An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance Special Report 265 (2002) / Chapter Skim
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Chatper 2: Vehicle Dynamics
Chatper 2: Vehicle Dynamics
Pages 21-37
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From page 21... ...
• Dynamic testing is performed on a test track and involves driving maneuvers. Although dynamic tests are potentially helpful in understanding the events immediately preceding rollover, they are expensive and require safety precautions for test drivers.
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From page 22... ...
The vehicle model leading to SSF characterizes the onset of rollover as a scenario in which the lateral forces become large enough that, if they continue long enough, rollover must result. As Figure 2-1 indicates, T is the track width (strictly, the average of the front and rear track widths)
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From page 23... ...
. (NOTE: Fy = lateral force; N = normal force; W = weight of vehicle.)
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that are calculated from measured vehicle parameters; and • Quantities derived from tests of entire vehicles -- notably the tilt table test, side pull test, and centrifuge test -- that depend on experimental results instead of measurements of vehicle dimensions and inertial properties. The following discussion addresses the advantages and disadvantages of these different static measures, with particular emphasis on SSF -- the metric that forms the basis for NHTSA's star ratings for rollover resistance.
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From page 25... ...
That is, the sum of the lateral forces is equal to the product of a tire–road friction coefficient, μ, and the weight of the vehicle. In this case, Relationship 1 yields the information that if, for a sustained period of time, μ > T/2H (5)
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From page 26... ...
In other scenarios not involving a smooth road surface, the large lateral forces resulting in rollover can be generated by interactions between the tire(s) and a curb, a pothole, a roadside slope, a furrow plowed during an off-road maneuver, or some other tripping mechanism.
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From page 27... ...
Thus, TTR yields a lower threshold of minimum lateral acceleration needed to pro Velocity, v Rigid obstruction FIGURE 2-4 Configuration for use of critical sliding velocity.
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From page 28... ...
In particular, best test results are obtained by having front and rear uphill wheels lift at the same time. This means vehicles with balanced front and rear roll stiffness will yield better TTR test results than otherwise similar vehicles with unequal roll stiffness, even though unequal roll couple distribution often produces improved dynamic performance (Federal Register 2001)
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From page 29... ...
SOURCE: Federal Register 2000.)
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From page 30... ...
Detractors point out that the side pull test has one of the same flaws as TTR: it is possible in some cases to obtain improved results by making suspension changes that degrade the vehicle's directional response. Furthermore, the side pull test is difficult to perform.
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From page 31... ...
During transient maneuvers involving high lateral acceleration, many vehicle design parameters have been shown to have an effect on vehicle handling behavior (e.g., front-to-rear roll couple distribution, roll axis location, tire behavior, suspension characteristics, and roll resonant frequency, to name a few)
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From page 32... ...
Phase 2 Phase 3 Phase 1 Transition region Control Region Out-of-control region x = vehicle heading Beginning loss of control Complete loss of control v = velocity vector Region of effectiveness of electronic stability control x x v v x,v Roadway surface v=0 x FIGURE 2-8 Phases of a rollover crash.
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From page 33... ...
Once the vehicle is in this configuration, SSF and the terrain over which the vehicle is moving are the dominant determinants of whether rollover will occur. The main motivation for dynamic testing is the need to understand the transition region of Figure 2-8, where driver actions, vehicle design, and such features as electronic stability control are important.
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From page 34... ...
DYNAMIC TESTING: FEATURES AND CHALLENGES Important Features The characteristics that distinguish dynamic testing from other vehicle tests are the transient nature of the controls applied to the vehicle and the vehicle's subsequent transient response. Dynamic tests can address one of three possible regimes: the normal driving range, which includes lateral accelerations up to about 0.3 g on a smooth, dry surface; the midrange of lateral acceleration, usually up to about 0.5 g on a smooth, dry surface; or the very high range of lateral acceleration.
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From page 35... ...
Given the complex trade-offs associated with vehicle design, NHTSA will be challenged to select one or more dynamic tests that will not have unintended detrimental consequences as manufacturers pursue a competitive advantage by designing to the test(s)
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From page 36... ...
Dynamic testing provides information on a vehicle's crash-avoidance characteristics; it discriminates among vehicles with a similar SSF but a different likelihood of getting into out-of-control situations. Both static measures and dynamic testing are needed to investigate a rollover crash in its entirety, from initiation to final outcome.
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From page 37... ...
2001. Consumer Information Regulations; Federal Motor Vehicle Safety Standards; Rollover Resistance; Request for Comments.
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