Automobile Inspection Technology

Chapter 30 Vehicle Inspection Technology (6)
(4) Braking time
The time elapsed in the braking process is the braking time.Braking time is generally not used as a separate evaluation index, but as an auxiliary evaluation index to evaluate braking performance.

2. Consistency of braking performance

The constancy of braking performance is mainly evaluated by the thermal fading resistance of the braking system.When the car is wading through water, the water fading resistance of the brake system can also reflect the constancy of the braking performance, but it is usually not used to specifically evaluate the constancy of the braking performance.

The structural form of the brake has a great influence on the resistance to thermal decay. Generally speaking, the braking efficiency of disc brakes is worse than that of drum brakes, but the stability is better than that of drum brakes.

3. Directional stability during braking

The ability of a car to maintain a straight line or a predetermined curve during braking is called the directional stability of the car during braking.Loss of directional stability causes the following problems:

(1) Brake deviation
Braking road deviation means that the car automatically drifts to the left or right when braking.It is mainly caused by the inconsistent increase in the braking force of the left and right wheel brakes of the car or the unequal braking force of the left and right wheels, especially the uneven braking force of the left and right wheel brakes of the steering wheel.

equal.In addition, factors such as the mechanical characteristics of automobile tires, the structure and stiffness of the suspension system, front wheel alignment, road conditions, and vehicle load distribution will also affect braking deviation.

(2) brake side slip
Braking side slip refers to the lateral movement of one or both axes of the car when braking.Mainly due to wheel brake lock caused.When braking at high speed, the rear wheel is locked and skidding is the most dangerous situation, and it is easy to cause a sharp turn, that is, the phenomenon of braking "tail flick"; Driving is very dangerous.

[-]. Bench test of automobile braking performance

1. Testing equipment

(1) Axle load testing platform
When using the brake test bench to test the braking performance of the car, the braking parameter standard is based on the percentage of the axle braking force in the axle load. Therefore, the axle braking performance must be evaluated only after the axle load and axle braking force are measured. Whether it meets the national standard requirements.

We call the equipment used to test the axle load quality of the axle as the axle load testing platform, and the axle load testing platform is also called the axle load meter.

The electronic axle load meter generally consists of a mechanical part (including a bearing device and a sensor device) and a display instrument.The double-load platform axle load meter, as shown in Figure 5-34, uses more detection lines, which can measure the wheel loads of left and right wheels.It has two upper scale bodies on the left and right, which are respectively installed in the left and right frames, and share a display instrument.

(2) Reaction roller brake test bench
Most of the brake testing equipment used in domestic and foreign automobile testing stations is in this form, especially the single-axis reaction roller brake test bench is the most commonly used.

The schematic diagram of the uniaxial reaction roller brake test bench is shown in Figure 5-35.It consists of frames,

It is composed of driving device, roller device, measuring device, lifting device and indicating and controlling device.In order to enable the brake test bench to detect the braking force of the left and right wheels at both ends of the axle at the same time, except for the frame, indicating and controlling devices, other devices are set independently.

(3) The flat-plate brake test bench is composed of a test plate, a measurement display system and a pedal dynamometer, and is generally divided into two test plates, a total of four, and are independent of each other.The schematic diagram of the flat brake test bench is shown in Figure 5-36.

The test plate is composed of a panel, a bottom plate, steel balls and force sensors.The bottom plate is fixed on the concrete ground as a base, the panel is supported on the bottom plate by pressure sensors and steel balls, and its longitudinal direction is connected with the bottom plate by tension sensors.The pressure sensor is used to measure the vertical force acting on the panel; the tension sensor is used to measure the horizontal force of the tire acting on the panel along the driving direction of the car. Changes in the output electrical signal.After the electrical signals output by the tension sensor and the pressure sensor are collected and processed by the computer, they are converted into braking force and wheel load and displayed on the display device respectively.If equipped with a wireless pedal pressure gauge, the flat brake test bench can not only measure the maximum braking force, but also provide information such as the curve of the braking force changing with time, the braking coordination time, etc., according to the fluctuation of the vertical force during the braking process condition, the performance of the suspension shock absorber can be tested.

The braking test process of the car on the flat brake test bench is relatively close to the braking process of the car running on the road.However, the flat-plate brake test bench has the disadvantages of poor test repeatability, troublesome repeated tests, large floor area, need for an auxiliary track, unfavorable for assembly line work, and unsafety. Therefore, its application is not as extensive as that of the reaction-force roller brake test bench.

2. The method of testing braking performance with a test bench

(1) Inspection with drum brake test bench

During the inspection, the surface of the drum of the brake test bench should be dry and free from loose matter and oil.The driver drives the vehicle onto the roller, adjusts the position, starts the roller, applies the brake, measures the required parameter values, and records whether the wheel is locked.

When measuring braking, in order to obtain sufficient adhesion to avoid wheel lockup, it is allowed to add sufficient additional mass to the vehicle or exert a force equivalent to the additional mass (additional mass or force does not count the axle load) Take measures to prevent the vehicle from moving (such as two-corner pads or traction).

After adopting the above methods, if the wheel still locks and slips on the roller or the whole vehicle moves backward with the roller rolling, and the braking force still fails to meet the qualified requirements, other methods specified in this standard should be used for inspection. .

Inspection with a flat brake test bench

The surface of the brake test bench plate should be dry and free from loose matter and oil.The driver aligns the vehicle with the platform at a speed of 5-10km/h and drives it onto the platform, puts the transmission in neutral, steps on the brake suddenly to stop the vehicle, and measures the required

parameter value.

3. Technical standards for bench testing

(1) Braking force requirements for braking performance bench test

See Table 5-7 for the braking force requirements of the braking performance bench test.

(2) Braking force balance requirements

The ratio of the maximum value of the braking force difference between the left and right wheels measured simultaneously during the whole process of braking force increase to the maximum value of the braking force of the left and right wheels of the axle measured during the whole process shall not exceed 20% for the front axle and 60% for the rear axle (and other axles) when the axle braking force is not less than 24% of the axle load, it shall not be greater than 60%. When the rear axle (and other axle) braking force is less than 8% of the axle load, the whole process of braking force growth The maximum value of the braking force difference between the left and right wheels measured at the same time should not be greater than [-]% of the axle load of the axle.

When the brake test bench is used to test the braking force of the vehicle's parking brake, the vehicle is unloaded, a driver is seated, and the parking brake device is used. 20% of the weight, and 1.2% for vehicles whose total weight is less than 15 times the curb weight.

(3) Wheel drag requirements

During the braking force test, the blocking force of each wheel shall not be greater than 5% of the axle load of the axle.

(4) Requirements for braking coordination time
In GB7258-2004 "Technical Conditions for Motor Vehicle Operation Safety", the braking coordination time is defined as: when the brake is stepped on suddenly, it starts when the foot touches the brake pedal (or the hand touches the brake handle) to the deceleration of the motor vehicle. (or braking force) to reach 75% of the average deceleration (or braking force specified in the standard) of the motor vehicle specified in the standard.Braking coordination time is an important parameter in braking performance testing.

It should not exceed 0.35s for vehicles with hydraulic brakes, and should not exceed 0.60s for vehicles with pneumatic brakes; the braking coordination time for automobiles, trains, articulated buses, and articulated trolleybuses should not exceed 0.80s.

(5) Testing requirements for emergency braking performance

Emergency braking is to stop the vehicle within a specified distance when a pipeline of the service brake system fails.

The new standard stipulates that first check whether the car has an effective emergency braking device.If the vehicle under test has an effective emergency braking device, it is not necessary to test its emergency braking performance.If the vehicle under inspection does not have an emergency braking device or if there is any doubt about its emergency braking performance, its emergency braking performance can be inspected according to the following regulations.

On the brake test bench, when one of the brake pipes of the brake system is artificially disabled, when the emergency brake works, the measured braking force should meet the requirements in Table 5-8.

(6) Parking brake performance requirements
When the brake test bench is used to detect the braking force of the vehicle's parking brake, the vehicle is empty, a driver is seated, and the parking brake device is used. 20% of the mass of the whole vehicle, and 1.2% for vehicles whose total mass is less than 15 times the mass of the curb.

[-]. Road test of vehicle braking performance

1. Brake performance road test inspection items

The main inspection items for road test braking performance testing include braking distance, fully released average deceleration, braking stability, braking coordination time, and parking brake slope.

2. Technical requirements for brake performance road test inspection items

See Table 5-9 to Table 5-15 for the technical requirements of the brake performance road test inspection items.

3. Road test braking performance test method

① The braking performance of the road test should be carried out on a flat (slope should not be greater than 1%), dry and clean hard road surface (the adhesion coefficient between the tire and the road surface should not be less than 0.7).

② On the test road surface, draw the sideline of the test channel with the width specified in GB7258-2004 "Technical Conditions for Motor Vehicle Operation Safety". A motor vehicle with variable speed can put the transmission in D gear), when coasting to the specified initial speed, step on the brake suddenly to stop the motor vehicle.

③ When using the braking distance to test the braking performance of the vehicle, use the speedometer, the fifth wheel meter or other test methods to measure the braking distance of the vehicle. For motor vehicles other than air braking, the pedal force (or hand control).

④ When using the fully issued average deceleration to test the service braking performance, use an instrument capable of measuring the fully issued average deceleration (MFDD) and braking coordination time to measure the fully issued average deceleration (MFDD) and braking coordination time of the vehicle For vehicles other than air brakes, the pedal force (or hand control force) should be measured at the same time.

4. Emergency brake performance test

When the vehicle is under no-load and full-load conditions, the emergency braking performance test shall be carried out according to the initial speed listed in Table 5-16, and the braking distance (or average deceleration) measured from the start point of the emergency braking operation to when the vehicle stops shall comply with Table 5-16 requirements.

Before the emergency braking test, one pipeline of the service braking system of the vehicle under test should be disabled.Then inspect according to inspection requirements.

5. Parking brake performance test

The so-called parking brake performance refers to the vehicle's ability to use the parking brake system on a certain slope to prevent the car from sliding down (slipping).

The national standard stipulates that the surface adhesion coefficient of the test ramp is greater than 0.7, and when the vehicle is unloaded, the total mass is less than 1.2 times the curb weight. The parking brake should be kept at 15% of the ramp. The parking brake performance of other vehicles It should be kept on a 20% slope, and the fixed time is 5 minutes, and both positive and negative directions should meet this requirement.

During the test, the service brake system cannot function.In order to meet the needs of parking brake detection, there should be at least two ramps with a slope of 15% and 20%.

Figure 5-37 The double-twisted line for measuring steering portability Figure 5-38 Steering wheel torque-steering wheel angle curve

In the test, the steering parameter measuring instrument records the steering wheel angle and steering wheel torque, and sorts out the steering wheel angle curve shown in Figure 5-38 according to the twisted-pair path for each round of driving.The steering portability is usually evaluated by the maximum torque of the steering wheel, the maximum force of the steering wheel, and the work of the steering wheel.

检验时的操纵力：手操纵时，对于座位数小于或等于9的载客汽车应不大于400N；对于其他车辆应不大于600N；脚操纵时，对于座位数小于或等于9的载客汽车应不大于500N，对于其他车辆应不大于700N。

(Section [-]) Vehicle Handling Stability Test

The vehicle handling stability test is mainly carried out on the special field of the vehicle proving ground.Before the test, pay attention to check whether the axle load distribution, tire inflation pressure and tread meet the requirements.

The parameters and instruments required for the on-road test of vehicle handling stability are: use a five-wheel instrument (contact or non-contact), a speedometer and a time signal generator to measure vehicle speed and time; use a steering parameter measuring instrument to measure the torque of the steering wheel And the angle of rotation; use the accelerometer to measure the lateral acceleration; use the two-degree-of-freedom angular velocity gyroscope to measure the yaw angle of the car.

[-]. Steering portability test

The car should have proper steering ease.If the steering is too heavy, the driver's labor intensity will be increased, and the driving safety will be affected because the steering cannot be turned correctly in time.If the steering is too light, it may cause the driver's road feeling to be too weak or the direction to fluctuate, which is also not conducive to driving safety.During the test, the steering parameter measuring instrument is installed on the steering wheel for detection, as shown in Figure 3-5.

At 0 to 0, the radius of curvature at the vertex of the twisted pair is the smallest, and its value is only 3.The minimum curvature radius of the twisted pair shall be determined by multiplying the minimum turning radius of the test vehicle by 1.1 times and rounding to an integer greater than this product.

[-]. Steady-state steering characteristic test

The purpose of the steady-state steering characteristic test is to measure the steady-state yaw response of the vehicle to the steering wheel angle input when the vehicle reaches a stable driving state.Our country mainly adopts the fixed steering wheel angle test method.

The test is carried out on a level field, and a circle with a radius of 15m is drawn on the field.The car drives along the drawn circle at the lowest stable speed. At this time, the steering wheel's rotation angle is the table to measure the initial speed to and the initial yaw rate AQ.Since the vehicle speed is very low and the centrifugal force is small, the tire slip angle is negligible.Using to and ringing can calculate the actual initial turning radius when ignoring the tire cornering, which is to maintain the steering wheel rotation angle w.Under the same conditions, let the car accelerate slowly, continuously and evenly (the longitudinal acceleration does not exceed 0.25m/s2), until the lateral acceleration of the car's center of gravity reaches 6.5m/s2.Continuously measure the vehicle speed U and the actual yaw angular velocity V during the acceleration period, and calculate the corresponding R and Ay values ​​according to the formula R=u/w, ay=wMr.Figure 5-39 shows the trajectory of the car in this test.For an understeering car, the turning radius becomes larger as the vehicle speed increases; conversely, the turning radius of an oversteering car becomes smaller and smaller.Pay attention to safety when the test reaches a large lateral acceleration.The difference between the front and rear wheel slip angles can also be converted according to the calculated value of the turning radius R:

[-]. Transient yaw response test

At present, the step test is commonly used to measure the transient response of the car to the input of the steering wheel angle.The test is carried out on a flat field. The car first drives in a straight line. After reaching the test speed, the steering wheel is suddenly turned at an angular velocity not less than 200°/s or not less than 500°/s (US ESV regulations).The steering wheel angle varies with the speed of the vehicle, but it is required to achieve a certain lateral acceleration of 0.4g during steady-state circular driving.The steering wheel remains unchanged after turning to the desired corner, and the accelerator remains unchanged, and the car enters the circle from a straight line.The test is required to be carried out at two speeds of 40km/h and 110km/h.Record the data such as vehicle speed to time L steering wheel angle 4W, lateral acceleration and yaw rate or and so on.According to the recorded data, sort out the time-varying curve of the yaw rate.Parameters such as response time, overshoot and stabilization time can be found out from the curve.The step test requires a large space, and special attention should be paid to safety during the test.

[-]. Turning back to positive ability test

The test of the ability to turn the car back to the center should be carried out on a flat field.Let the car drive along a circle with a radius of 15m, and adjust the steering wheel angle pulse test to be carried out on a flat field.The test speed is 70% of the maximum speed.The car runs at the test speed, and then an angle pulse is input to the steering wheel, as shown in Figure 5-41.The input pulse width of the steering wheel angle is 0.3-0.5s, and its maximum angle should make the maximum lateral acceleration of the vehicle 4m/s2.When the steering wheel angle pulse is input, the driving direction of the car will swing, and it will return to straight driving after a short time.Record the time Z and steering wheel angle of the test process

[-]. Steering wheel middle position handling stability test

The full vehicle speed makes the lateral acceleration reach 4m/s2, and then the steering wheel is suddenly released. Under the action of the righting torque, the front wheels will return to straight driving.Record the time 〖, vehicle speed M, and steering wheel angle of this process

For vehicles with a maximum speed of more than 100km/h, a high-speed return performance test is also required, and the test speed is 70% of the maximum speed.Let the car run straight at the test speed, and then the driver turns the steering wheel to make the lateral acceleration reach 2m/s2,

Then suddenly loosen the steering wheel and do the back-to-center test.

The return test is to characterize and measure the transition process of the car from the curve back to the straight line, and it is the basic performance test to measure the free operation force input.The ability to return to center is an important aspect of vehicle handling stability. A car without return to center may not be able to return to the center at all (that is, it has a relatively large residual yaw rate), or the driving direction may reciprocate during the process of returning to center. With a wobbly car, the driver and passengers are unhappy.

[-]. Steering wheel angle pulse test

(End of this chapter)