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40 The researchers compared the characteristics of vehicle types more likely to roll over on slopes, as identified from this crash data analysis, to MASH test vehicles (33). MASH, however, does not specify a particular make and model of the test vehicles. It gives a range of design parameters that the vehicle should fall within, including mass, dimensions, center of mass location, location of engine, location of drive axle, and type of transmission. Thus, the comparison between MASH design vehicles and those identified as most likely to rollover on slopes in Chapter 3 was made by extracting dimensional, weight, and other physical characteristics data. Expert AutoStats software, which currently contains over 42,000 cars, pickup trucks, vans, and utility vehicles from the 1940s model years to present, was used to extract the vehicle data of interest (31). The Expert AutoStats software database is commonly used in accident reconstruction problems. Most of the design parameter values in the database are measured values, except the roll, pitch, and yaw moments of inertia. The moments of inertia values are approximate and are based on analytical calculations. The comparison of the MASH vehicles was made with the top five vehicles likely to roll over on slopes for each of the eight vehicle classes. These 40 vehicles were presented previously in Table 3.8. Comparisons were made with the MASH 1100C vehicle (2425-lb small passenger car) and 2270P vehicle (5000-lb pickup truck). Vehicles selected for the 1100C and 2270P were Kia Rio (2006) and Dodge Ram (2005), respectively. These are the most commonly used MASH test vehicles used by the testing labs. The researchers collected data for the following vehicle design parameters: â¢ Curb Weight â¢ Curb Weight, Front Axle â¢ Curb Weight, Rear Axle â¢ Drive Wheels (front or rear) â¢ Total Length â¢ Wheelbase â¢ Bumper to Axle Distance (front) â¢ Bumper to Axle Distance (rear) â¢ Bumper to Windshield (front) â¢ Bumper to Window (rear) â¢ Maximum Width â¢ Front Track Width â¢ Rear Track Width â¢ Static Stability Factor â¢ Height of Front Bumper Top â¢ Height of Hood Top â¢ Height of Rear Bumper Top C H A P T E R 4 Comparison of Vehicle Characteristics
Comparison of Vehicle Characteristics 41 â¢ Height of Trunk Top â¢ Turning Cycle (diameter) â¢ Angle of Tires at Maximum Steering â¢ Break Type (ABS or not) â¢ Transmission Type â¢ CG Inches Behind Front Axle â¢ CG Inches from Side â¢ CG Inches from Ground â¢ Yaw Moment of Inertia â¢ Pitch Moment of Inertia â¢ Roll Moment of Inertia The researchers considered various design parameters and compared them to the MASH pickup trucks and small passenger car vehicles. The researchers also considered how meaning- ful comparisons could be made between the design parameters and the vehicleâs propensity to roll over. It was concluded that using the SSF and the roll moment of inertia to compare stability and rollover propensity was most meaningful. Shown in Figure 4.1 are the ranges of SSF for vehicles most likely to roll over on slopes, for each vehicle class. The higher SSF value implies greater stability. The SSF values for the MASH pickup trucks and small cars are also shown for comparison purposes. Similarly, shown in Figure 4.2 are the ranges of roll moments of inertia for vehicles most likely to roll over, along with the roll moments of inertia values for MASH vehicles. As presented in Chapter 3, the researchers had determined the RRs of slope-related rollovers of one particular vehicle body type relative to the rest of the vehicle types using the FARS data. The results of this crash data analysis are presented again in Figure 4.3. Various similarities can be observed between the crash data analysis results (Figure 4.3) and the results of the vehicle SSF and roll movement of inertia ranges (Figures 4.1 and 4.2). For instance, the 2-door and Figure 4.1. SSF comparison of vehicles most likely to roll over on slopes.
42 Guidelines for Traversability of Roadside Slopes Figure 4.2. Roll moment of inertia comparison of vehicles most likely to roll over on slopes. (Data Source: FARS, 2004-2010; Posted Speed Limit: 45-75 mph) 0.0 1.0 2.0 3.0 4.0 5.0 2-Dr Sedan 4-Dr Sedan Compact Utility Large Utility Minivan Large Van Compact Pickup Standard Pickup Vehicle Body Type R el at iv e R is k (v s. A ll O th er T yp es C om bi ne d) Figure 4.3. RR of slope-related rollovers for one particular vehicle body type to that of the rest of the body types and associated confidence intervals, FARS data.
Comparison of Vehicle Characteristics 43 4-door sedans have the lowest RR of slope-related rollovers (see Figure 4.3), are the most stable because of higher SSF (see Figure 4.1), and have lower roll moment of inertia (see Figure 4.2). Compact utility vehicles and large utility vehicles have high RR of slope-related rollovers (see Figure 4.3), are less stable because of low SSF (see Figure 4.1), and have a higher roll moment of inertia. Similar trends exist for other vehicle types. The exception is the large van, which has a small RR of rollover from the actual crash data analysis (see Figure 4.3), but is much more unstable (i.e., lower SSF, see Figure 4.1) and has a relatively high roll moment of inertia (see Figure 4.2). Trends generated by comparing vehicle design features such as SSF and roll moment of inertia do not take into account many other factors that are somewhat accounted for in the crash data. So differences are expected. Rollover studies have been conducted for decades by NHTSA, NAS, and other research institutes. Over the years, many âvehicle factorsâ and âdriver maneuvering behavioral factorsâ have been researched. NHTSA has consistently defended the use of SSF as a key âvehicle factorâ in rating vehicle rollover propensity through various studies. See Special Report 265: The National Highway Traffic Safety Administrationâs Rating System for Rollover Resistance: An Assessment (2002) for a historical review (34). NHTSAâs current rollover estimates are based on rollover risk models developed in its New Car Assessment Program (32). There are many other factors that can influence the vehicleâs stability. There is no straight- forward way to establish a relationship between the various design parameters and the propensity for rollover on slopes. Furthermore, if the traversability guidelines are developed using just MASH vehicles, it is not possible to make meaningful adjustments to the guidelines based on differences in vehicle design parameter of other vehicle classes in comparison to MASH vehicles. Because of these concerns, the researchers eventually performed detailed simulation analyses using the MASH vehicles (small passenger cars and pickup trucks) and some of the vehicle types identified as likely to roll over from the crash database. This was considered that most efficient and sound methodology for incorporating different types of vehicles in the develop- ment of the traversability guidelines. The exercise of comparing difference vehicle properties to MASH vehicles did, however, guide the researchers in selecting vehicle makes and models of the non-MASH vehicles for use in the simulation analyses. Results of these simulations will be presented in following chapters.