Commercial Vehicle Air Brake Testing by Timothy J. Reust The vehicle used for this testing was a 1982 GMC 7000 series, two axle, dump truck. The truck was tested in an empty condition. The testing was conducted on a flat and dry asphalt road surface. The instrumentation consisted of a data acquisition system, Datron optical 5th wheel, Vericom VC2000, pressure transducers, string potentiometer and a tape switch. The instrumentation provided data for time, distance, speed, acceleration, brake chamber air pressure, pushrod travel and brake pedal application. The test results were used to evaluate deceleration rates, brake lag times, pushrod travel, pre-visible tire mark energy loss and the effect of different pushrod adjustments. The vehicle was also weighed to provide information on the total weight and the weight distribution. A passenger vehicle was also skid tested on the same road surface to provide a comparison of deceleration. The rubber hardness of the tires on the truck and passenger vehicle was tested using a tire durometer. The results presented in the main text of this paper are for the tests conducted with a pushrod adjustment of approximately 1.0 inch. The readers are encouraged to look at the test summary pages (at the rear of the paper) for additional details and for the test results of different pushrod adjustments. Test Equipment A Datron V1 optical 5th wheel was used to provide time based data of speed and distance. This unit was attached to the rear surface of the fuel tank with a system of suction cups and brackets. Pressure transducers were used to provide brake chamber air pressure data. The transducers that were used had a range of 0 to 250 psi. The transducers were placed in the air line using a “T” fitting at the service side of the brake chamber. Transducers were installed at the right side of each axle. A tape switch was used to provide a signal to indicate the initial application of the brake pedal. The tape switch was mounted on the surface of the brake pedal with two-sided tape. A string potentiometer was used to provide data for the pushrod travel distance. The potentiometer was mounted to a bracket and placed near the brake chamber and the string was attached to the pushrod. The orientation of the potentiometer and the string was in line with the pushrod travel direction.

An Edelbrock QuikData, data acquisition system was used to collect the test data. The 5th wheel, pressure transducers, string potentiometer and the tape switch were connected to the data acquisition system. The system was set at a sample rate of 500 hertz (500 samples per second) and a filter rate of five, resulting in a final data collection rate of 100 hertz (100 samples per second). This unit was placed inside the cab of the tractor. The software with this unit provided a graphical display of the collected data. The data was exported into Excel for further analysis. A Vericom VC2000 was used to provide deceleration data for the skid tests. After a skid test, the output of this unit provided an average deceleration value. This unit was attached to the inside surface of the tractor‘s windshield by means of suction cups. A set of Intercomp PT300 digital wheel weight scales were used to weigh the vehicle. These scales had a rating of 20,000 pounds and a resolution of 20 pounds. Deceleration Rates The deceleration rates were determined by two methods. The first method was a direct reading from the Vericom VC2000, the second method was by calculation using the Datron V1 optical 5th wheel data. The Vericom reported an average deceleration rate of 0.547 g for the truck test with the pushrod adjustments of one inch The Datron data was used to calculate the average deceleration rate by using the recorded speed at the time the visible tire mark began and the measured length of that tire mark. The calculations resulted in an average deceleration rate of 0.498 to 0.510 g for the truck test with the pushrod adjustments of one inch. Comparison of Commercial Vehicle and Passenger Vehicle Deceleration Rates Additional skid testing was conducted on the same road surface using a 2001 Buick Century Custom (passenger vehicle). The average deceleration rates were obtained using the Vericom VC2000. The vehicle was tested with the ABS system “on” and “off”. The purpose of this testing was to provide a comparison of commercial vehicle and passenger maximum braking deceleration rates using the same road surface. The tires on this vehicle had a tire durometer reading (rubber hardness) of 68 to 72. The recorded average deceleration rate for the Buick with the ABS “on” was 0.848 g. The recorded average deceleration rate for the Buick with the ABS “off” was 0.789 g. Based on the test results, the truck achieved 64.5 % of the Buick ABS “on” deceleration rate and 69.3 % of the Ford Buick ABS “off” deceleration rate.

Brake Lag Time Typically brake lag time is considered as the time from initial brake application, up to when the visible tire mark begins. The brake lag time for this testing was placed into two categories; when the impending tire mark began and when the locked wheel skid mark began. The brake lag time associated with the beginning of the impending tire mark ranged from 0.27 to 0.29 seconds. The brake lag time associated with the beginning of the locked tire mark ranged from 0.61 to 0.62 seconds. With the test data it was also possible to look at the time duration from initial brake application, up to when the air pressure first began to increase in the brake chamber. The tests resulted in a lag time of 0.04 to 0.05 seconds for axle 1 and 0.08 to 0.10 seconds for axle 2. A series of graphs have been prepared to illustrate the air pressure in the brake chamber as a function of time. Time 0.0 (zero) on the graphs represents the time when the brake pedal is first applied during the brake test. Pushrod Travel Distance This testing measured the brake system pushrod stroke distance during the application of the brake. The pushrod travel testing took place with the truck stopped and with the air system fully charged. The brake pedal was applied abruptly (similar to an emergency stop). The information gathered for this testing included time, brake chamber air pressure and pushrod stroke distance. A series of graphs have been prepared to illustrate the several combinations of the data that include pushrod travel vs. air pressure, time vs. air pressure, and time vs. pushrod travel. . Time 0.0 (zero) on the graphs represents the time when the brake pedal is first applied during the brake test. The data shows a total stroke length of approximately 0.888 inches for the rear brake.

Pre-Skid Energy Loss Previous authors have discussed the energy loss that takes place from the time the brake pedal is first applied, up to when the visible skid mark appears on the road surface. A paper by Reed [1] on passenger vehicles indicated, “an additional 15 to 30 percent increase in initial energy would be required due to this assumption to accurately compute initial vehicle velocity”. A second paper by Reed [2] indicated “whereas a substantial percentage of initial energy is dissipated prior to skid mark initiation for a passenger car, little or no energy dissipation has been observed in trucks prior to skid mark initiation”. Based on the measurements from the tests with the dump truck, initial energy loss was approximately 5.0 to 6.0 percent leading up to the beginning of the impending tire mark and 31.2 to 39 percent leading up to the beginning of the locked tire mark. A reason for the minimal energy loss prior to the beginning of the impending tire mark is due to the rate at which the air pressure increases in the brake chamber. The calculated rate of air pressure rise in the brake chamber was approximately 480 psi per second. Initially when the brake pedal is applied there is very little speed loss because there is no air pressure in the brake chamber, but once the pressure begins to increase, it does so very rapidly. The rapid air pressure increase allows for very little speed loss and energy loss prior to the beginning of the impending tire mark. Finding the beginning of the impending tire mark after each test can be affected by factors such as the contrast of the road surface and the tire mark and also the lighting condition. A reason for the significant energy loss prior to the beginning of the locked tire mark is due to the transition time from the beginning of the impending to the beginning of the locked phase. During this transition time (approximately 0.38 seconds), significant deceleration of the vehicle takes place. Within this 0.38 second transition time, it takes approximately 0.22 seconds for the wheel rotation to slow enough to begin to generate a locked tire mark after the air pressure inside the brake chamber has reached it’s maximum. REFERENCES [1]

Reed, W. and Keskin, T., “A Comparison of Emergency Braking Characteristics of Passenger Cars” Paper 880231, SAE, 1988.

[2]

Reed, W. and Keskin, T., “Vehicular Deceleration and Its Relationship to Friction” Paper 890736, SAE, 1989.

The author would like to thank Neal Clark, Mike Frain, Del Hansen, Bob Ockey, Chet Snyder and Tim Staab for their participation in this truck testing project. Questions and comments on this paper are welcomed and can be forwarded to: Timothy J. Reust Accident Science 27811 Avenue Hopkins, Suite #1 Valencia, California 91355 (661) 254-8084 [email protected] www.accidentscience.com Test Vehicle 1982 GMC 7000 series 2 Axle Dump Truck Engine: Detroit Diesel 8.2 L Transmission: Allison MT643 Automatic VIN #1GDM7D1G0CV587402 Odometer 13200 Wheelbase 167 inches GVWR: 27,500 lb Front Axle GAWR: 9,000 lb Rear Axle GAWR: 18,500 lb Unladen Weight: 11,680 lb Test Weight: 13,080 lb (54 % front, 46 % rear) Front Brake Chambers: 20 Rear Brake Chambers: 30/30 Front Tires: Firestone T559 Radial, 10.00 R 20 14 PR Tire Durometer Reading (rubber hardness): 72 to 78 Rear Tires: Firestone T546 Radial, 10.00 R 20 16 PR Tire Durometer Reading (rubber hardness): 74 to 82

Test Summary Test # b1 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 1 inch Pushrod travel, axle 2 brakes: 1 inch Tire mark from rear axle used for evaluation Speed when brakes first applied: 42.21681 mph Speed when impending tire mark begins: 41.00 mph Brake chamber air pressure (rear axle) at begin of impending phase: 79.03817 psi Time from initial brake application to begin of impending phase: 0.27 sec. Travel distance from initial brake application to begin of impending phase: 16.6336 feet Length of impending tire mark: 21.3009 feet Calculated deceleration rate of impending phase: 0.507 g Percent energy loss, begin of brake application to begin of impending mark: 6.0% Speed when locked tire mark begins: 36.85042 mph Brake chamber air pressure (rear axle) at begin of locked phase: 104.4657 psi Time duration of brake chamber air pressure (rear axle) over 104 psi, prior to begin of locked phase: 0.23 sec. Time from initial brake application to begin of locked tire phase: 0.62 sec. Travel distance from initial brake application to begin of locked tire phase: 38.1336 feet Length of locked tire mark: 88.5337 feet Calculated deceleration rate of locked phase: 0.512 g Percent energy loss, begin of brake application to begin of locked mark: 31.2% Calculated deceleration rate of impending and locked phases combined: 0.510 g Total brake distance: 126.6336 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.05 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.24 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.10 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.23 sec. Comments: No Vericom VC2000 data available for this test. The brakes were “cold” for this test (i.e., not at a normal operating temperature). There were visible skid marks from both rear tires from this test.

Test # b2 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 1 inch Pushrod travel, axle 2 brakes: 1 inch Tire mark from rear axle used for evaluation Speed when brakes first applied: 36.46218 mph Speed when impending tire mark begins: 35.57815 mph Brake chamber air pressure (rear axle) at begin of impending phase: 89.45802 psi Time from initial brake application to begin of impending phase: 0.29 sec. Travel distance from initial brake application to begin of impending phase: 16.3667 feet Length of impending tire mark: 17.4333 feet Calculated deceleration rate of impending phase: 0.592 g Percent energy loss, begin of brake application to begin of impending mark: 5.0% Speed when locked tire mark begins: 30.92941 mph Brake chamber air pressure (rear axle) at begin of locked phase: 104.4657 psi Time duration of brake chamber air pressure (rear axle) over 104 psi, prior to begin of locked phase: 0.24 sec. Time from initial brake application to begin of locked tire phase: 0.61 sec. Travel distance from initial brake application to begin of locked tire phase: 33.80 feet Length of locked tire mark: 67.50 feet Calculated deceleration rate of locked phase: 0.473 g Percent energy loss, begin of brake application to begin of locked mark: 39.0% Calculated deceleration rate of impending and locked phases combined: 0.498 g Total brake distance: 101.30 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.04 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.21 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.08 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.20 sec. VC2000 average deceleration: 0.547 g Comments: Driving with the brakes lightly applied warmed up the brakes for this test. There were visible skid marks from both rear tires from this test.

Test # b3 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 1.5 inch Pushrod travel, axle 2 brakes: 1.5 inch Tire mark from rear axle used for evaluation Speed when brakes first applied: 38.78656 mph Speed when impending tire mark begins: 35.68739 mph Brake chamber air pressure (rear axle) at begin of impending phase: 104.4657 psi Time from initial brake application to begin of impending phase: 0.47 sec. Travel distance from initial brake application to begin of impending phase: 27.1001 feet Length of impending tire mark: 11.3666 feet Calculated deceleration rate of impending phase: 0.472 g Percent energy loss, begin of brake application to begin of impending mark: 18.1 % Speed when locked tire mark begins: 33.36303 mph Brake chamber air pressure (rear axle) at begin of locked phase: 104.4657 psi Time duration of brake chamber air pressure (rear axle) over 104 psi, prior to begin of locked phase: 0.29 sec. Time from initial brake application to begin of locked tire phase: 0.69 sec. Travel distance from initial brake application to begin of locked tire phase: 38.4667 feet Length of locked tire mark: 83.7333 feet Calculated deceleration rate of locked phase: 0.444 g Percent energy loss, begin of brake application to begin of locked mark: 35.2 % Calculated deceleration rate of impending and locked phases combined: 0.447 g Total brake distance: 122.20 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.03 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.23 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.06 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.20 sec. VC2000 average deceleration: 0.464 g Comments: Driving with the brakes lightly applied warmed up the brakes for this test. There were visible skid marks from both rear tires from this test.

Test # b4 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 2 inch Pushrod travel, axle 2 brakes: 2 inch Speed when brakes first applied: 38.89748 mph Calculated deceleration rate (from the time brake chamber air pressure reaches 100 psi, until the vehicle comes to a stop): 0.291 g Total brake distance: 261.60 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.03 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.26 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.07 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.25 sec. VC2000 average deceleration: 0.170 g Comments: Driving with the brakes lightly applied warmed up the brakes for this test. There were no visible skid marks from this test. Test # b5 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 1 inch Pushrod travel, axle 2 brakes: 2 inch Speed when brakes first applied: 38.78656 mph Calculated deceleration rate (from the time brake chamber air pressure reaches 100 psi, until the vehicle comes to a stop): 0.474 g Total brake distance: 118.2999 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.05 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.24 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.09 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.26 sec. VC2000 average deceleration: 0.474 g Comments: Driving with the brakes lightly applied warmed up the brakes for this test. There were no visible skid marks from this test.

Test # b6 Vehicle: 1982 GMC C7000, 2 axles Road surface: Asphalt, dry, 0.2 degree downgrade Pushrod travel, axle 1 brakes: 2 inch Pushrod travel, axle 2 brakes: 1 inch Tire mark from rear axle used for evaluation Speed when brakes first applied: 38.73109 mph Speed when impending tire mark begins: 35.63193 mph Brake chamber air pressure (rear axle) at begin of impending phase: 104.4657 psi Time from initial brake application to begin of impending phase: 0.50 sec. Travel distance from initial brake application to begin of impending phase: 28.7334 feet Length of impending tire mark: 8.80 feet Calculated deceleration rate of impending phase: 0.381 g Percent energy loss, begin of brake application to begin of impending mark: 18.2 % Speed when locked tire mark begins: 34.19328 mph Brake chamber air pressure (rear axle) at begin of locked phase: 104.4657 psi Time duration of brake chamber air pressure (rear axle) over 104 psi, prior to begin of locked phase: 0.27 sec. Time from initial brake application to begin of locked tire phase: 0.67 sec. Travel distance from initial brake application to begin of locked tire phase: 37.5334 feet Length of locked tire mark: 136.80 feet Calculated deceleration rate of locked phase: 0.285 g Percent energy loss, begin of brake application to begin of locked mark: 28.3 % Calculated deceleration rate of impending and locked phases combined: 0.291 g Total brake distance: 174.3334 feet Time from initial brake application to begin of pressure increase in front brake chamber: 0.04 sec. Time from initial brake application to front brake chamber reaching 60 psi: 0.27 sec. Time from initial brake application to begin of pressure increase in rear brake chamber: 0.08 sec. Time from initial brake application to rear brake chamber reaching 60 psi: 0.20 sec. VC2000 average deceleration: 0.300 g Comments: Driving with the brakes lightly applied warmed up the brakes for this test. There were visible skid marks from both rear tires from this test.

Sample Calculations Commercial type dump truck brakes to a stop. Locked skid length: 88.5 feet Deceleration: 0.512 g Pre-skid energy loss: 31 % Calculate the speed of the vehicle when the visible locked skidding began. Use the formula √ (30 x friction x distance) = speed in mph √ (30 x 0.512 x 88.5) = 36.9 mph Calculate the speed of the vehicle when the brakes were first applied. √ (36.92 x 1.31) = 42.2 mph Note that the .31 in the 1.31 increases the energy by the 31 percent.

Commercial type dump truck brakes to a stop. Length of impending and locked skid combined: 109.8 feet Deceleration: 0.510 g Pre-visible tire mark energy loss: 6.0 % Calculate the speed of the vehicle when the impending tire mark began. Use the formula √ (30 x friction x distance) = speed in mph √ (30 x 0.510 x 109.8) = 41.0 mph Calculate the speed of the vehicle when the brakes were first applied. √ (41.02 x 1.06) = 42.2 mph Note that the .06 in the 1.06 increases the energy by the 6.0 percent.

Brake Test b2 impending mark at 0.29 sec. Locked mark at 0.61 sec axle #1, 1" stroke axle #2, 1" stroke average deceleration 0.498 g

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Brake Test b4 1982 GMC C7000 Dump Truck axle #1, 2" stroke axle #2, 2" stroke average deceleration 0.291 g

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