By Jim Kerr

Vehicle stability is affected by many factors: tire pressures, tire design, spring rates, shock absorber damping rates, sway bar strength and track width (the distance between the tires on the same axle). They are all carefully selected by the engineers during the vehicle’s design to maximize the vehicle’s stability and ride. If the body rolls or leans on corners, stability is reduced. Suspension angles can be set to keep the roll centre – the point around which the body rotates – as low as possible. Of course, the higher centre of gravity of tall vehicles such as pickup trucks and SUVs creates more body roll. To counteract this, there are now many SUVs, some pickups and even passenger cars that are using electronics to help the suspension keep the body stable. This feature is often called roll stability control.

Roll stability control starts with the antilock brake system (ABS). The wheel speed sensors from the ABS send the wheel speed to the ABS computer, which is then used by the roll stability program to determine the direction the vehicle is turning. When the vehicle turns, the tires on the outside of the corner have to travel further to get around the corner, so they have to rotate faster. The computer can measure this difference to determine vehicle direction.

Other sensors are used as well. A steering angle sensor determines where the driver wants to go, or at least where he is steering. A yaw sensor measures the rate of directional change of the vehicle. This sensor is usually located closer to the centre of the vehicle and uses an electronic strain gauge to measure angle change rates. All of these sensors are used for vehicle stability control systems, which control the brakes to reduce the possibility of the vehicle spinning out on a corner, or the driver losing control during a sudden evasive manoeuvre. One more sensor is needed for roll stability control. This is a roll sensor, which measures the angle of the body from vertical and the rate of change of that angle.

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