Final Report for the Methodology Study of the Consumer Braking Information Initiative

Work Performed by U.S. Army Aberdeen Test Center, Fall 1998

Note: The following document includes the exective summary and main text of the report. To view the complete report including appendices, go to the Department of Transportation docket website at http://dms/search/ and use Docket Number 6583.

The report is also available from the docket website in TIFF format, with somewhat better image quality than the PDF version, but it is a much larger file at 10,632k.

Executive Summary

NHTSA is investigating the feasibility of developing a braking performance measurement test procedure for light vehicles. The development of a suitable test procedure to evaluate the braking performance of light vehicles would enable NHTSA to provide braking performance information such as stopping distance, in addition to crash test performance information, as part of the agency's New Car Assessment Program (NCAP), on those new vehicles that are purchased for use in crash tests under the NCAP.

The Aberdeen Test Center, a division of the U.S. Army Material Command, in Aberdeen Maryland, was contracted by NHTSA to conduct this research effort. Tests were conducted during the Fall of 1998 on ten light vehicles, using straight line stops on dry and wet asphalt, from an initial speed of 62 mph, with each vehicle in both lightly-loaded and fully-loaded conditions. The purpose of the tests was to determine if variability in stopping distance could be minimized, to collect sufficient data to permit statistical analysis of the results, and provide direction in developing a test procedure.

Braking tests were conducted on five passenger cars, two passenger mini vans, one full-size cargo van, one full-size sport utility vehicle, and one full-size pickup truck. All of the vehicles were equipped with a four-wheel antilock braking system (ABS), except for the pickup truck which had a rear-wheel only ABS. The vehicles were leased and were either 1998 or 1999 model year vehicles, with mileages between 2,300 and 18,000 miles. The tires on each vehicle were replaced with new tires of the same make, model, and size as the original tires. Each vehicle's brakes were inspected for normal wear, but were not replaced or subjected to conditioning other than from normal, as-received use. The new tires were conditioned by driving at 50 mph for 50 miles.

Selecting vehicles that were equipped with four wheel ABS was a decision intended to minimize the variability in stopping tests. If a vehicle does not have ABS, then the test driver must skillfully apply the brakes to attain minimum stopping distance without locking the vehicle's wheels. Conversely, it was reasoned that a vehicle with ABS acting on all wheels could be braked sufficiently hard to activate the ABS (i.e., at least some of the wheels would lock up if the ABS was not present), and as long as the brake pedal force remained high enough to keep the ABS activated for the duration of the stop, then the ABS would keep the vehicle at its optimal level of braking. The pickup truck that only had rear-wheel ABS was acquired inadvertently and could not be included in the final results, but did provide useful information on brake pedal force at the threshold of front wheel lockup.

A peak brake pedal force of 112 lbs. (500 N) was targeted to be consistent with pedal forces specified for certain tests in Federal Motor Vehicle Safety Standard No. 135, Light Vehicle Brake Systems. However, brake applications as high as 450 lbs. were experienced during early testing, generally with the peak brake pedal force occurring at the top of the initial pedal force ramp-up. Subsequent efforts were made to target a steady pedal force of 150 lbs., with emphasis on rapid achievement of this force. Exceeding the target pedal force was not found to affect the stopping distance, however, since the ABS took control of the braking forces to prevent wheel lockup. For the pickup truck that was equipped with rear-wheel ABS, pedal forces in the 25 to 35-lb. range were found to be the pedal force just prior to front wheel lockup, and the peak pedal forces could not be achieved as rapidly as for the vehicles that had four-wheel ABS.

For each condition of load (lightly-loaded and loaded to Gross Vehicle Weight Rating [GVWR])and road condition (wet and dry asphalt), ten stops were made for a total of forty stops per vehicle. The driver was permitted to first make several test stops to become familiarized with each vehicle, and to warm up the brakes. After each stop, the vehicle was driven around the test area to cool the brakes, and then the brake rotors and drums were checked with a hand-held pyrometer to check that front rotor temperatures (which were always hotter than the rear brake drums/rotors) were below 212 degrees F before the next stop was conducted. One of the passenger cars was used as a control vehicle to provide comparative stopping data throughout the test program, and this vehicle was instrumented with thermocouples in the front brake linings to provide additional lining temperature data throughout the testing.

Road friction measurements of the test area were made eight times during the test period using a skid trailer. On each day that road friction was measured, ten measurements of the dry asphalt and ten measurements of the wet asphalt were made, and average dry and wet values were derived. The average peak coefficient of friction ranged from 0.89 to 0.95 for dry pavement and 0.85 to 0.88 for wet pavement. These measurements indicate that the asphalt surface was in good condition.

For each set of ten stops, the mean stopping distance was calculated along with the standard deviation and 95^th percentile stopping distance. Analysis of the pedal force attained during the first 0.3 seconds of brake application was used to develop the classification of a stop as Class A, B, C, and D, with Class D representing the slowest ramp-up of pedal force. Elimination of the slowest, Class D stops was found to have some effect on reducing the standard deviation (and hence 95^th percentile stopping distance) for some of the vehicles, while for other vehicles there was not an appreciable difference in eliminating the Class D stops. Appendix D provides an analysis of the effects on eliminating each successively slower class of stops from the ten stops for each condition of road and load. Appendix E provides final statistics for each vehicle with Class D stops removed. Note that in some cases, the remaining number of Class A, B, and C stops is small thus statistical significance of the mean and standard deviation is reduced. Also note that the Class A through D stop classifications do not apply to the pickup truck since much lower pedal forces were maintained in order to prevent front wheel lockup. Future research will be useful in determining what class of stop (e.g., Class C or better) can be consistently attained for most or all light vehicles equipped with four wheel ABS, now that these classifications have been identified.

NHTSA does not intend this report to provide comparative stopping distance information for the vehicles tested. Rather, the research effort is a preliminary effort to develop a test protocol that could be used in the future to measure the braking performance of NCAP vehicles. Further research is anticipated to further develop the test protocol, and determine, for example, if stopping tests can be replicated at other test facilities with consistent results. NHTSA is also coordinating this effort with European and Japanese governments with a goal of having a harmonized, international method that could be used to rate the braking performance of light vehicles.