DEVELOPMENT OF A GRIP AND THERMODYNAMICS SENSITIVE PROCEDURE FOR THE DETERMINATION OF TYRE/ROAD INTERACTION CURVES BASED ON OUTDOOR TEST SESSIONS

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1 DEVELOPMENT OF A GRIP AND THERMODYNAMICS SENSITIVE PROCEDURE FOR THE DETERMINATION OF TYRE/ROAD INTERACTION CURVES BASED ON OUTDOOR TEST SESSIONS Flavio Farroni, Aleksandr Sakhnevych, Francesco Timpone 20TH AND 21ST APRIL 2015 UNIVERSITY OF SURREY, GUILDFORD, UK

2 Designers and technicians, involved in vehicle dynamics, face during their daily activities with the need of reliable data regarding tyres and their physical behaviour. The solution is often provided by bench characterizations, rarely able to test tyres in real working conditions as concerns road surface and the consequential thermal and frictional phenomena. The aim of the developed procedure is the determination of the tyre/road interaction curves basing on the data acquired during experimental sessions performed employing the whole vehicle as a sort of moving lab, taking into account effects commonly neglected.

3 The target of the research activity described in this presentation is the development of a procedure able to estimate tyre interaction characteristics, taking into account the fundamental friction and thermal phenomena concerning with tyre road interaction and reproducing them in simulation environments. The grip and thermodynamics sensitive procedure is based on the cooperation of : TRICK tool; TRT model; GrETA model; generating a many sided and powerful instrument of analysis and simulation, useful: to identify the range of temperature in which grip performance is maximized; to predict the tyre behaviour on different tracks, evaluating the performance at compound variation, in order to define the optimal vehicle setup to reach optimal grip/thermodynamic conditions.

4 TRICK Tyre/Road Interaction Characterization and Knowledge T.R.I.C.K., an acronym for Tyre/Road Interaction Characterization and Knowledge, is based on a vehicle model and it is able to provide the tyre interaction experimental characteristic curves directly from the outdoor test sessions. Once characterized the vehicle, filtering and sensors output, following a specifically defined track driving routine, the TRICK tool is able to generate a virtual telemetry. Following a specifically defined track driving routine, the procedure is employed to reproduce the real interaction with road, taking into account the primary phenomena involved, commonly neglected in bench data, with the aim to investigate the tyre behaviour in the widest possible range of working conditions.

5 TRICK Tyre/Road Interaction Characterization and Knowledge T.R.I.C.K. tool basically comprises of an 8 DOF quadricycle vehicle model, able to take into account roll and aerodynamics effects and it is able to process directly the signal acquired from CAN bus and from devices 1 estimating sideslip angle. The procedure has been validated by comparing the results to the measurements of the interaction forces, made using the dynamometric wheel instrumentation, showing good agreement, in particular as concerns the evaluation of the steady state dynamics effects. [1] Datron, GPS, virtual sensors

6 TRICK Tyre/Road Interaction Characterization and Knowledge T.R.I.C.K tool is able to provide force and slip estimations, useful to generate experimental tyre interaction curves. Tyre experimental interaction characteristics 1 obtained by means of TRICK tool for a rear wheel drive sport vehicle [1] The colour of the markers varies from blue to red at increasing vertical load values

7 TRT Thermo Racing Tyre T.R.T. is physical-analytical tyre three-dimensional thermal model, developed with the aim to predict the temperature of the different tyre layers, basing on telemetry data processing techniques. TRT is able to provide temperature local distribution, with particular reference to the deep layer, usually not reachable by measurement instruments. Heat Generation due to tyre/road tangential interaction and tyre cyclic deformation during the rolling; Heat Exchange with the external environment due to thermal conduction between the tread and the road, convections of the surface and the inner liner layers respectively with outside and inner air; Heat Conduction between the tyre layer due to the temperature gradient.

8 TRT Thermo Racing Tyre TRT Initialization Thermodynamic properties Contact patch Extension

9 TRT Thermo Racing Tyre T.R.T. model provides in output the temperature circumferential distribution inside the different tyre layers (surface, bulk, inner liner), as well as all the heat flows concerning the various tyre working conditions, allowing to carry out thermal analysis in transient conditions, involving the tyre temperature effects on vehicle dynamics.

10 GrETA Grip Estimation for Tyre Analyses GrETA is a tyre/road friction physical-analytical model, able to provide an effective calculation of the power dissipated for hysteresis by road asperities indented in tyre tread and taking into account the phenomena involved with adhesive friction. Tread elementary volume is molded as a square-based parallelepiped, in contact with the asphalt modelled as a sum of sinusoidal waves distributed in the space characterizing the different roughness scales.

11 GrETA Grip Estimation for Tyre Analyses The dimensions of the above parallelepiped along the vertical and the longitudinal directions are respectively equal to the tyre tread thickness and to the macroroughness wavelength λ MACRO, whereas the best-fitting sinusoidal waves corresponding to macro and micro profiles able to reproduce the wavelength λ and roughness indices R a are experimentally acquired by laser scan on different dry tracks.

12 GrETA Grip Estimation for Tyre Analyses GrETA Initialization DSC/DMA test Laser scan

13 GrETA Grip Estimation for Tyre Analyses The main advantage, the employment of the grip model represents, is the possibility to predict the variations induced by different tread compounds or ground textures on vehicle dynamics. Once characterized road and tread polymers, a Hysteretic friction model, applied in innovative conditions thanks to 3D stress/strain states by means of properly adapted Kuznetsov equations, has been adopted. The friction maps generated, as GrETA output, are in good agreement with data available in literature and with the ones provided by specifically designed experimental activities.

14 Results Different solutions of employment Models, procedures and methodologies discussed are able to describe and analyse different aspects of the phenomena concerning with tyre/road interaction, but their cooperation can constitute even more powerful instrument to extend the comprehension of such complex theme. A general overview of the developed models and procedures are reported. There are different possibilities to make the model cooperate, providing various solutions of their interlinked employment.

15 TRICK & TRT Temperature/Grip range The employment in conjunction of TRICK and TRT constitutes a valid instrument for carrying out the tyre thermal analysis. In particular, the range of temperature, in which grip performances are maximized, can be identified. Once carried out the track experimental session and, thus, acquired data useful to be processed by TRICK procedure, a forces and consequently grip estimation with the virtual telemetry are provided. Speed, slip, camber and force channels are used as input for TRT, whose results, in particular the estimation of tyre bulk temperature, are very useful for grip analysis discussed in the following.

16 TRICK & TRT Temperature/Grip range Thermal test session has been carried out following a specific procedure, developed with the aim to collect tyre data at different thermal conditions.

17 TRICK & TRT Temperature/Grip range Common analysis concerning the relationship between tyre friction coefficient and temperature is base on the only thermal data experimentally available: external and in few cases internal surface temperature. A typical correlation between lateral grip and measured external surface temperature appears like the following, from which very few information can be deducted.

18 TRICK & TRT Temperature/Grip range Thanks to the availability of bulk temperature, it is possible to provide much more correlations, from which thermal range can be identified. The reason for which bulk temperature offers better results can be attributed to the fact that surface temperature varies with fast dynamics, not able to modify in a so short time polymers characteristics in order to have a response on the whole tyre frictional behaviour.

19 TRICK & TRT Temperature/Grip range As a further validation of the described procedure, DMA experimental tests have been carried out on tyre rubber compounds, at fixed frequency and displacement (1Hz, 1%) making temperature increase at 1 C per minute from -50 C up 100 C, with the aim to acquire data useful to properly model the behaviour of SBR copolymers constituting tread: storage modulus E and tan(δ). GT tyres, differently from passenger ones, are characterized by a well distinct relative maximum at about 42 C and by higher values of tan(δ) at the usual employment temperatures.

20 TRICK & TRT Temperature/Grip range When both the frequency and temperature vary, it is possible to make use of the property whereby an appropriate shift operation is combining the effect of them, making use of the temperature-frequency equivalence principle, that the components of the stiffness dynamic modulus at any reference frequency and temperature (f 1, T 1 ) are identical to the ones observable at any other frequency f 2 at a properly shifted value of temperature α(t 1 ): E f1, T1 E f2, T1 The most widely used relationship to describe the equivalence principle is the Williams- Landel-Ferry (WLM) transform, that will be employed in order to determine the unknown equivalent temperature T*=α(T 1 ) and, consequently to validate the procedure: f T T log f 1 T * 2 1 in which a common ΔT value for SBR copolymers is about 8 C.

21 TRICK & TRT Temperature/Grip range Remembering that DMA test has been carried out at 1Hz frequency, definitely different from common tread stress frequencies, a quick calculation, hypothesizing an average road macro-roughness equal to 0.01 m and a range of sliding speed of [1 5] m/s, allows to estimate the real temperature at which tan(δ) maximum can be experienced by driver: [v ΔT = 8(log 10 ( s ] [1 5] λ ))=8(log 10 ( 0.01 )) [16 22] C that, added to the starting 42 C, gives a temperature range in accordance with the experimental value of the shown procedure.

22 TRT, GrETA & MF Performance Optimization Employing TRT output as an input for GrETA, the dependence from temperature, tyre working variables, road roughness and compound characteristics can be introduced in MF interaction model. The advantages coming from the interaction of these models, to be employed in simulation environments, can be summarized in the following three points: prediction of tyre behaviour on different tracks, each one characterized by different road roughness and weather conditions; performance evaluation at compound characteristics variation in order to establish a dialog channel with tyremakers; definition of optimal vehicle setup in order to reach optimal grip/thermodynamic conditions.

23 TRT, GrETA & MF Performance Optimization Being not available local velocity and pressure distributions within the contact patch, GrETA is supposed to work in this case basing on global variables, as well measured on track as calculated in simulation. Thus, the longitudinal and lateral friction estimated values are considered as sort of normalized scaling factors for longitudinal and lateral interaction forces, coming as an output from Pacejka formulation. It can be noticed that grip model employment produces better results, in particular as concerns longitudinal interaction in traction phase, thermally stressful for high performance tyres and able to generate heat friction power mechanism that induces not negligible effects in tyre/road interaction modelling.

24 TRT, GrETA & MF Performance Optimization Longitudinal rear tyre forces modelled by MF, compared with telemetry track data (in red) Coulomb friction law GrETA friction law

25 TRT, GrETA & MF Performance Optimization Lateral rear tyre forces modelled by MF, compared with telemetry track data (in red) Coulomb friction law GrETA friction law

26 Conclusions So the main features that the integration of the described model can provide are: full geometric, thermodynamic, viscoelastic and structural characterization of tyres; estimation of tyre interaction curves from experimental outdoor test data (TRICK); definition of a standard track driving procedure that employs tyres in multiple dynamic and thermal conditions; estimation of surface, bulk and inner liner tyre temperatures for variable working conditions and real-time reproduction of tyre thermodynamic behaviour in simulation applications (TRT); correlation of tyre thermal conditions with friction phenomena observable at the interface with road; prediction of tyre frictional behaviour at tread compound and ground roughness variations (GrETA); modelling of tyre interaction by means of MF innovative formulations able to take into account grip and thermodynamic effects on vehicle dynamics; definition of the optimal wheels and vehicle setup in order to provide the maximum possible performance improvement.

27 Further Developments MF model is not the most flexible and physical method to describe tyre/road interaction local phenomena, but represents a very robust and intuitive solution to obtain the hardly achievable aim to model tyre tangential forces. For this reason, further developments will focus on the integration with a completely physical interaction model [1], that, cooperating with the models described, could become a fundamental part of an analytic and predictive instrument to be employed in a widest possible rage of automotive applications. A final step in the field on tyre/interaction modelling and analysis could be represented by the development of a global wheel model [2], able to cooperate with the presented ones, constituting an ideal instrument for the prediction and the simulation of the real tyre dynamics. [1] Capone G., Giordano D., Russo M., Terzo M., Timpone F., Ph.An.Ty.M.H.A.: a Physical Analytical Tyre Model for Handling Analysis - the Normal Interaction, Vehicle System Dynamics, Vol. 47, N. 1, pp , 2009 [2] Calabrese F., Farroni F., Timpone F., A Flexible Ring Tyre Model for Normal Interaction, International Review on Modelling and Simulations (I.RE.MO.S.),Vol. 6, N. 4, 2013

28 Thanks for your attention

29 Contact Information Timpone Francesco Assistant Professor University of Naples Vehicle Dynamics Chair Holder University of Naples Flavio Farroni Postdoctoral Research Fellow University of Naples Vehicle Dynamics Technical Consultant Ferrari S.p.A Aleksandr Sakhnevych PhD Bursary Holder University of Naples