Consumer Braking Information

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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 95th 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 95th 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.

THE MAIN TEXT OF REPORT BEGINS BELOW

AUTOMOTIVE INSTRUMENTATION TEAM

 

 

USATECOM PROJECT NO.: 1-VG-620-000-015

DATE: 3 March 1999

REPORT NO.: 99-AIT-17

 

 

 

 

 

 

 

 

 

METHODOLOGY STUDY

FOR

THE CONSUMER BRAKING INFORMATION INITIATIVE

 

 

 

 

 

DATES OF TEST: 20 September 1998 through 20 November 1998

 

 

PREPARED BY: Gregory A. Schultz, U.S Army Aberdeen Test Center
  Michael J. Babinchak, Dynamic Sciences Incorporated

 

 

APPROVED:
(original signed)
JOHN R. WALLACE
Leader, Automotive Instrumentation Team

 

 

 

 

U.S. ARMY ABERDEEN TEST CENTER

ABERDEEN PROVING GROUND, MARYLAND 21005-5059

 

TABLE OF CONTENTS 

I.	BACKGROUND

II.	OBJECTIVE

III.	TASK 1  Perform Braking Performance Tests and Investigate the Causes of Stopping Distance Variability

	Procedure
	Test Results
	Analysis
	Conclusion
	
IV.	TASK 2  Provide Details on Methodology to Address Variability

	Pedal Effort
	Vehicle Parameters
	Environmental Test Conditions
	Instrumentation and Measurement Techniques
	Test Sample Size
	
V.	TASK 3  Develop a Test Protocol for the Braking Initiative

	General Test Conditions
	Procedural Conditions
	Required Test Data
	Measurement Techniques
	Road Test Procedures

VI.	TASK 4 - Identify a Method to Report Braking Performance to Consumers
	
VII.	TASK 5  Develop a Test Report Format

	APPENDIX A	Vehicle Photographs
	APPENDIX B	Individual Brake Stop Results
	APPENDIX C	Sample Pedal Effort Plots
	APPENDIX D	Brake Stop Statistics with Pedal Effort Breakdown
	APPENDIX E	Final Performance Statistics for Each Vehicle
	APPENDIX F	Brake and Tire Temperature Data Sheets
	APPENDIX G	ATC Meteorology Data
	APPENDIX H	ASTM Frictional Skid Resistance Test Data
	APPENDIX J	Sample Wind Force Calculation
	APPENDIX K	Consumer Performance Measures
	APPENDIX L	Test Report Format

i

  1. BACKGROUND
  2. The U.S. Army Aberdeen Test Center (ATC) has performed a methodology study on passenger vehicle brake testing in support of an effort by the National Highway Traffic Safety Administration (NHTSA) to develop an effective consumer braking information program. With the implementation of this program, consumers would have access to brake performance information obtained from standardized test procedures, in addition to the collision safety information currently available.

     

  3. OBJECTIVE
  4. The objectives of this methodology study were the following:

    Task 1 - Perform braking performance tests and investigate the causes of stopping distance variability.

    Task 2 - Provide details on a test methodology to minimize variability.

    Task 3 - Develop a test protocol for the braking initiative.

    Task 4 - Identify a method to report braking performance to consumers.

    Task 5 - Develop a test report format.

     

  5. TASK 1 -Perform Braking Performance Tests and Investigate the Causes of Stopping Distance Variability
    1. Procedure
    1. General Procedure

Service brake effectiveness tests were conducted on 10 vehicles with anti-lock brake systems (ABS). Testing consisted of straight-line brake stops from 100 km/hr (62 mph). The brakes were applied so that the ABS was activated as quickly as possible and fully invoked throughout the brake stop until the vehicle came to rest. Vehicle speed, stopping distance and pedal force were measured and recorded during each stop event. The vehicles were operated by professional test drivers with brake test experience ranging from low to high. Each vehicle was tested under two payload configurations on both wet and dry asphalt surfaces.

The initial criteria for vehicle selection was for each test item to be less than one year old with between 8,000 and 16,000 km (5,000 and 10,000 miles) of usage. However, some exceptions to this rule were allowed based on vehicle availability. A list of the vehicles used during testing is presented in Table 1 and a photograph of each vehicle is included in Appendix A. All of the vehicles were equipped with four-wheel ABS except for the Dodge Ram 1500 4x4, which was equipped with only rear ABS. Additionally, each vehicle selected had an automatic transmission.

TABLE 1. SUMMARY OF TEST VEHICLES

 

Vehicle No.

Make

Model

Year

Mileage

Brake specifications

ABS

Front

Rear

1

Pontiac

Grand Am

1998

9,483

4-wheel

rotors

drums

2

Ford

Expedition

1998

5,050

4-wheel

rotors

rotors

3

Toyota

Camry

1998

18,020

4-wheel

rotors

drums

4

Chevrolet

Malibu

1998

8,436

4-wheel

rotors

drums

5

Cadillac

DeVille

1998

2,283

4-wheel

rotors

rotors

6

Chevrolet

Express (1-ton)

1999

3,200

4-wheel

rotors

drums

7

Dodge

Ram 1500 4x4 (shortbed)

1998

14,840

rear-wheel

rotors

drums

8

Dodge

Caravan

1998

15,200

4-wheel

rotors

drums

9

Chevrolet

Astro

1998

8,500

4-wheel

rotors

drums

10

Pontiac

Bonneville

1998

5,100

4-wheel

rotors

drums

 

Vehicle No. 1 (Pontiac Grand Am) was also used as a baseline vehicle throughout testing. This vehicle was subjected to three instrumented brake stops each day of testing. These data were used to investigate variations in stopping distance caused by changes in environmental test parameters such as road surface friction, wind speed and ambient temperature.

Prior to testing, the OEM tires on each vehicle were replaced with new tires of the same make, model and size as the originals. Tire inflation pressures were set and maintained at the suggested levels shown on the tires. In retrospect, the inflation pressures recommended by the vehicle manufacturers should have been used. This change is reflected in Task 3, the test protocol. Following the tire replacement, each of the vehicles was operated for 80 km (50 miles) at approximately 80 km/hr (50 mph) to provide a limited break-in for the tires. No additional brake burnish procedure was conducted.

The weight distribution of each test item was determined with the vehicle empty and after being payloaded. The payload was configured based on the recommended gross vehicle weight (GVW) and maximum axle ratings reported on the driver-side door. Sand bags and body weight simulators, as shown in Figure 1, were used as payload.

Figure 1. Body Weight Simulator.

 

Testing was conducted at ATC's Phillips Army Airfield facility, near the intersection of Runways 17 and 22. The longitudinal test course grade was 0.1 percent, with each brake stop performed upslope. Road surface frictional coefficients were measured and recorded by the Eastern Federal Lands Highway Division of the Federal Highway Administration per ASTM E1337, both with and without water delivery. Frictional data were taken prior to testing and re-measured periodically to ensure consistent conditions throughout testing. The test rig is shown in Figure 2.

 

 

Figure 2. Frictional Coefficient Test Rig.

The test matrix for the brake performance test was as follows:
Vehicle TestBaseline Test
Vehicle #Configuration# stopsVehicle #Configuration# stops
1a,b,c,d40
2a,b,c,d401a3
3a,b,c,d401a3
4a,b,c,d401a3
5a,b,c,d401a3
6a,b,c,d401a3
7a,b,c,d401a3
8a,b,c,d401a3
9a,b,c,d401a3
10a,b,c,d401a3

 

Four test configurations were implemented:

    1. No payload, dry asphalt
    2. Full payload, wet asphalt
    3. No payload, wet asphalt
    4. Full payload, dry asphalt

Test data collected included:

The test instrumentation installed on each vehicle consisted of ATC's Advanced Onboard Computer System (ADOCS), a pedal force transducer, a rolling fifth-wheel, driver displays and brake-lining thermocouples (on vehicle No. 1 during baseline testing). A GSE Inc. model 114350 pedal effort transducer, Serial No. 90, was installed on the brake pedal to measure pedal force. A Nucleus model NC8 fifth-wheel, Serial No. 8479, shown in Figure 3, was used to measure vehicle speed and rolling distance. The resolution of the fifth-wheel and the force transducer were 0.01 m (0.03 ft) and 1.0 N (0.23 lb), respectively.

Figure 3. Grand Am Vehicle with Fifth-Wheel.

 

Other test instrumentation consisted of a hand-held, thermocouple-type pyrometer for measuring brake component temperature, tire temperature and ambient roadway temperature. Average wind speed, peak wind speed, average wind direction and wind direction standard deviation were obtained in 15-minute intervals using an anemometer provided by ATC's Meteorology Team (MET).

Stopping distance, vehicle speed and brake pedal force data were sampled dynamically during each brake stop event. The stopping distance measurement was triggered by the vehicle brake light circuit and ended when the vehicle came to rest. The sampling rates for the fifth-wheel and pedal force transducer were 200 Hz and 10 Hz, respectively. To account for variability in the target speed at brake application, the measured stopping distances were normalized to 100 km/hr (62 mph) in accordance with SAE J299 (August 1987). All other vehicle-related measurements were obtained statically.

The following three sections describe the procedures used for brake application, brake temperature measurements and cool-down, and water application on the test surface. Each of the three procedures evolved to some degree during early testing and therefore, are being given separate consideration. While it is not typically desirable to modify procedures during testing, early results showed that some modifications were required.

 

  1. Brake Application Procedure

Test drivers were initially instructed to perform brake stop events in a manner simulating a panic stop, with the transmissions left in drive. The goal was to fully invoke the ABS as quickly as possible, exceed the 500-N (112-lb) force limit used for compliance testing in FMVSS 135 and maintain a steady application until the vehicle came to rest. This brake application method emphasized vehicle performance, as opposed to driver performance, and ensured that all vehicle brake systems were controlled with sufficient force for peak ABS performance.

While testing the first two vehicles, average steady-state application forces typically varied from 1100 to 1500 N (250 to 350 lb), with peak forces as high as 2000 N (450 lb). Immediate generation of these high forces produced high initial application rates, generally exceeding 500 N (112 lb) in 0.1 seconds. While these high rates were desirable, the high steady-state forces were considered excessive. Therefore, a 660-N (150-lb) target was established for the steady-state force.

A different brake application method was required for the Dodge Ram 1500 4x4, since the vehicle was equipped with only rear ABS. In order to avoid lock-up of the front wheels, drivers had to perform brake stops with less pedal force than with the other vehicles, while still achieving optimum brake performance. This limitation resulted in brake stops with significantly lower initial ramp-up rates and subsequent steady-state force levels.

 

  1. Brake Temperature Measurement and Cool-Down Procedure

Prior to performing each brake stop, brake lining temperatures were required to be kept below 100 oC (212 oF). Since the use of thermocouples within the brake linings was not included in the scope of this test, thermocouple-type pyrometers were used to measure temperature. For disk brakes, the lining temperature on the exposed side of the outer brake pads was recorded, and for drum brakes the reading was taken on the outer surface of the drums, adjacent to the swept area of the brakes. The initial assumption was that the temperature gradient across the lining material and drum material was relatively small.

As brake temperature data was collected during testing of vehicle No. 1, brake rotor temperatures were also measured and recorded. A substantial difference was noted between the temperature of the front rotors and the temperature of the back of the pad linings. While the temperatures on the exposed side of the brake pads were found to be below the 100 oC (212 oF) limit, the rotor temperatures rose above 100 oC (212 oF) and reached as high as 196 oC (385 oF).

To gain a better understanding of the heat transfer across the brake pads, thermocouples were installed in the front brake pads and rear brake shoes of vehicle No. 1 prior to its use as a baseline vehicle. The thermocouples were placed approximately 1/16 inch below the lining surface adjacent to the rotor. During testing of vehicle No. 2, brake temperatures of the baseline vehicle were monitored and recorded using the thermocouples as well as manually with the pyrometer.

An examination of the temperature results showed that the thermocouple data closely matched the temperatures obtained from the rotors with the pyrometer. The findings revealed that the temperature at the outer surface of brake pads was not an accurate representation of the brake lining temperature. It was noted that the rotor and brake pad combination could be modeled as a classic heat equation problem, with the rotor temperature assigned as one boundary condition of the pad. Using this approach, a continuous temperature gradient would be expected across the pad with the temperature on the rotor side of the pad being equal to the rotor surface temperature.

Based on this model and an analysis of the test data, it was concluded that measurements of the rotor surface temperatures yielded relatively accurate measurements of the lining temperatures at the lining/rotor interface. It was also noted that the rear brake shoe temperatures of vehicle No. 1, obtained using thermocouples, were significantly lower than the temperatures of the front brakes.

As a result of these findings, the brake temperature measurement procedure was modified. Throughout the remainder of testing (starting with the third test vehicle), the temperatures of the front brake rotors were measured with the pyrometer and these readings were used as the temperature indicator to keep below 100 oC (212 oF). Rear brake temperatures were also recorded, but were always significantly cooler than the front.

Typically, after each brake stop, the front rotor temperatures were above the 100 oC (212 oF) limit and the next stop could not be initiated. To cool the brakes, the vehicle was operated at approximately 80 km/hr (50 mph) for a short period of time after each brake stop. Experimentation showed that the temperatures could be controlled and stabilized with the cool-down procedure lasting between 6 to 10 minutes, depending on the ambient temperature.

 

  1. Water Application Procedure

For wet asphalt testing, water was applied to the road surface using the water tanker rig shown in Figure 4. The truck was operated at approximately 32 km/hr (20 mph), while water from the tanker was placed over the test surface using the distribution pipe shown on the back of the tanker. The water was released through holes placed along the longitudinal axis of the pipe with pressure generated from the pressure head in the tanker.

Prior to wet surface testing, three passes were made with the water tanker traveling longitudinally along the test area, as shown in Figure 5. The first two passes were made side-by-side, and the third pass was made overlapping the center of the lane created by the first two passes. The total length of the wet area was approximately 150 m (500 ft). Prior to each brake stop event, an additional pass was made with the water tanker along the center lane where the brake stops were conducted. Water was distributed with the intent to fully wet the asphalt surface without creating excessive standing water.

 

 

Figure 4. Water Tanker Rig.

 

Figure 5. Water Application Procedure.

When vehicles No. 2 and 4 (Ford Expedition and Chevrolet Malibu) were tested on wet asphalt, some hydroplaning was experienced. An inspection of the test area revealed that standing water as deep as 1/4 inch had collected in minor depressions on the test course. To avoid this condition in later testing, the test area was displaced approximately 45 m (150 ft) farther up the runway, while still remaining within the area where the frictional measurements were taken. During subsequent testing, the water depth generally remained below 3 mm (1/8 inch).

 

    1. Test Results
  1. Weight Distribution

The weight distribution of each vehicle without payload is presented in Table 2. The weight distribution of each vehicle when fully payloaded and its corresponding gross vehicle weight (GVW) rating is presented in Tables 3 and 4, respectively. All weights were taken with driver and ADOCS included and with the vehicle fully fueled.

  1. Center of Gravity (CG)

The longitudinal CG locations of each vehicle both empty and fully payloaded are presented in Tables 5 and 6, respectively. All tests were conducted with driver weight (using sand bags) and ADOCS included and with the vehicle fully fueled.

TABLE 2. VEHICLE WEIGHT DISTRIBUTIONS WITHOUT PAYLOAD

 

Weight

 

Front axle

Rear axle

Total

Vehicle

kg

lb

kg

lb

kg

lb

Pontiac Grand Am SE

910

2000

580

1280

1490

3280

Ford Expedition

1360

3000

1220

2700

2580

5700

Toyota Camry LE

910

2000

610

1340

1520

3340

Chevy Malibu LS

940

2080

550

1220

1490

3300

Cadillac DeVille

1200

2640

760

1680

1960

4320

Dodge Caravan SE

1110

2440

860

1900

1970

4330

Dodge Ram 1500 4X4

1440

3180

960

2120

2400

5300

Chevrolet Express (1-ton)

1260

2780

980

2160

2240

4940

Chevrolet Astro

1120

2460

970

2140

2090

4600

Pontiac Bonneville

1080

2380

630

1380

1710

3760

TABLE 3. VEHICLE WEIGHT DISTRIBUTIONS, FULLY PAYLOADED

 

Weight

 

Front axle

Rear axle

Total

Vehicle

kg

lb

kg

lb

kg

lb

Pontiac Grand Am SE

1020

2260

800

1760

1820

4020

Ford Expedition

1440

3180

1820

4000

3260

7180

Toyota Camry LE

960

2120

920

2020

1880

4140

Chevy Malibu LS

1020

2240

790

1740

1810

3980

Cadillac DeVille

1260

2770

1070

2370

2330

5140

Dodge Caravan SE

1220

2700

1210

2660

2430

5360

Dodge Ram 1500 4X4

1450

3200

1450

3200

2900

6400

Chevrolet Express (1-ton)

1500

3300

1710

3780

3210

7080

Chevrolet Astro

1260

2780

1420

3140

2680

5920

Pontiac Bonneville

1140

2510

940

2070

2080

4580

 

 

 

 

TABLE 4. GVW MANUFACTURER RATING

 

Weight

 

Front axle

Rear axle

Total

Vehicle

kg

lb

kg

lb

kg

lb

Pontiac Grand Am SE

1028

2266

796

1755

1824

4021

Ford Expedition

1564

3450

1872

4128

3266

7200

Toyota Camry LE

1088

2400

1088

2400

1896

4180

Chevy Malibu LS

1008

2223

800

1764

1808

3987

Cadillac DeVille

1259

2776

1076

2372

2335

5148

Dodge Caravan SE

1245

2746

1245

2746

2430

5360

Dodge Ram 1500

1726

3806

1726

3806

2902

6400

Chevrolet Express (1-ton)

1633

3600

1799

3968

3220

7100

Chevrolet Astro

1270

2800

1428

3150

2698

5950

Pontiac Bonneville

1141

2516

942

2078

2083

4594

TABLE 5. CENTER OF GRAVITY, WITHOUT PAYLOAD

 

Measurement

 

Longitudinal (forward from rear axle)

Vehicle

cm

in

Pontiac Grand Am SE

168.4

66.3

Ford Expedition

159.0

62.6

Toyota Camry LE

159.8

62.9

Chevy Malibu LS

170.4

67.1

Cadillac DeVille

177.5

69.9

Dodge Caravan SE

171.5

67.5

Dodge Ram 1500 4x4

204.5

80.5

Chevrolet Express (1-ton)

193.0

76.0

Chevrolet Astro

150.9

59.4

Pontiac Bonneville

177.3

69.8

 

 

 

 

 

TABLE 6. CENTER OF GRAVITY, FULLY PAYLOADED

 

Measurement

 

Longitudinal (forward from rear axle)

Vehicle

cm

in

Pontiac Grand Am SE

153.7

60.5

Ford Expedition

133.9

52.7

Toyota Camry LE

134.9

53.1

Chevy Malibu LS

151.1

59.5

Cadillac DeVille

155.7

61.3

Dodge Caravan SE

151.9

59.8

Dodge Ram 1500 4x4

170.9

67.3

Chevrolet Express (1-ton)

160.0

63.0

Chevrolet Astro

130.8

51.5

Pontiac Bonneville

153.2

60.3

 

  1. Braking

Brake stop results from each vehicle in all four test configurations are presented in Table 7. Results from day-to-day baseline testing with the Pontiac Grand Am are presented in Table 8. Stopping distances and deceleration rates shown for each vehicle configuration are averages of all stops conducted that were considered to follow the guidelines presented in the test procedure. Brake stops not conducted properly were removed from the data set.

Results from each individual brake stop for each vehicle can be found in Tables B-1 through B-11 in Appendix B. Sample plots of applied pedal effort versus time can be found in Appendix C in Figures C-1 through C-10. Each figure contains pedal force plots from all brake stops conducted within a specific test configuration. One group of plots from each test vehicle is included.

TABLE 7. AVERAGE BRAKE STOP RESULTS FROM 100 KM/HR (62 MPH)

 

Dry surface

Wet surface

 

Stopping distance

Deceleration rate

Stopping distance

Deceleration rate

Vehicle

m

ft

m/sec2

ft/sec2

m

ft

m/sec2

ft/sec2

without payload

Pontiac Grand Am SE

45.1

147.9

8.0

26.2

58.0

190.1

6.2

20.4

Ford Expedition

52.0

170.4

6.9

22.7

60.6

198.9

5.9

19.5

Toyota Camry LE

48.7

159.7

7.4

24.2

53.6

175.7

6.7

22.0

Chevy Malibu LS

43.1

141.3

8.4

27.4

45.8

150.3

7.9

25.8

Cadillac DeVille

47.7

156.4

7.5

24.8

49.9

163.8

7.2

23.6

Dodge Caravan SE

48.7

159.8

7.4

24.2

50.7

166.3

7.1

23.3

Dodge Ram 1500 4x4

60.7

199.2

5.9

19.4

63.9

209.6

5.6

18.5

Chevrolet Express (1-ton)

50.7

166.4

7.1

23.3

54.7

179.3

6.6

21.6

Chevrolet Astro

51.9

170.2

6.9

22.7

53.3

174.9

6.7

22.1

Pontiac Bonneville

47.8

156.7

7.5

24.7

49.2

161.3

7.3

24.0

fully payloaded

Pontiac Grand Am SE

46.3

152.0

7.8

25.5

52.3

171.5

6.9

22.6

Ford Expedition

51.5

168.8

7.0

22.9

67.0

219.9

5.4

17.6

Toyota Camry LE

49.2

161.5

7.3

24.0

53.1

174.3

6.8

22.2

Chevy Malibu LS

47.0

154.0

7.7

25.1

50.0

164.1

7.2

23.6

Cadillac DeVille

50.4

165.2

7.1

23.4

50.0

163.9

7.2

23.6

Dodge Caravan SE

52.8

173.1

6.8

22.4

58.1

190.6

6.2

20.3

Dodge Ram 1500 4x4

57.5

188.5

6.3

20.5

62.6

205.2

5.8

18.9

Chevrolet Express (1-ton)

55.0

180.4

6.5

21.5

56.3

184.7

6.4

21.0

Chevrolet Astro

55.9

183.4

6.4

21.1

57.7

189.1

6.2

20.5

Pontiac Bonneville

49.7

162.9

7.2

23.8

50.5

165.5

7.1

23.4

 

TABLE 8. AVERAGE BRAKE STOP RESULTS FROM 100 KM/HR (62 MPH),

PONTIAC GRAND AM BASELINE TESTING

   

Stopping distance

Deceleration rate

Date of testing (1998)

Vehicle tested same day

m

ft

m/sec2

ft/sec2

7 October

Expedition

45.7

149.8

7.9

25.8

8 Octobera

Expedition

47.9

157.1

7.5

24.6

9 October

Expedition

43.7

143.3

8.2

27.0

13 October

Camry

44.3

145.4

8.1

26.6

14 October

Camry

45.5

149.4

7.9

25.9

15 October

Camry

42.6

139.8

8.4

27.7

19 Octoberb

Malibu

47.0

154.3

7.7

25.1

20 October

Malibu

44.4

145.7

8.1

26.6

22 October

DeVille

42.9

140.6

8.4

27.5

23 October

DeVille

42.8

140.3

8.4

27.6

30 October

Caravan

43.6

143.0

8.3

27.1

2 November

Caravan

43.5

142.6

8.3

27.2

3 November

Caravan

43.7

143.4

8.2

27.0

6 November

Ram 1500 4x4

44.6

146.4

8.1

26.4

12 November

Express (1-ton)

44.6

146.2

8.1

26.5

18 November

Astro

43.8

143.7

8.2

26.9

20 November

Bonneville

43.3

142.1

8.3

27.2

aTesting was conducted on damp pavement with no free standing water.

bTires were rotated before testing.

 

  1. Analysis

An analysis of the data was conducted to investigate the variability in braking performance of each vehicle and to determine the sensitivity of the brake stops to variables such as pedal effort, brake temperatures, surface conditions, environmental variations and payload. The analyses consisted of the following:

Initially, stopping distance results from each vehicle test configuration were compiled and the mean and standard deviation (sn-1) were calculated for each data set. One-sided, 95% confidence interval estimates were also determined for each data set assuming a normal distribution of the measured stopping distances. The reported one-sided confidence intervals of each data set indicate to a 95% confidence that the actual average stopping distance is below this value. The results are shown in Appendix D.

The data obtained from Dodge Ram testing were excluded from the following analyses, since the vehicle was equipped without front ABS. The rear ABS was effective at eliminating rear wheel lock-up, and thus yaw, during the brake stops. However, brake applications had to be performed with significantly less pedal effort than the other vehicles in order to eliminate front wheel lock-up. Therefore, there was no basis for comparison.

The effects of pedal effort and brake temperature on individual brake stops were examined first using all of the brake stop data found in Appendix B. Criteria were then established for each variable based on trends found within the data that adversely affected the validity of the brake stop results. These criteria will be discussed in the following sections. Individual brake stops not meeting the established criteria were then removed and the average stopping distance and standard deviation of the data set were recalculated. The final statistics for each vehicle after removing brake stops not meeting established criteria are presented in Appendix E. A summary of the final results are shown in Tables 9 and 10 and Figures 6 through 9.

 

TABLE 9. FINAL STATISTICS FOLLOWING REMOVAL OF CLASS D AND COLD STOPS

 

Dry surface

Wet surface

 

Average

stopping distance

Standard

Deviation

Average

stopping distance

Standard

deviation

Vehicle

m

ft

m

ft

m

ft

m

ft

without payload

Pontiac Grand Am SE

45.1

147.9

0.5

1.6

58.0

190.1

2.1

6.9

Ford Expedition

52.0

170.4

2.5

8.1

60.3

197.8

2.7

8.7

Toyota Camry LE

48.8

160.0

0.6

1.9

53.6

175.7

1.6

5.3

Chevy Malibu LS

43.1

141.3

0.4

1.4

45.8

150.3

0.9

2.9

Cadillac DeVille

47.7

156.3

0.9

2.9

49.9

163.6

0.6

1.9

Dodge Caravan SE

48.7

159.7

0.6

2.0

50.5

165.5

1.0

3.2

Chevrolet Express (1-ton)

50.5

165.6

0.8

2.7

54.4

178.3

0.6

1.9

Chevrolet Astro

52.0

170.5

0.4

1.2

53.1

174.1

0.5

1.5

Pontiac Bonneville

47.8

156.7

0.6

1.9

49.2

161.3

0.5

1.7

fully payloaded

Pontiac Grand Am SE

46.3

152.0

0.5

1.6

52.3

171.5

2.6

8.5

Ford Expedition

50.4

165.4

0.9

3.1

67.2

220.4

3.0

10.0

Toyota Camry LE

49.2

161.5

0.8

2.6

53.1

174.3

0.7

2.2

Chevy Malibu LS

47.0

154.0

0.7

2.4

50.4

165.2

3.1

10.2

Cadillac DeVille

50.4

165.2

1.2

4.1

50.0

163.9

0.5

1.6

Dodge Caravan SE

52.8

173.1

1.5

4.8

58.1

190.6

1.3

4.2

Chevrolet Express (1-ton)

54.6

179.1

1.8

5.8

56.1

184.1

1.0

3.2

Chevrolet Astro

55.8

183.0

0.8

2.7

56.4

185.1

0.3

0.9

Pontiac Bonneville

50.1

164.2

1.3

4.4

50.4

165.3

0.9

3.0

 

 

TABLE 10. BASELINE VEHICLE FINAL STATISTICS FOLLOWING REMOVAL OF CLASS D AND COLD STOPS

   

Stopping distance

Deceleration rate

Date of testing (1998)

Vehicle tested same day

m

ft

m/sec2

ft/sec2

7 October

Expedition

45.7

149.8

7.9

25.8

8 Octobera

Expedition

47.9

156.7

7.5

24.7

9 October

Expedition

43.7

143.8

8.2

26.9

13 October

Camry

44.3

145.4

8.1

26.6

14 October

Camry

45.5

149.4

7.9

25.9

15 October

Camry

42.6

139.8

8.4

27.7

19 Octoberb

Malibu

47.0

147.4

8.0

26.3

20 October

Malibu

44.4

145.7

8.1

26.6

22 October

DeVille

42.9

140.7

8.4

27.5

23 October

DeVille

42.8

140.3

8.4

27.6

30 October

Caravan

43.6

143.0

8.3

27.1

2 November

Caravan

43.5

142.6

8.3

27.2

3 November

Caravan

43.7

143.4

8.2

27.0

6 November

Ram 1500

44.3

145.5

8.1

26.6

12 November

Express (1-ton)

44.6

145.1

8.1

26.7

18 November

Astro

43.8

143.7

8.2

26.9

20 November

Bonneville

43.3

142.1

8.3

27.2

aTesting was conducted on damp pavement with no free standing water.

bTires were rotated before testing.

 

 

 

 

 

 

Figure 6. Comparison of Vehicle Stop Results on Dry Surface

Empty Versus Payloaded.

 

 

 

 

 

Figure 7. Comparison of Vehicle Stop Results on Wet Surface

Empty Versus Payloaded.

 

 

 

 

Figure 8. Comparison of Vehicle Stop Results without Payload

Dry Versus Wet Surface.

 

 

 

 

Figure 9. Comparison of Vehicle Stop Results with Payload

Dry Versus Wet Surface.

  1. Brake Pedal Effort Effects

Throughout testing, brake stops were performed by applying a specified force instantaneously upon the brake pedal and maintaining a target pedal force until the vehicle came to rest. A typical plot of pedal force application versus time can be seen in Figure 10. The effect of pedal force on vehicle stopping distance when applied in this manner was analyzed throughout testing. Specifically, two factors were examined closely to determine if variations in applied pedal effort led to deviations in stopping distances. First, the initial spike application was analyzed to determine if slower rates in achieving the target pedal force led to greater deviation between individual brake stops for each test configuration. Second, the pedal force after the initial spike was examined to determine if the magnitude of the steady-state pedal effort led to variations in stopping distances.

 

Figure 10. Typical Pedal Effort Application, Pedal Force versus Time.

 

In assessing the initial pedal force application rate recorded during testing, each stop was placed into one of four classes - A, B, C or D - based on the applied pedal force recorded at 0.1, 0.2 and 0.3 seconds for each brake stop. The applied pedal force range at each time interval that define the four classes are shown in Table 11. All brake stops had to fall within one of the four classes to be considered a valid brake stop. Stops with pedal forces falling below class D were concluded to have too slow a rise time and not considered valid. A sample plot of pedal force versus time for each class is shown in Figure 11.

 

TABLE 11. PEDAL EFFORT CATEGORY BREAKDOWN

 

Force measurement

 

at 0.1 seconds

at 0.2 seconds

at 0.3 seconds

Class

N

lb

N

lb

N

lb

A

over 445

over 100

over 445

over 100

over 445

over 100

B

334 445

70 100

over 445

over 100

over 445

over 100

C

222 334

50 70

over 445

over 100

over 445

over 100

D

0 - 222

0 50

222 - 445

50 100

over 445

over 100

 

 

Figure 11. Sample Pedal Effort Application, Classes A through D.

 

The data recorded from the pedal transducer showed an initial pedal force present at the brake event start time (t0). The presence of this force can be attributed to the initial acceleration of the effective mass of the brake pedal and pedal force transducer. This observation is an application of Newton's 2nd Law. Simply stated, the pedal can not move unless a force is applied to it. Under class A stop application rates, initial pedal accelerations of several g's were present. Brake applications at these accelerations to the effective mass of the brake pedal and transducer resulted in the observed forces.

A statistical analysis of the data obtained from each vehicle under each configuration was done to assess the effect of the initial force spike on stopping distance. The statistical data can be found in Appendix D. The average, standard deviation and 95% one-sided confidence interval were determined for each group with all stops included in the population. The same analysis was conducted with stops from less desirable classes removed (one class at a time) from the population until only stops in class A remained.

The brake stop data and the corresponding statistical data showed that, in the majority of cases, improvement in standard deviation and average stopping distance was evident with the removal of stops included under class D. Figures 12 through 15 compare the average stopping distance and standard deviation calculated both with and without class D stops included for each vehicle configuration set that contained at least one "D" in the population. Of the 18 data sets, 14 sets showed a decrease in average stopping distance and 15 sets showed a decrease in standard deviation with the removal of class D stops. Only the Pontiac Bonneville on dry surface with payload had an increase in both categories. An overall analysis of the class D stops supports the trend in improved average stopping distance and standard deviation. Of the 43 total class D stops conducted during testing, 67 percent (29 of 43) placed in the longest three stops of a data set. Furthermore, of the 54 longest three stops from the 18 data sets containing at least one class D stop, 54 percent were class D. Based on these findings, class D stops were excluded from the final statistics data presented in Table 9 and Appendix E.

The statistics were recalculated after removing stops under class C and then class B . Generally, the removal of these stops produced no consistent trends in braking performance or left a population too small in size to examine statistically.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 12. Comparison of Individual Data Sets with and without Class D Stops

Dry Surface without Payload.

 

 

 

 

 

 

 

 

 

Figure 13. Comparison of Individual Data Sets with and without Class D Stops

Wet Surface without Payload.

 

 

 

 

 

 

 

 

Figure 14. Comparison of Individual Data Sets with and without Class D Stops

Dry Surface with Payload.

 

 

 

 

 

 

 

 

 

 

Figure 15. Comparison of Individual Data Sets with and without Class D Stops Included

Wet Surface with Payload.

 

 

 

The steady-state pedal force after the initial spike was also examined to determine if its magnitude influenced stopping distance. In the analysis, 20 brake stops from Grand Am baseline testing were randomly selected and examined. Only stops included in the final results that met the class A pedal effort criterion were selected. The relevant data from each stop and the average stopping distance and standard deviation of the entire group is presented in Table 12.

TABLE 12. RESULTS FROM 20 BASELINE BRAKE STOPS

   

Measurement

   

Stopping distance

Average pedal force

Datea

Stop No.

m

ft

N

lb

18 November

1

45.2

148.3

520.9

117.1

18 November

3

43.0

141.1

537.3

109.1

22 October

2

42.8

140.3

1722.3

387.2

23 October

1

42.7

140.0

1180.1

265.3

13 October

1

43.9

144.0

1197.4

269.2

13 October

6

44.1

144.6

894.5

201.1

14 October

2

45.3

148.6

970.6

218.2

15 October

1

42.5

139.5

1053.7

236.9

20 November

3

43.4

142.5

696.6

156.6

19 October

3

44.9

147.4

626.7

140.9

2 November

1

43.7

143.4

780.6

175.5

3 November

2

43.4

142.3

514.2

115.6

7 October

2

45.1

147.9

1553.2

349.2

7 October

5

45.8

150.1

1376.2

309.4

9 October

1

43.8

143.7

1536.8

345.5

20 November

1

42.9

140.8

780.6

175.5

30 October

2

43.3

142.0

471.9

106.1

20 October

1

43.8

143.7

645.4

145.1

14 October

4

44.7

146.8

888.3

199.7

23 October

3

42.9

140.7

1054.6

237.1

Average stopping distance 43.9 m (143.9 ft) Standard deviation 1.0 m (3.3 ft)

aNo stops from 8 October were included due to surface condition.

 

An analysis of the data presented in Table 12 revealed that no significant difference in stopping distance was evident with varying levels of steady-state applied pedal effort. The average stopping distance of the six brake stops with average pedal efforts under 670 N (150 lb) was 43.9 m (144.1 ft), compared to 44.0 m (144.3 ft) for stops with average pedal efforts over 1110 N (250 lb). The remaining eight stops had an average stopping distance of 43.7 m (143.4 ft).

A further examination of Table 12 showed that the longest stop in the set [45.8 m (150.1 ft) on 7 October] had a higher average pedal force than the shortest stop [42.5 m (139.5 ft) on 15 October]. This observation supports the conclusion that the magnitude of the steady-state pedal force was independent of stopping distance.

 

  1. Brake Temperature

No trends between recorded brake temperatures and brake performance data were noted when considering stops in which cool-down runs were conducted prior to the brake stop. However, initial stops performed with vehicles that sat stationary for extended periods of time, allowing brakes to cool to ambient temperatures, produced unfavorable results in some cases. Table 13 shows results from the eight stops conducted in which the brake temperatures were measured within 6 oC (10 oF) of ambient temperature before testing.

 

 

TABLE 13. RESULTS FROM BRAKE STOPS PERFORMED WITH COLD BRAKES

(NEAR AMBIENT TEMPERATURE)

 

Vehicle

 

Configuration

Stopping

distance of run

Average stopping

distance of set

Standard

deviation of set

Standard deviations from average

Ambient

temperature

m

ft

m

Ft

m

ft

oC

oF

Ford Expedition

Wet/Payloaded

60.4

198.3a

67.0

219.9

3.3

10.8

2.0

20

68

Toyota Camry

Dry/No payload

47.6

156.2

48.7

159.7

0.6

2.1

1.7

19

66

Chevrolet Malibu

Wet/Payloaded

46.8

153.4

50.0

164.1

3.1

10.3

1.0

18

64

Dodge Caravan

Dry/No payload

46.8

153.7

48.7

159.8

1.1

3.5

1.7

12

54

Grand Am

Baseline (10/9)

42.9

140.9

43.7

143.3

0.4

1.4

1.7

17

62

Grand Am

Baseline (10/21)

46.9

153.8

45.0

147.6

1.5

4.9

1.3

15

59

Grand Am

Baseline (10/22)

42.8

140.3

42.9

140.6

0.2

0.5

0.6

10

50

Grand Am

Baseline (10/30)

48.7

159.8a

43.6

143.0

0.3

0.9

18.7

18

64

aBrake stop was not included in the original data set population.

 

The data shows that eight out of the nine stops resulted in stopping distances at least one standard deviation from the average stopping distance of the data set. Based on these findings, stops with cold brakes were excluded from the final statistics presented in Tables 9 and 10 and Appendix E, in addition to the class D pedal effort stop exclusion. The remainder of the analysis was conducted with these revised statistics. All recorded brake temperature data can be found in Appendix F.

 

  1. Tire Temperature Effects

Tire temperature was measured to determine its effect on braking performance both within each individual data set and from day-to-day baseline testing. However, tire temperature varied little from the actual ambient temperature throughout the beginning stages of testing and therefore, no correlations between tire temperature and vehicle performance or variability could be established. Accordingly, tire temperature measurements were not recorded after testing was concluded with vehicle No. 3 (Toyota Camry). Recorded tire temperatures during testing of the first three vehicles can be found in Appendix F.

 

  1. Day-to-Day Variation Effects

An analysis of the results from baseline testing with the Pontiac Grand Am was conducted to investigate day-to-day performance variations due to factors such as environmental changes and frictional coefficient changes. A summary of the average brake stop results from each day and the recorded environmental data is presented in Table 14. All recorded meteorology data can be found in Appendix G. The average wind direction in Table 14 is presented relative to the direction of vehicle travel, so that a value of 90 degrees represents a crosswind coming from the right of the vehicle.

Road surface frictional coefficients measured and recorded weekly by the Eastern Federal Lands Highway Division of the Federal Highway Administration are presented in Table 15. All measurements shown are averages of 10 individual chirp tests conducted on the date provided. Results from each individual chirp test can be found in Appendix H. The peak frictional coefficient results for dry surface testing generally decreased with temperature by approximately 4 percent over an ambient temperature range of 15 oC (28 oF). The only deviation with these results was the measurements made on 15 October. No significant variation in the wet surface frictional measurements was found.

 

TABLE 14. AVERAGE STOPPING DISTANCE AND ENVIRONMENTAL DATA

FROM BASELINE TESTING, PONTIAC GRAND AM

 

 

Test date (1998)

Measurement

Average stopping distance

Standard

deviation

Ambient

temperature

Average

wind speed

Peak speed

Avg wind

direction

(degrees)

m

ft

m

ft

oC

oF

km/hr

mph

km/hr

mph

7 October

45.7

149.8

0.6

2.1

19

66

5

3

8

5

108

8 Octobera

47.8

156.7

0.6

2.1

20

68

5

3

8

5

136

9 October

43.8

143.8

0.2

0.8

17

62

3

2

8

5

303

13 October

44.3

145.4

0.5

1.7

20

68

6

4

10

6

184

14 October

45.5

149.4

0.5

1.7

18

64

6

4

13

8

245

15 October

42.6

139.8

0.2

0.7

14

57

6

4

10

6

225

19 Octoberb,c

44.9

147.4

----

----

21

71

5

3

10

6

267

20 October

44.4

145.7

0.7

2.4

18

64

5

3

11

7

265

22 October

42.9

140.6

0.2

0.5

10

50

6

4

13

8

300

23 October

42.8

140.3

0.1

0.4

11

52

5

3

8

5

251

30 October

43.6

143.0

0.3

0.9

18

64

8

5

13

8

241

2 November

43.5

142.6

0.2

0.7

12

54

5

3

8

5

278

3 November

43.7

143.4

0.3

1.0

9

48

2

1

3

2

264

6 Novemberc

44.3

145.5

----

----

7

45

5

3

10

6

319

12 November

44.2

145.1

0.5

1.6

14

57

5

3

11

7

280

18 November

43.8

143.7

1.0

3.2

8

46

3

2

6

4

318

20 November

43.3

142.1

0.3

1.1

16

61

3

2

3

2

225

Average stopping distanced 44.3 m (145.4 ft) Standard deviationd 1.4 m (4.5 ft)

Average stopping distancee 44.0 m (144.5 ft) Standard deviatione 1.0 m (3.4 ft)

aTesting was conducted on damp pavement with no free-standing water.

bTires were rotated before testing.

cOnly one brake stop was valid for the final statistics.

dCalculated with all dates included.

eCalculated without 8 and 19 October dates included.

 

TABLE 15. AVERAGE RESULTS FROM CHIRP TESTING

 

Measurement

 

Dry surface

Wet surface

 

Test date (1998)

Frictional coefficient

Test speed

Ambient temperature

Frictional coefficient

Test speed

Ambient temperature

km/hr

mph

oC

oF

km/hr

mph

oC

oF

17 September

0.949

64.2

39.9

27

81

0.869

64.0

39.8

29

84

22 September

0.937

64.4

40.0

25

77

0.882

65.0

40.4

25

77

1 October

0.936

64.2

39.9

26

78

0.859

64.2

39.9

24

75

15 October

0.893

64.5

40.1

17

62

0.847

64.4

40.0

19

66

19 October

0.932

65.0

40.4

22

71

0.878

64.2

39.9

21

70

29 October

0.923

65.2

40.5

14

57

0.875

64.7

40.2

15

59

9 November

0.905

64.5

40.1

12

53

0.868

65.3

40.6

12

53

24 November

0.916

64.5

40.1

15

59

0.871

64.0

39.8

15

59

 

In the limited environmental conditions in which these tests were conducted, no consistent trends were evident between the average stopping distance of the Grand Am and the changes in the ambient temperature, associated frictional coefficients or wind speed. Likewise, no trends were evident when considering the calculated standard deviations.

To better quantify the impact of wind conditions on brake testing, an engineering analysis was conducted to study the effect of aerodynamic drag on a test vehicle experiencing a head or tail wind during a brake stop. The analysis and sample calculation, presented in Appendix J, are based on a representative brake stop from the data set and appropriate values for the coefficient of drag, vehicle frontal area and air density. From this analysis, Figures 16 and 17 are provided to show the possible differences in stopping distance results due to various wind conditions. The drag coefficients (CD) and vehicle frontal areas (AP) used represent the expected upper and lower limits for the vehicles tested.

Figure 16. Wind Effects with CD= 0.3 and AP= 2.3 m2.

 

Figure 17. Wind Effects with CD= 0.6 and AP= 2.8 m2.

 

 

Based on the analysis, a representative worst case scenario with a head wind traveling at the peak wind speed measured during testing [12.9 km/hr (8.0 mph)] resulted in approximately a 0.4-m (1.4-ft) difference in stopping distance. Hence, the analysis supports the observation that the winds experienced during testing had minimal impact on the stopping distance results.

  1. The Effect of Free Standing Water on Wet Stops

Another variable considered to impact the variability in braking performance was the depth of the free standing water during wet brake stops. As previously mentioned in the procedure, standing water as deep as 1/4 inch occurred in portions of the brake area during testing of the first four vehicles. Because some test runs resulted in noticeable hydroplaning, the test area was moved a short distance to an area where the water depth remained under 3 mm (1/8 inch) and significant water collection was avoided.

Although it is difficult to assess the effect of water depth on stopping distance without further testing, a comparison of the standard deviations of the data sets before and after the test area was moved does indicate a greater variability in stopping distance with water depth over 3 mm (1/8 inch). A comparison of each vehicle's standard deviation on wet pavement is presented in Figure 18.

Figure 18. Standard Deviations of Vehicle Data Sets on Wet Surface.

 

As shown in Figure 18, all four vehicles tested before the test area was moved (Grand Am, Expedition, Camry and Malibu) experienced a significantly higher standard deviation in at least one payload configuration than the vehicles tested after the test area was moved. This trend supports the observations made concerning hydroplaning during the testing of vehicles No. 2 and 4, and lends evidence that other instances of hydroplaning may have occurred during wet brake stops before the test area was moved.

 

  1. Test Configuration Effects

An analysis was conducted to investigate the sensitivity of the measured stopping distances and their associated variations to the four test configurations shown below:

This analysis does not differentiate between the variations due to vehicle performance versus test methodology. However, it does support some intuitive notions related to brake testing results.

For the analysis, a test occasion refers to the set of data for a particular vehicle under any one of the four test configurations shown in the test matrix. Within-occasion data refers to the mean and standard deviations for a particular test occasion. Occasion-to-occasion data refers to a comparison across test occasions of the within-occasion means.

The within-occasion data for each vehicle, presented earlier in the report, were grouped according to the four test configurations. Within each configuration grouping, all nine vehicles were represented. For each configuration, the within-occasion standard deviations were statistically combined to determine the pooled within-occasion standard deviations. Next, the within-occasion means for each configuration were compared to determine occasion-to-occasion means and standard deviations. Finally, the within-occasion and occasion-to-occasion standard deviations were combined to determine the total system dispersion for each configuration. A similar approach was taken to analyze the four standard test parameters independently. The results are presented in Table 16.

TABLE 16. STATISTICS OF VEHICLE INDEPENDENT, CONDITION DEPENDENT DATA SETS

 

Average stopping distance

System dispersion

Data set

m

ft

m

ft

Dry asphalt, no payload

48.4

158.7

3.1

10.3

Dry asphalt, with payload

50.7

166.4

3.4

11.1

Wet asphalt, no payload

52.7

173.0

4.7

15.5

Wet asphalt, with payload

54.9

180.0

5.8

19.0

Dry asphalt, both vehicle configurations

49.6

162.6

3.4

11.2

Wet asphalt, both vehicle configurations

53.8

176.5

5.2

17.2

No payload, both asphalt conditions

50.5

165.8

4.5

14.8

With payload, both asphalt conditions

52.8

173.2

5.1

16.7

 

As would be expected intuitively, the largest system dispersion resulted from testing under the full payload, wet asphalt configuration, followed by testing under the wet, no payload configuration. The results also indicate that the brake test results are slightly more sensitive to wet asphalt conditions than to payload configuration.

 

    1. Conclusion

With the exclusion of Class D stops and cold brake stops from the data sets, the final statistics show relatively small standard deviations in measured stopping distance for the four configurations tested. Exceptions were noted within the wet surface results for the Pontiac Grand Am and Ford Expedition in both payload configurations, and the Chevrolet Malibu in the full payload configuration. The results for the Expedition on dry pavement without payload also showed a relatively high variation in stopping distance.

The hydroplaning experienced early in testing may have caused the higher standard deviations observed with the Grand Am, Malibu and Expedition during wet surface testing. Once the wet surface brake stop location was moved to avoid hydroplaning, the wet surface results improved considerably. However, the cause of the relatively high standard deviation observed for the Expedition tested without payload on dry surface is unknown. Additional testing of the Expedition is necessary to determine if the results were vehicle-related or the consequence of an unexplained test condition.

Baseline test results also showed relatively small variation in stopping distance within individual test occasions. However, some spread in the average stopping distance from occasion to occasion was observed during early testing, with results becoming more consistent during the last two thirds of the tests conducted. Two of the data sets were noted to have irregular test conditions, and should not be compared with other data. The inconsistencies in average stopping distances observed in early testing could not be attributed to changes in ambient temperature, frictional coefficient or wind conditions, but may have been a result of an initial break-in period of the brake components or curing of the tires. However, within the framework of this test and the analyses conducted, no definite cause was evident.

  1. TASK 2 Provide Details on Methodology to Address Variability

The results obtained from 100-km/hr (62-mph) brake effectiveness testing with vehicles equipped with ABS were analyzed in Task 1 to investigate the sources of stopping distance variability. The analysis showed that certain parameters significantly affected the performance results, while other test variables had little or no effect on the variability of the data. This section of the report will review the findings from Task 1 and provides details on methods to reduce test variability. Test factors not addressed in Task 1 that were beyond the scope of the test matrix will also be discussed.

Many of the test conditions and procedures outlined in FMVSS 135 were utilized throughout testing. Details provided on test methodology in this section are intended to supplement the test procedures of FMVSS 135. These supplements are recommended after giving appropriate consideration to the test findings in Task 1 in areas such as pedal force application, brake temperature and water application.

 

  1. Pedal Effort

The test results from Task 1 were analyzed to investigate the effects of the initial brake pedal force application rate and the subsequent steady-state pedal force on stopping distance variability.

As outlined in Task 1, each brake stop was placed into one of four classes of initial brake pedal application - A, B, C or D - with class A having the highest pedal force rate and class D having the lowest. An analysis of the results showed that data sets including class D stops generally had a higher variability in stopping distance than the same data sets with class D stops removed. It was concluded that slow pedal force rates may have delayed the initiation of the ABS system and consequently, increased the stopping distance variability of the data set. To provide a more accurate measure of ABS braking performance for each vehicle, class D stops were excluded.

Further analysis of the effect on stopping distance variability by successive exclusion of class C and B stops from the data sets was inconclusive because of the small sample sizes associated with class B and C stops. Therefore, the impact of the inclusion of class B and C stops on stopping distance variability is not fully known.

ATC recommends that all brake stops meet the class A pedal force criterion. The addition of this criterion will assist in reducing stopping distance variability caused by differences in the initial pedal effort input and ensure repeatable ABS initiation.

An analysis was also conducted to investigate the effects of steady-state pedal force on stopping distance. Using the results from baseline testing, it was verified that stopping distances were not affected by significant differences in the average pedal force of the brake stops [ranging from 440 to 1730 N (100 to 390 lb)]. Therefore, under the test requirement that the ABS remain activated throughout the entire brake stop, the analysis showed that excessive pedal forces are not required to obtain consistent and representative performance results.

Considering these findings, ATC recommends that steady-state pedal forces fall between 500 and 800 N (112 and 180 lb) during testing. The range exceeds the requirement presented in FMVSS 135, while defining an acceptable and easily achievable upper and lower target limit for the test driver. Steady-state pedal forces in this range also ensure that the ABS will remain fully invoked throughout the entire brake stop.

 

  1. Vehicle Parameters

The variability in braking performance results caused by test parameters such as brake temperature, tire temperature and payload characteristics was examined in Task 1. All three parameters were considered to be vehicle test variables that could be controlled prior to the start of each test.

As discussed in Task 1, nine brake stops were conducted with brake temperatures measured within 6 oC (10 oF) of ambient temperature. An analysis of these stops showed that in eight of the nine cases, the stopping distance placed at least one standard deviation from the average stopping distance of the data set. Therefore, because inconsistent test conditions may have led to variability in stopping distances, brake stops performed under this condition were removed. The exclusion of these brake stops is further supported by the fact that brake temperatures typically exceed ambient temperatures during normal operation, and that brake stops performed with "cold brakes" could be considered unrealistic.

To address variability in brake performance caused by "cold brakes", ATC recommends that all stops be conducted with front brake rotor temperatures between 65 and 100 oC (149 and 212 oF), as required in FMVSS 135. Rotor temperatures falling below this range should be heated by making one or more brake applications as outlined under Section S6.5.6 in FMVSS 135. All brake temperatures measured above 100 oC (212 oF) should be cooled by driving the vehicle without brake application at speeds up to 100 km/hr (62 mph) until falling into the acceptable temperature range.

No consistent trends between braking performance and tire temperature were noted. Tire temperatures were generally comparable with road surface temperatures and therefore, no correlations between tire temperature and variations in stopping distance could be established. However, it should be noted that testing was conducted with various brands and models of tires, which may have affected stopping distance performance, especially under wet conditions. For this reason, the brand and model of the tires on test vehicles should be identified in the test results provided to the consumer.

Vehicle payload was also analyzed in Task 1. As expected, the results showed that vehicles demonstrated increased variability in stopping distance when fully payloaded versus empty. However, since only one full-payload configuration was tested for each vehicle, the impact of varying the vehicle's center of gravity location on stopping distance variability could not be determined. Because insufficient data was obtained to make recommendations regarding payload procedure, those described in FMVSS 135 should be followed.

 

  1. Environmental Test Conditions

Test conditions such as wind speed, ambient temperature and road surface friction coefficient were recorded and analyzed in Task 1. Observations were also made concerning the delivery of water to the test area for wet surface testing. Of these environmental test conditions, only the depth of the water on the test area during wet surface testing was found to significantly increase variability in brake performance.

Ambient temperature varied little throughout testing [ranged from 7 to 21 0C (45 to 75 0F)], meeting the criteria outlined in FMVSS 135 [between 0 and 40 oC (32 and 104 oF)]. Since no testing was conducted with an ambient temperature above 21 0C (75 0F), conclusions regarding test results at temperatures approaching 40 0C could not be drawn. Therefore, given the data obtained from this study, there is no basis to deviate from the ambient temperature criteria specified in FMVSS 135.

Average and peak wind speed also showed little variation throughout testing, with an average speed ranged from 2 to 8 km/hr (1 to 5 mph) and peak speed not exceeding 13 km/hr (8 mph)]. As a result, no insight could be gained from the test data regarding the effect of wind conditions on stopping distance. Therefore, an analytical investigation was conducted in Task 1 to determine the sensitivity of wind conditions on stopping distance. Based on the engineering analysis performed, it was concluded that the wind speed criteria provided in FMVSS 135 [(not greater than 5 m/s (11.2 mph)] is adequate.

The peak surface friction coefficient also showed little variation throughout testing on both wet and dry surfaces. As a result, variations in stopping distance could not be attributed to this parameter. Testing was conducted at a single location with peak friction ranging from 0.89 to 0.95 and 0.85 to 0.88 on dry and wet pavement, respectively. Considering the range of dry surface fiction coefficients experienced during this study, the 0.9 nominal value specified in FMVSS 135 appears to be adequate for future testing.

Wet surface testing is not addressed in FMVSS 135. Based on information from NHTSA representatives, the typical peak friction value for wet surface testing at other test sites is nominally 0.8, which is lower than those experienced during this study. Since adequate results were obtained with a wet peak friction value of 0.85, it seems reasonable to expect adequate results with a friction coefficient of 0.8. However, ATC recommends that the specified nominal value be no lower than 0.8, so as not to deviate too far from the results of this study.

An analysis of the results confirmed observations that hydroplaning occurred early in testing and was responsible for increased variability between stopping distances. To minimize variations in brake test results during wet surface testing, ATC recommends that courses should be free of collection areas, and water thickness should be monitored and kept below 3 mm (1/8 inch). The results presented in Task 1 show that testing under these surface conditions resulted in significantly less variability in stopping distances without any noticeable incidents of hydroplaning.

 

  1. Instrumentation and Measurement Techniques

Standardization of the instrumentation and measurement techniques used to determine stopping distance is necessary to ensure consistency and accuracy of the reported results between test agencies that may perform the brake tests. The information in the following paragraphs is based on the results of this effort as well as prior test experience.

ATC recommends that a rolling fifth-wheel sensor with quadrature capability be used to measure stopping distance and vehicle speed. Non-contact sensors should be avoided for this application due to the increased potential for error as vehicle speed approaches zero, particularly on wet pavement. The use of the quadrature technique will account for fifth-wheel directional changes resulting from the pitching motion of the vehicle as it comes to rest. The sensor should be located on the vehicle such that the wheel does not leave the pavement surface at any time during the brake event. Prior to use, the fifth-wheel should be calibrated by operation over a known distance.

The brake event start time should be initiated by activation of the brake light circuit. The switch at the brake pedal and the electrical circuit should be inspected to ensure the circuit is activated by minimal movement of the brake pedal. While more elaborate techniques and sensors could be used, the results from this effort indicate that use of the brake light circuit is adequate. The event should be concluded when the vehicle comes to rest.

Pulse counts from the rolling fifth-wheel should be summed throughout the entire braking event. With quadrature capability enabled, counts resulting from the rocking of the vehicle as it comes to rest will be nullified. Once the vehicle comes to rest, the total pulse count should be multiplied by the scale factor determined during calibration to calculate the actual stopping distance. Vehicle speed should be determined by dividing the pulse count during each sample period by the sample time interval.

The pedal force tranducer should have adequate resolution to determine whether a pedal application meets the class A stop criteria. A maximum transducer output range of 200 to 300 lb is recommended. Although a 10 Hz sample rate was used for the pedal force transducer in Task 1, the 40 Hz minimum sample rate required in FMVSS 135 is recommended.

 

  1. Test Sample Size

For statistical purposes, it is always desirable to have as large a sample size as possible. However, program constraints such as cost and time often dictate a reduced sample of the population. Based on the results from Task 1, a sample size of 10 stops per test condition is practical. Tests on a single vehicle were achievable within one day and the results showed small variations in stopping distance, ensuring reasonable confidence in the data.

 

 

  1. TASK 3 Develop a Test Protocol for the Braking Initiative

ATC recommends the following protocol for the consumer braking program. The protocol is based on the test procedures and results from this study and relevant sections of the Federal Motor Vehicle Safety Standard (FMVSS) No. 135, Passenger Car Brake Systems. Although Tasks 1 and 2 only addressed testing for vehicles equipped with ABS, the procedures recommended here are more general and include non-ABS equipped vehicles.

 

  1. General Test Conditions

Adhere to Section 6 of FMVSS 135, with the following modifications:

    1. Change S6.2.1. Pavement Friction to require a nominal peak frictional coefficient (PFC) of 0.9 for dry pavement.
    2. Replace S6.4. Instrumentation with the following:
    3. Brake temperature measurement. The brake temperature is measured at the surface of the front brake rotors with a calibrated hand-held pyrometer.

      Vehicle speed and stopping distance measurement. The vehicle speed measurement is performed using a calibrated rolling fifth-wheel transducer with quadrature capability. Prior to testing, an accuracy not exceeding 0.5 percent shall be verified on a pre-measured 60-m (200-ft) test lane.

      Brake pedal effort measurement. The pedal effort measurement is performed with a calibrated transducer on the brake pedal. This transducer should not interfere with normal brake application.

      Anemometer. The ambient temperature, wind speed and wind direction measurements are to be performed with a calibrated anemometer located at the test site.

    4. Add the following:

Wet surface condition. For wet surface testing, the test area shall be fully wet with standing water not deeper than 3 mm (1/8 inch). Water shall be re-applied to the test surface prior to each brake stop event.

  1. Procedural Conditions

Adhere to the following sections of FMVSS 135, with the noted exceptions:

    1. S6.5.3. Stopping Distance; S6.5.3.1 Only.
    2. S6.5.4. Vehicle position and attitude.
    3. S6.5.5. Transmission selector control; Testing is to be conducted with the transmission in gear. Adhere to S6.5.5.2.
    4. S6.5.6. Initial brake temperature (IBT).

 

 

  1. Required Test Data

Test data to be collected includes:

 

  1. Measurement Techniques

Stopping distance. A rolling fifth-wheel transducer with quadrature capability shall be mounted on the vehicle and used to measure vehicle stopping distance. The brake stop event start time shall be initiated by activation of the brake light circuit and stopped when the vehicle is at rest. The switch at the brake pedal and the electrical circuit shall be inspected and adjusted to ensure the circuit is activated by minimal movement of the brake pedal. Stopping distance shall be determined by summing pulses from the fifth-wheel during the brake event, and multiplying this sum by the appropriate scale factor. A minimum sample rate of 40 Hz is required.

Vehicle speed. A finite-difference technique shall be applied to the pulse counts from the fifth-wheel over each sample period to determine the vehicle speed.

Brake pedal force. A force transducer shall be applied to the brake pedal to measure pedal effort. A minimum sample rate of 40 Hz is required.

Stopping Distance Normalization. All stopping distance measurements shall be normalized in accordance with SAE 299 (August 1987) based on an initial vehicle speed of 100 km/hr (62 mph).

 

  1. Road Test Procedures

Adhere to the following sections of FMVSS 135 and the noted additions and exceptions:

  1. S7.1. Burnish.
  2. Exception: Omit the temperature requirement from S7.1.3 (g) Interval between runs and base the interval strictly on the distance requirement.

  3. S7.5 Effectiveness Test. Change to the following:
    1. S 7.5.1. Vehicle Condition
      1. Vehicle load: GVWR and LLVW.
      2. Transmission position: In gear.
    2. S 7.5.2. Test Conditions
      1. IBT: 65 oC to 100 oC (149 oF to 212 oF).
      2. Test Speed: 100 km/h (62 mph).
      3. Wheel lockup: No noticeable lockup of any wheel allowed.
      4. Number of runs: 10 at each weight configura
      5. tion.
      6. Test Surface: Nominal PFC value of 0.9 for dry pavement and 0.8 for wet pavement.
    3. Brake Pedal Application Procedure
    4. ABS: The brake pedal is to be applied so that the pedal effort exceeds 445 N (100 lb) in 0.1 seconds or less, while targeting a steady-state application force of 670 N (150 lb). The allowable range for the pedal force is greater than 500 N (112 lb) and less than 800 N (180 lb). The target force is to be held constant until the vehicle comes to rest.

      Non-ABS: The brake pedal is to be applied so that the vehicle is stopped in the shortest possible distance, while avoiding any instances of wheel lock-up.

    5. Water Application Procedure
    6. For wet surface testing, water shall be applied using a water tanker truck that is equipped to distribute water evenly across the width of the test lane. Prior to wet surface testing, three passes shall be made with the water tank traveling longitudinally along the test area (shown previously in Figure 5 in Task 1). The first two passes shall be made side-by-side, and the third pass shall be made overlapping the center of the lane created by the first two passes. The total length of the wet area shall be at least 100 m (330 ft). Prior to each brake stop event, an additional pass shall be made with the water tank along the center lane where the brake stops are to be conducted. Water shall be distributed to fully wet the asphalt surface while keeping the water depth in any area of the test lane below 3 mm (1/8 inch).

     

  1. TASK 4 - Identify a Method to Report Braking Performance to Consumers

The goal of the consumer braking program is to present accurate, unbiased brake performance information that the consumer can find useful and informative. Brake performance measures should not be skewed in any way to present the best stopping distance for a specific vehicle, but should include the results from all brake stops conducted under the required test conditions.

To assist in the selection of a reporting method to the consumer, the final results from each vehicle are presented in Appendix K in terms of the mean, standard deviation, 95% one-sided confidence interval and 95th-percentile (1.645 standard deviations above the mean) and 99th-percentile (2.320 standard deviations above the mean) stopping distances. Of these performance measures, the concept of standard deviation and 95% confidence interval may not easily be understood by the average consumer, and should probably be avoided.

Two measures of braking performance that may effectively inform the consumer of a vehicle's braking performance are average stopping distance and 95th-percentile stopping distance. The average stopping distance represents a valid mean of the vehicle's brake performance over the 10 stops performed during testing, with all stops included in the calculated average. The 95th-percentile stopping distance provides a measure of brake performance based on the average stopping distance and the variability of the data set.

The 95th-percentile stopping distance informs the consumer of the distance within which the vehicle should stop 95 percent of the time. Vehicles with high variability will have 95th-percentile stopping distances significantly higher than the reported average, while those with small deviations between individual stopping distances will have values closer to the reported average. This concept is illustrated by comparing the following two sets of data:

 Avg. stopping distance (ft)Standard deviation (ft)95th-percentile stopping distance (ft)
Vehicle A171.5 8.5 185.5
Vehicle B 174.1 1.5 176.6

 

Considering the average stopping distance, vehicle A showed better braking performance. However, because the variability of vehicle A was significantly higher than vehicle B during testing, vehicle B had a shorter 95th-percentile stopping distance, and therefore, provided better performance reliability.

Overall, the average stopping distance and 95th-percentile stopping distance values provide the consumer with a measure of the vehicle's stopping distance and stopping consistency. The consumer should be informed that the findings were based on 10 stops performed under the same test conditions, and a normal distribution was assumed when determining the 95th-percentile stopping distance value. The consumer should also be informed that the conditions under which these tests were conducted do not necessarily match the conditions found in all real-world brake events, and that the information is based on testing performed under procedural requirements.

 

  1. TASK 5 Develop a Test Report Format

A format for reporting tests conducted in support of the consumer braking program is provided in Appendix L. The format is structured in outline form in an effort to standardize the method in which brake stop results are reported to NHTSA. Tables to report the test findings and to provide analysis of the data are included.

 

Appendices are available below in PDF version only. If you cannot browse PDFs, please go to Adobe and download the free Adobe Acrobat Reader.
AppendixDownload Time
APPENDIX A Vehicle Photographs49 k
APPENDIX B Individual Brake Stop Results223 k
APPENDIX C Sample Pedal Effort Plots88 k
APPENDIX D Brake Stop Statistics with Pedal Effort Breakdown86 k
APPENDIX E Final Performance Statistics for Each Vehicle50 k
APPENDIX F Brake and Tire Temperature Data Sheets754 k
APPENDIX G ATC Meteorology Data250 k
APPENDIX H ASTM Frictional Skid Resistance Test Data249 k
APPENDIX J Sample Wind Force Calculation102 k
APPENDIX K Consumer Performance Measures37 k
APPENDIX L Test Report Format52 k

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