Test method of automobile braking performance

Vehicle braking performance inspection is mainly based on bench test, and road test can be carried out for vehicles that cannot be tested or have doubts about bench test. The following is an introduction to the test method of automobile braking performance, welcome to read!

Test method of automobile braking performance 1 The ability of an automobile to stop in a short time and keep the driving direction stable, as well as the ability to keep a certain speed when going down a long slope, is called automobile braking performance. Since the birth of the automobile, the braking performance of the automobile has become very important, and with the development of automobile technology and the improvement of driving speed, it has become more and more important. Braking is directly related to traffic safety, and major traffic accidents are often related to long braking distance and side slip during emergency braking.

1 automobile braking performance evaluation index

There are three main indexes to evaluate automobile braking performance.

(1) Braking efficiency, that is, braking distance and braking deceleration, refers to the braking distance from braking to stopping at a certain initial speed and the deceleration when braking on a good road surface, which is the most basic evaluation index of braking performance.

(2) The constancy of braking efficiency. When a car is driving at high speed or braking continuously on a long slope, the degree of maintaining braking efficiency is called thermal recession resistance. In addition, when the car wades, the water enters the brakes, and the reduction of braking efficiency in a short time is called water retreat. The car should quickly restore its original braking efficiency in a short time.

(3) The directional stability of the automobile during braking, that is, the performance that the automobile does not run off, sideslip or lose its steering ability during braking.

2 Comparison of test methods of automobile braking performance

When evaluating automobile braking performance, the inspection station mainly tests braking efficiency and directional stability during braking. For the constancy of braking efficiency, the test method is complicated, so it is generally not tested. When evaluating automobile braking performance, the parameters detected by the testing station mainly include braking force, braking deceleration, braking distance and braking coordination time. The detection methods of braking performance by the detection station are divided into bench test (that is, through the braking detection platform) and road test (that is, through the five-wheel instrument or deceleration instrument). Road test inspection can detect braking deceleration, braking distance, braking coordination time and stability; Bench test can detect braking force, braking force balance and wheel braking force.

2. 1 Road test inspection of braking performance

According to Article 7, Article 10 and Article 3 of Technical Conditions for Motor Vehicle Operation Safety (GB 7258-20 12), if there is any objection to the test results of braking performance of bench test, the road test shall be conducted again in accordance with Article 7 and Article 10 (road test braking performance test) under no-load conditions. If there is any objection to the re-inspection result of no-load state, the re-inspection result of full-load road test shall prevail. This shows that the national standard regards the road test of braking performance as the final method to judge braking performance. Braking Performance Road test can truly reflect the dynamic braking performance such as the influence of axle load transfer in the actual driving process of the vehicle, and can also comprehensively reflect the influence of the structural performance of other automobile systems on the braking performance of the vehicle, such as the influence of steering mechanism and suspension system structure on the braking direction stability, without large equipment and workshops. However, the road test of braking performance has the following disadvantages.

(1) is not conducive to the assembly line and has low efficiency. With the rapid increase of car ownership, today's testing stations have to undertake hundreds of testing tasks every day, and not only testing braking performance. The road test inspection needs a large site that is isolated from other areas and meets the requirements. It is impossible to accomplish such a task without assembly line and high efficiency.

(2) It can only be analyzed as a whole. The braking road test takes the actual driving and the whole vehicle as the research object, which can only quantitatively evaluate the braking performance of the whole vehicle, but can't detect the braking situation and braking force distribution of each wheel, and can't accurately judge the fault position, which is not conducive to making a correction plan for unqualified vehicles and diagnosing the specific parts where the fault occurs.

(3) It is very dangerous. Because the braking road test is a dynamic high-speed test that is close to the actual driving, especially the full-load road test, if the tested vehicle fails to brake, there may be problems such as too long braking distance and poor directional stability, so there are certain safety hazards.

2.2 Brake performance bench test

According to the motion state of the inspected vehicle relative to the ground, the bench test of braking performance can be divided into static test method and dynamic test method. The static detection method adopts roller reaction brake test bench, and the dynamic detection method adopts flat brake test bench.

2.2- 1 drum reaction brake checklist

At present, the reaction roller brake test-bed is considered to be the most commonly used instrument and equipment for testing automobile braking performance in automobile inspection stations. Its advantages are: lower vehicle speed and higher detection safety factor; Convenient assembly line operation, economical and rapid detection; Good repeatability, which can quantitatively measure the whole process of braking growth, and is beneficial to judge the fault location and formulate the correction scheme for unqualified vehicles; In addition to the need for equipment and workshops, it is less restricted by external conditions, which is convenient for testing the layout of the site. However, it has the following disadvantages.

(1) The detected state is inconsistent with the actual braking state of the car. When the vehicle is braking at high speed, the braking force of the front axle of the vehicle is greatly improved due to the transfer of the dynamic axle load, while the structure of the drum brake testing platform belongs to static testing and cannot simulate the transfer of the axle load.

(2) It is difficult to detect the braking stability of automobiles scientifically. When braking, the four wheels are stressed, which determines the stability of the car when braking. The evaluation of braking stability by using drum brake detection platform is to consider the front and rear axles separately, and judge whether the braking stability is qualified by measuring the maximum braking force difference between the left and right wheels and the ratio of the maximum braking force between the left and right wheels of the axle measured in the whole process. This evaluation method simply considers the influence of the braking force difference on the stability in the process of single-axle braking, but does not consider the influence of the interaction between the front wheel and the rear wheel axle on the braking stability of the automobile. When the drum brake test-bed is used, the influence of axle load transfer on braking stability can not be truly reflected.

(3) It is difficult to accurately detect the braking performance of the automobile equipped with ABS. With the development of electronic technology and automobile technology, the application of ABS greatly improves the braking performance of the automobile, so that the automobile can prevent the wheels from completely locking in emergency braking, and it is in a semi-locking and semi-rolling motion state with the greatest longitudinal adhesion and lateral adhesion, and the slip rate is kept at 65,438+00% ~ 20%. When testing the braking performance of the car on the drum brake test bench, the relative speed of the car is only 3 km/h~5 km/h, and ABS is ineffective at this speed. In fact, this kind of test only tests the braking performance of the car when ABS doesn't work, not the braking performance of the car when it actually brakes.

2.2-2 Inspection Platform for Flat Brake

The flat brake inspection table used in dynamic method has the advantages of simple structure, less daily maintenance workload, strong working reliability and high detection efficiency, and is now more and more favored by inspection stations. Let's talk about some common misunderstandings about the flat brake inspection table at present.

(1) The faster the speed, the greater the kinetic energy, so the braking force measured on the flat brake platform at different speeds is different, resulting in poor repeatability. When cars with different speeds implement emergency braking, the parking time is different. When braking, cars with different speeds have different braking distances. The faster the speed, the longer the braking distance and the longer the braking time. According to GB 7258-20 12, the detection of driving braking performance is only related to the whole process of braking force growth, that is, the initial rising section of the brake pedal, and the duration of braking force has nothing to do with detection. Higher vehicle speed will make the wheel move a long distance on the brake platform, and prolong the stress time of the load cell, but it will not affect the measurement results.

(2) The measuring accuracy of drum-type reaction brake test-bed is higher than that of flat-plate brake test-bed. The biggest difference between the two is that the flat brake test-bed measures the braking force of the car, while the roller reaction brake test-bed measures the braking force of the wheel. The existing methods for measuring automobile braking performance can't say who is more accurate, only which method can reflect the braking performance of the tested automobile more accurately. For example, for front-wheel passenger cars, the braking force can reach 1.40% of the static axle load in actual braking, but it can't exceed 1.000% in the roller reaction braking test bench, so it is normal that the braking force error obtained by different measurement methods reaches several tens of percent. We can only judge whether the measured data of the inspected vehicle is better than the limit of the national standard according to the requirements of the national standard. For example, if the braking force of the same vehicle is sufficient, it can be seen that the braking force measured by the flat brake detection platform is greater than that measured by the roller reaction brake detection platform. If the results obtained by the two measurement methods are both greater than the limits specified by the national standard, we say that the two sets of measurement data are correct, but the data obtained by the roller reaction brake detection platform is only a force related to the braking force of the automobile, not the real braking force of the automobile, but the braking force excluding other factors.

(3) The flat brake test bench cannot test the wheel braking force. When the vehicle is at rest and in neutral, the resistance when the vehicle is pushed by external force to start moving from rest is considered as wheel blocking force. When tested with a flat brake test bench, the vehicle slides on a neutral flat plate under the action of inertial force. At present, the length of flat brake test-bed is generally long and the sliding time is relatively long, so the wheel locking force is measured during sliding. The roller reaction brake inspection table tests the wheel locking force because the roller rotates to drive the wheel, while the flat brake inspection table measures the wheel locking force through the movement of the vehicle itself, that is to say, the vehicle wheel rotates to drive the flat plate to measure the wheel locking force. It can also be understood that there is relative movement between the vehicle and the flat plate. We think that the vehicle is stationary and the flat plate moves, so that the wheel locking force can be measured, but it cannot be denied that it is really inconvenient to test the wheel locking force on the flat plate brake detection platform. The wheel locking force is deleted from the list of motor vehicle safety technical inspection items in the Items and Methods of Motor Vehicle Safety Technical Inspection (GB 21861-2014), that is to say, it is not necessary to detect and judge the wheel locking rate of all vehicles, which clears the obstacles for the wide application of flat brake inspection platform.

3 abstract

Vehicle braking performance inspection is mainly based on bench test, and road test can be carried out for vehicles that cannot be tested or have doubts about bench test. The braking process of automobile in actual driving is influenced by road conditions, speed, loading, personnel operation, vehicle condition and other factors. As the final test of braking performance, road test is only a method to simulate the braking performance of automobile. It can be said that there is no test method that is completely consistent with the actual braking. It is the purpose of our automobile inspection to correctly judge whether the braking performance of the automobile is qualified or not. Only by choosing the correct measurement method and fully understanding all kinds of measurement methods can we reduce the misjudgment in inspection.

Vehicle braking performance test method II. Development background of vehicle braking performance test.

With the development of economy in China, the number of cars is increasing day by day. In order to ensure highway safety, manufacturers need to carry out vehicle safety inspection before leaving the factory. With the development of electronic technology and machining industry, modern automobile diagnosis and detection technology is gradually developed on the basis of traditional detection methods. Automobile inspection usually refers to the use of modern inspection technology and equipment, computers, automatic control and other high-tech to detect the status quo of automobile technology, which is a comprehensive applied science.

The braking performance of automobile is one of the main performances of automobile, and the detection of braking performance is extremely important for all vehicles. The braking performance of a car is related to personal safety and is an important guarantee for the safe driving of the car. The data shows that road traffic accidents caused by poor braking account for 1/3 of the total number of accidents. The braking performance of an automobile depends not only on the performance of the braking system, but also on the driving performance of the automobile, the mechanical characteristics of the tire, the adhesion conditions of the road surface and the ergonomic characteristics related to the braking operation.

The braking performance of an automobile is determined by its braking system, and its braking process is very complicated, which is related to the general layout of the automobile and the selection of various parameters of the braking system. The car itself is a complex system. During the operation, all components are moving. With the passage of time, the technical status of each system will change, and its technical status will continue to deteriorate, which will lead to the decline of the performance of the car, thus increasing the possibility of its failure and causing a lot of traffic safety hazards. With the improvement of road quality and the development of high-grade highways and expressways, the speed of cars is getting faster and faster, so the requirements for braking performance of cars are getting higher and higher.

Second, the research significance of automobile braking performance test-bed

We know that the road test method is the most intuitive and true detection method. However, the road test method needs professional test sites, and there are not many professional test sites in China. There are only four automobile test sites in China: Hainan Automobile Test Site, Xiangfan Automobile Test Site, China Dingyuan Automobile Test Site and Beijing Tongzhou Automobile Test Site. Therefore, it is difficult to popularize, which is not suitable for China's national conditions. In addition, the road test will cause certain wear and tear to the car. During the road test, various sensors must be installed on the wheels or axles of the car every time to collect the braking data of the car during the road test, so it is difficult to avoid the errors caused by loading and unloading these sensors every time. In 2004, it was stipulated that when the braking performance of motor vehicles was questioned after bench inspection, road test inspection could be used for re-inspection, and the results of full-load road test would prevail. In the existing testing equipment, there is no testing equipment that can use bench test instead of road test. The dynamic braking test system is based on the design principle of simulating road test, and finds a method as close as possible to the road test. It has happened before that the road test passed and the platform test failed. The final test result is subject to the test result of the road test. This reflects the inconsistency between road test and bench test. The dynamic braking detection system is based on the road test method and adopts the bench test method to detect the road test. In this way, the automobile inspection industry in China will be more scientific.

The bench test of automobile brake is a compulsory verification item of brake, which simulates the braking process of automobile. Through bench test, the braking efficiency, thermal stability, lining wear and brake assembly strength are tested, revealing its inherent statistical law, finding out its existing problems, and proposing solutions to ensure road traffic safety. Its advantages are that it can quickly and accurately test the braking performance, is not limited by climatic conditions and has good test repeatability. The braking force or braking distance of each wheel can be expressed quantitatively, which is beneficial to analyze the distribution of braking force of front and rear axles, the balance state of braking force of each axle, braking coordination time and other parameters, and provides reference for fault diagnosis.

Provide reliable basis. At present, bench test has become the most commonly used method for automobile diagnosis and detection.

At present, reaction drum brake test-bed and flat brake test-bed are widely used. Due to the small space required, low equipment cost and convenient testing, the reaction drum brake test-bed has become the main equipment for measuring the braking performance of motor vehicles in various testing stations in China. The flat brake test-bed is close to the actual road condition, and the force on the wheel when braking on the platform is similar to that when braking on the road, which makes the test results closer to the actual situation. However, the flat brake test-bed has strict requirements on sensors, verification tools and test methods, which increases the cost and makes the test more difficult. Considering all kinds of factors, the reaction roller test-bed is selected in this experiment.

Three, vehicle braking performance testing standards:

Braking force requirements: the total braking force accounts for the percentage of vehicle gravity, with no load ≥60% or full load ≥ 50%; The percentage of the braking force of the main bearing shaft to the load of the sleeve shaft is ≥60% under no load or ≥50% under full load. exist

In GB7258- 1997, the requirement that the percentage of total braking force in vehicle gravity is not less than 60% or the full load is not less than 50% is still maintained. Because the understanding of the main bearing shaft is easy to make mistakes, the ratio of the braking force of the main bearing shaft to the shaft weight of this shaft is changed to the braking force of the front axle not less than 60%.

Requirements for braking force balance: the original standard takes axle load as the standard, that is, the braking force difference between the left and right wheels of the front axle shall not be greater than 5% of the axle load, and the braking force difference between the left and right wheels of the rear axle shall not be greater than 8% of the axle load. Because this regulation can't accurately reflect the actual situation that the difference of braking force should change in direct proportion with the increase of braking force, it was revised in GB7258- 1997.

In the whole braking force growth process, the ratio of the braking force difference between the left and right wheels to the braking force of the left and right wheels of the axle should not be greater than 20% for the front axle and 24% for the rear axle. Compared with the original standard, the front axle requirements are relaxed appropriately, and the rear uranium requirements are basically unchanged.

Fourth, the significance of automobile braking performance testing standards

Ensure driving safety and give full play to the power of the car.

Five, some problems that should be paid attention to in the process of braking performance testing.

(A) ABS braking vehicle detection

At present, there are mainly flat brake platform and drum brake platform for testing vehicle braking performance in motor vehicle testing station. Both of these brake platforms have some shortcomings in detecting vehicles with ABS. At present, there is a view in the automobile inspection industry that the braking performance of vehicles with ABS cannot be accurately evaluated by drum brakes. The reason is that the braking performance of vehicles with ABS can not be detected by drum brake platform, because the wheels of vehicles with ABS can not be locked and the maximum braking force can not be reflected. Under normal circumstances, the ABS system of vehicles generally only works when the driving speed is above 25 ~ 40 km/h and emergency braking is taken. Although it is not enough to meet the conditions of ABS braking on the drum brake platform, the test result is a comprehensive evaluation of the normal braking force and braking force balance of the vehicle, and the braking test platform reflects the braking performance parameters except the emergency braking situation of the vehicle. According to the author's experience, generally speaking, as long as it is a vehicle that passes the braking test under the condition of normal braking, the braking performance during emergency braking is better. The ABS system of a vehicle only works when the vehicle speed is above 30 ~ 40km/h, and as far as its braking control process is concerned, the index to evaluate its braking effect should be the degree of braking deceleration and braking deviation, not the braking force generated.

At present, the flat brake platform we use only reaches the detection speed of 5 ~ 10 km/h according to the standard, and its detection results can not accurately reflect the braking performance of ABS vehicles. Therefore, the author thinks that the flat brake platform can detect ABS vehicles, but the rolling brake platform can't control and detect them, which is a misunderstanding. It is difficult for ABS system to simulate emergency braking conditions when working in bench test, and the braking performance can only be approximately evaluated in the test, which is a limitation of bench test. According to the author's experience, the braking performance of vehicles that pass the brake bench performance test is qualified in emergency braking.

(b) Detection of braking force of multi-axle vehicles

Type I trucks are all multi-axle vehicles, and the rear axles are mostly double drive axles. The semi-trailer of heavy truck train is also a multi-axle vehicle. The length, width and height of these multi-axle vehicles make full use of the size limit of the outer corridor stipulated in GB 1589-2004. Due to the long dimensions of vehicles, many inspection stations are limited by the layout of field equipment, so it is impossible to carry out on-line inspection of these vehicles. The requirements of on-line inspection of heavy vehicles were fully considered when the inspection line was completely reformed. In addition to the uniaxial load of the selected equipment reaching 15t, the station spacing of the detection line is also set to 7m, which meets the vehicle inspection requirements. However, some practical problems have been encountered in the inspection: at present, the roller reaction brake inspection platform widely used in the inspection line can only be used for single-axis inspection, while the brake platforms with front and rear free rollers can meet the requirements of double-axis inspection, but they cannot be used for simultaneous inspection of all axles; In the bench test of automobile train, the braking force and braking force balance of each axle of tractor and semi-trailer meet the requirements of GB7258-2004 and GB 18565-2004, which does not mean that the overall braking performance of automobile train is up to standard.

At present, due to the limitation of conditions, most checkpoints are not strict enough or do not check this control. A method for detecting the braking coordination time of the whole vehicle on the drum-type reaction braking detection platform is proposed. The braking coordination time should refer to the braking coordination time of the whole vehicle. The braking force detected by the drum reaction braking control and inspection platform is measured separately for each axle, and the relevant parameters are evaluated according to the provisions of the standard. As the braking coordination time, it should refer to the braking coordination time of the whole vehicle, not just the coordination time of the front axle or the rear axle, which is in line with the original intention of the standard. Because there is no clear restriction on the braking force of the original axle of the vehicle, combined with the braking coordination time stipulated in the actual braking situation standard for safe driving of the vehicle, it can be understood that the braking coordination time of the whole vehicle is reasonable. The detection method is to use the brake pedal dynamometer to measure the action time, and in the whole process of braking force detection, the braking force value corresponding to each recording point (time) of each wheel braking force sampling is stored, and the total braking force value of each wheel at the same time reaches the product of the percentage of the total weight of the tested vehicle and the standard median value. Therefore, using the brake pedal dynamometer, as long as the corresponding content is added to the software, the braking coordination time can be detected on the roller reaction brake test bench.

Application of intransitive verb flat brake test-bed

The flat brake test-bed is a low-speed dynamic inertia brake test-bed. During the inspection, the car will drive the motor grader at a speed of 5 ~ 10 km/h, put the transmission in neutral and brake urgently. Under the action of inertia, a force equal to and opposite to the braking force is applied to the wheels of the car to make the flat plate move longitudinally along the platform. The braking force of each wheel is measured by the sensor, and the detection result is displayed by the indicator. The test-bed has the advantages of simple structure, few moving parts, low power consumption and less daily maintenance workload, which improves the working reliability. The test process is close to the actual road test conditions, which reflects the actual braking performance of the vehicle, that is, it can reflect the influence of braking on axle load transfer and the influence of other automobile systems on vehicle braking performance. The test-bed does not need to simulate the moment of inertia of the vehicle, and it is easy to combine the brake test-bed with the wheel weight meter and sideslip meter, making the vehicle test convenient and efficient.

(a) poor test repeatability

In the case of insufficient external environment, the repeatability is mainly affected by the tester's braking habits. Theoretically, the initial braking speed has no direct influence on the measured value. According to the second law of Newton's mechanics (f = Ma, force is proportional to acceleration), in the braking process, the braking force is not directly related to the initial speed, but to the change of speed because the mass is constant and the force reaches the maximum when the acceleration is maximum. Of course, the initial speed should not be too high or too low, and the high speed is not safe. Moreover, because the braking distance is too long, it is not easy to park the car on the flat plate, which will affect the test accuracy. When testing the braking performance of the flat brake platform, the tester is required to brake quickly. The point of maximum deceleration is the point of maximum braking force, and the change process of vehicle deceleration during braking is the change process of vehicle braking force. Therefore, the poor repeatability of the test is closely related to the inspector's emergency braking process, and the inconsistent actions of the inspector will lead to poor repeatability of the test results.

(2) The influence of the detection sensor on the detection result.

With the advantages of flat brake test-bed being gradually recognized by relevant domestic management departments and testing departments, the testing method of flat brake test-bed has been clearly used in GB7258-2004. When the test line is upgraded, the flat brake test bench is selected for the sliding line. At the initial stage of use, the test results are unstable and have poor repeatability. After analysis and research, we found that the flat brake test-bed has higher requirements for inspectors' operation, and its actions should be standardized and accurate. In addition, the sensor accuracy of the brake platform also has a great influence on the test results.