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) • Rollover ratings should be on dynamic tests • Rollover ratings will confuse customers
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
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
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
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
Tilt Table Angle (TTA)Minimum table angle at which a vehicle on the table will tip over.
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
Critical Sliding Velocity (CSV)Theoretical minimum lateral speed for tripped rollover: Vehicle Motion
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
Static Stability Factor (SSF) - t/2hFirst order estimate of steady state lateral acceleration at wheel lift
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?
Adjusted RO/SVC; 220,000 SVC; R2 = 0.88Adjusted to National Avg. Road Use and for Differences in State Reporting
Phase II Rollover Testing * ABS Failure
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
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
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
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
Defined Steering of Fishhook Maneuver Close to full lock Approx. 270 degrees
Defined Steering Test - Fish Hook Vehicle 1 Path Vehicle 2 Path
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?
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