Method for determining the grip coefficient of a tyre on a wet surface

Abstract

The invention relates to a method for obtaining the grip coefficient Mu of a tyre on a wet surface in usage condition on a vehicle, comprising the following steps: - obtaining (S2) a reference longitudinal rigidity KXRef of the tyre on a reference surface in damp conditions; - obtaining (S1) a grip coefficient MuRef of the tyre on the reference surface in damp conditions; - determining weather parameters (S3) during the use of the tyre on the vehicle; - if the surface is in wet condition, determining (S4) a longitudinal rigidity KXMes of the tyre on the running surface; - evaluating (S5) the grip coefficient Mu of the tyre on the wet surface using formula (I) wherein the coefficient n is a real number between 0.2 and 2.0.

Description

METHOD FOR DETERMINING THE COEFFICIENT OF GRIP OF THE TYRE ON WET GROUND Field of invention

[0001] The present invention relates to the determination of the driving conditions of a tire, in particular on wet ground, in order to improve the active safety of vehicles by improving the information relating to the tire in real time. Technological background

[0002] The present invention relates to methods for determining the coefficient of adhesion of the tire on the ground at the scale of the contact patch, i.e. the contact surface of the tire with the ground when the tire, mounted on a rim, is loaded and possibly inflated. Indeed, the overall coefficient of adhesion of the tire, generally called Mu, makes it possible to evaluate the potential adhesion of the tire on the ground. This makes it possible to optimize the active safety devices of the vehicle in order to prevent random behavior of the vehicle leading to potentially risky trajectories. This coefficient of adhesion of the tire Mu obviously depends on the ground, both its nature, i.e. an asphalt road or sandy ground, and its condition, i.e. dry, damp, wet, snowy.For example, the same tire in terms of usage conditions, i.e. the same inflation pressure and the same applied static load, will see its adhesion coefficient Mu change depending on whether it is rolling on soft ground such as packed sand or snow or on rigid ground such as an asphalt road. But its adhesion coefficient will also change depending on the condition of the ground. Thus, the adhesion coefficient Mu of a tire is higher on dry ground than on wet ground, due to the presence of water which modifies the adhesion with the tire. Furthermore, for example, one of the conditions strongly influencing the adhesion coefficient of the tire is the height of residual water on the ground which can cause a partial or complete separation of the contact between the tire and the ground leading to aquaplaning of the tire.

[0003] In addition, external factors such as ambient temperature can also influence the behavior of the tire's rubber compounds and therefore its adhesion potential. Therefore, obtaining information on the tire's adhesion coefficient according to weather conditions makes it possible to adapt the trigger thresholds of the vehicle's active safety devices. This real-time adaptation of The trigger thresholds of active safety devices make driving the vehicle more peaceful. In particular, on black ground, including asphalt or bituminous roads for example, one of the conditions that strongly influences the tire's grip coefficient is the residual water level on the ground, which can cause partial or complete separation of the contact between the tire and the ground, leading to aquaplaning of the tire.

[0004] The following objects of the invention aim to determine the evolution of the overall grip coefficient Mu of the tire in real time on the vehicle due to the sole presence of liquid water on the road surface, thereby determining the critical aquaplaning speed of the tire regardless of its condition and defining a safe linear rolling speed of the vehicle under the conditions experienced by the vehicle serving for example as trigger thresholds for active safety devices. Description of the invention

[0005] The invention relates to a method for obtaining the adhesion coefficient Mu of the tire on wet ground in conditions of use on a vehicle comprising the following steps: • Obtaining a KXR reference longitudinal rigidity e f of the tire on a reference ground in wet conditions; • Obtaining a MuRef grip coefficient of the tire on the reference ground in wet conditions; • Determination of meteorological parameters when using the tire on the vehicle to identify the condition of the ground • If the ground is in wet condition, determination of longitudinal rigidity KXMÊS of the tire on the rolling ground; • Evaluation of the tire's coefficient of adhesion on wet ground using the following formula: • [Math 1] , where the coefficient n is a real number between 0.2 and 2.0.

[0006] Thus, the process first consists of obtaining reference quantities of the tire. These reference quantities correspond to use of the tire on so-called wet ground. Here, the term "wet" means that the liquid water, although present on the road, does not constitute a film between the ground and the tire. Therefore, the quantity of water is below a threshold such that the water can infiltrate into the roughness of the ground without remaining generally on the surface of the same ground. Of course, this threshold is a function of the grain size of the ground, but the fact of being in wet conditions ensures two conditions: the presence of water at the ground level, which intrinsically modifies the adhesion between the ground and the tire, and that the water is stored at ground level below its maximum height.That is to say, the interface between liquid water and air is located below the maximum height of the ground at the scale of the contact patch, that is to say the contact surface between the ground and the tire in use conditions. Conversely, a wet ground is a ground where liquid water is present and for which the interface between water and air is greater than the maximum height of the ground at the scale of the contact patch. Thus, a film of water is interposed between the ground and the tire. Therefore, these data can be obtained on various types of soil by controlling the quantity of liquid water applied to the reference soil.

[0007] The necessary reference quantities are, on the one hand, the tyre's grip coefficient on wet ground MuRef and, on the other hand, the longitudinal rigidity of the tyre on this ground KXRef. These quantities can be fixed quantities. But they can be linked to the tyre, for example the seasonality of the tyre, i.e. a tyre typed as a summer tyre, winter tyre or 4 seasons, or to the state of wear and aging of the tyre but they are independent of the nature of the ground, more precisely, their dependence on the nature of the ground will be negligible compared to that of the tyre.

[0008] The MuRef coefficient of adhesion of the tire on wet ground corresponds to the maximum level of the ratio between the shear forces and the normal force applied by the tire on the ground at the level of the contact patch. Beyond this threshold, the tire begins to slide on the ground. Here, we are talking about a slip of the tire at the scale of the contact patch and not at the scale of an element of material of the tire which will be similar to a microslip. The ground is in a wet state, that is to say in the presence of liquid water whose interface with the air is located below the maximum altitude of the ground at the scale of the contact patch.

[0009] KXR longitudinal stiffness e f corresponds to the slope at the origin of the curve linking the shear forces of the tire on the ground to the slip rate g% of the assembled assembly. For reasons of convenience and ease of calculation, shear forces are generally limited to the longitudinal force FX, i.e. in the direction of movement of the tire when it rotates around its natural axis of rotation.

[0010] The measurement of longitudinal stiffness KX requires both measuring the forces FX at the wheel center of the mounted assembly and the slip rate g% of the mounted assembly relative to the ground. Thus, it is necessary to obtain reliable information on these two quantities in real time and at the same time.

[0011] For the longitudinal forces FX at the wheel center of the mounted assembly, these can be estimated, for example, through the torques applied around the axis of rotation of the mounted assembly, whether motor or brake when the vehicle is moving in a straight line. This implies being able to access these data via the vehicle characteristics.

[0012] They can also be obtained, for example, through the static load of the vehicle and the longitudinal acceleration of the vehicle's center of gravity coupled with the distribution of engine and braking forces between the front and rear axles. Optionally, the physical model used to calculate the longitudinal forces FX at the wheel center of the mounted assembly takes into account various parameters including the road gradient, the vehicle's forward speed, the vehicle's aerodynamic drag and the rolling resistance of the tire casing.

[0013] But the FX forces can also be obtained using more direct measurements at the level of the mounted assembly. It will be noted by way of non-limiting illustrative example that the processing of at least two measurements of circumferential extension or contraction in at least one sidewall of the tire at two fixed points in space, located at different azimuths along the circumference makes it possible to estimate the forces at the wheel center. This circumferential contraction or extension of the sidewalls is advantageously estimated by measuring the distance between the cords of the carcass ply of the sidewalls. Reference will be made to patent document WO-A-03 / 014693 in the name of the applicants for a detailed description of this measurement of the characteristics of the mounted assembly.

[0014] The other essential characteristic for evaluating the longitudinal stiffness KX is the slip rate g% of the assembly mounted at the wheel center. This quantity can be estimated directly by the data provided by the electronic systems on board the vehicle such as the ABS system.

[0015] But it can also be evaluated through three elementary parameters which are the rotation speed W of the assembly mounted at the wheel center, the rolling radius Re of the mounted assembly and the forward speed V0 of the vehicle. The rotation speed W can be simply obtained by a wheel revolution encoder coupled to a clock. The rolling radius Re of the mounted assembly, which is not very sensitive to wear, is obtained using the distance traveled by the vehicle and the number of revolutions made by the mounted assembly to cover this distance. Finally, the forward speed V0 of the vehicle is obtained via a high frequency measuring device such as the RT 3000 for example to have high precision or a GPS linked to the vehicle in elementary mode.

[0016] Obtaining the reference longitudinal rigidity then consists of carrying out a measurement or a simulation of the longitudinal rigidity of the mounted assembly comprising the said tire on a vehicle or on a measuring bench on the scale of the mounted assembly provided that the rolling ground at the time of this measurement is in a wet state.

[0017] Recording the curve of the longitudinal shear force FX as a function of the slip rate makes it possible to obtain on the one hand the slope at the origin which is similar to the value of the KXR e f and on the other hand, the maxima of the curve with the highest slip rate defines a value of the reference adhesion coefficient MuRef of the said tire by combining it with the static vertical force applied to the mounted assembly.

[0018] Furthermore, the method requires obtaining the weather conditions at the time of the assessment in order to decide whether the wet ground condition is respected. This must determine whether precipitation is occurring at the time of the measurement and with what intensity, that is to say, what is the precipitation density. A high intensity, having exceeded a certain threshold, indicates that the condition is likely wet. The nature of this precipitation must also be defined: snow, hail, rain which influence the state of the road. Indeed, driving on snow, hailstones or liquid water are not entirely comparable. Generally, in the case of hail or snow, the reduced visibility which results from it encourages the driver to naturally reduce his speed.On the other hand, depending on the intensity of the rain and the size of the water drops, the evaluation of the quantity of water which will concentrate on the roadway is more random, which requires the method of the invention to objectify this state.

[0019] The method requires a measurement of the longitudinal stiffness KXM CS on the road surface if the road surface is considered to be in a wet condition, which is determined by the intensity of precipitation and its nature.

[0020] Finally, the method evaluates in absolute terms the coefficient of adhesion of the tire on the rolling ground, which is wet, Mu by the ratio of the reference and measurement longitudinal rigidities according to the given formula and the adhesion potential of the tire in the absence of a water film interposed between the ground and the tire. Thus, the evaluation of the coefficient of adhesion of the tire on the rolling ground in a so-called wet state is carried out directly following the measurement of the measurement longitudinal rigidity, which makes its availability instantaneous.

[0021] The observed variation is mainly driven by the presence of a film of water at the interface between the ground and the tire. This estimation of the wet grip coefficient of the tire makes it possible to estimate, for example, the maximum longitudinal forces to be applied to the vehicle's wheels to optimize its braking distance. For the stability of the vehicle, it is preferable that no wheel slips on the ground or if it must slip that this is symmetrical between the two wheels of the same axle to guarantee the stability of the vehicle.

[0022] Preferably, the coefficient n is between 0.5 and 1.0.

[0023] The inventor found that the range of 0.2 to 2.0 on the coefficient n of the formula makes it possible to obtain a good estimate of the coefficient of adhesion of the tire on wet ground on the scale of the contact patch according to the seasonality of the tires, their ranges and their dimensions. The restriction of the range to the interval between 0.5 and 1.0 is particularly relevant for passenger car and van tires.

[0024] Advantageously, the meteorological parameters are included in the group including the outside temperature, the level of intensity of precipitation.

[0025] The outside temperature is a quantity accessible on most transport vehicles at the vehicle level relating to its direct environment. Taking this quantity into account makes it easy to discriminate whether the precipitation seen by the vehicle is similar to snow or rain.

[0026] Very advantageously, the level of precipitation intensity is assessed by sound measurement, vibration measurement, activation of vehicle devices sensitive to precipitation such as rain detectors on windshields or the speed of windshield wipers.

[0027] The precipitation intensity level allows us to estimate the amount of precipitation in terms of volume through an impact number on a sensitive area of ​​the vehicle such as a rain detector. But the precipitation intensity can also to assess the nature of precipitation using an assessment of the shocks of these impacts in relation to thresholds to discriminate the presence of hail or heavy raindrops. Of course, the quantity of precipitation can be assessed for example by the speed of the windshield wipers which determine certain thresholds according to the quantity of rain received by the windshield for example. Finally, vibration or acoustic measurements at the level of the vehicle body, and in particular at the level of the cavity receiving the vehicle's mounted assemblies, also make it possible to assess the quantity of mobile particles on the road but also their nature by analyzing the vibroacoustic signals they produce when they impact the vehicle.

[0028] According to a particular embodiment, the method comprises a step of determining a longitudinal rigidity KX'MÊS of the tire on the rolling ground when the state of the rolling ground is wet.

[0029] The wet condition can easily be identified using different thresholds than the wet condition using the same means of determination. It is then possible to carry out the same longitudinal stiffness measurement as in the wet condition using the same data except that the result obtained is a longitudinal stiffness of the tire on wet ground at the scale of the contact patch.

[0030] Advantageously, the reference longitudinal rigidity KXR e f of the tire on wet ground is evaluated as the average of the longitudinal stiffnesses KX'MÊS on wet ground obtained on the tire mounted on the vehicle for a duration T.

[0031] Obtaining the reference longitudinal stiffness can then be done by averaging the longitudinal stiffnesses on wet ground that are achieved at the vehicle level. This makes it possible to adapt the fixed value taken initially, which generally corresponds to a new condition of the tire, to the life cycle of the tire, that is to say by taking into account its wear and ageing implicitly.

[0032] According to another particular embodiment, the method comprises a step of identifying the tire mounted on the vehicle comprising at least the seasonality of the tire.

[0033] Preferably, the step of identifying the tire fitted to the vehicle includes obtaining the wear of the tire and / or the aging of the tire.

[0034] Advantageously, the coefficient n depends on the tire.

[0035] Very advantageously, the reference longitudinal rigidity KXRe f depends on the tire.

[0036] Standard measurements of the reference quantities are always possible, in particular by entering values ​​corresponding to a standard tire in new condition. However, information on the seasonality of the tires is necessary to improve the level of estimation of the tire's coefficient of adhesion on wet ground. Of course, on a secondary scale, knowledge of the tire brand, its range and / or its dimensions makes it possible to improve the prediction of the coefficient of adhesion by adapting the standard values ​​more precisely to the identity of the tire. This degree of precision no longer makes sense if the nature of the ground varies too greatly compared to an average ground commonly encountered by the tire. Indeed, for example, the variations in the coefficient of adhesion on wet ground Mu generated by the nature of the ground become essential compared to the variations induced by the complete identity of the tire.

[0037] But taking into account the state of wear and the aging of the tire makes it possible to adapt the standard values ​​of the reference quantities, in particular the reference longitudinal rigidity KX^ which is more sensitive to these parameters than the reference coefficient of adhesion of the tire on wet ground, for which a standard estimate is often sufficient. These two parameters influence at least one of the reference quantities and their consideration can be easy on the vehicle. Thus, the analysis of data such as the number of kilometers traveled and the time since the tire was installed on the vehicle, allows a good estimation of these parameters and therefore an update of the standard values ​​for the reference quantities, which improves the quality of evaluation of the coefficient of adhesion of the tire on wet ground and guarantees a better adaptation of the trigger thresholds of the vehicle's active safety devices.

[0038] In addition, the identification of the tire also makes it possible to adapt the coefficient n of the formula by modifying its value, which is initially taken as a flat rate by calibrating for example on a standard tire in new condition.

[0039] The invention also relates to a method for obtaining a critical aquaplaning speed v cr of a tire on wet ground, mounted on a vehicle in driving condition, comprising the following steps: • Determination of a longitudinal speed v of movement of the vehicle; Obtaining the MuRef grip coefficient of the tire on a reference surface in wet conditions; • Obtaining the tire's grip coefficient Mu on wet ground; • Evaluation of the critical aquaplaning speed Ver of the tire using a function F comprising the parameters v, Mu and MuRef of the form: . [Math

[0040] Preferably, the function F is of the form: • [Math is a real number between 0.1 and 1.0.

[0041] Very advantageously, P is between 0.2 and 0.4.

[0042] From the estimation of the tyre's wet grip coefficient at the Faire de contact scale, an intrinsic quantity of the tyre by the reference wet grip coefficient of the tyre and the linear speed of movement of the vehicle on which the tyre is mounted, it is possible to evaluate the critical aquaplaning speed of this tyre on the rolling surface. This critical aquaplaning speed then corresponds to the speed of movement of the vehicle which leads to a total loss of contact between the tyre and the wet surface by the saturation of the tread groove network of the tyre due to an excessive quantity of liquid water present above the maximum height of the macro-roughness of the ground at the Faire de contact scale. The groove network is then no longer able to evacuate the quantity of water which appears on the wet road.The higher the travel speed, the greater the flow of water to be evacuated and therefore the potential saturation of the tread groove network increases, leading to a progressive loss of contact between the tire and the ground, up to and including aquaplaning of the tire.

[0043] To estimate this critical hydroplaning speed, it is necessary to have access to the linear displacement speed v of the vehicle on wet ground, a value of the reference adhesion coefficient MuRef of the tire on a standard wet ground and an evaluation of the adhesion coefficient of the tire on the wet ground on which the vehicle is traveling at speed v. From these quantities, the critical aquaplaning speed v is evaluated. cr for the said tire on the said wet ground. This allows the vehicle's active safety devices to be informed in real-time to activate the vehicle's speed limiter if necessary and warn the driver of the situation, if there is one, or inform the on-board systems so that they adapt the vehicle's speed to the conditions. The formula identified on F is well suited for passenger car / light truck tires. The range of P values ​​covers all tire seasonalities, i.e. summer, winter and all-season tires. The preferred range of P is well suited for summer seasonal tires in premium ranges.

[0044] The invention finally relates to a method for obtaining a safe rolling speed V on wet ground for a vehicle equipped with a pneumatic tire comprising the following steps: • Determination of a minimum tyre grip coefficient on wet ground MuMin • Obtaining a MuRef grip coefficient of the tire on a reference ground in wet conditions; • Determination of the critical aquaplaning speed v cr of the tire on wet ground according to one of claims 11 to 13; • Evaluation of the safe rolling speed V using a function H comprising the parameters v cr , MuMin and MuRef of the form: • [Math

[0045] Preferably, the function H is of the form: • [Math 5] , where y is a real number, preferably y is between 0.1 and 1.0.

[0046] Finally, real-time knowledge of the critical aquaplaning speed v crof the tire on wet ground, the determination of a reference coefficient of adhesion of the tire on wet ground generally encountered by the tire and a desired minimum coefficient of adhesion MuMin makes it possible to define a maximum rolling speed making it possible to avoid on the one hand any risk of aquaplaning and on the other hand, to ensure emergency maneuvers by minimizing the risks. These emergency maneuvers on the vehicle can be for example an untimely change of lane or a trajectory on a tight bend, that is to say with a small radius of curvature. This ensures a peaceful driving for the driver, that is to say without stress, if there is any, by directly controlling the vehicle's active safety devices such as cruise control and speed limiter to optimize the vehicle's stability devices. Brief description of the drawings

[0047] The invention will be better understood on reading the following description, given solely as a non-limiting example and with reference to the appended figures in which the same reference numbers designate identical parts throughout and in which: • Fig. 1 shows a synopsis of the process for determining the grip coefficient Mu of the tire on wet ground, as well as the determination of the critical speed v cr aquaplaning and determination of the safe rolling speed V according to the invention; • Fig. 2 shows the evolution of the adhesion coefficient Mu of tires on wet ground as a function of the ratio of their longitudinal rigidity KX on wet ground and on a reference ground in a wet state; • Fig. 3 shows the evolution of the critical aquaplaning speed of various tires on wet ground as a function of the tire's grip coefficient Mu on wet ground; • Fig. 4 shows the evolution of the safe rolling speed V of tires on wet ground as a function of the minimum grip coefficient MuMin of each tire on wet ground. Detailed description of embodiments

[0048] Fig. 1 is a general synopsis of the methods according to the invention. First of all, the method for determining the adhesion coefficient Mu of the tire on wet ground is defined through steps S1 to S5 with, optionally, steps 01, 02 and 03.

[0049] Step S2 consists of determining the reference longitudinal stiffness KXR ef of the tire fitted to the vehicle on a ground in a wet state. This value can be obtained as a flat rate initially, typically a value in a range between 20000 N / g% and 500000 N / g% is entirely acceptable.

[0050] However, the determination of this fixed value may also depend on the identity of the tire through step 01 in order to be more precise. The identity information priority for this dependency comes from the seasonality of the tire at order zero. It is therefore necessary to know whether it is a "summer", "winter" or "4 seasons" type tire to refine the fixed value. But, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will be rather of order 1 in terms of influential factors. Finally, a last embodiment consists, as an option, in measuring longitudinal rigidities KX'MÊS of the tire mounted on the vehicle corresponding to straight line rolling on wet ground and this whatever the nature of the ground.Thus, by averaging the measurements carried out KX'MÊS over a short period on the scale of the change in wear of the tire or its aging, we obtain a possibly more relevant value taking into account all the factors influencing the longitudinal rigidity KX of the tire without the optional step 01 of tire identification.

[0051] The SI step consists of determining the reference grip coefficient MuRef of the tire on wet ground. This value can be obtained as a standard initially; typically, a value in the range between 0.5 and 1.3 is entirely acceptable.

[0052] But, the determination of this fixed value can also depend on the identity of the tire through step 01 in order to be more precise. The priority identity information for this dependency comes from the seasonality of the tire at order zero. It is therefore necessary to know if it is a "summer", "winter" or "4 seasons" type tire to refine the fixed value. But, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will rather be of order 1 in terms of influential factors.

[0053] These first two steps S1 and S2 can potentially be carried out before evaluating the various quantities of the invention in real time, which makes it possible to feed these values ​​into the calculator. The calculator can be installed in the vehicle or removed from the vehicle. In this second solution, the data is fed via a cloud, as is the restitution of the evaluations which will be transmitted to the vehicle.

[0054] During the vehicle's driving, the weather conditions must be determined during a step S3. This consists, for example, of estimating the outside temperature of the vehicle and estimate the level of precipitation. The first meteorological parameter, the outside temperature, makes it possible to estimate whether the precipitation is potentially snow or water in a liquid state. The second meteorological parameter, the level of precipitation, makes it possible, depending on the sensors used on the vehicle, to evaluate the quantity of precipitation in terms of volume and / or to evaluate the quantity of precipitation in terms of mass falling on the vehicle. Thus, the speed of the automatic windshield wipers on the vehicle makes it possible to distinguish the quantity of precipitation in volume encountered by the rain sensor. And sound or vibration sensors on the vehicle make it possible to evaluate the quantity in volume of precipitation but also the quantity by mass of precipitation by taking into account, if necessary, the longitudinal speed of the vehicle through the optional step 03.Thus, collecting all this data and analyzing it allows us to estimate whether the vehicle is encountering a wet or damp road surface. In the latter case, the amount of water stagnating above the road surface is irrelevant in terms of the contact surface between the tire and the ground.

[0055] In the case where the analysis of the meteorological results defines a ground in a wet state, it is appropriate to move on to the next step S4. This consists of determining, during the rolling of the vehicle, the longitudinal rigidity KXM CSof the tire fitted to the vehicle on the rolling ground which is in a wet state. This measurement is to be carried out in a straight line, through the indication of the steering angle for example, an evaluation of the slope of the points defined by the slip rate g% of the tire and the force FX applied to the tire, measured at the wheel center for example. The accumulation of several points each representing the force FX at the wheel center for a given slip rate g% makes it possible to estimate the slope at the origin of the cloud of points which is similar to the longitudinal rigidity KX of the tire on the ground in a wet state.

[0056] Finally, an estimate of the tire's wet grip coefficient Mu can be obtained through step S5. The inputs of this step S5 are, on the one hand, the reference quantities MuRef and KXR ef which are the outputs of steps S1 and S2 and on the other hand the determination of the longitudinal stiffness KXMÊS obtained in step S4. Thus, it is possible in real time on the vehicle and during driving on wet ground to estimate the remaining grip potential of the tire linked to the wet state of the ground through the grip coefficient Mu of the tire. An alert can then be sent to the driver or to the on-board driving systems to adapt the driving conditions of the vehicle according to this grip coefficient Mu. For example, by measuring the remaining grip potential of the tires of the front axle, which will be the most sensitive to the quantity of liquid water on the When driving forward on the road, it is possible to avoid loss of contact between one of the tires at the front of the vehicle and the ground, or even the entire front axle.

[0057] Then the method of determining the critical speed aquaplaning v cr of the tire on wet ground is defined through steps S and S5 to S7.

[0058] Steps S1 and S5 are those which have already been explained during the process of determining the adhesion coefficient Mu of the tire on wet ground.

[0059] In the case where the ground is in a wet state, it is possible that the tire may lose rubber / ground contact by creating an interlayer film of water between the two solid elements that are the tire and the ground, which corresponds to aquaplaning. In order to determine the limit speed from which rubber / ground contact is lost, which is called the critical aquaplaning speed, it is first necessary to know the rolling speed of the vehicle through step S6.

[0060] This step S6 takes the result of the optional step 03 if this was carried out for the determination of the adhesion coefficient Mu of the tire on wet ground in step S5. Otherwise, step S6 consists of determining the rolling speed v of the vehicle when the state of the ground has been identified as wet. This determination of the speed can take various possibilities such as the speed defined by a GPS system (acronym in English for "Global Positioning System"), the rotation speed of the wheels of the vehicle by estimating a crushed radius Re associated with the measured wheel. Generally, the crushed radius Re of the tire is a function of the dimensions and the range of tires as well as the static load applied and the inflation pressure of the tire. The determination of the speed can also be obtained through the on-board systems of the vehicle and in particular the instruments of the dashboard of the vehicle such as the speedometer.

[0061] Finally, the determination of the critical aquaplaning speed v cr is obtained through step S7. The inputs of this step S7 are on the one hand the reference quantity MuRef of step SI, the output of step S5, i.e. the coefficient of adhesion of the tire on wet ground Mu and, on the other hand the determination of the rolling speed of the vehicle v obtained in step S6. Thus, it is possible in real time on the vehicle and during rolling on wet ground to estimate the critical aquaplaning speed v cr of the tire fitted to the vehicle linked to the wet state of the ground. An alert can then be sent to the driver or to the on-board driving systems to adapt the vehicle's driving conditions, in particular the driving speed according to this critical aquaplaning speed v cr . For example, by informing the driver or the vehicle's on-board driving systems to comply with a speed limit of X% or Y speed units below the critical aquaplaning speed v cr .

[0062] Finally, the method for determining a safe rolling speed V on wet ground is defined through steps S1 and S7 to S9.

[0063] Steps S1 and S7 are those which have already been explained during the process of determining the critical aquaplaning speed v cr of the tire on wet ground.

[0064] The ground being in a wet state, it is possible that the tire may lose rubber / ground contact by creating an intermediate film of water between the two solid elements that are the tire and the ground, which corresponds to aquaplaning. In order to drive in complete safety with respect to the presence of this film of water on the road, it is advisable to define for a minimum tire grip level that one wishes to maintain, which is called MuMin, the threshold rolling speed V not to be exceeded to guarantee this minimum grip level MuMin.

[0065] Step S8 consists of determining the minimum grip coefficient MuMin of the tire according to the condition of the ground which has been identified as wet. This determination of the minimum grip coefficient MuMin of the tire on wet ground serves to guarantee easy and safe driving of the vehicle despite the potential presence of large quantities of liquid water on the roadway and whatever the maneuvers of the vehicle including cornering for example.

[0066] This determination can be obtained initially on a flat-rate basis; typically, a value in a range between 0.4 and 0.8 is entirely acceptable.

[0067] But, the determination of this fixed value which is linked to the vehicle can also depend on the identity of the tire through step 01 in order to be more precise. The priority identity information for this dependency comes from the seasonality of the tire at order zero. It is therefore necessary to know if it is a "summer", "winter" or "4 seasons" type tire to refine the fixed value. But, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will rather be of order 1 in terms of influential factors.

[0068] Finally, the determination of the safe rolling speed V is obtained through step S9. The inputs of this step S9 are on the one hand the reference quantity MuRef of step SI, the output of step S7, i.e. the critical aquaplaning speed v crof the tire on wet ground and, on the other hand, the determination of the minimum adhesion coefficient MuMin of the tire on wet ground obtained in step S8. Thus, it is possible in real time on the vehicle and during driving on wet ground to estimate the safe driving speed V of the tire fitted to the vehicle linked to the wet state of the ground. An alert can then be sent to the driver or the on-board driving systems to adapt the driving conditions of the vehicle, in particular the driving speed according to this safe driving speed V. For example, by informing the driver or the on-board driving systems of the vehicle to respect a speed limit, of X% or Y speed units below the safe driving speed V.

[0069] Fig. 2 shows a representation of the tyre's wet grip coefficient Mu as a function of the tyre's longitudinal stiffness KXMÊS measured on wet ground during driving. More precisely, the representation of the abscissas here is the ratio of the longitudinal stiffnesses KX of the same tyre between the wet ground measured on the vehicle and a reference ground in the wet state.

[0070] The two curves represented 101 and 201 each correspond to a tire. 100 and 200. Each tire in the figure belongs here to a tire category of different seasonality. The tire 100 represented by the curve 101 in solid line is a tire of category "summer" while the tire identified by the curve 201 in dotted line is a tire 200 of the category "4 seasons". Each tire 100 and 200 is characterized by a reference adhesion coefficient MuRef, here different. The mathematical representation of the two curves 101 and 201 is a function of the reference quantities MuRef and KXR e f of the tire as well as a power coefficient “n” which are all dependent on the tire 100 or 200.

[0071] Step S5 of the method consists of starting from a real-time KXMÊS, for a given 100 or 200 tire, identifying the point of adhesion coefficient Mu of the 100 or 200 tire on wet ground, regardless of the height of water on the ground.

[0072] For this purpose, at a KXM level CS Given the points K 100 and K200, a vertical line 102, 202 is drawn which intercepts the curves 101 and 201 of Fig. 2 at points 103 and 203 respectively. From points 103 and 203, an orthogonal line 104 and 204 is drawn which intercepts the ordinate axis at respective points Mu 100 and Mu200. These two points MulOO and Mu200 then represent the adhesion coefficient Mu of the 100 tire, respectively of the 200 tire, on wet ground for each of the tires and this whatever the height of water on the road.

[0073] Fig. 3 shows a representation of the critical aquaplaning speed v crof the tire on wet ground as a function of the adhesion coefficient Mu of the tire on wet ground when driving.

[0074] The two curves represented 111 and 211 each correspond to a tire. 100 and 200. Each tire in the figure belongs here to a tire category of different seasonality. The tire 100 represented by the curve 111 in solid line is a tire of category "summer" while the tire identified by the curve 211 in dotted line is a tire 200 of the category "4 seasons". Each tire 100 and 200 is characterized by a reference coefficient of adhesion MuRef, here different. The mathematical representation of the two curves 111 and 211 is a function of the reference quantity MuRef, the coefficient of adhesion Mu of the tire on wet ground, the speed v of movement of the vehicle on which the tire is mounted as well as a coefficient of power "P" which are all dependent on the tire 100 or 200.

[0075] Step S7 of the method consists of starting from a tire Mu on wet ground in real time, for a given 100 or 200 tire, to identify the critical aquaplaning speed point v cr of the 100 or 200 tire, regardless of the height of water on the ground.

[0076] For this purpose, at a given level of Mu, the points Mu 100 and Mu200, a vertical line 112, 212 is drawn which intercepts the curves 111 and 211 of Fig. 3 at points 113 and 213 respectively. From points 113 and 213, an orthogonal line 114 and 214 is drawn which intercepts the ordinate axis at a respective point v100 and v200. These two points v100 and v200 then represent the critical aquaplaning speed v cr of tire 100, respectively of tire 200, for each of the tires.

[0077] Fig. 4 shows a representation of the safe driving speed V as a function of the minimum grip coefficient MuMin of the tire on wet ground that we want to have in all driving circumstances.

[0078] The two curves represented 121 and 221 each correspond to a tire. 100 and 200. Each tire in the figure belongs here to a tire category of different seasonality. The tire 100 represented by the curve 121 in solid line is a tire of the “summer” category while the tire identified by the curve 221 in line dotted line is a 200 tire of the “4 seasons” category. Each 100 and 200 tire is characterized by a reference grip coefficient MuRef, here different. The mathematical representation of the two curves 121 and 221 is a function of the reference value MuRef of the tire on wet ground, of the critical aquaplaning speed v crof the tire, the minimum grip coefficient MuMin of the tire on wet ground as well as a power coefficient “y” which are all dependent on the tire 100 or 200.

[0079] Step S9 of the method consists of starting from a MuMin of the tire on wet ground in real time, for a given 100 or 200 tire, identifying the safe rolling speed point V of the 100 or 200 tire, regardless of the height of water on the ground.

[0080] For this purpose, at a given MuMin level, the points MUMIOO and MUM200, a vertical line 122, 222 is drawn which intercepts the curves 121 and 221 of Fig. 4 at points 123 and 223 respectively. From points 123 and 213, an orthogonal line 124 and 224 is drawn which intercepts the ordinate axis at a respective point V100 and V200. These two points V100 and V200 then represent the safe rolling speed V of the tire 100, respectively of the tire 200, for each of the tires and whatever the height of water on the roadway.

Claims

CLAIMS Method for obtaining the adhesion coefficient Mu of the tire on wet ground in conditions of use on a vehicle comprising the following steps: Obtaining (S2) a reference longitudinal stiffness KXR e f of the tire on a reference ground in wet conditions; Obtaining (SI) a MuRef adhesion coefficient of the tire on the reference ground in wet conditions; - Determination of meteorological parameters (S3) when using the tire on the vehicle; If the ground is in wet condition, determination (S4) of a longitudinal rigidity KXMÊS of the tire on the rolling ground; - Evaluation (S5) of the tire's adhesion coefficient Mu on wet ground using the following formula: [Math 1] , where the coefficient n is a real number between 0.2 and 2.

0. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 1 in which the coefficient n is between 0.5 and 1.

0. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 1 to 2 in which the meteorological parameters (S3) are included in the group comprising the outside temperature, the level of intensity of the precipitation. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 3 in which the level of intensity of the precipitation is evaluated by a sound measurement, a vibration measurement, an activation of devices of the vehicle sensitive to precipitation such as rain detectors on the windshields or the speed of wiping of the windshield wipers. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 1 to 4 in which the method comprises a step of determining a longitudinal rigidity KX'MÊS of the tire on the rolling ground (02) when the state of the rolling ground is wet. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 5 in which the reference longitudinal rigidity KXR ef (S2) of the tire on wet ground is evaluated as the average of the longitudinal stiffnesses KX'MÊS on wet ground (02] obtained on the tire mounted on the vehicle for a duration T. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to one of claims 1 to 6 in which the method comprises a step of identifying the tire (01) mounted on the vehicle comprising at least the seasonality of the tire. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to claim 7 in which the step of identifying the tire (01) mounted on the vehicle comprises obtaining the wear of the tire and / or the aging of the tire. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to one of claims 7 to 8 in which the coefficient n depends on the tire.Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to one of claims 7 to 9 in which the reference longitudinal rigidity KXRef (S2) depends on the tire. Method for obtaining a critical aquaplaning speed v cr of a tire on wet ground mounted on a vehicle in driving condition comprising the following steps: - Determination of a longitudinal speed v (S6) of movement of the vehicle; Obtaining a MuRef tire grip coefficient (SI) on a reference surface in wet conditions; Obtaining the adhesion coefficient Mu of the tire (S5) on wet ground according to one of claims 1 to 10; - Evaluation of the critical aquaplaning speed v cr of the tire (S7) using a function F comprising the parameters v, Mu and MuRef of the form: [Math 2] V Cr = F v, Mu, Mu Ref Method for obtaining a critical aquaplaning speed v cr of a tire on wet ground according to claim 11 in which the function F is of the form: [Math 3] , . v F{v,Mu,Mu Re f) = - g ((L Mu U 1 Mu Re f) l , where P is a real number between 0.1 and 1.

0. Method for obtaining the critical aquaplaning speed v cr of a tire on wet ground according to claim 12 in which P is between 0.2 and 0.

4. Method for obtaining a safe rolling speed V on wet ground of a vehicle equipped with a pneumatic casing comprising the following steps: - Determination of a minimum grip coefficient (S8) of the tire on wet ground MuMin; Obtaining a MuRef tire grip coefficient (SI) on a reference surface in wet conditions; - Determination of the critical aquaplaning speed v cr of the tire (S7) on wet ground according to one of claims 11 to 13; Evaluation of the safe rolling speed V (S9) using a function H comprising the parameters v cr , MuMin and MuRef of the form: [Math 4] Method for obtaining a safe rolling speed V on wet ground according to claim 14 in which the function H is of the form: [Math 5] , where y is a real number, preferably y is between 0.1 and 1.0.

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