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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

AgendaIntroduction and motivationThe high speed linear test rig HiLiTeSet upValidation with outdoor testsFriction mechanisms on iceBasicsMeasurements and Interpretation Conclusion

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Introduction and motivationDecreasing friction potential of the road results in an increasing probability of accidentReduction of development costs by Understanding relevant friction mechanisms of tread blocks on ice and snowTransfer of tests to the all year available labSource: ContinentalWinter 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Introduction and motivationABS system makes sure that the slip of a tire is reduced to a minimumCornering forces can still be transmitted to the groundBut 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

High speed linear test rig HiLiTeBedding on concrete block (m = 3 t) and styrofoamPowered by synchronous servo motor (T = 125 Nm)Travelling carriage is driven with tooth belt and performs linear motionChallenge: Compromise between mechanical stability and weightBeddingCarriageSynchronous servo motorTest TrackTooth beltRectangular profile

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

High speed linear test rig HiLiTeFriction surfaces Glass / Corundum (different grid) Asphalt / Concrete Ice / SnowAdditional equipment: IR-camera High speed cameraDesign 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Evaluation of measurementsSteady state time interval is characterized by constant velocityFriction coefficient (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 (t)

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Basic friction mechanisms on iceDry 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Compound evaluation with HiLiTe on iceComparison of different tread compounds (Ref, A, B, C, D): HiLiTe vs. OutdoorOutdoor test: car is accelerated, acceleration and slip are measured, -slip curve is calculated and evaluatedIndoor test: Several runs of each sample with different sliding velocitiesBest 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)

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Pattern evaluation with HiLiTe on iceIn general siped winter tires show better performance on ice than non siped tires (for standard winter compounds)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 blockPattern effects on ice depend on sliding velocity (increasing velocity increasing friction power increasing liquid layer, decreasing dry friction zone decreasing viscous friction forceSipes interrupt liquid layer new dry friction higher friction coefficient for siped samples or winter tires

Source: Continental

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Flexible TreadblockAllows flexible arrangement of block elementsAllows bordering single angles of block elementsDifferent distances can be createdHolder can be turned (0-360) side forces can be measured as well

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Variation of fixing method of tread elementsBlocked tread block sample: No element deformation can occurClamped sample: Simulation of bending deformation onlyFriction coefficient is significantly influenced by fixing method of sampleReason for significant difference becomes clear analyzing the contact area

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Evaluation of dynamic contact area on ice: Test set upTread 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 possibleCameraHiLiTe carriageGlass surface with ice layer

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Investigation of dynamic contact areaFN=72N, v=0,1m/s, =1,38FN=72N, v=0,1m/s, =0,41 bar3 barca. 22%ca. 38%Comparison of different contact areas (bordered (grey) vs. 20 clamped block element (black)) during friction measurement on ice at two different pressuresSignificant differences of contact area size can be observed! Clamped sample shows 60-80% smaller contact area sizeSignificant 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Pressure dependence of friction coefficientWith different fixing methods of block sample overall friction characteristic can be calculatedContact 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

ConclusionFN=72N, v=0,1m/s, =1,38FN=72N, v=0,1m/s, =0,4The 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

Stefan RipkaInstitut fr Dynamik und Schwingungen Gottfried Wilhelm Leibniz Universitt Hannover Appelstrae 11, 30167 Hannover ripka@ids.uni-hannover.de www.ids.uni-hannover.deThank you for your attention !

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Comparison of frictional properties of in house vs. natural snowIn 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 snowDifferences 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 rangenatural snow(CTI=90, r=525kg/m)in house produced snow(CTI=92, r=510kg/m)

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Influence of tread elements run out areaNeighbored elements / elements width influence friction on ice:Last element slides on track heated up by first elementBack part of larger elements slides on liquid layerDifferences between single element configurations caused by different block element widths resulting in different contact areasLiquid layer reduces friction potentialOptimum block width can be found

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Siped tread block model - Requirements - Properties of tread block elements: length and depthmass, stiffness, damping3D - model needs additional block widthNumber of sipesDistance between single elements possibility to generate two neighboured blocksFriction within the sipeLess computational effort compared to FEA

Output parameter: Deformation angle of each block element

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Simulation results of Timoshenko beam modelSimulation result for a beam with d =6 mm and l = 4 mmAfter the run-in process the steady state of the transversal and longitudinal deflection as well as the shear angle can be observedTransversal deflection w0,9mm 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

2010 Stefan Ripka, Leibniz Universitt HannoverTread block mechanics on ice and snow

Introduction and motivationWinter tires gain more and more attention due to safety reasonsRising 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 snowTransfer of tests to the all year available labSource: Continental

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