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

6. 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

7. 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

8. 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

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

10. Centrifuge Test Apparatus

11. 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

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

13. 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

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

15. 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?

16. Adjusted RO/SVC; 220,000 SVC; R2 = 0. 88 Adjusted to National Avg
Adjusted RO/SVC; 220,000 SVC; R2 = 0.88 Adjusted to National Avg. Road Use and for Differences in State Reporting

17. Phase II Rollover Testing
* ABS Failure

18. Star Rating Intervals - Summary (Linear) Approach

19. Measure of Rollover Risk Rollovers 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

20. Rewarding ESC A 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

21. Monitoring ESC Effectiveness Cadillac Seville, Deville, Eldorado data from 9 states Note: very scant data for Lexus LS 400 and M-B ML320 is more encouraging
Year
Model
ESC
Crash Total
SVC
SVC/#C
1996
STD NO
3073
187
6.1%
Sporty
616 56
9.1%
1997
2004
105
5.0%
YES
391 31
7.9%

22. Dynamic Maneuver Testing Two 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

23. Path Following Test Double Lane Change

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

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

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

27. 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?

28. How Best to Reward ESC? Yaw Stability Brake Intervention
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?
Treatment of ESC requires wisdom