Patent Publication Number: US-2009218023-A1

Title: Method for Controlling the Pressure in a Wheel for Vehicles and Wheel Having Controlled Pressure According to Said Method

Description:
The present invention relates to a method of controlling the pressure in a vehicle wheel. The invention also relates to a wheel having a controlled and compensated pressure, to be used for putting said method into practice. 
     A wheel for two-wheeled or four-wheeled vehicles generally comprises a rim coupled with a tyre that is inflated to a given operating pressure. 
     Said tyre generally comprises a carcass structure having at least one carcass ply, and at least one annular reinforcing structure associated with the carcass ply, a tread band of elastomer material at a radially external position to the carcass structure, a belt structure interposed between the carcass structure and tread band and a pair of sidewalls at axially opposite positions on the carcass structure. 
     In tubeless tyres, the tyre airtightness is ensured by the radially inner layer of said carcass structure generally called “liner”. When in use, due to the natural air loss through said radially inner layer (which layer at all events is never perfectly impervious to air), pressure within the tyre decreases thereby obliging the vehicle driver to carry out periodical restoration of same. 
     In an attempt to make the tyre pressure substantially constant over a rather long period of time, it was suggested use of rims internally housing a tank for gas under pressure at a higher pressure than the operating pressure of the tyre. By one or more valves suitably operated, pressure is restored when required. 
     U.S. Pat. No. 6,601,625 B2 discloses a wheel with a compressed air tank integrated into the rim. More specifically it is disclosed a high pressure tank to store compressed air from an outer source, a first mechanical valve allowing the compressed air to flow from a source external to the high pressure tank, a second mechanical valve allowing air passage from the high pressure tank to the inner tube of the tyre, a third valve releasing air from the inner tube of the tyre, and a fourth valve releasing air from the high pressure tank. The wheel described in said patent keeps the tyre pressure within a predetermined value in a mechanical manner, reducing the necessity for the vehicle driver to manually inflate the tyre to reach the desired pressure. When pressure within the tyre decreases under a predetermined threshold value, the air stored in the high pressure tank is released into the tyre maintaining the latter inflated to the desired minimum pressure; on the contrary, when pressure in the tyre increases beyond a predetermined threshold value, the air is released from the tyre to the surrounding atmosphere. 
     U.S. Pat. No. 4,067,376 depicts a system for automatic re-admission of the air lost from a tyre while the vehicle is running in order to minimise the effects of an explosion. The wheel is formed with an integrated annular bag adapted to store an amount of compressed air under high pressure. A pressure safety valve is placed between said bag and the tyre and is adapted to release air from the bag to the tyre each time pressure within the tyre decreases below a predetermined limit. 
     WO 2005/084967 in the name of the same Applicant proposes insertion of a valve between a tank of fluid under pressure associated with the rim of a wheel and the tyre mounted on said rim, which valve is designed to compensate for pressure variations with respect to variations in the room temperature by an elastic element having a variable elastic constant in response to temperature variations, so as to keep the valve to a closed position when the inner pressure of the tyre decreases due to a temperature reduction. In this way, restoration of the operating pressure in the tyre is carried out only when the pressure has gone down as a result of true air losses (micropunctures, lack of airtightness of the liner, etc.) and not for reasons connected with lowering of the room temperature. Thus pressure in the tyre is maintained constant over long periods of time, due to the fact that the operating duration of the tank of fluid under pressure is increased, the other conditions being the same. 
     With reference to the known devices described above, the Applicant has however perceived that in the absence of pressure in the tank, i.e. should pressure within the tank be the same as the atmospheric pressure, opening of the valve for restoration of the operating pressure would give rise, instead of the desired flowing of fluid from the tank to the tyre, to a sudden transfer of the fluid still present in the tyre itself towards the tank, which will result in an immediate deflation of the tyre. 
     The Applicant has found that this circumstance can be a source of danger, should a pressure loss accidentally occur in the tank, for example following breakage of same. In fact the tyre could suddenly become deflated, even while running to a high speed. 
     The Applicant has ascertained that by providing between the tank of the fluid under pressure and the tyre, a safety valve which is able to intercept the fluid outflow towards the tank without inhibiting passage of fluid from the tank itself to the tyre, a sudden deflation of the tyre is avoided in case of fluid losses in the tank, the functional operation of the tank itself being maintained for purposes of maintenance of the operating pressure in the tyre. 
     More particularly, in a first aspect, the invention relates to a method of controlling pressure in a vehicle wheel, in which said wheel comprises a tyre mounted on a rim, said method comprising the steps of: inflating an inner volume of the tyre to an operating pressure; admitting a fluid compressed to a first pressure higher that the operating pressure of the tyre, into a tank associated with the rim; opening a fluid communication, by at least one primary mechanical valve, between the inner volume of said tyre and said tank when pressure of the inner volume of said tyre is lower than said operating pressure; stopping the communication between said inner volume and tank when said tyre pressure is substantially equal to said operating pressure; stopping the communication between said inner volume and tank when during said opening step by means of the primary valve, the pressure value within the tank is lower than the pressure value in the tyre. 
     Thus there is a guarantee that in case of breakage of the tank, or also in the absence of pressure for any other reason, opening of the valve adapted to cause flowing of fluid to the tyre does not cause flowing back of the fluid itself towards the tank, which would bring about deflation of the tyre. 
     Stopping of the fluid communication in case of weak pressure in the tank is preferably carried out by a mechanical safety valve, preferably a one-way on-off valve. Advantageously, the safety valve operates independently of the primary valve, thus avoiding undesirable structural complications in the primary valve itself and ensuring a satisfactory accuracy of intervention of said primary valve for restoration of the operating pressure of the tyre during normal running. 
     Opening of the fluid communication between the inner volume of the tyre and the tank can be advantageously carried out by at least one primary mechanical valve opening of which is controlled by an elastic element with an elastic constant varying in a temperature range from −50° C. to +50° C. so as to maintain said primary valve to a closed position following a reduction in the inner tyre pressure due to a temperature reduction within said range. 
     In a preferred embodiment of said method, said elastic element controlling opening of said primary valve has a value of elastic constant measured at −50° C. (K −50° C. ) differing from the value of elastic constant measured at +50° C. (K +50° C. ) by at least 10% with respect to the value of elastic constant measured at +50° C. (K +50° C. ). 
     In a different embodiment, said elastic element controlling opening of said primary valve has a value of elastic constant measured at −50° C. (K −50° C. ) differing from the value of elastic constant measured at +50° C. (K +50° C. ) by no more than 40% with respect to the value of elastic constant measured at +50° C. (K +50° C. ). 
     Advantageously, in order to obtain wide time gaps between two manual reloading operations in succession, the ratio between said operating pressure of the tyre and said first pressure in said tank is included between about 0.1 and about 0.6. 
     By optimising the available volumes, in a further embodiment, the ratio between said operating pressure of the tyre and said first pressure in said tank is included between about 0.2 and about 0.4. 
     Advantageously, said method allows widely spread reloading devices to be used, due to the fact that said first pressure in said tank is included between about 8 and about 12 bars. 
     In a different preferred embodiment said first pressure in said tank is included between about 8.5 and about 10 bars. 
     To enable a precise maintenance of the operating pressure within the tyre, also in the presence of pressure variations within the tank also resulting from progressive consumption of the fluid stored therein, opening of the fluid communication between tyre and tank is advantageously carried out in response to a pressure gradient reduction between the tyre pressure and the ambient pressure. 
     To improve the steadiness of the wheel system, said step of bringing the inner volume of said tyre into communication with said tank takes place when the pressure of the inner volume of said tyre is lower than said operating pressure by at least 5%. 
     In a further aspect, the invention relates to a wheel having a controlled and compensated pressure, comprising: a rim associated with a tank adapted to be filled with a fluid to a first pressure; a tyre mounted on said rim and having an inner volume inflatable to an operating pressure, said operating pressure being lower than said first pressure; at least one primary mechanical valve adapted to regulate a communication between said tank and the inner volume of said tyre, to bring said tank into communication with said tyre, when the pressure of said tyre is lower than said operating pressure; at least one safety valve operatively interposed between the inner volume of the tyre and the tank to stop the communication between said inner volume and tank when the pressure value within the tank is lower than the pressure value of the tyre. 
     In more detail, said safety valve can be a mechanical valve, preferably a one-way on-off valve. 
     The safety valve preferably comprises a second closure member to be made in the form of an elastic diaphragm or a ball acting against an abutment seat defined by a perimetral edge of a communication opening between the tank and the inner tyre volume. 
     A second auxiliary elastic element can be advantageously associated with the safety valve to push said closure member towards the perimetral edge of said communication opening, so as to maintain the safety valve in a closed condition. 
     Said safety valve can be advantageously integrated into a valve body of said primary valve so as to be integrated with the latter in a compact assembly of reduced bulkiness capable of facilitating housing of same in the wheel rim. 
     In a preferred embodiment of the invention, the primary valve comprises at least one elastic element operatively associated with a closure member designed to open and close a port of said primary valve. Said elastic element preferably has an elastic constant (K) varying in a temperature range from −50° C. to +50° C. for example, so as to maintain the primary valve to a closed position following a reduction in the inner tyre pressure due to a temperature reduction within said range. 
     In a further embodiment, to divide the available volumes in an optimal manner, said tank provides for such a volume that the ratio of said volume of said tank to said inner volume of the tyre is included between about 0.1 and about 0.4. 
     In a different embodiment, said ratio is included between about 0.12 and about 0.25. 
     In a preferred embodiment, said elastic element comprises at least one spring. 
     In another preferred embodiment, said elastic element comprises a second spring operatively associated with said one spring. 
     In a different embodiment, said second spring has an elastic constant (K) that is substantially constant within a temperature range from −50° C. to +50° C. 
     Thus more accuracy can be obtained in the possibility of modulating intervention of said primary valve in response to temperature variations. Modulation of the intervention, in fact, can specifically rely on the spring, in which materials and expedients are capable of favouring a variation of the elastic constant against the temperature, while the thrust function relies on the second spring with an elastic constant that is substantially insensitive to temperature variations. 
     In a preferred embodiment, said second spring supports a major portion of the load of said elastic element. 
     Preferably, the load supported by the second spring is included between about 60% and about 95% of the load supported by said elastic element. 
     More preferably, the load supported by the second spring is included between about 70% and about 80% of the load supported by said elastic element. 
     In another embodiment the second spring is concentrically coupled with said one spring. 
     In a preferred embodiment the second spring is external with respect to said one spring. 
     To enable an embodiment in which a thrust spring is provided, said elastic constant (K) decreases on decreasing of the temperature in said temperature range. 
     To obtain an embodiment in which a pulling spring is provided, said elastic constant (K) increases on decreasing of the temperature in said temperature range. 
     To enable restoration of the inflating pressure in the tyre to take place in a correct manner without being affected by pressure variations in the tank, the primary closure member preferably comprises a diaphragm having a first surface exposed to the pressure of the inner tyre volume, and a second surface exposed to the ambient pressure, or other reference pressure in an environment hermetically insulated from the tank. 
     More particularly, the primary closure member is movable relative to the primary-valve port in response to a reduction in a pressure gradient between the tyre pressure and the reference pressure. 
     In a preferred embodiment, said wheel comprises an inflating valve operatively associated with said tank. 
     In a different embodiment, said wheel comprises a control and restoration valve associated with said tyre. 
     In a different aspect, the invention also relates to a valve assembly adapted to regulate a communication between a tank and an inner volume of a tyre to bring said tank into communication with said tyre when pressure in said tyre is lower than an operating pressure, said mechanical valve assembly comprising a primary mechanical valve associated with at least one safety valve, wherein said safety valve is operatively interposed between a duct adapted to be connected to the inner volume of said tyre and a communication opening to be connected to the tank, to stop communication between said duct and said communication opening should a pressure value within the duct be lower than a pressure value in the communication opening. 
     Further features and advantages of the invention will become more apparent from the detailed description of some preferred but not exclusive embodiments of a wheel having a controlled and compensated pressure in accordance with the present invention. 
    
    
     
       This description will be set out hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which: 
         FIG. 1  is a vertical view of a wheel in accordance with the invention; 
         FIG. 2  is a side view partly in section of the wheel shown in  FIG. 1 ; 
         FIG. 3  shows an enlarged portion of said side view; 
         FIG. 4  is an enlarged section of a detail of the wheel in accordance with the invention; 
         FIG. 5  is a partial vertical view of a second embodiment of the wheel in accordance with the invention; 
         FIG. 6  is a side view partly in section of the wheel shown in  FIG. 5 ; 
         FIG. 7  shows an enlarged portion of the side view shown in  FIG. 6 ; 
         FIG. 8  is a graph showing the variation of an elastic constant of an element in said wheel upon varying of the temperature. 
         FIG. 9  schematically shows a detail of a preferred embodiment in accordance with the invention. 
     
    
    
     As shown in  FIGS. 1 ,  2 ,  5  and  6 , wheel  1  for two-wheeled vehicles ( FIGS. 1 ,  2 ) or four-wheeled vehicles ( FIGS. 5 ,  6 ) in accordance with the invention, comprises a rim  2  on which a tyre  3  of an inner volume  3 ′ is mounted. Provided in rim  2  is a tank  4  associated with said rim and preferably integrated into the latter, said tank being suitable to contain a fluid under pressure, said fluid being air or a substantially inert gas such as nitrogen, for example. 
     In accordance with a preferred embodiment, the ratio between the operating pressure of tyre  3  and a first pressure existing in said tank  4  when fully loaded varies between about 0.1 and about 0.6, preferably between about 0.2 and about 0.4. 
     According to a further preferred embodiment, the ratio between the volume of said tank  4  and said inner volume  3 ′ of the tyre is included between about 0.1 and about 0.4, preferably between about 0.12 and about 0.25. 
     The rim  2  preferably houses a primary mechanical valve  5  at a radially internal position not far from the rotation centre of the wheel, which valve allows communication between tank  4  and the inner volume  3 ′ of tyre  3  to be regulated. 
     Preferably, said communication is obtained by providing, within rim  2 , a duct  6  connecting said primary valve  5  with the inner volume of said tyre  3 , said primary valve  5  further providing a connection with said tank  4  either directly through a communication opening  25  and/or or through a further duct  6 ′ ( FIG. 7 ). 
     Said primary valve  5  preferably comprises a valve body  7  housed in a suitable seat  8  formed in said rim  2 , which valve body has a first port  9  for connection with said tank  4  and a second port  10  for connection with said tyre  3  and therefore preferably connected with said duct  6 . 
     As shown in  FIGS. 2 ,  3 ,  6  and  7 , said valve body  7  is provided, internally of the axially external end, i.e. preferably at the opposite end of said first port  9 , with a base disc  11  on which an elastic element rests, which elastic element preferably comprises at least one spring  12  housed in a space  11   a.    
     Advantageously, said spring  12  is made of a material preferably selected from the so-called “shape memory alloy” (SMA) materials, in such a manner that its elastic constant K greatly depends on temperature. 
     For example, as shown in the graph in  FIG. 8 , it is possible to see that such dependence, in a graph Temperature (x axis)/Value of the elastic constant K (y axis) is substantially expressed by a straight line parallel to the x axis (chain line) for springs made of standard spring steel materials (i.e. the elastic constant is in this case substantially independent of temperature) within a predetermined temperature range, between −50° C. and +50° C. for example, which range, as better clarified in the following, can be coincident with the preferred temperature of use of wheel  1 . Said dependence within said range is on the contrary expressed by an increasing or decreasing function for the springs  12  in accordance with the invention made of the above specified materials. 
     Preferably, in accordance with the invention, provision is made for use of materials having a temperature range in which the elastic constant K of the springs made with use of said materials greatly varies between about −50° C. and about +50° C., said range being preferably included between about −30° C. and about +50° C., and more preferably included between about −30° C. and about +20° C. 
     In particular, in the last-mentioned temperature range (−30° C./+20° C.) the value of this constant K varies by approximately 26% with respect to the value found at the upper end of the range (+20° C.) for a spring made of a nickel-titanium steel (diameter of the wire 1.2 mm,  2  useful coils), more specifically from about 5,500 N/m (at 20° C.) to about 4,060 N/m (at −30° C.). 
     The employed materials are in any case selected in such a manner that said variation is included between about 10% and about 40%, preferably between about 20% and about 30% in a predetermined temperature range, at least included between −50° C. and 50° C. or narrower. 
     More specifically, spring  12  controlling opening of the primary valve  5  has a value of the elastic constant measured at the lower end of said range (at −50° C. (K −50° C. ) for example) differing from the value of the elastic constant measured at the upper end of said range (at +50° C. (K +50° C. ) for example) by at least 10% and preferably by no more than 40%, with respect to the value of the elastic constant measured at the upper end of said range (at +50° C. (K +50° C. ) for example), that is to say: 
     
       
         
           
             
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     According to said preferred solution, such dependence on temperature of the elastic constant is represented by a function increasing in said predetermined temperature range ( FIG. 8 ). 
     Finally, in the same  FIG. 8  it is possible to see that a spring made of a traditional spring steel, a UNI steel Class C for example, has a substantially constant value of the elastic constant K in the same temperature range (−30° C./+20° C.), said value being substantially equal to about 14,000 N/m at +20° C. and equal to about 14,200 N/m at −30° C., from which a variation ΔK equal to about 1.43% is drawn (diameter 1.2 mm, 3.5 useful coils). 
     In a preferred embodiment, as shown in  FIG. 4 , said elastic element  12 ,  12   a  comprises a second spring  12   a  operatively associated with spring  12 . In particular, the second spring  12   a  is a spring made of traditional spring steel (e.g. UNI steel Class C), the spring  12  being made of shape memory alloy (SMA) materials as above explained. Therefore, the elastic constant K of the second spring  12   a  is substantially constant in a temperature range included between −50° C. and +50° C. 
     Preferably, springs  12  and  12   a  are concentrically coupled, so that the second spring  12   a  is external with respect to spring  12 . Preferably, the load supported by said elastic element is divided between springs  12  and  12   a  in such a manner that the second spring  12   a  supports the major portion of said load. Consequently, valve  5  increases its sensitivity to temperature variation, because the spring  12  supports only a small load. Within this preferred embodiment and considering a Force/Elongation diagram, the load supported by spring  12  lies in a region of the diagram in which the curve is not asymptotic, i.e. a region in which the ratio between force and elongation is substantially linear. In a preferred embodiment, the load supported by said second spring  12   a  is included between about 60% and about 95% of the load supported by said elastic element, and more preferably between about 70% and about 80%. 
     As shown in  FIGS. 2 ,  3 ,  4 ,  6  and  7 , spring  12  is advantageously associated with a diaphragm  13  delimiting, by its axial position, the volume of a bag  21  communicating with said duct  6  and with the axially internal end of said valve body  7 , i.e. the portion thereof close to the first port  9 . 
     In more detail, the diaphragm  13  has a first surface  13   a  facing the bag  21  which is exposed to pressure of duct  6  and of the inner volume  3 ′ of tyre  3 , and a second surface  13   b  which is exposed to a reference pressure present in the space  11   a  housing the springs  12 ,  12   a  and confined between the diaphragm itself and the base disc  11 , which is hermetically insulated relative to tank  4 . The space  11   a  can communicate with the external environment, in which case the reference pressure will correspond to the ambient pressure present externally of the wheel  1 . Said diaphragm  13  is connected to a cap  14  extended in an axial direction, the other end of which can come into contact with a needle  15  housed in a bush  16  and terminating with a primary closure member  17  having an enlarged head, intervention of which allows passage of fluid through said first port  9  or not. Needle  15  is further maintained in place by another elastic element, such as a closing spring  18  for example. 
     To prevent any undesirable flowing back of fluid from tyre  3  to tank  4 , at least one safety valve  22 , preferably of the mechanical type, is advantageously provided, said valve being operatively interposed between the inner volume  3 ′ of the tyre and the tank itself. 
     The safety valve  22  preferably consists of a one-way on-off valve, advantageously integrated into the same valve body  7  housing the primary valve. In more detail, the safety valve  22  is preferably housed in one end of bush  16  axially facing tank  4 , in which provision is made for a cavity  23  containing the primary closure member  17  belonging to the primary valve  5 . 
     The safety valve  22  comprises a second closure member  24  housed in the holding cavity  23  as well and acting against an abutment seat defined by a perimetral edge  25   a  of a communication opening  25  connecting tank  4  to the inner volume  3 ′ of tyre  3  through the holding cavity  23  and port  9 . 
     The second closure member  24  may comprise an elastic diaphragm acting in an abutment relationship against the perimetral edge  25   a , as shown in  FIG. 4 . In a further preferred embodiment shown in  FIG. 9 , the second closure member  24  comprises a ball acting in abutment relationship with the perimetral edge  25   a  of the communication opening  25 . A further spring or other auxiliary elastic element  26  can be housed in the holding cavity  23  to urge the second closure member  24  towards the perimetral edge  25   a  to close the communication opening  25 . 
     Preferably, present on rim  2  is an inflation valve  19  directly in communication with tank  4 , whereas in a further preferred embodiment a control and restoration valve  20  is provided that is in communication with the inner volume  3 ′ of tyre  3 . 
     Pressure control and compensation within said wheel  1  take place as follows. 
     First, by a standard compressor for example, air is admitted into tank  4 , preferably through the inflation valve  19  to a given room temperature, of 15, 20, 25° C. or other value for example, said temperature being here and in the following identified as reference temperature TR. 
     Initially tyre  3  is deflated, so that spring  12 , preloaded to a given reference value in relation to the desired operating pressure within the tyre (that can generally vary from about 1.7 to about 5.5 bars, depending on the different tyre types), exerts pressure on the diaphragm  13  bringing cap  14  to act against needle  15 , which action leads the closure member  17  to open the passage through port  9  thereby connecting tank  4  to bag  21  and therefrom to duct  6  and tyre  3 . 
     When pressure within tyre  3  reaches the prescribed operating pressure, this pressure also exerts its action on the first surface  13   a  of diaphragm  13 . The gradient between the inner pressure of tyre  3  acting against the first surface  13   a  of the diaphragm  13 , and the ambient pressure or other reference pressure present in the space  11   a  and acting against the second surface  13   b  of the diaphragm itself, overcomes the spring  12  preload and causes separation of cap  14  from needle  15 . The positioning spring  18  brings needle  15  back to the rest position dragging along the primary closure member  17  therewith to a closed position, thus inhibiting passage of fluid under pressure from tank  4  to bag  21 . Then tank  4  is loaded to its rated capacity, generally included between 8 and 12 bars, more preferably between 8.5 and 10 bars. 
     During operation of the vehicle on which wheels  1  in accordance with the invention are mounted, small air losses occur due either to an imperfect airtightness of the radially inner layer of the tyre carcass structure, or to an imperfect adhesion between the tyre bead and the rim flange on which the bead bears, said pressure losses being quantifiable to about 0.1 bars/month. By reducing pressure in the inner volume  3 ′ of tyre  3 , i.e. the pressure gradient between the inner tyre pressure and the ambient pressure or other reference value present in the space  11   a , this reduction is transmitted to the bag  21  through duct  6 . Consequently spring  12  acts against diaphragm  13  and moves the primary closure member  17  as above described to the open position, until pressure in the tyre, through bag  21  and therefore diaphragm  13 , balances the force exerted by the calibration preload of spring  12 . 
     Advantageously, the primary valve  5  is calibrated so that it begins operating only after the reduction in pressure within the inner volume  3 ′ has reached at least 5% of the operating pressure; in other words, taking into account what previously stated, such a reduction must preferably be included between about 0.085 and about 0.275 bars. In this way steadiness to the wheel assembly is ensured, small reloading operations being avoided when minimum pressure losses occur. This is due to the fact that, when the primary valve  5  is closed, pressure in tank  4  acts on the respective port  9  over the whole contact area of the closure member  17 , while when the valve is open the action area of the same pressure corresponds to the only cross section of needle  15 . 
     When the vehicle is not running and the external temperature decreases, due to the known gas laws the pressure within the tyre starts decreasing as well, of about 0.1 bar on an average every 10° C. of reduction with respect to the reference temperature TR. However, the elastic constant K of spring  12  advantageously depends on the temperature in the terms previously illustrated (in the example shown in  FIG. 8  relating to a Ni/Ti steel, K decreases of about 5.24% every −10° C.), so that with a temperature reduction the constant too decreases, causing a reduction in the calibration preload as well. In this way the pressure reduction that is transferred to bag  21  from the inside of tyre  3  does not activate spring  12  because the spring preload is substantially decreased to such a value that it keeps balanced in spite of the reduced tyre pressure. 
     In this manner wheel  1  is not submitted to useless loading cycles due to possible high thermal ranges of the room temperature that would bring to a quick consumption of the fluid stored inside tank  4 , which fluid would then be discharged when the tyre temperature approaches the reference temperature TR again, by means of valve  20 , for example. Said valve  20  in a preferred embodiment is designed to avoid sudden overpressures, in case of failure of the primary valve  5  for example, and when it is necessary to control pressure within the inner volume  3 ′ of the tyre, also allowing the tyre inflation. 
     It is to be noted that the provided range in accordance with the invention within which said elastic constant varies substantially comprises the room temperature of normal operation of the tyre. This means that the concerned wheel  1  when it has to operate to such temperatures, has a temperature-compensated pressure control because the primary valve  5  does not start operation if the pressure reduction is only due to variations in the room temperature. 
     Also to higher temperatures with respect to the upper limit of said range an important variation in said elastic constant K does not occur but this fact is irrelevant as regards the correct operation of wheel  1 . In fact, when it is hot (at temperatures higher than TR) the tyre is self-balancing, i.e. the higher pressure due to the high temperatures is used to support it under those operating conditions. 
     It is further to be noticed that still when it is hot, the higher pressure existing within tyre  3 , being transferred into bag  21 , further compresses spring  12 , which spring avoids every reloading by increasingly moving away from cap  14 . 
     In case of pressure loss from tank  4 , following break or failure of the inflation valve  19 , the primary valve  5  keeps in a closed position until a pressure reduction within tyre  3  occurs such as to cause displacement of the closure member  17  upon command of spring  12 . In the absence of the safety valve  22  the consequent opening of port  9  for the purpose of achieving the desired restoration of the inner pressure of tyre  3 , would on the contrary cause escape of the whole fluid present in the tyre itself towards tank  4 , bringing about a consequent sudden deflation of same. 
     Advantageously, the safety valve  22  inhibits occurrence of the drawback described above. In fact, when the primary valve  5  opens in the absence of pressure or in the presence of an insufficient pressure within tank  4 , the second closure member  24  is pushed against the corresponding abutment seat  25  by the pressure itself that is present in tyre  3 , any flowing back of the fluid from the tyre to tank  4  being eliminated. Thus a safe running is ensured also in the absence of pressure in tank  4 . 
     Finally it will be recognised that the inner arrangement of the elements of the primary valve  5  can be easily modified so as to have a closure member that is opened by effect of a pulling action of spring  12  and not of a thrusting action as previously illustrated. In this case the elastic constant K of the elastic element must increase on decreasing of the temperature in the previously mentioned temperature ranges, in order to obtain the same operation of said primary valve  5 .