Consumers Union (CU), the publisher of Consumer Reports, is dissatisfied with the new system for rating the rollover risks of motor vehicles unveiled today by the National Highway Traffic Safety Administration (NHTSA). After an initial review of the new ratings system, CU repeated its concerns that NHTSA has based its rollover rating system solely on the vehicle's so-called static stability factor (SSF), rather than basing it on driving tests and the dynamic performance of the vehicles. "While we believe that providing consumers some information is preferable to providing no information at all, Consumers Union has serious reservations about using a static measure such as SSF to determine for comparative purposes the stability of vehicles," said Dr. R. David Pittle, CU's senior vice president and technical director. Dr. Pittle noted that SSF utilizes only two aspects of the vehicle, both of which are static measurements -- the center of gravity height and the track width. (SSF is calculated as T/(2H), T being the track width and H being height above the ground of the vehicle's center of gravity with one or more occupants in place). Important vehicle-design elements such as the suspension, tire design, and steering response affect a vehicle's stability and differ from vehicle to vehicle -- but are not accounted for by SSF. "There is no real way to know how a vehicle will act in an emergency situation by simply measuring its shape at rest," Pittle said. "We must see how it performs when it is driven, when the whole vehicle is acting as a complete dynamic system. We are encouraged that under the recent TREAD Act, Congress has directed NHTSA to develop a dynamic test for vehicle stability." CU has been in the forefront of testing vehicles for routine handling and emergency handling, as well as braking, acceleration, fuel economy, comfort and convenience. The test results appear in Consumer Reports and other CU publications. In 1996 CU petitioned NHTSA to develop a consumer information program that provided consumers comparative data on the rollover risks of SUVs based on dynamic testing -- that is, a program based on actual driving tests. NHTSA granted CU's petition. However, CU was disappointed last May when it learned that NHTSA had abandoned its plans for dynamic testing in favor of a static formula such as SSF, which CU believes is too coarse a measure to compare vehicle stability within the same class of vehicles. Last year, Congress passed an auto safety law called the TREAD Act (Transportation Recall Enhancement, Accountability and Documentation Act). The law directs NHTSA to develop a dynamic test for rollover risks to be used as the basis for its consumer information program. "We applaud Congress for its efforts to uphold and improve the government ratings for rollover risks. When NHTSA develops a valid dynamic test, the results should provide a far more accurate measure of vehicle stability than what is being offered today," said Sally Greenberg, senior product safety counsel for CU. In comments filed with NHTSA last August, CU noted that, despite its reservations about SSF, the static measure does have some positive attributes. "The SSF metric shows the importance of vehicle load conditions. We note that on NHTSA's website, the SSF rating for a given vehicle, particularly SUVs and minivans, may vary depending on whether the vehicle is carrying a full load of passengers and cargo. When loaded, many of these vehicles drop one star in NHTSA's rating. SSF could also be a useful element of evaluating the propensity for tripped rollovers, but needs to be used in conjunction with a dynamic stability test," said David Champion, director of CU's Automotive Testing Division.