Abstract:
A safety insert is designed to be mounted in an assembly including a tire and a rim of a vehicle, so that on bearing of the tire against the insert, the insert will generate multiple vibrating signals of the rotation frequency of the tire. In one embodiment, the vibrating signals are generated by a variation of at least one of the outer radius and radial stiffness as a function of azimuth α, which presents at least four maxima distributed in at least twice two different values M 1  and M 2.

Description:
This is a continuation of PCT/EP99/00525, filed Jan. 27, 1999. 
    
    
     BACKGROUND OF INVENTION 
     The invention concerns the use of tires equipped with safety insert and, in particular, the detection of bearing of the tire on the safety insert. It proposes an insert which warns the driver as soon as the tire bears on it after a flat or in case of substantial pressure loss. 
     These safety inserts are, in general, mounted on the rim inside the tire. Their function is to take up the load in case of tire failure. 
     The bearing of the tire on the safety insert is accompanied by a more or less marked degradation of its performance which may not be perceptible to the driver through the behavior and comfort of the vehicle. Furthermore, the operating lifetime of these inserts is limited. It is therefore essential for safety that the driver be warned as soon as a tire bears on its safety insert, so that the driver can follow the manufacturer&#39;s instructions. 
     Several safety inserts incorporating means for warning the driver of such bearing have previously been proposed. 
     U.S. Pat. No. 4,262,724 proposes a safety insert intended to be mounted in an assembly comprising a tire and a rim and radially outward from the rim. This insert has a radially outer surface which defines a radial support for the crown of the tire when the tire is deflated, as well as means for generating vibrating warning signals on a run-flat condition. These means are a variation of the run-flat radius of the insert between a minimum radius and a maximum radius or one or more bulges. 
     These solutions pose several problems. In order to be detected by the driver, the amplitude of the variations has to be high and that causes marked discomfort for the passengers of the vehicle, at least at certain speeds. On the other hand, it can greatly degrade the behavior of the tire and rim assembly concerned, particularly on acceleration and braking. All one can do is thus to avoid instant stopping of the vehicle in case of tire failure. On the other hand, when it is desired to use the vehicle, even at limited speed, over long distances, the warning transmitted by the insert has to be compatible with driving safety, not impairing the mechanical operation, while remaining perfectly perceptible to the driver either directly or by means of a suitable detection device. 
     Furthermore, patent application WO 94/03338 proposes a system for detecting the bearing of a tire on a safety insert. The system comprises a wheel accelerometer, placed on one of the wheel suspensions and measuring vertical accelerations. The accelerometers are connected to a central processing unit. The analysis is based on the detection on bearing of a resonance mode characteristic of a run-flat condition. 
     SUMMARY OF THE INVENTION 
     The invention concerns a safety insert which warns the driver, either directly or indirectly, upon bearing of the tire against the safety insert over a very wide range of speeds of the vehicle, while retaining a character tolerable to both the driver and the mechanics in the range of authorized speeds. 
     “Vehicle” is understood below to mean an autonomous running unit, passenger car, tractor or trailer, truck, motorcycle, etc. 
     A first safety insert according to the invention, intended to be mounted in an assembly comprising a tire and a rim of a vehicle and radially outward from the rim, has a radially outer bearing surface which defines a radial support for the crown of the tire when said tire is deflated and means for generating vibrating warning signals on a run-flat condition. This insert is characterized in that said means are a variation of a characteristic chosen from the group of outer radius and radial stiffness of said insert as a function of azimuth α, which presents at least four maxima distributed in at least twice two different values M 1  and M 2 . 
     In its simplest embodiment, characteristic M presents as a function of azimuth α two maxima M 1  diametrically opposite and two maxima M 2  diametrically opposite and placed at 90° from the maxima M 1 . 
     Such an insert has the advantage of producing on a run-flat condition a vibrating warning excitation distributed mainly over harmonics  2  and  4  of the turn of the wheel, while avoiding the frequency of the turn of the wheel. That facilitates detection of those signals directly by the driver, because the excitation of two harmonics, instead of only one, increases the range of speeds in which the vibrations are perceptible. Another advantage is to avoid the confusion always possible with, notably, the vibrations of the engine speed when the insert produces mainly a single excitation frequency. Furthermore, when the vehicle is equipped with a detection device, as, for example, described in application WO 94/03338, the peaks of the vibrating warning signals are very easily perceptible with very few risks of false alarms. 
     According to another embodiment, the characteristic M presents as a function of azimuth α two maxima M 1  diametrically opposite and three maxima M 2  distributed over the circumference and offset from the maxima M 1 . 
     Such an insert produces on a run-flat condition a vibrating warning excitation distributed mainly over harmonics  2  and  3  of the turn of the wheel, as well as by combination on harmonic  5 . 
     According to a third embodiment, the characteristic M presents as a function of azimuth α two maxima M 1  diametrically opposite and four maxima M 2  placed at 90° from one another and offset by 45° from the maxima M 1 . 
     This insert produces on a run-flat condition a vibrating warning excitation distributed according to harmonics  2 ,  4  and  8 . This has the advantage of an easier detection at low speed. 
     Preferably, on both sides of each maximum M 1  or M 2 , there is a single value minimum M 3  that increases the concentration of energy actually included in the harmonics principally excited. 
     The alarm means can also contain, in combination with the first variation, a variation as a function of azimuth α of the second characteristic of the group of the outer radius and radial stiffness of said insert. 
     According to another embodiment of the invention, the means are a combination of a variation of the outer radius and a variation of the radial stiffness of the insert as a function of azimuth α, so that the insert under load presents a crushed radius as a function of azimuth α containing at least four maxima. 
     The variations of radial stiffness as a function of azimuth α can range between 10 and 1000% and the variations of outer radius can range between 0.1 and 7 mm. 
     The variation of radial stiffness of the safety insert according to the invention preferably depends on the amplitude of the radial stress to which the support is subjected. For example, beyond a given radial stress threshold, the variation of radial stiffness as a function of azimuth α increases very markedly. 
     Such an insert can comprise a roughly cylindrical base intended to be fitted around the rim, a roughly cylindrical crown intended to come in contact with the tire in case of pressure loss and walls appreciably connecting the base and the crown radially. It can be made of flexible elastomeric materials and have arc-shaped walls. The variations of radial stiffness are then easily obtained by varying the thickness or shape of the walls and the variations of outer radius are preferably obtained by varying the lengths of the walls. 
     In one particular embodiment, the variations of radial rigidity and/or outer radius of the insert are applied axially only on a part of the insert intended to be placed toward the inside of the vehicle. That has the advantage of creating, on a run-flat condition, warning torques and, therefore, signals oriented both radially and transversely, which can facilitate their detection. Another advantage is that such inserts mainly transmit vibrating warning signals only on a straight run, while on a turn, the tires having a substantial camber, it is the “inactive” uniform part of the insert which mainly bears the load. The risks of disruption of vehicle behavior are thus reduced. 
     Finally, the variations of a radial rigidity and/or of outer radius of the insert can present a complementary axial development as a function of azimuth α. Such a development has the advantage of creating a vibration complementing the previous ones, a component of which is oriented in the axial direction. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Several embodiments of the invention are now described by means of the attached drawing, in which: 
     FIG. 1 presents, in meridian section, a tire and rim assembly equipped with a safety insert; 
     FIG. 2 a  presents a partial side view of an insert of elastomeric material and FIG. 2 b  presents a section of the same insert; 
     FIG. 3 presents, in meridian section, a schematic of an insert with a variation of the outer radius; 
     FIG. 4 presents, in meridian section, a schematic of a second insert with a variation in radial stiffness; 
     FIG. 5 presents, in meridian section, a schematic of a third insert with a combination of variations of radial stiffness and outer radius; 
     FIG. 6 presents, in partial side view, a variant of the insert of FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a wheel rim  1  equipped with an annular safety insert  2  resting on the bearing  6  of the rim  1 . The particular geometry of that wheel rim  1  is described in U.S. Pat. No. 5,749,982. It has two bead seats of different diameters and is particularly adapted for easy placement of said safety insert  2 . This assembly makes running possible in spite of a large pressure drop in the tire  3 . In case of such running, the insert of the deformed tire rubs on the outer surface of the insert, producing heating which limits the available radius of action: it is therefore important for the driver to be informed that the tire is bearing on its insert  2 . 
     For this purpose, a safety insert is advantageously used according to the invention, which contains means for generating harmonic vibrating warning signals of the turn of the wheel (that is, of the tire rotation frequency). 
     The insert shown in FIG. 2 is made of flexible elastomeric material. It comprises a generally ring-shaped base  10  reinforced by a ply (not represented) longitudinally oriented roughly at 0°, an appreciably annular crown  12  with longitudinal grooves  13  on its radially outer wall. It also has shaped walls joining the crown  12  and the base  10 . Between the walls  16  there are recesses  17  which, as shown in FIG. 2 b , completely cross the insert  2  axially. The base can include a stud  15  placed on the outer side near the tire bead. 
     Such an insert  2  can very easily have its radial stiffness modified by local widening or varying the shape of the walls  16 . Its outer radius can also be modified by local increase of the thickness of the base  10  or of the crown  12  and preferably by radial elongation of the walls  16 , which increases the weight of the insert only very slightly. Depending on the use of the insert, in cooperation or not with a detection device, and on the type of vehicle equipped, the amplitudes of variations of outer radius and radial stiffness can very markedly vary: between 0.1 and 7 mm for the outer radius and preferably between 1 and 3 mm and by 10 to 1000% for the radial rigidity. 
     FIG. 3 presents a diagram of an insert  20  according to the invention, which has a variation of outer radius among three values R 1 , R 2  and R 3 , such that R 1 &gt;R 2 &gt;R 3 , with a progressive variation of that radius between the maxima and the minima. The two zones of outer radii R 1  are at 180° from one another and so are the two zones of radii R 2 ; the four minima of radii R 3  are each between two maxima R 1  and R 2 . This results, on a run-flat condition, in a variation of crushed radius as a function of azimuth α with two principal harmonics, the first of frequency  2 , due to the first two maxima of radius R 1  and the second of frequency  4  due to the presence of the four maxima of radii R 1  and R 2  and of the four minima of radius R 3 . In that example, the R 1 −R 3  difference is equal to 5 mm and the R 2 −R 1  difference is equal to 3 mm. 
     FIG. 4 present a diagram of an insert  30  according to the invention, which has a variation of radial stiffness among three values K 1 , K 2  and K 3 , such that K 1 &gt;K 2 &gt;K 3 , with a progressive variation of that radius between the maxima and the minima. As previously, the two zones of stiffness K 1  are at 180° from one another and so are the two zones of stiffness K 2 ; the four minima of stiffness K 3  are each between two maxima K 1  and K 2 . This results, on a run-flat condition, in a variation of that stiffness as a function of a with two principal harmonics, the first of frequency  2 , due to the first two maxima of stiffness K 1  and the second of frequency  4  due to the presence of the four maxima of stiffness K 1  and K 2  and of the four minima of stiffness K 3 . 
     FIG. 5 shows a diagram of an insert  40  according to the invention, which presents a combination of a variation of outer radius and a variation of radial stiffness. Each characteristic presents two maxima (R 1 , K 2  respectively) and two minima (R 2 , K 1 , respectively). The two maxima of radius R 1  are offset from each other at an angle of 180° and are arranged at 90° from the two minima R 2 . Likewise, the two maxima of stiffness K 2  are offset from each other at an angle of 180° and are arranged at 90° from the two minima K 1 . Finally, each maximum of radius R 1  is offset at an angle of 90° from the two maxima of stiffness K 2 . The maxima of radial stiffness are sufficiently localized to produce in the insert assembly  40  a crushed radius on support containing four maxima. 
     Consequently, that insert also produces an harmonic excitation concentrated on harmonics  2  and  4 , but has the advantage of having a variable weighing as a function of speed. It has been found that the radial variations of stiffness were more perceptible at low speed and that the variations of outer radius were more perceptible at high speed. That increases the effective range of speeds of use of the insert. 
     FIG. 6 shows an insert  50  in partial side view. That insert, like the previous insert  40 , presents a combination of an outer radius variation and a radial stiffness variation as a function of azimuth α. It is characterized by having a radial stiffness variation as a function of azimuth α dependent on the radial stress the insert undergoes. For that purpose, the two maxima of stiffness K′ 2 (Q) have a radial stiffness dependent on the radial load or stress Q that the insert undergoes. 
     The insert represented in FIG. 6 is a variant also of the insert represented in FIG.  2 . It contains straight walls  51 . In the zone of maximum stiffness K′ 2  the base  10  is radially extended outward by a block of rubber  53 . The radial thickness of that block  53  is greater than half the radial height of the walls  51  joining the base  10  to the crown  12 . Consequently, the wall  52  joining the block  53  to the crown  12  has a very low radial height and the recesses  54  arranged between the block  53 , the crown  12  and the walls  51  and  52  have a volume more than half less than the volume of the recesses  17 . 
     As a result, when a load Q is applied on that zone of maximum stiffness K′ 2  as a function of azimuth α, wall  52  is crushed with a first given stiffness. Then, when the crown  12  comes in contact with the block  53 , the stiffness of the zone K′ 2  increases very markedly. Consequently, at low loads Q, the amplitude of variation of radial stiffness as a function of azimuth is comparable to that of the insert  40  of FIG. 5; on the other hand, when the load Q exceeds a given threshold, that amplitude of variation increases very markedly. It has been observed that this combination makes it possible to reach an excellent compromise for the effectiveness of excitation produced by the insert. When running over good roads, the slight variation of radial stiffness does not detract from comfort. And, when running over bad roads, the marked variation of radial stiffness makes it possible to maintain good excitation on a run-flat condition. 
     It is easily possible, without departing from the scope of this invention, to define inserts similar to those just described adapted for concentrating the energies of their warning signals on higher harmonic frequencies.