Electronically Controlled Braking Systems Test Program

Technical Report Documentation Page

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Table of Contents

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ELECTRONICALLY CONTROLLED BRAKING SYSTEMS PERFORMANCE EVALUATION

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Conducted Under the Auspices of:

The Cooperative Research Program

Of The Society of Automotive Engineers

February 2003

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Project Performed by:

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 Radlinski & Associates, Inc.

3143 County Road 154

East Liberty, Ohio 43319

937-666-5006

 ELECTRONICALLY CONTROLLED BRAKING SYSTEMS

PERFORMANCE EVALUATION

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INTRODUCTION

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This report presents results obtained from a test program involving Electronically Controlled Braking Systems (ECBS). This activity was conducted under the auspices of The Society of Engineers (SAE) Cooperative Research Program.  Funding for this activity was provided by the U.S. Department of Transportation (Purchase Order DTMC-01-P-00022) and in-kind contributions were made by Bendix (ECBS hardware), International Truck and Engine Co. (test vehicles), Arvin Meritor (brake components), Meritor WABCO (ECBS hardware and test vehicles) and Haldex (ECBS hardware and a test vehicle).  Testing was conducted by Radlinski & Associates, Inc., acting as a subcontractor to SAE.

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The plan for the testing was developed with the guidance of a joint government and industry working-group consisting of:

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<![if !supportLists]>·        <![endif]>SAE Members.

<![if !supportLists]>·        <![endif]>Motor & Equipment Manufacturers Association (MEMA)

<![if !supportLists]>·        <![endif]>Truck Manufacturers Association (TMA)

<![if !supportLists]>·        <![endif]>American Trucking Associations (ATA)

<![if !supportLists]>·        <![endif]>Federal Highway Administration (FHWA)

<![if !supportLists]>·        <![endif]>Federal Motor Carrier Safety Administration (FMCSA)

<![if !supportLists]>·        <![endif]>National Highway Traffic Safety Administration (NHTSA)

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BACKGROUND

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Air brake systems currently installed on commercial vehicles utilize pneumatic control circuits (air lines) and pneumatic valves to transfer brake commands (pedal inputs) from the driver to the pneumatic actuators that apply the foundation brake mechanisms.  If these components that control the actuators are replaced with electronic circuits and electro- pneumatic valves, brake actuation could be faster and more precise and, the development of advanced safety systems such as “smart” cruise control, rollover prevention and stability enhancement (all of which require automatic control of one or more brakes) would be facilitated. 

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ECBS has been developed and used extensively in Europe and it is being sold there in large quantities.  It is generally installed on vehicles that utilize air disc brakes.  Use of ECBS in the U.S., to date, is very limited.

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Federal Motor Vehicle Safety Standard (FMVSS) No. 121, which governs brake design, was originally developed in the late 1960’s and is based on control of the brakes on powered vehicles via dual pneumatic circuits.  If one circuit fails the other circuit can still control the brakes, although performance is permitted to degrade.  (Trailers need only one pneumatic circuit and use automatic application of the parking brakes as the backup). FMVSS No. 121 is somewhat design restrictive when it comes to the development of new, cost effective ECBS for trucks, tractors and buses.

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ECBS vehicles currently being sold in Europe and the U.S. incorporate one electronic circuit for primary control and two pneumatic circuits for backup.  Such systems (which essentially have three redundant control circuits) are complex, expensive and unattractive to commercial vehicle users.   More cost effective ECBS systems could be developed if changes are made to FMVSS No. 121, but before this can happen, the safety performance of these more simplified systems needs to be evaluated. 

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The purpose of the testing conducted here was to evaluate the braking performance of two trucks and two tractors that utilized less complex ECBS that had only a single pneumatic circuit for backup control.  This configuration is known as 1E/1P  (one electronic circuit for primary control and a single pneumatic circuit for back-up).  Such a system still needs two isolated air reservoir systems so that a single air leak does not take-out the entire air supply (which is needed to actuate the brakes even with ECBS).  Two ECBS equipped trailers were also tested.

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It should be emphasized that the 1E/1P ECBS systems that were evaluated in this test program were preproduction systems utilizing European style components.  They had not been tuned or optimized by the ECBS manufacturers for production on North American vehicle platforms.

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TESTING APPROACH

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In order to focus on the performance the 1E/1P ECBS and to be able to compare it with conventional, dual-circuit pneumatic control (2P PCBS), all comparative testing was conducted on vehicles with the same basic foundation brake system components.  All test vehicles utilized S-Cam drum-type foundation brakes which represent the most common foundation brakes in use in the US today.  Because European applications of ECBS have been coupled with disc brakes, most of the previous testing on ECBS that has been performed makes it difficult to use the data to separate ECBS (control system) performance from disc (foundation) brake performance.

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The basic philosophy behind the testing was to evaluate performance in accordance with FMVSS No.121, using the test conditions and performance criteria specified in this standard.

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Vehicles were first subjected to a comprehensive series of tests with conventional pneumatic control (2P PCBS) and then switched to 1E/1P ECBS and retested without changing any foundation brake components.  In an ideal world it would have been desirable to do this testing in a more pure back-to-back fashion where a few stops would be run with PCBS and then it would switched over to ECBS and the same test scenario repeated.  By doing this back-and-forth switching frequently (one or more times per day) and for each test segment, the influence in the data of any changes in foundation brake performance or environmental conditions would be minimized.

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Unfortunately, such frequent changes back and forth between ECBS and PCBS were not practical due to the time it took to make such a switch.  All tests were performed on each vehicle with PCBS and then the control system was switched to ECBS and all test conditions were repeated.

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To determine the magnitude of the foundation brake system changes that might have occurred during the testing, a switch back to PCBS was made after all of the ECBS tests were completed and a few stops were made in the full-load, full-system condition to see if the initial results with PCBS for this same condition could be repeated.

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In an attempt to minimize changes in the foundation brake performance during the testing, the brakes were pre-conditioned at the start of the testing by burnishing them with 500 snubs in accordance with FMVSS No. 121 and then making 10 stops from 60 mph with a full load.  All performance measurements were then made after this initial conditioning phase.

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TEST SEQUENCE

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The basic sequence of testing on each vehicle was as follows:

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<![if !supportLists]>1.      <![endif]>Prepare, instrument and load (vehicle equipped with PCBS)

<![if !supportLists]>2.      <![endif]>Burnish the brakes

<![if !supportLists]>3.      <![endif]>Conduct 10 - 60 mph conditioning stops

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<![if !supportLists]>6.      <![endif]>Conduct ECBS testing

<![if !supportLists]>7.      <![endif]>Convert back to PCBS

<![if !supportLists]>8.      <![endif]>Conduct limited testing to evaluate brake conditioning

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The truck tractors were loaded and burnished using an un-braked FMVSS No. 121 Control Trailer.  The trailers were loaded and then burnished by using the trailer brakes only.   By burnishing the units individually, more uniform results were obtained and the performance of the brakes on one unit did not affect the other, as would be the case if the vehicles were burnished in combination.

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The tractors were first tested with the control trailer and then tested bobtail.  Next, the tractors were coupled to the test trailers and the combinations were tested. 

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A detailed, step-by-step test sequence for each type of vehicle is included in Appendix A.

PERFORMANCE MEASURES UTILIZED

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The testing evaluated several different aspects of braking performance as follows:

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<![if !supportLists]>·        <![endif]>Pneumatic timing – Service brake apply and release

<![if !supportLists]>·        <![endif]>Minimum stopping distance with fully-intact brake systems

<![if !supportLists]>·        <![endif]>Minimum stopping distance with various single point failures in the braking systems.

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Pneumatic timing refers to the time it takes for the service brakes to apply and release and the definitions in FMVSS No. 121 were utilized.  Apply time is the time it takes for the brake actuators (brake chambers) and towing couplings (gladhands) to reach 60 psi after first movement of the brake pedal.  Release time is the time elapsed from first movement of the brake pedal (when the actuators and couplings are initially at 95 psi) until the actuators and couplings drop to 5 psi.  Timing should be faster with ECBS as the transmission time of electronic control signals is negligible compared to that of pneumatic signals.

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Minimum stopping distance was determined on dry pavement (concrete) from 60 mph by making stops with the brake pedal fully applied as rapidly as possible.  Tests were run with the test vehicles fully loaded as well as empty and the test conditions specified in FMVSS no. 121 were utilized. 

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Testing was performed first with fully intact braking systems and then various failures were introduced and the tests were repeated.  The failure modes were limited to single failures (as opposed to combined or multiple failures) and included pneumatic failures as well as electrical failures.  The pneumatic failures selected represented worst case conditions such as failed reservoirs and failed pneumatic control lines.  The electrical failures included disconnected connectors used in the system as well as open and shorted electrical circuits.   No failures within components (i.e. internal failures) were evaluated. 

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The worst-case failure found on each vehicle in the 2P PCBS configuration was used as the baseline to which the various failures in the 1E/1P ECBS configuration were compared.  One of the primary goals of this program was to determine if failures in 1E1P ECBS produced performance levels below that possible with 2P PCBS.

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In addition to the 60 mph, dry road stops, tests were also run on PCBS and ECBS vehicles (fully intact systems) to evaluate stopping performance on a slippery curve.  The test conditions for the Stability and Control Test specified in FMVSS No. 121 were utilized.  In this test the vehicle is driven around a 500 foot radius curve at 30mph (or 75 percent of the maximum drive through speed, whichever is lower) and then the brakes are fully applied.  The vehicle must stay in the 12 foot wide test lane but there are no stopping distance requirements in the standard. In the testing conducted here stopping distance was measured and utilized for comparative purposes.  The maximum possible initial speed for braking runs was also utilized as a comparative measure of performance.

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TEST VEHICLES

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Test vehicles included the following units:

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<![if !supportLists]>·        <![endif]>4x2 Single-unit Truck (Manufacturer “B” ECBS)

<![if !supportLists]>·        <![endif]>6x4 Single-unit Truck (Manufacturer “A” ECBS)

<![if !supportLists]>·        <![endif]>4x2 Truck Tractor (Manufacturer “B” ECBS)

<![if !supportLists]>·        <![endif]>6x4 Truck Tractor (Manufacturer “A” ECBS)

<![if !supportLists]>·        <![endif]>Single-axle semi-trailer (Manufacturer  “C” ECBS) -tested with 4x2 tractor

<![if !supportLists]>·        <![endif]>Tandem-axle semi-trailer (Manufacturer “A” ECBS) -tested with 6x4 tractor

<![if !supportLists]>·        <![endif]>FMVSS 121 Control Trailer (used with both tractors)

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More details on the test vehicles are included in the vehicle specification sheets included in Appendices B-D.  Note that three different ABS suppliers were represented among the various test vehicles.   Each powered vehicle has its own appendix that includes information about the vehicle and the detailed test results.  The semi-trailer information (except the Control Trailer) is included in the appendix for the tractor that was used to tow it for the testing.

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Photographs of the test vehicles are shown in Figures 1 through 5.  Figure 5 shows the FMVSS No.121 Control Trailer that was used with both the 4x2 and 6x4 tractors for selected portions of the testing.  The FMVSS 121 Control Trailer is a single-axle semi-trailer that does not have any brakes. Its primary purpose is to load the tractor to GVWR and all of its ballast is placed directly above the kingpin so that only the tractor load is increased as ballast is added.  The Control Trailer axle load is essentially fixed at 4,500 lbs as per FMVSS No. 121.  

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The 4x2 and 6x4 trucks were equipped with load racks and were tested empty (2,800 lb load rack only) and loaded to GVWR.  The 4x2 and 6x4 tractors were tested bobtail (empty without trailer) and loaded to GVWR (or as close as possible without exceeding GAWR’s) with a FMVSS No. 121 single-axle control trailer.  The 4x2 tractor was also tested with the single-axle van semi-trailer and the 6x4 tractor was tested with the tandem-axle flatbed semi-trailer.  In both cases, the combinations were loaded to maximum legal highway loads for all testing.  The maximum legal highway loading was less than the sum of the GAWRs in both combinations (2,000 lbs lower with the three-axle combination and 12,000 lbs lower with the five-axle combination).

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Tables 1 through 3 provide the axle loads and total loads for the various test conditions.  “Rear” refers to the single drive axle on the 4x2 vehicles and the drive tandem in the case of the 6x4 vehicles.  “Trailer” refers to the single trailer axle or tandem axles in the case of the semi-trailers.

Table 1 - Empty (Bobtail) Test Weights

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Table 2 - Loaded  Test Weights

Table 3 – Loaded Combination Weights

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Foundation Brakes - As mentioned earlier, the foundation brakes on all test vehicles were S-Cam drum-type.  The same brake linings and drums were used for all testing with ECBS and PCBS.  Prior to any testing the friction materials were thoroughly conditioned in an attempt to stabilize their performance (see TEST APPROACH section above).

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Pneumatic Control Systems – The PCBS used on the trucks and tractors was the typical dual-circuit type with ABS used on most air-braked vehicles today.  The trailers were equipped with typical single-circuit systems with ABS. (Dual-circuit systems are not used on trailers).

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In dual-circuit pneumatic control systems, the foundation brakes are split between the two circuits and the circuits work together operating all of the foundation brakes simultaneously until there is a failure.  The heart of the control system is the brake (foot) valve that incorporates two independent, isolated pneumatic circuits.  When a failure occurs in one circuit, service brakes controlled by that circuit stop working but the brakes on the other circuit continue to function providing a backup or emergency function.   Since only a portion of the brakes are working, stopping distance is increased significantly.

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In the case of the tractors with PCBS, the primary pneumatic circuit (“primary” usually refers to the circuit that has the greatest percentage of the braking) supplied air to, and controlled, the rear brakes; the secondary circuit supplied and controlled the front brakes.  Either system was capable of independently supplying and controlling the trailer brakes. 

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The PCBS used on both trucks also used the primary circuit to control and supply the rear brakes and the secondary circuit to supply and control the front brakes but in the event of a failure, the parking (spring) brakes on the rear axle were supplied and controlled by the secondary circuit through an inversion valve that is typically used on single-unit vehicles.  An inversion valve senses a loss of air pressure to the rear brakes and then reduces air to the spring brake chambers allowing them to apply force in proportion to the secondary control pressure.

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Inversion valves are usually needed on single-unit vehicles because a failure in the primary circuit renders the rear service brakes inoperative.  If the smaller front brakes were not supplemented with the parking brakes, the vehicle would not likely be able to meet the emergency braking requirements of FMVSS No. 121.  Inversion vales are not needed on tractors, since the trailer brakes are available to supplement the smaller front brakes in the event of a primary circuit failure.

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The single-circuit systems used on semi-trailers such as those tested in this program are designed so that if the single circuit fails and trailer reservoir pressure drops, the spring (parking) brakes on the trailer fully apply automatically.  There is no brake modulation capability as there is with-dual circuit systems used on powered vehicles.

ABS used with PCBS - The ABS used with PCBS on the trucks and tractors was of the four-sensor, four-modulator (4S/4M) type with sensors on the rear-most axles of the drive-axle tandems.  The ABS on the single-axle semi-trailer was of the 2S/1M type. The tandem-axle semi-trailer used side-by-side, 4S/2M ABS.

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ECBS on the two semi-trailers tested utilized brake control valves that were functionally similar to those used on the trucks and tractors.  ECBS on both semi-trailers had the provision to accept control signals from the tractor ECBS using the ISO 11992 databus but only the ECBS on the 6x4 tractor had provisions for providing such a trailer ECBS signal.

Both ECBS semi-trailers were configured to work in the electronic control mode when towed with tractors that do not have the ISO 11992 signal available.  They accomplish this by using a pressure sensor at the front of the trailer that monitors the control line pressure and relays the message to the trailer ECBS ECU.

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Figure 7 – Brake Control Valve for ECBS

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The single-axle trailer used a switch at the front of the trailer that activated at 5 psi telling the trailer ECBS ECU that the brakes were starting to be applied by the tractor.  This allowed the ECBS ECU to get a head start on the brake application.  Since it is only a switch, it does not provide, proportional electronic control but it does speed up the signal and improve trailer brake timing.

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 The tandem axle semi-trailer utilized a pressure transducer at the front of the trailer and as such did provide proportional electronic control of the trailer brakes by a tractor without the ISO11992 databus.

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The 6x4 truck, 6 x4 tractor and tandem axle trailer, utilized ECBS from Manufacturer A; the 4x2 truck and 4x2 tractor utilized ECBS from manufacturer B; and the single-axle trailer utilized ECBS from Manufacturer C.

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ECBS Load Sensing Function  - On trucks and tractors with air suspensions, ECBS typically utilizes a pressure sensor that monitors the air bag pressure in the rear suspension.  This enables the ECBS to tailor the braking for different loads.  ECBS on trailers with air suspensions typically work in the same manner.

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All of the vehicles tested in this program except the 4x2 truck utilized air suspensions and all of the ECBS configurations on vehicles with air suspensions, except the 6x4 tractor, incorporated this feature.  The 6x4 tractor ECBS did not use a pressure sensor; it calculated load from vehicle acceleration and engine torque data broadcast by the engine ECU.    The 4x2 truck ECBS had no method for sensing load and load sensing was not utilized on any of the vehicles in the PCBS configurations.

ABS and ECBS  -  ECBS provides the wheel slip control (ABS) function via its ECU, valves and wheel speed sensors and does not require a separate ABS.  On each test vehicle, the number of sensors (S) and modulator valves (M) used for wheel slip control with ECBS matched that of the stand-alone ABS configuration used with PCBS, with one exception:  on the single-axle trailer, the PCBS ABS was configured as 2S/1M whereas the ECBS ABS was configured as a 2S/2M system.

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Brake System Mixing on Combination Vehicles - When testing the tractors in combination with the semi-trailers, both PCBS and ECBS trailer brake configurations were utilized with each of the two different tractor brake control configurations.  Table 4 lists the combinations tested.

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Table 4 - Brake System “Mixes” Tested on Combination Vehicles.

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Test vehicles were equipped with digital data acquisition systems and instrumentation to measure and record vehicle speed, stopping distance, deceleration, brake pressures, brake temperatures and wheel speeds.  Speed and stopping distance were determined with a fifth wheel system.  Deceleration was measured with an accelerometer.  Brake pressures were measured with pressure transducers located in the control line and brake chambers so as to monitor each of the ABS/ECBS pneumatic circuits.  Thermocouples were installed in the primary shoe of each brake to monitor brake lining temperatures.

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Wheel speeds were either determined with dc tachometers mounted on each wheel hub or by tapping into the ABS or ECBS wheel speed sensor circuits.  When the ABS/ECBS wheel speed sensors were used, a frequency-to-voltage converter was used to provide a dc signal proportional to wheel speed as an input to the data acquisition system

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Speed and stopping distance were the primary measures of performance.  The other instrumentation provided supplemental information that was used to help analyze the results.

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TEST RESULTS

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Detailed test data and other vehicle information are included in the appendices and which are arranged in the following fashion  (Appendix A includes the detailed test sequence):

• Appendix B – 4x2 tractor, single-axle trailer and 

• Appendix C -  6x4 tractor, tandem-axle trailer and combinations of both
• Appendix D – 4x2 truck
• Appendix E – 6x4 truck

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Figure 9 shows the apply times for the trailers and the combination vehicles.  The trailers were first tested alone with a FMVSS No. 121 trailer test rig (mini-tractor) which simulates a “standard” tractor.  The single axle trailer was tested with a 50 in³ reservoir on its towing gladhand.  This trailer was equipped to tow another trailer in doubles operations.  Both trailers met FMVSS No. 121 and were faster with ECBS than with PCBS.

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Figure 9 shows that the 4x2 tractor and single-axle trailer (4x2-SA) combination is slower to apply the trailer brakes when the tractor is equipped with ECBS.  This is primarily due to the slow gladhand timing on this tractor with ECBS.  The trailer ECBS configuration was faster but this was negated by the slower ECBS tractor, making the all- ECBS combination slower than the all-PCBS combination.

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The 6x4 tractor and tandem axle trailer (6x4 -TA) combination showed significantly better trailer timing (0.16 sec faster) when both units were equipped with ECBS than when both were equipped with PCBS. 

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Figure 10 shows the brake release times for the trucks and tractors.  This graph indicates that ECBS is slower in some cases and faster in others.  With both tractors, ECBS release time at the gladhands was slower than with PCBS.  All four vehicles met FMVSS No. 121 in both configurations, however.

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Figure 11 shows release timing for the trailers (tested by them selves using the mini-tractor) and for the combinations.  Release time was not much different on the single-axle trailer, but the tandem-axle trailer exhibited some improvement with ECBS.  Both trailers meet FMVSS No.121 in both configurations.

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With the 4x2-SA combination, the trailer release timing did not change much with the addition of ECBS to the tractor or to the trailer but there did appear to be a slight trend to slower times with ECBS on either unit.

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The 6x4-TA combination showed an improvement in trailer release timing when the trailer had ECBS but with ECBS on the tractor, trailer release time increased, negating most of the improvement gained from the trailer ECBS.

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Figure 8 – Apply Times for Trucks and Tractors (0 to 60psi)

Figure 9 – Apply Times for Trailers and Combinations (0 to 60psi)

Figure 10 – Release Times for Trucks and Tractors (95 to 5psi)

Figure 11 – Release Times for Trailers and Combinations (0 to 95psi)

Full System Performance – Figure 12 shows the average stopping distance from 60 mph on dry concrete with a full brake apply for the trucks and tractors in both the PCBS and ECBS configurations without any pneumatic or electrical failures introduced (i.e. full system).  The top chart in Figure 12 is for the loaded condition.  In this case the tractors were loaded to GVWR using the un-braked FMVSS No. 121 control trailer.   The bottom chart is for the empty or bobtail loading condition.

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For reference, FMVSS No. 121 requirements are 310 feet for loaded trucks, 355 feet for loaded tractors and 335 feet for all empty trucks and bobtail tractors.  Figure 12 indicates that all distances shown were within FMVSS no. 121 requirements.

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In the loaded condition the vehicles were tested with PCBS, both at the beginning and the end of the testing, in order to have a measure of brake system effectiveness changes that occurred during the course of the testing.  It can be seen in Figure 12 that brake effectiveness on three of the four vehicles did not change significantly during the course of the testing.  The 4x2 tractor exhibited  a 50 foot (17 percent) increase in stopping distance, however.

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This was actually the second series of tests on this tractor.  In the first series of tests, where the tractor was equipped with a different brake lining material, testing was discontinued approximately half-way through the complete test series when it was discovered that effectiveness of the brakes had degraded by more than 25 percent and stopping distance was well above the level permitted by FMVSS No.121.  Loaded, full-system stopping distance had reached approximately 425 feet and there were no obvious problems found in the braking system components.

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 At that point, the decision was made to switch to a different brake lining material, install new brake drums and to rerun all of the tests (after completing a burnish and 10 conditioning stops at full load from 60 mph).  All tests were completed with this second lining material installed and the vehicle still met the FMVSS 121 requirement (355 ft.) at the end of the testing but the change in effectiveness was still significant.

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The impact of this change in foundation brake performance is that it makes it more difficult to compare PCBS and ECBS performance on this 4x2 tractor.  In fact, the increase in both empty and loaded stopping distance shown for this vehicle with ECBS in this figure cannot be considered significant and is most likely due to the brake effectiveness changing.

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Reviewing the loaded data (top chart) in Figure 12 for the other three vehicles indicate that the 4x2 truck with ECBS took longer to stop (52 ft.) whereas the 6x4 truck and 6 x4 tractor stopped slightly shorter with ECBS.

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The empty (bobtail) data on the bottom chart of Figure 12 indicate that the stopping distance of the 4x2 truck was significantly (90 feet or 50 percent) longer with ECBS.  than with PCBS.  Differences on the other vehicles were small.  The difference with the 4x2 tractor may be completely due to the change in brake effectiveness.  Even though the

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Figure 12 – Full System Performance – Tractors and Trucks (60mph Dry Road)

rear wheels were cycling the ABS and rear braking was limited by tire traction in this bobtail mode, the front brakes were operating at maximum capacity without cycling the ABS making changes in brake effectiveness a significant factor in determining stopping distance.

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Figure 13 shows the full-system stopping distance of the fully loaded vehicle combinations.  The differences with the 4x2 tractor and single-axle trailer (4x2–SA) combination were insignificant given the conditioning issue.  The differences with the 6x4 tractor and tandem-axle trailer (6x4-SA) combination were also relatively small but the general trend was for better performance with ECBS added to either the tractor or the trailer.  It is interesting to note that the trend is similar to the trend in the apply time data for this combination shown in Figure 9.

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FMVSS No. 121 does not have requirements for stopping distance of combinations but it is interesting to compare the performance of these loaded combinations to the requirements for tractors towing a FMVSS 121 control trailer.  The required distance is 355 feet with the un-braked Control Trailer and both combinations were well below this required level.  The combinations also stopped shorter than the tractors did when they were tested with the Control Trailer.  The 6x4-TA combination, which is by far the most common combination on the highway today, stopped from 60 mph in distances from 234 feet (all-PCBS) down to 207 feet (all-ECBS) when loaded to the maximum legal highway load (80,000 lbs).  Such five-axle combinations generally have brakes that are rated for 92,000 lbs (12K/40K/40K) giving them a significant reserve braking capacity and making their “highway” stopping distance much shorter than their “certified” stopping distance.

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Figure 14 shows stopping performance (maximum in-lane initial braking speed as a percent of maximum driving speed without braking) for the trucks and tractors tested on a 500 ft radius curve on a wet Jennite surface.  This is the surface typically used to conduct the Stability and Control Test specified in FMVSS No. 121.  This test is only currently a requirement for bobtail tractors and tractors loaded with a FMVSS No. 121 Control Trailer.  There are no stopping distance requirements in the Stability and Control Test, only the requirement that the vehicle stay in a 12 foot wide lane when making a full brake application from 30mph or 75 percent of the maximum drive-through speed, which ever is lower.  All four vehicles met this in-lane requirement both empty and loaded.  The ECBS equipped vehicles achieved initial braking speeds that were essentially equal to or higher than the PCBS vehicles.

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Figure 15 shows similar results for the two combination vehicles.  Both of these combinations met the in-lane requirements of FMVSS 121 as well.

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The 4x2 tractor and single-axle trailer did not show much difference in performance with the different brake system configurations but the 6x4 tractor and tandem-axle trailer performance was better (higher initial braking speeds) when ECBS was on either or both units in the combination.  The all-ECBS combination allowed the highest initial braking speed.

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Figure 13 – Full System Performance – Loaded Combinations (60mph Dry Road)

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Notes:

<![if !supportLists]>1)      <![endif]>FMVSS 121 requirements shown are for loaded tractors with a FMVSS 121 control trailer. There are no FMVSS 121 requirements for vehicle combinations.

<![if !supportLists]>2)      <![endif]>Brakes on the 4x2 tractor are designed for 52,000 lb. GVWR; brakes on the 6x4 tractor are designed for 92,000lb. GVWR.

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Figure 14 – Full System Performance – Tractors and Trucks (Wet Jennite Curve)

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Figure 15 – Full System Performance – Loaded Combinations (Wet Jennite Curve)

Failed System Performance – Figure 16 shows the stopping distance (60 mph, dry concrete) for the two loaded trucks with various pneumatic failures introduced into the braking systems.  The failures were introduced in both the PCBS and ECBS equipped trucks.  The failures were: failed primary reservoir, failed secondary reservoir and failed control line (primary).  These are generally considered to be the “worst-case” pneumatic failures and they are the ones specified in the current FMVSS No.121 test procedure document (TP121-04). 

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FMVSS No.121 requires that trucks be capable of stopping from 60 mph on a dry  road within 613 feet with any single leakage failure in the service brake system.  Figure 16 indicates that both trucks in both configurations (PCBS and ECBS) met FMVSS No. 121.

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Failing the secondary reservoir produces the longest stops on both trucks.  This leaves the trucks without front brakes.  The other failures did not result in complete loss of brakes on any of the axles and as such had a lesser impact on stopping performance.

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On the 4x2 truck, the impact of the secondary reservoir failure was greater on the ECBS configuration than on PCBS. This is consistent with the poor performance of ECBS in the full-system configuration on this truck when loaded.  On the 6x4 truck, the pneumatic failures always had a lower impact on the ECBS configurations.

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Figure 17 shows the stopping distances for the loaded combination vehicles with each of the brake configurations and with various pneumatic failures.  Failed gladhand refers to the disconnection of the trailer control line which renders the trailer brakes inoperative. 

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There are no stopping distance requirements in FMVSS No.121 for combination vehicles with failures nor are there requirements for loaded tractors with failures (i.e. with the control trailer).  The loaded truck requirement of 613 feet can be used for reference and based on Figure 17, all combinations met this requirement.

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Figure 17 indicates that the worst-case failures on both combinations were the failed primary reservoir and the failed trailer control line (disconnected control gladhand).  Failed primary reservoir resulted in complete loss of the tractor drive-axle(s) brakes; failed control line resulted in the complete loss of trailer brakes.  Since the level of braking at the front axles was less than that at the drives or trailer axles, a failed secondary reservoir has a lesser effect.  Generally, the pneumatic failures had the same or lesser effect on ECBS as they had on PCBS.  There was a significant difference on the 6x4-TA combination when the trailer control line was failed and both units had ECBS.  In this case performance was unaffected by this pneumatic failure since the “electronic control line” (ISO 11992) was still intact.  The 4x2-SA combination did not have this electronic trailer control line and relied solely on the pneumatic control line.

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Figure 18 shows the results for pneumatic failures on the empty trucks and bobtail tractors.  FMVSS No. 121 requires a distance at or below 613 feet and in all cases this was met.  The effect of the failures on ECBS was less than or equal to that for PCBS on the 6x4 tractor and 6x4 truck but was generally worse on ECBS on the 4x2 tractor and

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Figure 16 – Performance with Pneumatic Failures – Loaded Trucks (60mph Dry Road)

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Figure 17 – Performance with Pneumatic Failures – Loaded Combinations (60mph Dry Road)

Figure 18 – Performance with Pneumatic Failures – Empty Trucks and Tractors (60mph Dry Road)

4x2 truck.  On the truck this is consistent with previous results but on the tractor it is not clear why the failed secondary with ECBS distance was 16 percent longer than the PCBS distance.  It cannot be attributed to brake conditioning because in this case only the rear brakes were operational and they had more than enough torque for the very lightly loaded rear axle.

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Figure 19 shows the effect of various electrical failures in the ECBS system on the 4x2 truck, both in the empty and fully loaded condition.  Shown for reference on the chart are the stopping distances for the worst-case pneumatic failures (empty and loaded) for this same vehicle in the PCBS configuration.  It can be seen that all of the ECBS electrical failures resulted in performance that was equal to or better than that with the worst-case pneumatic failure with PCBS.

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Figure 20 shows the results for electrical failures in the 6x4 truck in a similar fashion to Figure 19.  The results were similar to those on the 4x2 truck.  The ECBS electrical failures had less effect on performance than did the pneumatic failures on the PCBS configured truck.

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Figure 21 shows the results for electrical failures in the ECBS on the 4x2 tractor in both the bobtail mode and when coupled to the loaded single-axle trailer.  Once again the ECBS electrical failures had a smaller impact on performance than did the pneumatic failures with PCBS.

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Figure 22 shows the results for ECBS electrical failures in the single-axle semi-trailer when the loaded trailer was coupled to the 4x2 tractor with ECBS.  None of these failures had a significant impact on braking performance of the combination compared to full-system performance.  The failures resulted in the loss of the ECBS antilock function on the trailer but the brakes did not have enough torque to lock the wheels so it did not matter.

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Figure 23 shows the ECBS electrical failure results for the 6x4 tractor in the bobtail mode and when coupled to the tandem-axle trailer with both ECBS and PCBS brakes.  The only failures that appeared to have caused significant problems were the open connector to the rear modulator and the open connector to the foot valve.  In both cases the rear braking on the tractor was lost but stopping distances were well within the FMVSS No. 121 requirements.  With these failures in the combination vehicle, the distances were well below the distance for the worst-case pneumatic failure with the PCBS tractor.  In the bobtail mode, the loss of the connection to the foot valve resulted in a few stops being longer than the worst-case failure with PCBS.

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Figure 24 shows the results for ECBS electrical failures in the tandem-axle semi-trailer when the loaded trailer was coupled to the 6x4 tractor with ECBS.  These failures basically resulted in the loss of the antilock function in the EBS, making it difficult for the driver to stop without locking the wheels, but the distances that resulted were far less that the worst-case pneumatic failures.

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Figure 19 – Performance with EBCS Electrical Failures – 4x2 Truck (60mph Dry Road)

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Figure 20 – Performance with EBCS Electrical Failures – 6x4 Truck (60mph Dry Road)

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Figure 21 – Performance with EBCS Electrical Failures – 4x2 Tractor (60mph Dry Road)

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Figure 22 – Performance with EBCS Electrical Failures – 4x2 Tractor w/ Single Axle Trailer (60mph Dry Road)

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Figure 23 – Performance with EBCS Electrical Failures – 6x4 Tractor (60mph Dry Road)

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 ADDITIONAL TESTING ON THE 4X2 TRUCK

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The full-system test results obtained on the 4x2 truck indicated a significant degradation in performance with ECBS compared to PCBS.  When equipped with ECBS the stopping distance from 60 mph on dry pavement was 50 percent longer when empty and 23 percent longer when loaded.  At both loads the vehicle still met the requirements of FMVSS No. 121 when equipped with ECBS. 

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The ECBS manufacturer expressed concern that the tests on ECBS and PCBS had been run several months apart.  Basically, all of the testing of PCBS on this vehicle occurred in the late fall and then poor weather set in and prevented resuming the ECBS testing until late winter.

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In order to investigate performance of this vehicle further it was retested with both ECBS and PCBS in the empty and loaded conditions over a three-day period.  Testing was limited to measuring full-system stopping distance on dry pavement.  This retesting, with the same vehicle and same components used previously, indicated a 67 percent increase in stopping distance when empty and a 16 percent increase when loaded with the vehicle equipped with ECBS.  The retesting basically confirmed the significant increases in stopping distances with ECBS that were experienced in the previous testing.

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The ECBS manufacturer was involved in this retesting and also conducted additional tests after making changes to the setup parameters in the ECBS.  One problem with the installation of ECBS on this vehicle was the lack of a load signal from the rear suspension or use of an algorithm in the ECBS ECU to compute load from acceleration and engine torque broadcast by the engine ECU. The other test vehicles all utilized one of these load-sensing approaches.

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The retesting with the modified setup parameters (one setting for the empty condition and a different setting for the loaded condition) was an attempt to simulate the use of a height-sensing device on the spring suspension to determine load.  Such devices are common on European vehicles.

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Figure 25 shows the results of the retesting.  Included in this figure are the results with PCBS, ECBS as configured in the previous tests and ECBS as modified with the different setup parameters    It can be seen that that the modifications to the setup parameters significantly improved empty performance but had little effect on loaded performance.  After modification, however, the stopping distance with ECBS was still 23 percent longer when empty and 15 percent longer when loaded than the distance with PCBS.

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The ECBS manufacturer has attributed the increase in stopping distance with the ECBS to the system not being properly tuned to a vehicle platform with a leaf-spring rear suspension.  The ECBS had not been previously released in North America in a vehicle platform with a leaf-spring rear suspension.  Comparable stopping distance to PCBS can be achieved with additional application testing for this particular vehicle platform.

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Figure 25- Results of Additional Testing on the 4x2 Truck (60mph Dry Road)

CONCLUSIONS

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Tests were performed to evaluate the braking performance of two trucks, two tractors and two semi-trailers equipped with prototype electronically controlled braking systems (ECBS).  These systems were configured with a single, primary electronic control circuit and a single pneumatic backup control circuit (1E/1P configuration).   For baseline performance, tests were also run on the same vehicles equipped with conventional pneumatically controlled braking systems (PCBS) with ABS.

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 When configured with either PCBS or ECBS, the vehicles evaluated in this test program met all of the vehicle performance requirements of FMVSS No. 121.  This included apply and release timing, full-system stopping distance, stability and control (braking in a turn) and partial-system stopping distance.  There were no cases where the introduction of either single-point electrical failures or single-point pneumatic failures into the ECBS or PCBS-equipped vehicles resulted in a non-compliance with the FMVSS No. 121 performance criteria.  In addition, none of the electrical failures that were introduced into the ECBS-equipped vehicles resulted in performance that was any worse than that which occurred when pneumatic failures were introduced into the PCBS-equipped vehicles.

Full-system stopping performance with ECBS was essentially equal to or better than that with PCBS on all test vehicles except the 4x2 truck, the only vehicle with a leaf-spring rear suspension (the rest of the vehicles utilized air suspensions).  With ECBS on the 4x2 truck, the empty and loaded stopping distance increased significantly (23 percent empty and 15 percent loaded) compared to the PCBS configuration on this same vehicle.  This increase is attributed to the ECBS not being properly tuned to the all leaf-spring rear suspension. 

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Stability and control with ECBS, determined by testing on a wet slippery curve with fully intact systems, was generally better than that obtained with PCBS.  When the individual vehicles and vehicle combinations were tested they we able to reach initial braking speeds that were essentially equal to or greater than that with PCBS. 

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Brake application time on individual trucks, tractors and trailers was found to be faster with ECBS than with PCBS.  However, on one of the two tractors that were tested (4x2), the timing at the tractor’s trailer control-line coupling was actually slower in the ECBS configuration than it was when configured with PCBS. When this 4x2 tractor with ECBS was coupled to the single-axle trailer with PCBS, the trailer apply time was longer than it was with PCBS on the tractor.  In addition, the 4x2 tractor ECBS was not equipped to communicate electronically with ECBS on the single-axle trailer and therefore the full benefits of an all-ECBS combination, in terms of brake timing, could not be demonstrated.  The 6x4 tractor and tandem-axle trailer combination was capable of demonstrating the benefits of having a faster brake application timing with ECBS on the trailer because the tractor and trailer ECBS were able to communicate with one another over the ISO 11992 databus. 

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Brake release timing was not always faster with ECBS.  On both tractors tested, the release times at the trailer control couplings were slower with ECBS.  It should be noted that the slower release times were likely due to fact that the ECBS valves were of European design and the fact that there are no timing requirements in the European braking regulations.  

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Development of a North American standard for electronic data communications between tractors and trailers will be necessary to insure that tractors and trailers with ECBS will be compatible with one another and to take full advantage of the capabilities of ECBS.  In order for ECBS-equipped vehicles to be compatible with vehicles not having ECBS, this standard will also need to take into consideration the fact that tractors and trailers built since March 1, 2001 all have a PLC communications capability which was installed in order to meet the requirements of FMVSS No. 121.  FMVSS No. 121 requires that a failure in the trailer ABS must be transmitted to the tractor and the tractor must activate a failure-warning indicator visible in the driver’s direct field of view.

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APPENDIX B

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VEHICLE INFORMATION AND DATA FOR 4x2 TRACTOR

Table 1 - Vehicle Information Sheet

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