Steering Column Movement in Severe Frontal Crashes and Its Potential Effect on Airbag Performance
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STEERING COLUMN MOVEMENT IN SEVERE FRONTAL CRASHES AND ITS POTENTIAL EFFECT ON AIRBAG PERFORMANCE David S. Zuby for measurements made on crash test dummies sub- Brian O’Neill jected to various crash and airbag tests. FMVSS 204, Insurance Institute for Highway Safety which specifies performance characteristics for the United States of America steering system, limits the amount of horizontal steer- Paper No. 136 ing column intrusion allowed in a 48 km/h crash against a rigid wall to 127 mm. An earlier standard, ABSTRACT FMVSS 203, limited the force on a block represent- ing a body impacting the steering wheel at 24 km/h; Excessive movement of steering columns in this standard no longer applies to vehicles with air- crashes can significantly degrade the performance of bags. These various standards have led to steering restraints, especially airbags. Although steering col- systems that generally include some form of collapsi- umn movement does not appear to be a major prob- ble, force-limiting steering column that is attached to lem in full-width rigid barrier crashes, it can be an the vehicle’s interior structure through so-called shear issue in other frontal crash types. Results from 106 capsules. These capsules allow the force-limiting frontal offset crash tests at 64 km/h (40 mi/h) were mechanism to act under the influence of a forward- used to characterize different patterns of steering directed force on the steering wheel but inhibit rear- column intrusion for different vehicle types. Large ward movement of the column. movements of the steering column often were associ- These design elements were found to be some- ated with the dummy’s head striking the steering what effective at preventing steering-system-induced wheel through the airbag. Some of the tested models injuries. Kahane (1982) found fewer deaths and inju- were redesigned over the course of this testing, and ries in vehicles whose steering systems complied comparisons with older designs showed that improv- with FMVSS 203 and 204, compared with vehicles ing the structural integrity of the occupant compart- that did not meet these standards. Nevertheless, many ment could lead to less longitudinal movement of the injuries were still associated with steering system steering column, but this was not necessarily the case contacts. A study of crashes occurring in the United for vertical column movements for some models in Kingdom found that many of the force-limiting the data set. Multipurpose passenger vehicles— mechanisms did not perform in real crashes the way pickups, utility vehicles and passenger vans—tended they did in regulatory tests because off-axis forces to have more vertical steering column movement than increased the columns’ resistance to collapse (Gloyns cars. Examples of fatal frontal crashes from the Na- et al., 1980). Concern about the shortcomings of these tional Automotive Sampling System/Crashworthiness designs was mitigated by the advent of driver airbags, Data System and serious frontal crashes from the which was associated with a significant reduction of Insurance Institute for Highway Safety’s Seven- injuries associated with steering system contacts. For County Crash Investigation Study were examined to example, the National Highway Traffic Safety Ad- better understand the real-world consequences of ministration estimates that the combination of an steering column movement. Both crash samples in- airbag plus a lap/shoulder belt reduces the risk of cluded cases in which serious and fatal injuries were serious head injury by 81 percent compared with 60 attributed to driver contact with the steering wheel percent for belts alone (Insurance Institute for High- despite the presence of a deployed airbag. Some way Safety (IIHS), 2001a). Still, injuries continue to examples of these injuries occurred in crashes with- be associated with steering wheel contact. Augenstein out catastrophic collapse of the occupant compart- et al. (1995) reported on liver and spleen injuries ment, and some of the drivers were belted. As crash sustained by drivers of airbag-equipped cars and test results suggest, movement of the steering column attributed some of these injuries to contact with the in frontal crashes can degrade real-world airbag ef- steering wheel, for instance. Such steering-system- fectiveness, and this phenomenon deserves more induced injuries to drivers of airbag-equipped vehi- attention than it has received in the past. cles may occur because movement of the steering column during the crash puts the airbag out of the INTRODUCTION position where it can provide protection. A laboratory re-creation of a 1991 National Federal Motor Vehicle Safety Standard (FMVSS) Automotive Sampling System (NASS) crash (case 208 specifies frontal crash protection requirements 79-021A) at IIHS’s Vehicle Research Center illus- for vehicles sold in the United States. The standard trates that even in a moderately severe crash, move- prescribes passenger protection by specifying limits ment of the steering column can be great enough to Zuby 1 prevent the airbag from protecting a driver’s chest average of column displacements in each of three (Arbelaez and O’Neill, 2001). In this re-creation, a directions: longitudinal (positive values represent 1991 Dodge Grand Caravan was crashed into the rear forward displacement), lateral (positive values repre- of a stationary 1958 Chevrolet Bel Air at 64 km/h. sent leftward movement), and vertical (positive val- Early in the crash sequence as the airbag inflated, the ues represent upward movement). steering column rotated upward and exposed the lower portion of the steering wheel rim to the un- Table 1. belted dummy’s chest. The dummy recorded moder- Steering Column Displacement (N=106) ately high sternum deflections, but all of its ribs were Longitudinal Lateral Vertical permanently deformed by this impact. The driver of (cm) (cm) (cm) the vehicle in the NASS crash died of massive chest Minimum -21 -11 -5 injuries, which were attributed by the NASS investi- Average -3.8 0.1 9.0 gators to bottoming out the airbag; however, the Maximum 11 29 27 crash re-creation indicates the more likely cause was contact with the steering wheel rim due to column Even in this severe frontal crash, rearward dis- rotation. Other NASS cases in which occupants with placement of the steering column is rarely greater airbags sustained fatal injuries also illustrate that than the limits imposed by FMVSS 204. Figure 1 airbag protection can be compromised when steering shows the distribution of the longitudinal steering columns move in crashes. Further evidence on the column displacements. Figure 2 shows that rearward key role of steering columns is provided by results displacement of the steering column is correlated from 64 km/h frontal offset crash tests—tests in with rearward displacement of the instrument panel which steering columns tend to move much more (R=0.81) and that large longitudinal steering wheel than in federally mandated 48 km/h rigid barrier tests. displacements tend to be associated with collapse of This paper examines the role of steering column the occupant compartment. Lateral displacements of movement in frontal crashes and the extent to which the steering wheel tend to be small and to the left in it can compromise restraint system performance. this crash condition. Vertical displacement of the steering column, although correlated with vertical CRASH TEST RESULTS movement of the instrument panel (R=0.81), is only weakly correlated with longitudinal movement of the Since 1995, IIHS has amassed a database of instrument panel (R=-0.30), as shown in Figures 3 more than 100 frontal offset crash tests at 64 km/h and 4, thus demonstrating that upward movement of involving new vehicle designs. All of the tests were the steering column can happen even in vehicles with crashes against a fixed barrier with the same deform- strong occupant compartments. able face specified for European regulatory tests Large vertical movements of the steering column (IIHS, 2000). The crashes were offset to the driver in frontal offset crash tests often were associated with side such that 40 percent of each vehicle’s overall the dummy’s head contacting the steering wheel width was aligned with the crash barrier. Measure- through the airbag or other indications of suboptimal ments from sensors in a belted Hybrid III 50th per- restraint system performance. Table 2 lists the crash centile male driver dummy and measurements of tests in which head contact with the steering wheel vehicle deformation, which included displacement of was observed, along with measurements of steering the steering column, were recorded for each test. The column displacement and the resulting head accelera- IIHS (2000) crashworthiness evaluation protocol tion and maximum head injury criterion (HIC). Some describes in detail how the tests were conducted and high head accelerations were recorded when the the measurements were made. dummy’s head hit the steering wheel, although the Measurements of steering column displacement, maximum HIC generally did not indicate an exces- which compare the precrash and postcrash positions sively high head injury risk. Nevertheless, these of the steering wheel center, do not reflect the maxi- steering wheel contacts would be associated with an mum displacement of the column end during the elevated risk of face or head injury compared with crash. Still, these measurements indicate the extent to tests in which the dummy’s head did not contact the which column movement is controlled by various steering wheel. The vertical steering column move- design features. The 106 tests used in this analysis ment among these models was greater on average represent the frontal crash performance of 102 unique (11.5 cm) than vertical displacement in vehicles in vehicle designs and show a large variation of steering which head contact with the steering wheel was not column displacement even among vehicles whose observed (8.1 cm).