Abstract:
A safety support includes a substantially cylindrical base configured to conform to the rim. The support further includes a crown with a radially outer wall configured to enter into contact with an internal wall of a crown of the tire in the event of a loss of pressure and to leave clearance therebetween at the rated operating pressure of the tire. The safety support further includes an annular body linking the base and the crown of the support. An envelope of the radially outer wall of the support crown is defined, when the support is mounted on the rim, with f being the deflection undergone by the support mounted on the rim under a load Z, the rated load of the tire, the envelope being confined between two cylinders of revolution of radius R max  and R min  satisfying the following relationship:
 
 0.2   f&lt;R   max   −R   min   &lt;2   f.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application PCT/EP2004/011706, filed Oct. 18, 2004, which claims priority to French Patent Application 03/12499, filed Oct. 24, 2003, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to safety supports for vehicle tires, intended to be mounted on the rims thereof, inside the tires, to support the load in the event of tire failure or abnormally low inflation pressure. The present invention relates, more particularly, to structural safety supports, generally made from an elastomeric material. 
     2. Description of the Related Art 
     U.S. Pat. No. 5,891,279 (counterpart to EP 0 796 747), which is incorporated herein by reference, describes a safety support in which the crown has an outer wall which takes the form substantially of a cylinder of revolution and comprises longitudinal grooves. U.S. Pat. No. 6,564,842 (counterpart to WO 00/76791), which incorporated herein by reference, also describes such a safety support. 
     To improve the endurance of the safety support and tire assemblies when running flat or at reduced inflation pressure, lubricating compositions or gels are usually incorporated on the inner face of the tire. These gels are intended to reduce the friction between the support and the inner face of the tire surrounding the support. Such gels usually comprise a lubricant such as glycerol and a thickener such as silica. 
     SUMMARY OF THE INVENTION 
     The invention provides a safety support whose endurance performance when running flat is improved relative to prior supports. The safety support includes a substantially cylindrical base configured to conform to the rim. The support further includes a crown with a radially outer wall configured to enter into contact with an internal wall of a crown of the tire in the event of a loss of pressure and to leave clearance therebetween at the rated operating pressure of the tire. The safety support further includes an annular body linking the base and the crown of the support. An envelope of the radially outer wall of the support crown is defined, when the support is mounted on the rim, with f being the deflection undergone by the support mounted on the rim under a load Z, the rated load of the tire, the envelope being confined between two cylinders of revolution of radius R max  and R min  satisfying the following relationship:
 
0.2 f&lt;R   max   −R   min &lt;2 f  and
 
preferably, this difference satisfies:
 
0.3 f&lt;R   max   −R   min   &lt;f. 
 
     It should be noted that the surface of revolution E is defined solely in the zones of the outer wall of the crown intended to come into contact with the internal wall of the crown of the tire when running flat. That is to say that account is taken only of the parts of the wall which actually come into contact with the internal wall of the crown of the tire when running flat. In particular, account is not taken of the radius of the bottom of the grooves disposed along the crown. These groove bottoms do not come into contact with the internal wall of the crown of the tire when running flat. 
     This surface of revolution E may be determined in practice by geometric measurements of the support mounted on its operating rim. E is also very close to the theoretical profile E′ of the envelope of the outer wall of the crown of the support. In the following, the surface of revolution E will be known as the envelope of the outer wall of the crown of the support. The differences between the two surfaces of revolution E and E′ are associated in particular with the many uncertainties of support manufacture, contraction of the materials after molding, cross-linking or vulcanization thereof, untrue roundness of the operating rim etc. 
     It is advantageous to arrange the zone of minimum radius of the surface E, the envelope of the outer wall of the crown, axially at at least one lateral end of the crown of the support. 
     The zone of minimum radius R min  is preferably disposed on the side of the support intended to be positioned on the outside of the vehicle. 
     The envelope E of the outer wall of the crown of a safety support according to the invention may also comprise a second zone of radius R′ min , greater than or equal to R min , disposed axially on the other side of the circumferential median plane P from the zone of radius R min  and satisfying the relationship:
 
0.2 f&lt;R   max   −R′   min &lt;2 f 
 
and preferably
 
0.3 f&lt;R   max   −R′   min   &lt;f. 
 
     The presence of these zones of smaller radius disposed at least at one lateral end of the crown of the support makes it possible to limit damage to the internal wall of the tire as well as to the crown of the support when running flat and thus to increase significantly the endurance of the tire/support assembly under such flat-running conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A number of embodiments of safety supports according to the invention will now be described with reference to the attached drawings, in which: 
         FIG. 1  is a side view of a safety support; 
         FIG. 2  is a section AA as indicated in  FIG. 1  of an example of an annular body of a safety support according to the invention; 
         FIG. 3  is a view in axial section of an assembly made up of a safety support according to the invention mounted on a wheel rim and of a tire; 
         FIG. 4  is a schematic side view of a safety support mounted on a wheel rim, the assembly being subjected to a load Z; 
         FIG. 5  is an axial section of a second example of a safety support according to the invention; and 
         FIG. 6  is an axial section of a third example of a safety support according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a side view of a safety support  1  corresponding to those in U.S. Pat. No. 6,564,842. This support comprises three parts: a base  2 , of generally annular shape; a substantially annular crown  3 , having longitudinal grooves  5  (see  FIG. 3 ) on the radially outer wall thereof; and an annular body  4  connecting the base  2  and crown  3 . 
       FIG. 1  also specifies the geometric conventions used in the present application. The axis X passing through O is the axis of rotation of the support (axis X is perpendicular to the plane of  FIG. 1 ). After mounting the support in the cavity of a tire and around a rim, the axis X is also the common axis of rotation of the support, the tire and the rim. The direction T is a radial direction, that is to say passing through the axis X and perpendicular thereto. The direction C is a circumferential direction; at any point of the support, the tire or the rim, this circumferential direction is perpendicular to the radial direction passing through this point as well as to the axis X. 
       FIG. 2  is a section AA as indicated in  FIG. 1  of an example of an annular body of a safety support according to the invention. This example is described in U.S. Pat. No. 6,564,842. This annular body  4  comprises radial partitions  42  distributed over the circumference of the support and extending axially on either side of plane P (the circumferential median plane) and radial junctions  43  and  44  extending substantially circumferentially and connected at each of their ends to two adjacent partitions  42 . In the example shown, the junctions  43  and  44  have different circumferential lengths. The longer circumferential junctions  44  are preferably positioned on the side of the support designed to be disposed towards the inside of the vehicle. The junctions may be disposed along the lateral edge of the annular body (e.g.,  43 ) or set back therefrom (e.g.,  44 ). The geometry of this  FIG. 2  is shown only by way of example and a very large number of other geometries of the annular body may be used for the safety supports according to the invention. 
       FIG. 3  is a view in axial section of an assembly made up of a safety support according to the invention mounted on a wheel rim and of a tire. This Figure shows the “outer” (EXT) and “inner” (INT) sides of the assembly, that is to say those intended to be disposed towards the outside of the vehicle or towards the inside of the vehicle. As described above, the safety support  10  comprises a base  2 , an annular body  4  and a crown  30 . 
     The wheel rim  6  is described in particular in U.S. Pat. No. 6,470,936 (counterpart to WO 00/05083), which is incorporated herein by reference, and comprises two rim seats  61  and  61 ′, outer and inner respectively, of unequal diameters and whose generatrices are inclined towards the outside. The inner seat  61 ′ disposed on the inner side has a diameter greater than that of the outer seat  61 . The two seats are extended externally by protrusions or humps  62  and  62 ′. The outer seat  61  is extended axially towards the inside by a first bearing surface  63  followed by a circumferential channel  64 , and a second bearing surface  63 ′. The second bearing surface  63 ′ is provided at its end facing towards the inside of the vehicle with a positioning stop  65 . As the Figure indicates, the safety support  10  comes to bear radially on the two bearing surfaces  63  and  63 ′ and axially against the stop  65 . It should be noted that the circumferential junction element  44  is disposed axially opposite the bearing surface  63 ′ to ensure good transmission of forces between the support and the rim. Likewise, the junction element  43  is preferably disposed opposite the bearing surface  63 . The inner seat  61 ′ is extended axially towards the outside of the vehicle by a rim flange  66 , the flange defining with the positioning stop  65  a mounting channel  67 . The rim  6  also comprises a valve hole  68  disposed in the outer sidewall of the circumferential channel  64 . A valve and an inflation pressure measuring device  69  are fixed to this valve hole. 
     A tire  7  is mounted on the rim  6 . This tire  7  comprises in particular two beads  71  and  71 ′ surrounding the rim seats  61  and  61 ′ and a crown  72  whose internal wall  73  is designed to come to rest against the crown  30  of the support in the event of significant inflation pressure loss and flat running. 
     The support  10  is a support according to the invention. Its crown  30  comprises three parts, a central portion  31  whose outer envelope reaches the maximum radius R max , an outer lateral part  32  whose outer envelope has a radius R min  and an inner lateral part  33  whose outer envelope has a radius R′ min . The outer and inner lateral parts  32  and  33  respectively have substantially the geometry of a cylinder of revolution. The central portion  31  comprises axially from the inside towards the outside of the vehicle a substantially conical zone  310  whose radius varies from R′ min  to R max , a zone  311  whose outer envelope is a cylinder of revolution of the radius R max , and then a second transitional zone  312  forming a transition to the outer zone  32 . The zones  311  and  312  comprise circumferential grooves  5 . Such grooves  5  may also be disposed in the lateral zones  33  and  32 . 
     The crown  30  of the support  10  thus comprises at its two lateral ends a zone of a radius smaller than the maximum radius of the central portion. 
     The value of these differences between R max  and R min  and R′ min  is explained with reference to  FIG. 4 , which is a schematic representation of an assembly of a wheel (composed of a disc  9  and a rim  6 ) and safety support  10  coming to bear on a flat surface S. 
     The wheel/safety support assembly  10  is mounted on a chuck (not shown) with an axis X passing through O. In the unloaded state, the outer radius R of the assembly corresponds to the radius R max  of the central portion  31  of the crown  30  of the support  10 . On the other hand, at the center of the contact zone between the wheel/safety support assembly and the surface S, when a load Z is applied at O, the radius diminishes and becomes R 0 . The value of the difference between R and R 0  is known as the deflection f, the rated load of the tire, which is the maximum load which the tire can bear in operation, having been selected as the load Z. This value is defined by European Tyre and Rim Technical Organization (ETRTO) standards. 
     The value of f is easily determined experimentally by following the above mode of operation. This is done at 20° C. It is of course important to choose as the wheel and rim the safety support operating wheel and rim, that is to say, the wheel and rim for which the safety support has been designed. 
     The Applicants have surprisingly noted that selecting for the values R max , R min  and R′ min  values which satisfy the following relationship:
 
0.2 f&lt;R   max   −R   min &lt;2 f  and preferably
 
0.3 f&lt;R   max   −R   min   &lt;f 
 
significantly improves endurance performance during flat running for a given quantity of lubricant introduced into the inner cavity  8  (see  FIG. 3 ) of the tire.
 
     It should be noted that the main constituent material of the safety supports may vary widely. The mechanical properties of these supports and, in particular, the deflection f under a given load and for a given rim geometry will also vary widely. By way of example, the modulus of elasticity of a rubber mix may vary between 8 and 40 MPa, whereas that of a polyurethane elastomer or a thermoplastic elastomer may vary between 20 and 150 MPa. We shall take as an example the modulus of extension at 10% deformation and 20° C. 
     Tests have been performed with a 120×440−40 safety support of rubber material, where 120 corresponds to the width in mm of the support, 440 to its diameter in mm and 40 to its height, also in mm. 
     It has been noted that the value of the deflection undergone by the support and operating rim assembly under the rated load Z=450 kg is 6 mm. The difference between the values R max  and R min  or R′ min  was zero for the control (crown geometry substantially that of a cylinder of revolution) and 2 mm for the support according to the invention. 
     The support/rim/tire assemblies were tested at 100 km/h, after the introduction of 90 grams of a lubricant composed mainly of glycerol and silica. The test vehicle was a Renault Scenic 2 and the tire/wheel/support assembly tested during flat running was disposed at the rear of the vehicle. The test circuit was a circuit of the motorway type. 
     The control assembly traveled for a distance of 70 km running flat, that is to say with a relative inflation pressure of zero between the cavity of the tire and the ambient air, before damage occurred to the internal wall  73  of the tire in particular opposite the outer end of the crown of the support. Stoppage was caused by abrasion between these two opposing surfaces. This abrasion may be deemed to be due to a lack of lubrication between these surfaces. 
     The assembly comprising a support  10  according to the invention traveled a distance of 180 km before stopping. This result shows the very clear benefit in modifying the external geometry of the crown of the supports in order to optimize the flat-running endurance performance of the safety support/wheel/tire assemblies. 
       FIG. 5  shows a second example of a safety support. The crown  35  comprises an outer wall curved axially such that the outer radius of the support is R max  at the center of the crown and R min  at the two lateral ends. As above, the order of magnitude of the deflection undergone by this support under flat-running operating conditions on its operating rim is of the order of 6 mm and the variation in radius is of the order of 2 mm. 
       FIG. 6  shows a third example of a safety support. The crown  37  comprises an outer wall with, axially from the outside towards the inside, a first zone  373  in the form of a cylinder of revolution, of the radius R min , a conical zone  372 , then a zone  371  curved substantially constantly such that at the inner end of the crown there is a radius R′ min  very close to the radius R min . 
     In the Figures showing the three illustrated examples of safety supports according to the invention, the surfaces E and E′ have been merged. 
     The Applicants have also noted that, when the variation in radius is less than 0.2 f, the consequences with regard to performance were not significant given the variation inherent in the manufacture of supports and in the endurance tests. On the other hand, when this variation in radius is increased beyond 2f, the opposite effect from that desired is observed. 
     The three crown geometries described have been described only by way of example and numerous other possibilities are feasible without going beyond the scope of this invention.