November 27, 2002

Volvo's Roll Stability Control
by Jim Kerr

Concerns about the stability of SUV's have made the news across North America. It only makes sense. They are high off the ground, have a tall body structure, and are usually designed to hold a lot of cargo. On many of the SUV's built today, you will find warning labels notifying drivers of the SUV's high centre of gravity handling characteristics.

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Stiffer suspensions, lower roll centre designs, and higher tire pressures help, but the driver is still the biggest influence on vehicle stability. Corner too fast, make erratic steering inputs, or hit a curb or soft road surface while turning and you could find yourself in trouble. Now Volvo has introduced a new system to help overcome this.

Volvo calls it 'Roll Stability Control' and it is an active safety technology introduced on Volvo's first SUV, the 2003 XC90. Even with the higher centre of gravity of the XC90, especially with cargo loaded on the roof rack, Volvo wanted to retain the stability, safety, and handling of their sedans and station wagons. Thus, the roll stability control system.

The key to this system is a gyroscopic sensor located along the centerline of the vehicle, just below the cup holders. It calculates both roll angle and the speed at which the roll angle is changing. For example, if you were driving slowly off-road, the roll angle might be high but because the rate of change is slow, there is no need for the system to intervene. However, if you were cornering too fast, the roll angle might be much lower, but it is increasing rapidly so the system would intervene to help prevent a roll over.

Information from the gyroscopic sensor is fed into Volvo's Dynamic Stability and Traction Control (DSTC) computer. The computer uses information from the ABS wheel speed sensors, the throttle position sensor, steering wheel angle, steering wheel rate of change, and a 'yaw' rate sensor that measures the change in vehicle direction. If the computer determines from all these inputs that conditions exist that could be a potential roll-over, the system applies one or more brakes and reduces engine torque to make the vehicle more stable.

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The reaction of the Roll Stability Control system is similar to what a skilled driver would do if the vehicle were in a potential roll-over situation: induce understeer. Of course, the computer controls can do it much quicker and more accurately because it can apply individual brakes. Understeer means the vehicle steers less than the driver turns the steering wheel. It feels like the front end of the vehicle is 'pushing' outward in the corner.

When cornering too fast, the Roll Stability Control brakes to induce understeer and reduce the rate of body roll. At 80 kph, this will increase the vehicle's turning radius about 1/2 metre, not a significant amount, but enough to prevent a roll-over. While most drivers would brake to slow the vehicle, just as the computer does, it feels unnatural to steer in a different direction than the road is going, but that is what is necessary to reduce body roll. The computer does this precisely and quickly to enable the vehicle to still turn the corner and remain stable.

Many vehicles on the market already are equipped with stability control systems and it would be a relatively easy adaptation to include the gyroscopic sensor and advanced computer programming similar to Volvo's. The XC90 is the first in the market with Roll Stability Control. Developed jointly with Ford Motor Company, we can expect to see this type of control system on other SUV's in the near future.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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