Simple Measurements Can Predict Real World Dynamic Rollover Performance

Rollovers have been a big problem for the last 40 years, resulting in one-third (9,000) of the annual automotive fatalities and a comparable number of catastrophic (brain damage and paralysis) injuries from head and neck trauma. The problem has been traced to inadequate roof strength, considering the proximity of the head and neck to roof intrusion.

Regulations implemented in the last five years have doubled the roof strength and reduced the number of new production vehicles which rollover, but with more than 250 million vehicles on the road the mix of the fleet changes slowly.

The regulations are not without problems. Recent studies and experimental tests have shown that increased roof strength of tall or square vehicles, like the Ford F-150 and the Scion xB, have little effect in reducing roof crush and injury.

A simple new car measurement procedure has been developed, but not yet adopted to assess and rate vehicle dynamic rollover performance in three categories: good, acceptable and poor. A solution to the problem requires a change in roof geometry to a rounded cross-section at the windshield and is compounded by the popularity of tall pickups and SUVs which constitute about 40 percent of current production.

The measurement and predictive rating system consists of identifying vehicles roof strength to weight ratio (SWR), as published by the Insurance Institute for Highway Safety (IIHS), roof elasticity and measuring the distance across the front of the roof at the windshield and comparing it to the published height of the vehicle and its center of gravity. That measurement and calculation is called determining the major radius (MR).

One simple solution to the tall vehicle problem is to round the roof at the windshield in such a way that in a rollover the vehicle rolls and pitches and contacts the ground with forces taken up by the top of the fenders, hood and the strong pillar at the rear of the front door.

The geometry change would improve passenger car performance and put it into the “good” instead of “acceptable” category, but is a styling change that manufacturers find threatening to sales appeal.

The table above lists the measured parameters and compares the normalized performance (identified in dynamic rollover tests by the Jordan Rollover System) with the predicted performance. The results are also characterized by good, acceptable and poor performance.

The two identical Volvo XC-90s and Ford Explorers exhibit a measurement error of about an inch due to manufacturing tolerances, and testing variations. Therefore one inch is the minimum margin of prediction error.

The table also indicates that a tall high (MR) aspect ratio vehicle, like the F-150, despite having an improved roof SWR of 4.7, has “poor” performance. Manufacturers believe that the high aspect ratio affords improved vision, is a major selling point and are reluctant to reduce the height of pickups. The reduced height (MR) of the Toyota Highlander SUV improves its injury risk performance. The Honda CR-V illustrates an alternate design with an SWR of 2.6 and “acceptable” performance (an investigation revealed a thin, structurally spaced, inner panel glued to the outer roof panel).

In summary, this low cost procedure is a way of calculating and predicting new vehicle dynamic injury risk within the margin of test error and advising the public of the results.