The Take Away on Rollover Drag Factors
Dolly rollover tests suggest that the appropriate drag factor range for use in rollover reconstruction, excluding special circumstance, is 0.38 to 0.50. The finding is from the calculated results of 81 dolly rollover crash tests statistically trimmed to exclude the upper and lower 15 percent.
Reevaluation of roll phase analysis of rollover tests on an actual highway found lowered average roll phase drag factors. The average roll phase drag factor as published in the papers was 0.53 g (min = 0.39, max = 0.74) and the average reevaluated drag factor was 0.45 g (min = 0.36, max = 0.52). Natural rollover tests suggest the appropriate drag factor range, excluding special circumstances, is 0.39 to 0.50. The finding is from the calculated results of 21 naturally occurring rollover crash tests statistically trimmed to exclude the upper and lower 15 percent.
Getting Rollover Drag Factors Right
A comparison was conducted of numerous historical studies by reexamination of the original works, analysis of their data, and centralized compilation and analysis of their results. In total 81 dolly rollover crash tests, 24 naturally occurring rollover crash tests, and 102 reconstructed rollovers were identified. Of the 24 naturally occurring tests 18 were steer induced rollover tests.
The range of drag factors for all examined dolly rollovers was 0.38 g to 0.50 g with the upper and lower 15 percent statistically trimmed. The average drag factor for dolly rollovers was 0.44 g (Standard Deviation = 0.064) with a reported minimum of 0.31 g and a reported maximum of 0.61 g. The range of drag factors for the set of naturally occurring rollovers was 0.39 g to 0.50 g with the upper and lower 15 percent statistically trimmed. The average drag factor for naturally occurring rollovers was 0.44 g (Standard Deviation = 0.063) with a reported minimum of 0.33 g and a reported maximum of 0.57 g.
Reevaluation of roll phase analysis published in two papers reporting results of rollover tests on an actual highway (Asay, 2009 and 2010) found lowered average roll phase drag factors as shown in table 1. The average roll phase drag factor published in the papers was 0.53 g (min = 0.39, max = 0.74) and the average reevaluated drag factor was 0.45 g (min = 0.36, max = 0.52). Reevaluation was performed from data published in the papers. A final analysis should be conducted using the actual test data.
Published results of reconstruction derived roll phase drag factors (Hight, 1972) between 0.40 g and 0.65 g was confirmed as the range representing the middle 60% of pre-1972 reconstructed rollover crashes on flat ground, figure 3. The reconstructed drag factors were in a range of 0.04 g to 1.20 g for all 102 plotted results, including downhill rollovers and rollovers with vertical drops. For rollovers on flat ground the reconstructed range was 0.21 g to 0.83 g.
Sudden Acceleration in Reverse
I ran across an interesting recall this morning. It is interesting because it points to potential for problems with software in the control modules of key safety systems. The recall summary is:
“Honda is recalling certain model year 2011 CR-Z passenger cars with manual transmissions, manufactured from January 8, 2010, through June 27, 2011. Should the engine stall while the brake pedal is not pressed, there is a possibility that the engine control unit (ECU) software may cause the electric motor of the hybrid system to move the vehicle unexpectedly in the opposite direction of the selected gear.”
Essentially this Honda recall identified a defect that results in sudden acceleration in the opposite direction of intended travel – sudden acceleration in reverse.
This is not the only recent recall on this issue: Buick Lacrosse’ were recalled because an improper diagnosis:
“may cause the ESC to falsely activate, resulting in sudden changes in vehicle handling and deceleration, particularly at higher speeds, which may cause the driver difficulty in maintaining the vehicle’s desired path of travel and desired vehicle speed, and could result in a crash without warning.”
Essentially the Lacrosse recall identified a defect that may result in brakes application on one side of a vehicle while it is traveling at highway speeds – sudden acceleration to the left (or right).
NTSB Reports Important Steer Axle Tire Failure Testing Results
After conducting testing of natural tire delamination and simulated tire blowout failures on the steer axle of a motorcoach, the US National Transportation Safety Board (NTSB) reported important results that questioned pervasive thought and conventional guidance to drivers regarding such events. Conclusions derived from the report include:
- Tire delaminations produced rotation and torque at the handwheel,
- Some tire failures trials produced lateral force impulse that presented considerable difficulty for maintaining control, while others were “no problem” and presented little challenge,
- Tire delaminations produced sudden, though different from event to event, turning of the vehicle,
- Detection of underinflated tires by visual inspection or “thumping” was not accurate and not advisable. A tire gauge must be used to evaluate tire pressure,
- Progressive loss of air pressure – even to less than 50% of manufactures recommended pressure – were imperceptible while driving and
- Braking, contrary to conventional wisdom and guidance, was shown to not degrade, but improved vehicle control.
The testing was commissioned in an effort to explain factors that may have contributed to a driver’s loss of control of a motorcoach. To this end the NTSB conducted a literature review and ultimately testing regarding effects of steering axle tire failures on handling characteristics and dynamics of a bus or motorcoach. The Board cited numerous studies it found instructive, but no published studies examining the dynamics of buses or motorcoaches in response to tire failure. For this reason, the NTSB undertook its tests to evaluate the effects of steering axle tire delamination or blowout failures on a driver’s ability to maintain control of a motorcoach. Although it was not part of the original plan, the effect of braking on vehicle control was also specifically evaluated.
The NTSB tests were supported by the parties to its investigation and with the active participation, advice and assistance of individuals associated with: Continental Tire, Federal Motor Carrier Safety Administration, Motor Coach Industries, Inc., Greyhound Lines, Inc., The Goodyear Tire & Rubber Company, SmarTire Systems, Inc., Continental AG, Detroit Diesel/Allison Transmission Distributor and TRW, Inc.
Test conditions and response were recorded via the Last Stop Record which provided a snapshot of vehicle data for the 104-second interval preceding engine shutdown, including the vehicle speed (mph), engine speed (rpm), engine load percentage, throttle percentage, and whether the brakes were applied at the time of the tire failure and/or in the intervening period until the vehicle was brought to a stop. Sensors to monitor tire pressure and temperature and steering wheel torque were used. Global Positioning System (GPS) data were recorded to track the location of the test vehicle on the track.
New Test Results: A Breakthrough in Understanding Front Tire Failure Crashes
Filed under: Crash Reconstruction, Random, Testing, Tread Separation
By Mark Arndt
Not all tire tread separations are equal and new testing documents previously unknown differences between a front tire failure and a rear tire failure. Almost universally, tread separation event testing is limited to rear tire failures. Most of the Ford Explorer/Firestone Tire crashes involved rear tires and the causes of these crashes are attributed to a variety of vehicle factors – the largest factor relates to adverse changes in vehicle controllability.
So why do vehicles that have front tire tread separations get into crashes?
The answer, in part, is explained because despite decreased sensitivity to steering the failure event is startling, produces violent vibration and loud noise and pulling. Pulling is turning of the vehicle without the driver turning the steering wheel. Of course, the vehicle steering characteristics also changed suddenly and nonsymmetrically, complicating the driving task. New testing of front tire tread separation demonstates for some vehicles a substantially increased pulling response comparable to equivalent rear tire failure. New testing also documents a torque response transmitted through the steering wheel that may jerk the steering wheel from the driver’s grip.
As a rule of thumb, when a rear tire experiences a tread separation the resulting change in the vehicle’s understeer gradient, a key measure of the vehicle turning characteristics, is roughly three degrees per G (3 deg/G) . Where, G is equal to the acceleration of gravity. And, when a rear tire experiences a tread separation event all vehicles ever tested respond in dynamic maneuvers with oversteer – in other words, they spin-out.
It is perplexing that the same changes at the tire that makes a vehicle spin-out when there is a rear tire failure also makes a vehicle less likely to spin-out when there is a front tire failure – in other words, when there is a front tire failure the vehicle will understeer more and become less sensitive to steering. The new testing results show that an external disturbance may play a greater role that previously understood.
Black Box Proven Accurate and Valuable to Crash Reconstruction
By Mark Arndt
A paper recently published at the 2009 SAE World Congress demonstrates the accuracy and utility of speed data collected by the Powertrain Control Module (PCM) of late model Ford vehicles. Testing described in the paper was completed in conjunction with an evaluation of Electronic Stability Control (ESC) systems supported by Tab Turner of the law firm Turner & Associates.
An instrumented 2005 Ford Explorer was used to evaluate speed data provided from its PCM at high slip angles and other dynamic maneuvers. The slip angle is the angle between the heading of a vehicle and its velocity direction –- a vehicle that is side-slipping or spinning out has a high slip angle.
PCM speed was compared to speed and slip angle collected from a calibrated velocity sensor. In addition to speed, slip angle and other standard handling test measurements the vehicle brake switch and throttle were recorded so PCM data could be synchronized. After each test run the vehicle ignition was turned off and the PCM was downloaded using commercially available Bosch hardware and software. The principal maneuver was the National Highway Traffic Safety Administration (NHTSA) sine-with-dwell test consisting of a 0.7 HZ sinusoidal steer with a 0.5 second dwell at the steer reversal peak.
Runs were conducted with the vehicle’s Electronic Stability Control (ESC) disengaged so that the test vehicle would achieve large slip angles. Other dynamic maneuvers included: NHTSA’s sine-with-dwell with ESC engaged; 100% accelerator to 80 mph with 0.5G braking to stop; and acceleration to 50 mph with maximum ABS braking to stop.
Results demonstrate agreement between the speed recorded by the calibrated instrumentation and speed recorded by the vehicle’s PCM for conditions when the vehicle slip angle and rear wheel slip were near zero. PCM speed was lower than instrumented speed in high slip angle maneuvers. PCM on average underreported during maximum ABS braking and at medium to high speed in 0.5G braking. In acceleration the PCM speed had no detectable under-reporting error except at the highest speeds with 100% accelerator application.
Mark W. Arndt
