NHTSA Published Update of ESC Analysis
The August 10, 2011 Federal Register contained NHTSA’s updated statistical analysis on its existing Safety Standard 126, Electronic Stability Control Systems. The report’s title is: Crash Prevention Effectiveness in Light-Vehicle Electronic Stability Control: An Update of the 2007 NHTSA Evaluation. The Notice stated:
“Statistical analyses based on data for calendar years 1997 to 2009 from the Fatality Analysis Reporting System (FARS) and the General Estimates System (GES) of the National Automotive Sampling System (NASS) estimate the long-term effectiveness of electronic stability control (ESC) for passenger cars and LTVs (light trucks and vans). Safety Standard 126 establishes standards for electronic stability control systems manufactured for use in light vehicles. This report is an update of a previous NHTSA analysis of ESC effectiveness (72 FR 41582) published in 2007.”
“The principal findings are that ESC was associated with a six percent decrease in the likelihood that a vehicle would be involved in any police reported crash and an 18 percent reduction in the probability that a vehicle would be involved in a fatal crash. For passenger cars, the reductions are 5 percent and 23 percent, respectively; for LTVs, 7 percent and 20 percent. Each of these reductions is statistically significant except for the 5 percent overall effect in cars.”
Comments from the public are solicited and must be received by December 8, 2011.
Fire Suppression in Buses and Coaches
According to the brandposten (English edition) the number of fires in buses and coaches has more than doubled since the end of the 1990s. As a result many Buses and Coaches are now equipped with automatic fire suppression systems in the engine compartment. The brandposten goes on to describe a test method for evaluating fire suppression systems. There was an interesting recall on this topic in which a fire suppression system in certain Glaval Buses was incorrectly made with the discharge nozzles incorrectly placed in the driver’s area. The consequence was described: “In the event of a system discharge, a significant cloud of powder could be released into the driver’s compartment possibly obstructing the driver’s view. This situation could result in a vehicle crash.”
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.
Fuel Fill Pipe Damage, Leakage and Fire by Tire Tread Separation
Filed under: Fill Pipe, Fuel Tank, Testing, Tread Separation
By Mark Arndt
Given knowledge of the dangers, the notion that tire failures occur on vehicles traveling at highway speeds is frightening. Yet, such failures occur and the general acceptance of tire failure is so deeply rooted in vehicle performance that spare tires are standard equipment.
A specific rollover incident in which a rear tire tread separation caused tearing apart of a fuel tank fill pipe routed just behind the wheel well was recreated in a controlled test (watch video). The incident resulted in fuel spillage, fire and burn injuries. The October 2001 incident involved a 1995 Land Rover Discovery with a General AmeriStar tire failure. The vehicle manufacture’s corporate representatives were shown the test in late 2002 during depositions.
High speed video captured contact between tire tread and reinforced rubber fuel hose as the flailing end of the tread separates from the tire. Repeated contacts by the tread to the fill pipe fuel hose and vehicle underbody result in fill pipe failure and release of liquid from the fuel tank. Tests of a peer vehicle did not produce a failure or leakage.
Regarding vehicle handling and tire tread separation, failures a relationship between vehicle design and loss of control is scientifically documented. Only recently has an incremental improvement in vehicle handling following tire tread separation been demonstrated with Electronic Stability Control (ESC). Regarding other aspects of vehicle performance in tire failures, specifically including tire tread separations, dangers exist that can enhance the chance of harm.
It is a well know consequence of tire failures that the tire tread can damage the vehicle. In tire tread separations substantial damage to the wheel well sheet metal is probable. Tire tread failure induced damage has been documented to hydraulic brake lines, parking brake cables, tail lights, fuel fill pipes, wiring and bumpers. Parts of a car, not to mention the tire tread, can be knocked free and onto the road surface. Vibrations from a tire failure have tripped inertially activated fuel pump cut off switches resulting in unexpected engine cut-off.
Engineers can readily foresee similar scenarios for a variety of safety equipment that is taken for granted in motor vehicles. For example:
- A tire tread separation occurs at night, damages wiring that routes near the wheel well and renders driver’s suddenly blind to the road or hazards on the road,
- A tread separation causes permanent damage to hydraulic brake lines or parking brake cables resulting in brake failure or compromised performance.
- A tread separation causes damage to a light cover or reflector, often rear taillight breakage is observed.
That these are important events in a vehicle’s safety performance is simply supported by the fact that Federal Motor Vehicle Safety Standards (FMVSS) regulate the performance of vehicle systems that are directly dependent upon the key components described above.
A tire failure event could be analogous to the Part 581 Bumper Standard. Low speed, often parking related, bumper contacts occur in normal driving. Comparatively, tire failures are also expected – driving manuals instruct how to react to a tire failure and most cars have spare tires. Anticipating low speed contacts, the Part 582 Bumper Standard covering all passenger motor vehicles sold in the United States prescribes protective criteria for: lamps, reflective devices and head light alignment; operation of doors; fueling and cooling systems; propulsion, suspension, steering and braking systems; impact energy absorbers; fasteners and joints; and, even separations of surface material, paints and coatings and permanent deviations of original contours. Comparatively, following a tire failure a vehicle should be capable of performing at the minimum level of safety prescribed by applicable FMVSS. Vehicle design interventions can effectively eliminate dangers from tire failures induced vehicle component damage.
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.
Fuel Tank Fill Pipe Valve Prevents Leakage in Crash
A recent crash test conducted by the San Francisco Law Firm Lieff Cabraser Heimann & Bernstein (under the direction of Transportation Safety Technologies, Inc.) has provided an important measure of the feasibility and performance of fuel tank fill pipe valves in crash situations when the fill pipe is severed or damaged.
The valve positioned inside the fuel tank at the end of the fuel tank fill pipe was part of the Onboard Refueling Vapor Recovery (ORVR) system. ORVR was a safety feature first dictated for passenger cars in 1998 by the US Environmental Protection Agency (EPA) to limit fuel emissions. The ORVR required that fuel vapor generated during vehicle refueling be stored on the vehicle instead of at the gas station. Stored refueling vapors are burned in the vehicle engine after refueling.
A common feature of ORVR systems are mechanism that minimize fuel atomization as it enters the fuel tank and limits to migration of vapors out of the tank when the fuel cap is off. Most manufactures utilize a one way valve on the fuel tank fill pipe. The valve is located either in the fuel tank or in-line of the fill pipe. Some vehicle manufactures have utilized valves that serve a dual purpose of vapor barrier for ORVR and liquid fuel barrier for crashworthiness. The dual purpose valve is a preferred and logical choice given the vulnerability of some fuel tank fill pipes and fuel caps in crashed. Sport Utility Vehicles with their high rates of rollovers are natural benefactors of valves that prevent spilled gasoline from fuel tank fill pipes.
The crash test involved a 2001 Kia Sportage with a modified rear suspension and fuel tank shield. The vehicle was stuck by a Federal Motor Vehicle Safety Standard 214/311R (FMVSS214/301R) moving deformable barrier ballasted to 5011 pounds. The Sportage fuel tank was filled to 15.4 gallons which is about 92 percent of its refill capacity. The Sportage was hit squarely at the rear with the left edge of the barrier aligned 16.5 inches to the right of the Sportage centerline – a right offset rear impact.
As a result of the crash the modified rear suspension posed no threat of puncture to the fuel tank and the fuel tank shield worked. There were no punctures of the fuel tank, but there was a separation of the fuel tank fill pipe. A reinforced rubber interconnecting fill pipe hose tore apart in the crash. The fill pipe failure was observable only as the fuel tank was removed. A static rollover conducted after the crash test pursuant to the rollover test requirement s of FMVSS301 demonstrated no fuel leakage from any portion of the fuel tank – proof that the one way flow valve in the fuel tank fill pipe worked.
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
