Identification of A Vehicle Pull Mechanism

Transcription

1 Seoul 2000 FISITA World Automotive Congress June 12-15, 2000, Seoul, Korea F2000G353 Identification of A Vehicle Pull Mechanism Sang-Hyun Oh*, Young-Hee Cho, Gwanghun Gim Vehicle Dynamics Research Team, R&D Center, Hankook Tire Co., Ltd., 23-1, Jang-dong, Yusung-gu, Taejon, S. Korea A vehicle pull problem is directly related to safety and comfort. Major parameters causing the pull problem are the PRAT and conicity of tires, the, cross camber, cross caster and other manufacturing uniformity of a vehicle, and road crown. The effects of these parameters are described in this paper. The significance of and PRAT is explained as pull matching parameters. Especially, conicity sensitivity and sensitivity with respect to the changes of cross camber and cross caster are given. To minimize the pull problem, it is shown that a co-work between vehicle and tire companies is necessary from the beginning of a vehicle development to the end KEYWORDS : Vehicle Pull, Drift, RAT,, PRAT, Conicity, Camber, Caster, Road crown INTRODUCTION A vehicle pull is defined as a drift of a vehicle toward the shoulder of road regardless of a driver s intention. It is directly connected with safety, and it makes a driver feel getting tired. Sometimes it is focused to improve handling and ride performance without considering the vehicle pull at the initial stage of a vehicle development. Therefore, most activities for the vehicle pull are carried out when the pull problem is occurred after the mass production of the vehicle. In reality, many field claims raised by consumers are related to the vehicle pull. Afterwards, many carmakers are trying to improve the vehicle pull problem based on consumers claims. It means that carmakers have a huge risk to lose their brand image and to pay additional cost. In order for tire companies to improve the vehicle pull problem, additional approval process may be needed. For the purpose of a vehicle pull problem, first of all, we need to identify a vehicle pull mechanism by considering both influences of a vehicle and tires. This paper describes the influences of vehicle wheel alignment and tire properties affecting the vehicle pull. The magnitudes of some vehicle pull parameters must be reduced to improve the vehicle pull problem such as cross camber and cross caster of the vehicle and conicity of the tire. Cross camber is defined as a value of difference between left and right camber angles shown in Figure 1. Cross caster is defined a value of difference of left and right caster angles shown in Figure 2. In addition, it is necessary to consider pull matching parameters between a vehicle and tires. (Vehicle Residual Aligning Torque) is the main parameter of the vehicle and PRAT (Plysteer Residual Aligning Torque) is that of the tire. is defined as an average value of steering wheel torque required for straight driving shown in Figure 3. The value of represents a vehicle pull tendency; negative value indicates right drift tendency and positive value stands for left drift tendency. PRAT represents a tire pull tendency and is calculated as an average value of residual moments (RATcw, RATccw) determined from a 1

2 Cross camber = ( γ fl - γ fr ) Lateral force Fy γ fl γ fr Front left Front right Figure 1 Definition of cross camber. Cross caster = (ν fl - ν fr ) RATcw RATccw Aligning Torque Mz PRAT = slip angle Fy (cw) Fy (ccw) Mz (cw) Mz (ccw) RATcw RATccw 2 ν fr Figure 4 Plysteer Residual Aligning Torque. ν fl Front left Front right Figure 2 Definition of cross caster. Fixed Steering Fy_front axle Mz_steering wheel slip angle functional relation between lateral force and self-aligning moment depending on driving directions (refer to Figure 4) [1]. To improve the vehicle pull performance, both of a car maker and a tire company have to carry out systematic activities at every stage of development process. In the early stage of the development, they need to analyze a matching characteristic between the of a control vehicle and the PRAT of a control tire, and the characteristic of a marketing place where the vehicle is to be sold. Through a vehicle pull sensitivity study, afterwards, the vehicle and the tires should be retuned. This paper explains fundamental questions about the vehicle pull such as, what is a vehicle pull, how to test, and which parameters affect in which way. The SAE coordinate system is here selected as the reference coordinate system. C.G Figure 3 Vehicle pull with a fixed control test. VEHICLE PULL MECHANISM A vehicle pull is a phenomenon characterized by the magnitudes of residual force and moment, which are determined based on a functional relation between the lateral force of a front axle and the moment of a steering wheel. There are two kinds of test methods for the 2

3 evaluation of the vehicle pull; one is a free control test referred to Figure 5, and another is fixed control test shown in Figure 3. With a free control test, a vehicle is maintained at a constant speed of 80km/h or 100km/h. With release of a steering wheel, the vehicle may start to drift. The amount of lateral movement of the vehicle after 100m driving distance is used as a criterion for vehicle pull performance. In general, the allowable range of the drift is from 1 m to 1 m on the public road with about 1 degree of road crown. The fixed control test measures the residual moment of a steering wheel using a steering torque sensor. Since it is a kind of an objective test, the method is expected to result in data with high accuracy and repeatability. But it takes much time to complete a test from sensor mounting to data analysis. Therefore, it is mainly used as a systematic vehicle pull sensitivity study. Figure 6 shows a co-relation between the two tests. Figure 7 represents the main parameters of a vehicle pull; the cross camber and cross caster of a vehicle, the conicity and PRAT of a tire, and road crown. Conicity can be characterized as the difference of lateral forces produced during clockwise and counter-clockwise rotations of a tire at zero slip angle shown in Figure 8. In general, these parameters can be classified as two groups such as for manufacturing uniformity and design characteristic. Cross camber, cross caster and conicity are belong to the manufacturing uniformity, and and PRAT are design characteristic parameters. A crucial point for a good vehicle pull performance is to improve the matching characteristics of a vehicle and tires. It is needed to know the desired and its proper PRAT. Figure 9 shows how is related to PRAT with respect to the road crown. Once designed for a given road crown is determined, PRAT can be properly produced by adjusting tire design. In this way, vehicle pull characteristic can be optimized. Free Steering Center 1 m Deviation path 1 m 100 m Figure 5 Vehicle drift with a free control test. Vehicle Drift (Nm) Vehicle Pull (m) Figure 6 Relation between vehicle pull and vehicle drift. Cross camber Cross caster Other Manufacturing manufacturing uniformity uniformity PRAT Conicity Road Crown Figure 7 Parameters causing vehicle pull. 3

4 Direction Profile LFcw LFccw Center groove Shoulder groove Direction LFcw LFccw Conicity = 2 Figure 8 Tire conicity. Belt Bad Good PRAT effect Road crown Figure 11 Design parameters affecting PRAT. Tire drift tendency changes from right to left as PRAT varies from a plus value to a negative value with respect to its neutral value. For an example, Figure 10 shows a vehicle having a neutral drift tendency with -2.0Nm of tire PRAT. If tires are changed to have -4.5Nm of PRAT, it will cause a left drift tendency. If the vehicle is equipped with tires having 0.5Nm of PRAT, it will show a right drift tendency. PRAT is characterized by tire profile, center groove, PRAT (Nm) Figure 9 PRAT as a matching characteristic with. PRAT -4.5 Nm -2.0 Nm 0.5 Nm shoulder groove, belt, etc. (refer to Figure 11). Conicity is another core parameter from tire side. Conicity effect on a vehicle pull has already been discussed by many authors [2-3]. The first one of Figure 12 is a conceptual plot of conicity sensitivity on RAT. The slope of the graph, as shown in the second one of Figure 12, is defined as conicity sensitivity. Generally, tires with a low RAT sensitivity drift drift drift drift show low pull sensitivity. Cross camber, cross caster and other manufacturing uniformity with each vehicle are also major factors. In Figure 13, it is shown that varies by changing cross camber from -1 to 1 degree. A vehicle having lower camber sensitivity shows better performance for a vehicle pull. The control of the cross camber is very important to reduce the vehicle pull like as the conicity with tires. Figure 10 Effect of PRAT. Figure 14 shows test results with different cross caster angles. 4

5 Conicity sensitivity RAT (Nm) 10 RAT (Nm) 5 Conicity (kg) Bad Good Conicity Caster sensitivity = Cross caster Figure 12 Tire conicity sensitivity. Figure 14 Effect of cross caster. Camber sensitivity = Cross camber Figure 13 Effect of cross camber. Figure 15 Effect of manufacturing uniformity I. The magnitude of caster angle itself with zero cross caster angle can affects the vehicle pull sensitivity of a vehicle. More dominant parameter is however the cross caster angle between the left and the right wheel. A negative cross caster angle causes a vehicle with a relative left pull tendency and a positive value induces a right pull tendency. The cross caster angle should be controlled to be low to enhance the vehicle pull performance and should be tightly managed because it is also a dominant factor affecting on value like as a cross camber. Figures 15 and 16 show the effect of manufacturing uniformity directly related to a vehicle itself. For Figure 15 and 16, a fixed steering test and a free steering test are employed respectively. The same sample sets of test tires are used throughout all tests. Figure 15 shows the result of for the vehicles having the same alignment values of cross camber angle and cross caster angle. Figure 16 shows the results of vehicle drift for the randomly sampled mass production vehicles. 5

6 Vehicle Drift (m) Vehicle 1 Vehicle 2 Vehicle 3 Vehicle 4 Vehicle 5 Vehicle 6 These test results show that manufacturing uniformity on each vehicle is one of crucial parameters on vehicle pull performance. Each vehicle shown in Figure 15 has the different magnitude and sensitivity depending on conicity. Tire A Tire B Tire C Tire E Tire F Tire G Figure 16 Effect of manufacturing uniformity II. Australia England Ja pan. Center Germany Korea USA.. Figure 17 Road crown difference with respect to region. Public Road There are other vehicle pull parameters beyond vehicle and tire parameters such as road crown, lateral wind and weight distribution. The most important parameter is the road crown among these parameters. The road crown is not a design parameter but has a meaningful effect on the vehicle pull. That is why the road crown should be considered as an important factor for the development of a vehicle and tires. As shown in Figure 17, the road crown varies in a lefthand-side driving region (UK, Australia, Japan, etc.) and a right-hand-side driving region (Korea, Germany, USA, etc.). Therefore, vehicle pull problems from different market places can be greatly reduced by understanding road crown effect. A test criterion can also be reasonably modified by consideration of the road crown of a test track as shown in Figure 18. In this case, the test criterion should be Center Line appropriately shifted to the left because the road crown of the test track is smaller compared with that of public road. Test P/G Figure 19 shows the effect of the road crown. Road crown - Road crown Road crown sensitivity = Road crown 1.5m 0.5 m Figure 18 Apply of road crown effect. Figure 19 Effect of road crown. 6

7 CONCLUSION Several vehicle pull parameters have been discussed; the PRAT and conicity of tires, the, cross camber, cross caster and other manufacturing uniformity of a vehicle, and road crown. In addition, it is needed to investigate vehicle pull sensitivity to validate vehicle pull performance. Especially, conicity sensitivity and sensitivity with respect to cross camber and cross caster must be checked during the development of a vehicle. The most important thing to guarantee the vehicle performance is that the variance of pull parameters should be retained. To enhance vehicle pull performance, vehicle and tire makers should do a co-work from the initial stage of a vehicle development to improve matching property between the vehicle and the tires. REFERENCES [1] Pottinger M., 1990, Tire/Vehicle Pull : An Introduction Emphasizing Plysteer Effects, Tire Science and Technology, TSTCA, Vol.18, No3, July-September, [2] Frank E. Matyja, 1987, Tread Design and Belt Angle Effect on Residual Aligning Torque, SAE Paper No [3] Richard W. Topping, 1975, Tire Induced Steering Pull, SAE Paper No