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Analysis of influence of tire F&M on improvement of vehicle on-center steering.

ABSTRACT

In this research, the influence of tire force and moment (F&M) characteristics on vehicle on-center steering performance was analyzed and then how to improve vehicle on-center performance was studied through controlling tire structure design parameter, tread pattern shape and tread grip characteristics.

First, the relationship between vehicle on-center steering performance and tire F&M characteristics was identified by comparing vehicle steering measurements and tire F&M measurements. It was found that key factor of tire related with on-center performance is aligning torque at lower slip angles. As the aligning torque at slip angle 1[degrees] increases, on-center feel is improved.

Second, the influence of tire design parameters on tire aligning torque was studied through F&M finite element (FE) analysis and measurement. It was found that the aligning torque at lower slip angle increases as stiffness of the tread and sidewall decreases. However, it shows trade-off with tire cornering force stiffness. It is necessary to optimize cornering force stiffness and aligning torque stiffness in tire structural design. In addition, as tread contact area to ground and viscosity of tread compound increase, both cornering force and aligning torque at lower slip angles increase together.

Finally on-center steering performance of an Europe mid-size wagon was improved by increasing tire aligning torque at lower slip angle based on these research results.

CITATION: Yum, K., "Analysis of Influence of Tire F&M on Improvement of Vehicle On-Center Steering," SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):2016.

INTRODUCTION

Sophisticated on-center steering response is one of important factors in premium vehicle driving. Recently vehicle manufacturers have been considering improving on-center steering response. On-center steering response is mainly dependent on steering system and suspension geometry. Steering system specification and suspension geometry in the initial stage of vehicle development are very important for better on-center steering. Tire dynamic characteristics also have an effect on on-center steering so on-center steering feel is evaluated in vehicle during tire development.

The first objective of this research is to identify the influence of tire dynamic force & moment (F&M) characteristics on vehicle on-center steering feel. First, vehicle steering response was measured using two tires with different F&M characteristics. Then the relationship of tire F&M characteristics and on-center steering response was analyzed and the tire aligning torque at low slip angle was found to control vehicle on-center steering response.

The second objective is to investigate tire design parameters to control aligning torque at a slip angle of 1[degrees]. F&M FE analysis was performed for several tire models with different structure design. Then F&M were measured for tire samples with different tread compound and different tread pattern in order to study the effect of viscoelastic properties of tread compound and tread pattern shape on aligning torque.

Finally the tire design to optimize on-center steering response and limited handling was suggested and verified through tire F&M measurement and vehicle R&H test.

RELATIONSHIP BETWEEN VEHICLE ON-CENTER STEERING RESPONSE AND TIRE F&M CHARACTERISTICS

Vehicle On-Center Measurement

In order to study vehicle on-center steering response with respect to the tire, two kinds of tire samples with different specifications were prepared for subjective vehicle steering test. Tire A and Tire B have same tire size 215/55R17 and different all season tread pattern.

A Sine Wave Test method was used for vehicle on-center measurement. Yaw response and lateral acceleration of mid-size sedan were measured by inputting steering angle [+ or -]28[degrees] as an sine wave with 0.3 Hz at the vehicle speed 80 kph. Figure 1 shows evaluation results of steering wheel torques relative to steering wheel angle and lateral acceleration for two tires.

Steering torque at 0[degrees] slip angle is related with tightness of on-center steering response. Steering torque at 0[degrees] slip angle of Tire A is higher than that of Tire B by 64%. Steering stiffness is defined as the slope from -0.02g to +0.02g in the graph of lateral acceleration and steering wheel torque. Steering torque slope of Tire A is also higher than that of Tire B by 48%. Namely Tire A is better than Tire B in overall objective on-center response measurement. In addition, Tire A showed better on-center feel than Tire B by one step in subjective vehicle steering test.

Relationship between On-Center Steering Response and Tire Aligning Torque

Cornering force and aligning torque relative to slip angle were measured by using Flat Trac for two tire samples which were used in vehicle on-center steering measurement. Here four load conditions (60%, 100%, 140% and 180%) were applied at the rolling speed 80 kph. Figure 2 shows the cornering force and the aligning moment in 100% load condition for both Tire A and Tire B.

Tire A has higher cornering force by 2~5% up to 4[degrees] slip angle than Tire B. However Tire B shows higher aligning torque by 9~25% up to 4[degrees] slip angle than Tire A. Especially cornering force at 1[degrees] slip angle (cornering stiffness) of Tire A is higher by 5% than Tire B but aligning torque at 1[degrees] slip angle (aligning stiffness) of Tire A is lower by 25%. In addition, Tire B is higher than Tire A in H-Function and G-Function which shows change rate of torque and force according to load variation. Here Tire A which shows better on-center steering performance is superior to Tire B only in aligning stiffness at lower slip angles. Tire A is considered to have on-center tightness by providing steering system with greater aligning torque at the ground contact area.

The case that aligning torque in lower slip angle is higher than Tire A was not been tested in this measurement. So it cannot be clearly defined that on-center steering response increases as aligning torque at lower angles increases. However, it is necessary to have equivalent or higher aligning stiffness based on the performance reference tire (Control Tire) during tire development.

INFLUENCE STUDY OF TIRE DESIGN PARAMETER ON ONCENTER PERFORMANCE

It is necessary to understand how tire design parameters have an effect on aligning torque stiffness to control on-center steering response during tire development. In this research, several studies to show the effect of tire structure, tread pattern and tread compound property were performed by using F&M FE analysis and F&M measurement.

Effect of Tire Structural Design Parameter

Several tire tunings to control tire structural design parameters in tread and sidewall are performed in order to match with vehicle R&H (ride and handling) and NVH performance during tire development. Table 1 shows available range of structural design parameters in tread and sidewall in 215/50R17 tire development for Europe mid-size wagon.

In order to study the effect of structural design parameters on aligning stiffness, the base FE model was built by using Min design parameters in Table 1 at first. Additional five FE models were built by changing one parameter with Max value among five parameters from the base model. Then F&M FE analysis was performed for six tire models. F&M analysis method is as follows.

1. Three dimensional (3-D) patterned model was built by using SMG (Symmetric Model Generation) option in ABAQUS.

2. Footprint analysis was performed.

3. Rolling analysis was performed with a speed 80 kph and [+ or -]6 slip angle.

4. Cornering force and aligning torque at each slip angle were calculated.

Figure 3 shows 3-dimesion patterned model and rolling analysis model in F&M FE analysis. Figure 4 shows the cornering force and aligning torque at 1[degrees] slip angle calculated in FE analysis for six tire models. Here the cornering stiffness and the aligning stiffness for the base model was defined as 100% and shown as violet line in Figure 4. Each graph in Figure 4 shows sensitivity of each structural design parameter. It was found that aligning torque increases when belt width, bead filler height and carcass turn-up height decrease and belt angle increases. It means that aligning stiffness increases when tread longitudinal stiffness and sidewall vertical stiffness decrease. However it results in cornering stiffness deterioration. It was found that aligning stiffness and cornering stiffness have trade-offs when controlling tire structural design parameters. In addition, belt width, bead filler height and bead filler reinforcement are relatively effective parameters to control aligning stiffness.

Effect of Tire Tread Pattern Shape

In order to study the influence of tire tread pattern on aligning stiffness, three kinds of tread shape were modeled based on the slick (non-patterned) tire with tire size 215/55R17 through hand carving. Three tire samples (Tire C, D and E) have different contact area ratio in lateral direction by changing block and groove width. Here contact area ratio means the ratio of real contact area to ground and total contact area including groove. Table 2 shows tread pattern shape and contact area ratio of Tire C, D and E. Tire C has wider groove and Tire D has grooves with different groove width. Tire E shows the narrowest groove among three tire samples.

Then cornering force and aligning torque relative to slip angle for three tire samples shown in Table 2 were measured by using Flat Trac. Table 3 shows F&M measurement results in 100% load condition. Here F&M results of Tire C were defined as 100%. It was found that both cornering force and aligning torque increase as tread contact area increases. Unlike structural design parameter, tread pattern shape contributes to increase both F&M.

Effect of Tread Viscoelastic Property

Viscosity of tire tread compound plays an important role in grip performance in vehicle handling and braking. In order to study the effect of tread viscoelastic property on aligning stiffness, two kinds of tire samples (Tire F and Tire G) with different tread compound were built by using same summer tread pattern mold and applying same inner structure. Then, test specimens were extracted from the top tread compound of each tire sample in order to measure viscoelastic property of tread compound. Elastic loss modulus (E') and elastic storage modulus (E") for each specimen were measured according to ASTM D623 by using the GABO meter. Finally viscoelastic property, tan[delta], was calculated. Table 4 shows viscoelastic characteristics of three tire samples with different tread compounds. Tan [delta] is the ratio of elastic loss modulus and elastic storage modulus and tan [delta] values at 0[degrees]C and 20[degrees]C are related to grip performance on the wet and dry surface. Tan [delta] values at 0[degrees]C and 20[degrees]C of tread compound H are higher than those of compound F.

Then F&M for Tire F and Tire G was measured and Table 5 shows cornering force and aligning torque in 100% load condition. F&M of Tire F was defined as 100%. Cornering force and aligning torque of Tire G are higher than those of Tire F. It was found that both cornering force and aligning torque increase as viscosity (tan [delta]) of tread compound increases.

OPTIMIZATION OF ON-CENTER STEERING AND HANDLING PERFORMANCE

Previous parameter study shows influence of tire design parameters on aligning torque at lower slip angles related to on-center steering performance and cornering stiffness related to handling performance. When changing tire inner structure, it results in trade-offs between cornering force and aligning torque. However controlling design parameters, pattern shape and compound properties of the tread, which contact ground directly, shows increase of both cornering force and aligning torque.

In order to verify previous parameter study and to improve both oncenter steering and limited handling performance, three kinds of tire samples (Tire I, J and K) were built based on the reference tire (Tire H) by changing tread pattern, tread compound and inner structure. Tire size was 215/50R17 and all tread patterns were summer. Table 6 shows tire design parameters applied to Tire H ~ K. Tire I has a concept to improve on-center steering by changing tread pattern shape and tread compound viscosity based on Tire H. Tire J focuses on handling performance by modifying inner structure compared with Tire I. Tire K has a concept to improve both on-center steering and handling performance rather than Tire I and Tire J by increasing tread compound viscosity.

Then cornering force and aligning torque relative to slip angle for four tire samples shown in Table 6 were measured by using Flat Trac. Figure 5 shows aligning torque at 1[degrees] slip angle related to on-center steering and cornering force at 4[degrees] slip angle related to limited handling. The cornering force and the aligning torque of the base model, Tire H, were defined as 100%. As expected, Tire I shows higher aligning torque than Tire H and Tire J has higher cornering force than Tire H and Tire I. Finally both aligning torque and cornering force of Tire K are higher than those of other tire models.

In addition, handling and steering performance was evaluated for four tire samples, Tire H through Tire K, by using the mid-size wagon vehicle. Table 7 shows subjective evaluation results of limited handling and on-center feel for Tire H through Tire K. The performance of Tire H was defined as score 7 and performance of other tire models were evaluated relatively. As expected, based on F&M results of four tires, Tire I shows better on-center feel rather than Tire H and Tire J shows better handling rather than Tire H and Tire I. In addition, both performances were improved in Tire K based on Tire H.

However it was found that rolling resistance of Tire K degraded relative to the base model, Tire H. In order to apply Tire K, additional consideration to improve rolling resistance were required.

SUMMARY/CONCLUSIONS

In this research, the influence of tire force and moment (F&M) characteristics on vehicle on-center steering performance was analyzed. Then how to improve vehicle on-center performance was studied through controlling tire structure, tire tread pattern shape and tire tread grip characteristics.

First, the relationship between vehicle on-center steering performance and tire F&M characteristics was identified by comparing vehicle steering measurement and tire F&M measurement. It was found that on-center steering performance is improved as the aligning torque at 1[degrees] slip angle increases.

Second, the influence of tire design parameters on tire aligning torque was studied by F&M FE analysis and measurements. It was also found that the aligning torque increases as tire tread and sidewall stiffness decreases. However it shows trade-off with tire cornering force stiffness, so it is necessary to optimize cornering stiffness and aligning stiffness in a codependent manner.

Third, the effect of tire tread viscosity and tread contact area was studied. As viscosity of tread compound and tread contact area ratio increase, cornering force and aligning torque at lower slip angles increases together.

Finally on-center steering and handling performance of mid-size wagon were optimized by controlling tire inner structure, tread pattern shape and tread viscosity.

REFERENCES

[1.] Norman, K., "Objective Evaluation of On-Center Handling Performance," SAE Technical Paper 840069, 1984, doi:10.4271/840069.

[2.] Johnson, P., Khami, R., Bauman, J., Goebel, T. et al., "Carbon Canister Development for Enhanced Evaporative Emissions and On-Board Refueling," SAE Technical Paper 970312, 1997, doi:10.4271/970312.

[3.] Higuchi, A. and Sakai, H., "Objective Evaluation Method of On-Center Handling Characteristics," SAE Technical Paper 2001-01-0481, 2001, doi:10.4271/2001-01-0481.

[4.] Smith, N.D., "Understanding Parameters Influencing Tire Modeling," Formula SAE platform. Colorado State University, 2004.

[5.] Seo,J., "A Study of Steering Performance by On Center Handling Test on Small Passenger Car," Proceedings of KSAE Spring Conference, 2010(11):1300-1305, 2010.

CONTACT INFORMATION

For additional information concerning this article and related questions, please contact Kiho Yum via kihoyum@hyundai.com

DEFINITIONS/ABBREVIATIONS

F&M - cornering force and aligning torque.

E' - elastic loss modulus.

E" - elastic storage modulus.

Kiho Yum

Hyundai Motor Company

Table 1. Tire design parameters to control aligning stiffness

          design parameter           Min           Max

tread     belt width                 190 mm        198 mm
          belt angle                  23[degrees]   27[degrees]
          bead filler height          20 mm         50 mm
sidewall  carcass turn-up height      50 mm         80 mm
          bead filler reinforcement  not applied   applied

Table 3. F&M measurement results for different tread pattern tires

                 slip angle  Tire C  Tire D  Tire E

cornering force  1[degrees]  100%    101%    105%
aligning torque  1[degrees]  100%    102%    106%

Table 4. Viscoelastic characteristics of different tread compounds

                              Tire F  Tire G
tread compound                  F       G

                 0[degrees]C  100%    139%
tan [??]        20[degrees]C  100%    151%
                60[degrees]C  100%    125%

Table 5. F&M results of two tires with different tread compounds

                 slip angle  Tire F  Tire G

aligning torque  1[degrees]  100%    105%
cornering force  1[degrees]  100%    103%
                 4[degrees]  100%    102%

Table 7. Subjective handling and steering evaluation results

                  Tire H  Tire I  Tire J  Tire K
                  (base)

limited handling    7     7-(-7)  7~7+    7~7+
on-center feel      7     7~7+    7~7     7(~7+)
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Article Details
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Title Annotation:force and moment
Author:Yum, Kiho
Publication:SAE International Journal of Passenger Cars - Mechanical Systems
Article Type:Report
Date:Jun 1, 2016
Words:2823
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