Transcript Document 7302495

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
Overview of Vehicle Effect
Rollovers
as a % of
All Crashes
Rollovers
as a % of
Single Veh.
Crashes
Pickup
Trucks
SUVs
Cars
Vans
1.6 %
2.0 %
3.7 %
5.1 %
13 %
14 %
24 %
32 %
0.50
Rollovers per Single-Vehicle Crash
Linear Regression Model, Summary Data
Logistic Regression Model, Individual Variables
Logistic Regression Model, Scenario-Risk
0.40
0.30
0.20
0.10
0.00
0.90
1.00
1.10
1.20
1.30
SSF
1.40
1.50
1.60
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.
Centrifuge Test Apparatus
Test vehicle
Stationary
s upport pole
Counter
weight
Rotating centrifuge
Minimal
foundation
required
Electric
drive
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:
CSV =
ö
2 gI oxx æ TW 2
2
ç
+ hcg - hcg ÷÷
2 ç
Mhcg è
4
ø
Vehicle Motion
I oxx
æ TW 2
ö
2
= I xx + M ç
+ hcg ÷
è 4
ø
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/2h
First 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 = 0.88
Adjusted to National Avg. Road Use and for Differences in State Reporting
1.00
Rollovers per Single-Vehicle Crash
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.90
1.00
1.10
1.20
1.30
SSF
1.40
1.50
1.60
minor
moderate
Two
Wheel Lift major
Limit Condition
Phase II Rollover Testing
no limit condition or
wheel lift for test
conditions
Explorer
Ranger
E150 Club Wagon
Astro
Tahoe
Tracker
Chevrolet S-10
Chevrolet C1500
Caravan
Metro
Lumina
Neon
* ABS Failure
SSF
1.07
1.08
1.11
1.13
1.13
1.14
1.16
1.23
1.26
1.28
1.37
1.44
J-Turn
Fishhook1
Fishhook2
No Brake
Pulse Brake Right Left to Right Left to
Right Left Right Left to Left Right to Left Right
oversteer oversteer oversteer
*
debead
oversteer
oversteer
oversteer
oversteer
oversteer
oversteer
oversteer
oversteer
oversteer
debead
oversteer understeer oversteer
debead
n/a
oversteer
understeer understeer
oversteer
oversteer
Probability of Rollover per Single Vehicle Crash
Star Rating Intervals - Summary (Linear) Approach
50%
40%
30%
20%
10%
0%
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
Static Stability Factor
1.4
1.45
1.5
1.55
1.6
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
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
Monitoring ESC Effectiveness
1996-7 Cadillac Seville, Deville, Eldorado
1996-9 data from 9 states
Note: very scant data for Lexus LS 400 and M-B
ML320 is more encouraging
Year
Model
ESC
SVC
SVC/#C
NO
Crash
Total
3073
1996
STD
187
6.1%
1996
Sporty
NO
616
56
9.1%
1997
STD
NO
2004
105
5.0%
1997
Sporty
YES
391
31
7.9%
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
Path Following Test
Double Lane Change
50'
L1
L2
50'
Comparison of Double Lane Change
Steer Input for Two Drivers
500
400
Steer Angle (deg)
300
200
100
KS Input
EAP Input
0
-100
-200
-300
-400
0
0.5
1
1.5
Time (sec)
2
2.5
3
HANDWHEEL ANGLE
Defined Steering of Fishhook
Maneuver
Close to full
lock
Approx. 270
degrees
TIME
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