a look at how traction and stability control are achieved in a modern vehicle.
Today’s traction control systems wouldn’t be possible without anti-lock brakes. ABS, or Anti-lock Braking System, is a system which prevents wheel skid during braking. The brakes are most efficient just before the wheel locks up. This is where the wheel is able to provide the most friction between itself and the road. If you’ve ever driven a vehicle without ABS, you’re probably familiar with having to pump the brakes to prevent the wheels from locking up under heavy braking or on slick roads. ABS essentially does the same thing, except ABS is able to pulse the brakes several times a second, which is much faster than any mere mortal ever could. Standard ABS systems consist of 3 main components:
- Wheel Speed Sensors: This consists of a tone ring or reluctor, which rotates with the wheel. A fixed sensor reacts to evenly spaced segments on the tone ring/reluctor. The signal can provide either an analog or digital signal to the computer.
- Hydraulic Unit: This contains a pump motor and modulator valve assembly. Separate valves for each wheel are switched on and off to apply and release the brakes.
- ABS Control Module: This processes inputs from the wheel speed sensors and provides output activation of solenoids to activate the modulator valves. The ABS control module is often attached to the hydraulic unit.
ABS uses speed sensors at each wheel to detect if a wheel begins to decelerate too rapidly, indicating a potential skid. A hydraulic pump is used to help provide a constant supply of pressure to the hydraulic unit. Solenoids are pulsed inside the modulator valve assembly, which apply and release the brake at each wheel during adverse braking conditions. This keeps the wheel right on the verge of locking up where stopping power is the greatest.
Traction control is active under speeds of 25-30 mph and employs the same wheel speed sensors and hydraulic unit as the ABS. The traction control function is usually integrated in the same computer as the ABS. If the computer detects a wheel accelerating more rapidly than the others, the brakes will be pulsed at that wheel to prevent it from slipping. In addition to pulsing the brakes, the ABS computer may tell the engine to reduce the throttle angle, injector pulse, or spark to reduce the engine power contributing to the wheel slippage. On vehicles with automatic transmissions, the ABS computer may also instruct the transmission to shf8d3ift up a gear in order to reduce torque created by the engine.
A traction control indicator lamp is included in the instrument cluster and flashes when the traction control system is operated. The lamp will remain illuminated at all times if there is a fault in the traction control system. Some vehicles provide a button to deactivate the traction control system. Deactivating the traction control may be advantageous in situations where the vehicle is stuck in mud or snow, during dynamometer testing, or when you feel the need to rip a sick burnout. During deactivation, the lamp will remain illuminated to indicate the system is inactivated.
Stability control further builds on the principles of traction control. It monitors additional sensor inputs to inform the computer about the vehicle dynamics. Stability control is active at all vehicle speeds, and takes corrective action if it is determined that there is a loss of control based on these added inputs:
- Yaw Rate Sensor: Yaw is a term describing rotation around a vehicle’s vertical axis. When you imagine spinning a top, the speed of the top spinning around its vertical axis would be an example of yaw rate. This input helps detect the actual direction of the vehicle.
- Lateral Acceleration Sensor: Lateral acceleration occurs when a force acts upon the vehicle causing it to move sideways to the direction of travel. Lateral acceleration is most often noticed when cornering, as centrifugal force pulls the vehicle to the outside of the curve. This input helps detect if the vehicle is sliding.
- Steering Angle Sensor: The rotational angle and velocity of the steering wheel is measured in degrees. This input detects the driver’s intended direction of the vehicle.
- Brake Pressure Sensor: The hydraulic pressure of the brake fluid can vary due to several factors. This input detects the instantaneous brake pressure in order to calculate braking force to the wheels.
If a corrective action is necessary, the stability control computer must determine which direction the driver is steering and which direction the vehicle is moving. The driver’s intended course is determined from the wheel speed sensor and steering angle sensor inputs. The vehicle’s actual direction is determined from the yaw rate sensor and lateral acceleration sensor inputs. There are two main situations in which stability control will take corrective action:
- Understeer: This occurs when the front wheels are turned too abruptly during cornering, causing them to slide. The direction of the vehicle cannot be controlled if the front wheels are sliding, and thus, the vehicle continues in a forward direction, rather than turning the corner.
- Oversteer: This occurs when the rear of the vehicle becomes unstable during cornering, causing the rear wheels to lose traction and swing to the outside of the curve. The vehicle begins to rotate in such a way that the driver is facing the inside of the corner.
If an understeer situation is detected, stability control takes corrective action by activating the rear brake on the inside of the corner and intervening with the engine and transmission management systems. The throttle angle is adjusted accordingly, and the automatic transmission can be shifted into an appropriate gear. If an oversteer situation is detected, stability control takes corrective action by activating the front brake on the outside of the corner and intervening in the engine and transmission management systems to attempt to regain control of the vehicle.
ABS, traction control, and stability control all work to help maintain control of the vehicle, but no system can replace good judgement. Be aware of situations where these systems have little or no effect on a loss of control, such as sliding on ice. How do you know if you vehicle even has traction or stability control? Traction and stability control are referred to by various names depending on the vehicle manufacturer. Anti Slip Regulation (ASR), Dynamic Stability Control (DSC), Electronic Stability Program (ESP), and AdvanceTrac are all examples of the various trade names associated with traction and stability control. Check your owner’s manual to determine which options your vehicle may be fitted with.