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Background to NHTSA NCAP Ratings for Rollover Resistance

Background to NHTSA NCAP Ratings for Rollover Resistance Why are they based on Static Stability Factor? Criticism of Rollover Ratings Vehicle properties have little effect Static Stability Factor (SSF) is too simplistic SSF does not reward Electronic Stability Control (ESC)

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Background to NHTSA NCAP Ratings for Rollover Resistance

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  1. Background to NHTSA NCAP Ratings for Rollover Resistance Why are they based on Static Stability Factor?

  2. Criticism of Rollover Ratings • Vehicle properties have little effect • Static Stability Factor (SSF) is too simplistic • SSF does not reward Electronic Stability Control (ESC) • Rollover ratings should be on dynamic tests • Rollover ratings will confuse customers

  3. What This Presentation Addresses • Why we think vehicle properties affect rollover • Requirements for a rollover rating system • Evaluation of several laboratory metrics • Our measure of rollover risk – ro/svc • The problem of rewarding ESC • Observations about dynamic maneuver tests

  4. Overview of Vehicle Effect

  5. Requirements for Rollover Rating System • Represent tripped and untripped rollovers • Correlation with rollover crash statistics • Clear causal relationship to rollover • Objective and repeatable measurements • Low potential for unintended consequences • Understandable by consumers

  6. Tripped and Untripped Rollover • 82% of Rollovers are in SVC • 95% are tripped (curb, soft soil, ditch, guard rail, side slope, rim dig, etc.) • <5% are untripped (external forces provided by pavement friction) • Driving maneuvers test only for untripped • Low tire traction, massive understeer and some ESC strategies that have no effect on a vehicle that encounters a tripping mechanism will prevent wheel lift in maneuver tests. • Laboratory Metrics better relate to tripped rollover

  7. Laboratory Metrics • Tilt Table Ratio or Centrifuge Test • Critical Sliding Velocity • SSF • Highly cross correlated because of the importance of cg height and track width • Each correlate with crash statistics

  8. Tilt Table Angle (TTA)Minimum table angle at which a vehicle on the table will tip over.

  9. Centrifuge Test Apparatus

  10. Why Not Tilt Table or Centrifuge? • Their advantage is measurement of suspension and tire deflection effect • Test performance increases when both tires lift simultaneously • Roll stiffness ratios for best score cause more oversteer than current practice • Potential for unintended consequences

  11. Critical Sliding Velocity (CSV)Theoretical minimum lateral speed for tripped rollover: Vehicle Motion

  12. Why Not Critical Sliding Velocity? • CSV adds the effect of roll moment inertia on tripped rollovers • The rigid body model causes CSV to be less than realistic (range 10 to 15 mph) • Increase in CSV through higher roll moment causes theoretical loss of maneuver test performance • Consumer perception is the problem

  13. Static Stability Factor (SSF) - t/2hFirst order estimate of steady state lateral acceleration at wheel lift

  14. SSF is the Best Choice? • Represents 1st order causal influences on rollover - overturning and restoring moments • C.G. height measurement accurate to 0.5% • Least possibility of bad trade-offs • Simple concept - intuitive to consumers • Remaining Questions • What is its correlation to real rollover crashes? • How important is its effect? • What about untripped rollover?

  15. Adjusted RO/SVC; 220,000 SVC; R2 = 0.88Adjusted to National Avg. Road Use and for Differences in State Reporting

  16. Phase II Rollover Testing * ABS Failure

  17. Star Rating Intervals - Summary (Linear) Approach

  18. Measure of Rollover RiskRollovers per Single Vehicle Crash (ro/svc) • single veh. ro/ 10k register vehicles = (# c/10k rv) X (svc/# c) X (ro/svc) • # c/10k rv: driven by driver/road effects • svc/# c: influenced by driver/road; also will show effect of ESC • ro/svc; least sensitive to driver/road effects • Better to consider factors separately

  19. Rewarding ESCA Problem for SSF • What is ESC? • Treatment in consumer info web-site • Expected to reduce (svc/total crashes) • Expected to reduce untripped rollover • Too new for much statistical evidence • NHTSA monitoring ESC effectiveness

  20. Monitoring ESC Effectiveness1996-7 Cadillac Seville, Deville, Eldorado1996-9 data from 9 statesNote: very scant data for Lexus LS 400 and M-B ML320 is more encouraging

  21. Dynamic Maneuver TestingTwo Main Types • Path following – Double Lane Change • Plus – Face Validity • Minus – Objectivity and Repeatability • Defined Steering- Fishhook • Plus – Objectivity and repeatability • Minus – Less Face Validity • Information added by maneuver tests • Roll momentum effect at steering reversal • Operation of ESC

  22. Path Following TestDouble Lane Change

  23. Comparison of Double Lane Change Steer Input for Two Drivers

  24. Defined Steering of Fishhook Maneuver Close to full lock Approx. 270 degrees

  25. Defined Steering Test - Fish Hook Vehicle 1 Path Vehicle 2 Path

  26. Difficulties Common to All Driving Maneuver Tests • Driver safety • High cost • Effect of outriggers • Effect of tire wear • Complexity of ratings • Correlation to crash statistics unlikely • Effect of pavement friction variation • May be overwhelmed by the brake intervention aspect of ESC- good or bad?

  27. How Best to Reward ESC? • Yaw Stability • Original purpose of ESC • Cannot be duplicated by driver action • May not be rewarded by maneuver tests • Future data needed to know effectiveness • Brake Intervention • Not different from driver input • Biggest vehicle attribute in maneuver test? • Future data needed to know effectiveness • Treatment of ESC requires wisdom

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