By Jim Kerr
My father always told me to drive on slippery roads like there was an egg sitting between my foot and the gas or brake pedals. That advice has kept me out of a lot of driving trouble, but for drivers that can’t always have that subtle touch on the pedals, there are electronics that can help. There are many manufacturer-specific acronyms and names, including PSM (Porsche Stability Management), GM’s StabiliTrak, or Toyota’s VSC (Vehicle Stability Control).
Although there are some differences, all these systems help to keep the vehicle under control. We can refer to them simply as “stability controls.”
Stability control typically does two things: first, it reduces engine power and secondly, it slows a wheel so the vehicle moves in a direction you want. To do this, the stability control computer needs information from several sensors.
First, the system needs to know where you wish to go. A steering wheel angle sensor indicates this to the computer. Keep in mind, however, that stability controls help keep the vehicle going where the steering wheel is pointed: steer towards the ditch, and that is where you will end up. Knowing where your wheels are pointing is very important. Don’t saw the steering wheel back and forth, or try to oversteer to correct for a slide – just steer in the desired direction.
A yaw sensor is also used to help determine the angle of the vehicle in relation to the steering wheel angle. Imagine a vertical pivot point right in the middle of the vehicle. The vehicle can rotate around this pivot. During a slide, skid or avoidance manoeuvre, the vehicle angle changes. The stability control computer looks at the difference in the steering input and vehicle angle to determine how to put the vehicle back in line again.
For example, if the vehicle is oversteering, such as when the driver is going around a left-hand turn and the back end of the vehicle is sliding out to the right, the computer will signal the engine computer to reduce power and then brake the right front wheel to pull the vehicle back in line again. If the vehicle is understeering in the same situation, meaning it doesn’t want to turn the corner, then the computer would again reduce power, but this time it would brake the inside or left front wheel to pull the vehicle into the corner. Once the vehicle is going the direction the steering input requests, power is restored and the brake is released.
Lateral G sensors are also used on some vehicles. These sensors measure the sideways cornering force affecting the vehicle, so that stability control braking pressures can be modified in corners. As the vehicle rolls or leans slightly on a corner, more downward force and traction occurs on the outside tires, with less traction on the inside tires. Modifying brake pressure reduces the possibility of a wheel locking up if the stability control system intervenes.
Wheel speed sensor input can also be used to determine if the vehicle is turning a corner. The tires on the outside of the corner have to travel further, so they rotate faster than the tires on the inside of the corner. This difference in speed is calculated by the computer to help modify stability control operation in corners.
Not all vehicles reduce power and apply brakes to correct for oversteer or understeer situations. In 2005, the Acura RL introduced SH-AWD (Super Handling All Wheel Drive). Instead of braking a wheel on one side of the vehicle to make it turn, the SH-AWD system accelerates the opposite rear wheel to push the car back in line without a decrease in speed or performance. It feels a little different, but works extremely well.
Stability control systems are continually improving. Suspension management and roll stability systems are being integrated into stability controls. Some systems, such as those from Subaru, have a sportier feel, while other systems such as Mercedes-Benz are more conservative and intervene quicker. Regardless of the manufacturer, stability control systems are great. They help keep vehicles under control on slippery roads, but even more impressive, they help keep the vehicle under control in sudden collision avoidance manoeuvres, too. It can assist even the best drivers in some situations, and you don’t have to clean egg yolk off the floor.