Stefan Ripka, Hagen Lind, Matthias Wangenheim, University Hannover

1. Investigation of friction mechanisms of siped tire tread blocks on snowy and icy surfaces Stefan Ripka, Hagen Lind,Matthias Wangenheim, University Hannover Klaus Wiese, Burkhard Wies, Continental Akron, 20.09.2010

2. Agenda • Introduction and motivation • The high speed linear test rig “HiLiTe” • Set up • Validation with outdoor tests • Friction mechanisms on ice • Basics • Measurements and Interpretation • Conclusion

3. Introduction and motivation • Decreasing friction potential of the road results in an increasing probability of accident • Winter tires gain more and more attention due to safety reasons • Rising requirement of customers concerning tire performance • Effort of the tire industry for improving the tire characteristics • Reduction of development costs by • Understanding relevant friction mechanisms of tread blocks on ice and snow • Transfer of tests to the all year available lab Source: Continental

4. Introduction and motivation • ABS system makes sure that the slip of a tire is reduced to a minimum •  Cornering forces can still be transmitted to the ground • But there is still a zone left, where the tire slips over the road surface! • During a braking manoeuvre a wide range of sliding velocities can thus be observed •  Test rig needs to cover a wide parameter range for closer investigations

5. High speed linear test rig „HiLiTe“ • Bedding on concrete block (m= 3 t) and styrofoam • Powered by synchronous servo motor (T= 125 Nm) • Travelling carriage is driven with tooth belt and performs linear motion • Challenge: Compromise between mechanical stability and weight Synchronous servo motor Rectangularprofile Tooth belt Bedding Test Track Carriage

6. High speed linear test rig „HiLiTe“ • Design Parameters • Rubber sample: Tread block • Sliding velocity v= 0.1 … 10 m/s • Normal force FN = 23 … 1000 N • Length of friction surface l= 5 m • Located in environmental chamber • Temperature T= -25 °C … + 60 °C • Friction surfaces • Glass / Corundum (different grid) • Asphalt / Concrete • Ice / Snow • Additional equipment: • IR-camera • High speed camera

7. Evaluation of measurements • Steady state time interval is characterized by constant velocity • Friction coefficient µ(t) is calculated for each time step from friction force FR(t) divided by normal force FN(t) • Characteristic friction coefficient is calculated via the average of the steady state time interval of µ(t)

8. Basic friction mechanisms on ice • Dry run in area of tread block e  Due to friction energy P ice is heated up  Liquid layer: • Friction force FR consists of dry friction Fd and viscous friction Fv • Length of dry area depends on friction energy  Influenced by sliding velocity v, normal force FN, temperature T, … • Low friction level on ice caused by viscous friction due to liquid layer

9. Compound evaluation with HiLiTe on ice • Comparison of different tread compounds (Ref, A, B, C, D): HiLiTe vs. Outdoor • Outdoor test: car is accelerated, acceleration and slip are measured, µ-slip curve is calculated and evaluated • Indoor test: Several runs of each sample with different sliding velocities • Best fit of results for a sliding velocity of v= 5 m/s but correct ranking of compounds for all sliding velocities •  Compound evaluation is velocity independent (Different friction levels at higher velocities caused by reduction of adhesion / dry friction zone)

10. Pattern evaluation with HiLiTe on ice G0 G3 Source: Continental • Investigation of pattern effects with HiLiTe: Siped (G3) vs. non siped (G0) sample for different compounds (REF, A, B, C, D) • A ratio larger than 100% means a higher friction coefficient for siped block • Pattern effects on ice depend on sliding velocity (increasing velocity  increasing friction power  increasing liquid layer, decreasing dry friction zone  decreasing viscous friction force • Sipes interrupt liquid layer  new dry friction  higher friction coefficient for siped samples or winter tires • In general siped winter tires show better performance on ice than non siped tires (for standard winter compounds)

11. Flexible Treadblock • Allows flexible arrangement of block elements • Allows bordering single angles of block elements • Different distances can be created • Holder can be turned (0-360°)  side forces can be measured as well

12. Variation of fixing method of tread elements • Blocked tread block sample: No element deformation can occur • Clamped sample: Simulation of bending deformation only • Friction coefficient is significantly influenced by fixing method of sample • Reason for significant difference becomes clear analyzing the contact area

13. Evaluation of dynamic contact area on ice: Test set up • Tread block sample is observed while sliding on ice • Clear ice surface to observe the tread block sample from the bottom up • Ice surface is prepared on glass • Camera takes picture from contact area •  Detailed analysis of contact area size possible HiLiTe carriage Camera Glass surface with ice layer

14. Investigation of dynamic contact area 1 bar 3 bar ca. 22% ca. 38% • Comparison of different contact areas (bordered (grey) vs. 20° clamped block element (black)) during friction measurement on ice at two different pressures • Significant differences of contact area size can be observed! •  Clamped sample shows 60-80% smaller contact area size • Significant influence of the contact area on the friction process: Higher local pressure  higher friction energy density  increasing liquid layer, decreasing dry friction zone  decreasing friction coefficient FN=72N, v=0,1m/s, µ=1,38 FN=72N, v=0,1m/s, µ=0,4

15. Pressure dependence of friction coefficient G0 • With different fixing methods of block sample overall friction characteristic can be calculated • Contact pressure is calculated from global vertical load and contact area:  non deformed sample (p1=1 bar and p2=3 bar)  clamped sample (p3=4,5 bar and p4=7,9 bar) • Friction coefficient (from clamped sample) fits into global decreasing pressure characteristic •  Local pressure and therefore contact area size influence friction process significantly

16. Conclusion • The friction level of the road has a significant influence on the accident probability • High speed linear test rig allows investigating typical pressures and sliding velocities which prevail at passenger car tires • Basic tread block friction mechanisms on ice have been introduced • The validation of HiLiTe measurements on ice with outdoor traction test was successful • A method for analyzing the contact area during the sliding process was presented • The influence of the contact area on the friction process of tread block elements was demonstrated and explained FN=72N, v=0,1m/s, µ=1,38 FN=72N, v=0,1m/s, µ=0,4

17. Thank you for your attention ! Stefan Ripka Institut für Dynamik und SchwingungenGottfried Wilhelm Leibniz Universität HannoverAppelstraße 11, 30167 Hannoverripka@ids.uni-hannover.dewww.ids.uni-hannover.de

18. Comparison of frictional properties of in house vs. natural snow natural snow (CTI=90, r=525kg/m³) 2 cm in house produced snow (CTI=92, r=510kg/m³) • In the measured velocity range friction coefficient is relatively constant compared to ice friction •  Dominant friction effect is snow milling which can be observed on natural and in house snow • Differences of friction level results from different surface hardness (from sintering process of the snow) • PC-traction measurements from outside with similar tread pattern show same result: Traction force is nearly constant over a wide slip range

19. Influence of tread elements run out area • Neighbored elements / elements width influence friction on ice: • Last element slides on track heated up by first element • Back part of larger elements slides on liquid layer • Differences between single element configurations caused by different block element widths resulting in different contact areas • Liquid layer reduces friction potential • Optimum block width can be found

20. Siped tread block model - Requirements - • Properties of tread block elements: • length and depth • mass, stiffness, damping • 3D - model needs additional block width • Number of sipes • Distance between single elements possibility to generate two neighboured blocks • Friction within the sipe • Less computational effort compared to FEA  Output parameter: Deformation angle of each block element

21. Siped tread block model-Timoshenko beam • Timoshenko beam considers longitudinal (x, u) andtransversal (z,w) movement, also includes shear deformation b • Contact problems (bar-bar/bar-surface) solved via penalty method • The friction Force FR is calculated via the law of Coulomb, friction coefficient µ can be adopted to every friction characteristic • Friction characteristic considers all friction phenomena like pressure and velocity dependence, contact area, surface material and surface texture

22. Simulation results of Timoshenko beam model • Simulation result for a beam with d= 6 mm and l= 4 mm • After the run-in process the steady state of the transversal and longitudinal deflection as well as the shear angle can be observed • Transversal deflection w 0,9 mm matches very well in experiment and simulation • Shear angle b 1,7 ° is much too small  Tire tread specific parameters for simulation to be defined in the future •  In general the Timoshenko beam provides a satisfying approach for simulating the bending angle of single tread blocks

23. Introduction and motivation Source: Continental • Winter tires gain more and more attention due to safety reasons • Rising requirement of customers concerning tire performance • Effort of the tire industry for improving the tire characteristics • Reduction of development costs by • understanding relevant friction mechanisms of tread blocks on ice and snow • Transfer of tests to the all year available lab