Studded tyre The present invention relates to a tyre.
The invention is particularly well-suited to passenger vehicles and utility — vehicles.
Studded tyres have undeniable advantages in terms of the way in which they behave under winter driving conditions, such as, for example, driving on icy road surfaces.
Contact with ice, and more specifically the penetration of the said stud into the ice, makes it possible to compensate for the reduction in grip observed at the tread pattern elements of the tyre tread.
Specifically, the studs bite into the ice and are able to generate additional force on the ice.
For more details on studded tyres of this type, reference may be made for example to patent applications FR 2 131 913, EP 0 813 981, GB 1 546 780 and DE 23 04 036 which relate to studded tyres for heavy goods vehicles.
One of the difficulties in using such studded tyres is that these tyres, when used on a road that is not covered in ice or snow, damage the state of the road surface and lead to premature wearing of the roadway.
This is why a certain number of countries have banned the use of studded tyres or have limited their use to certain types of vehicles and/or limited winter
— periods.
Now, increasing the effectiveness with which a studded tyre grips on ice is generally characterized either by a greater abrasiveness of each stud with respect to the roadway, for the same number of studs, or by an increase in the number of studs, with the abrasiveness of each stud remaining constant.
This generally leads to an increase in the detrimental effect that the stud- ded tyre has on a roadway that is not covered in ice or snow.
The present invention seeks to provide a studded tyre that exhibits excel- lent grip on ice while at the same time having a reduced impact on a roadway that is not covered in ice or snow.
The invention relates to a tyre comprising a tread having a tread surface, and a plurality of studs anchored in the tread and projecting out from the tread surface.
According to one general feature, the mean surface density of studs on the tread surface is at least equal to 6.7 studs per square decimetre (dm?). According to another general feature, the maximum cross section Smax of each stud, considered perpendicular to the axis of elongation of said stud, of the plurality of studs is at most equal to 35 square millimetres (mm?).
A “tyre” means all types of resilient tread, whether or not it is subjected to an internal pressure.
A “tread surface” of a tyre means that surface of the tread that comes into contact with the roadway when this tyre is running, inflated to its service pressure,
and considered without the studs.
The area of the “tread surface” is calculated from the width and diameter of the tread of the tyre in the unconstrained state, namely not mounted on its rim.
What is meant here by “mean surface density” is the ratio between the total number of studs and the area of the tread surface of the tyre expressed in dm”.
In other words, the studs are distributed in the tread with a mean density of 6.7 studs to 1 dm? of tread surface area.
What is meant by “maximum cross section” is the maximum cross section of the stud considered perpendicular to the axis of elongation of the stud.
In the case of a cylindrical stud, this maximum cross section is defined by the diameter of the said stud.
Tests carried out by the Applicant Company have made it possible to demonstrate that the particular surface density of studs, combined with such a max- imum cross section makes it possible to increase the grip performance of the stud- ded tyre on an icy roadway while at the same time limiting its abrasive action on a
— dry roadway.
The increase in the mean surface density of studs with respect to conven- tional studded tyres makes it possible to spread the load over a greater number of studs in the contact patch in which the tread is in contact with the roadway.
The grip of the tyre on icy roadways is improved.
Furthermore, stud wear is limited.
Furthermore, the maximum cross section of the studs which has been de- termined by the Applicant Company makes it possible to obtain a good compromise between improved tyre grip on ice, and the limited abrasiveness of the studs on the roadway. According to one optional feature, the mean linear density of studs on the tread surface is at least equal to 115 studs per metre. What is meant here by “mean linear density” is the ratio between the total number of studs and the circumference of the tread surface of the tyre expressed in metres. In other words, the studs are distributed in the tread with a mean density of 115 studs to 1 metre of tread circum- ference. According to another optional feature, the projecting height Hs of each stud of the plurality of studs is at most equal to 1.6 millimetres (mm), and prefera- bly comprised between 0.8 mm and 1.2 mm. This makes it possible to further limit the abrasiveness of the studs on the roadway. The projecting height Hs of each stud of the plurality of studs may be at most equal to 20% of the total height Hc of the said stud. What is meant by the “projecting height” of a stud is the radial distance between the radially outermost point of the stud and the portion of tread surface surrounding this stud, for example out to a distance of 1 centimetre with respect to the axis of the stud. A “radial” direction is a direction corresponding to a radius of the tyre. The radial direction is therefore a direction which is perpendicular to the axis of rotation of the tyre. The radially outermost point of the stud is thus the point of this stud that is furthest away from the axis of rotation of the tyre. According to an advantageous optional feature, the static striking force of each stud of the plurality of studs is comprised in a range from 120 to 170 Newtons
(N). In one preferred embodiment, the static striking force of each of the studs of — the tyre is comprised in the range from 120 to 170 N. Alternatively, it is neverthe- less possible to make provision for just some of the studs each to have a static striking force comprised within this range. What is meant by the “static striking force” of a stud is the vertical load applied by this stud when the tyre is compressed onto a flat roadway under an in- ternal inflation pressure equal to 2 bar and under a load corresponding to 70% of the maximum load capacity of the tyre. This maximum load capacity is usually indicated by a load index inscribed on at least one of the sidewalls of the tyre.
Such a striking force further encourages a good compromise between im- proved grip of the tyre on ice and the limited abrasiveness of the studs on the road-
way. The total height of each stud of the plurality of studs may be comprised between 8 mm and 11 mm, and preferably equal to 10 mm. One stud of the plurality of studs generally comprises a jacket anchored in the tread and a stud pin intended to come into contact with the roadway. The jacket and the stud pin may be made from different materials. For preference, the stud pin is made from tungsten carbide and the jacket is made from metal alloy, preferably steel. Alternatively, the jacket and the stud pin may be made from the same material. The studs of the plurality of studs may comprise a base anchored in the tread and connected to the jacket, the maximum cross section Smax correspond- ing to the maximum cross section of the said base. The maximum cross section of the pin part of the said stud may advanta- — geously be comprised between 3 mm? and 3.5 mm”. The cross section is considered perpendicular to the axis of elongation of the stud. This makes it possible to limit still further the abrasiveness of the studs on the roadway. The mass of each stud of the plurality of studs may be comprised between
0.7 g and 12 g. For preference, the surface void ratio of the tread of the said tyre when new is comprised between 30% and 50%. What is meant by the “surface void ratio” of the tread is the ratio between, on the one hand, the difference between the total surface area of the tread and the area of those parts of the tread pattern elements that are intended to come into con- tact with the ground during running and, on the other hand, this total surface area of the tread. Alternatively or in combination, the volume void ratio of the tread of the said tyre when new is comprised between 25% and 50%. What is meant by the “volume void ratio” of a tread is the ratio between the volume of voids in the tread, these being made up of the grooves and the sipes, and the total volume of the tread. In one embodiment, the height of the tread patterns of the tread may be comprised between 6 mm and 12 mm.
In one embodiment, the tread comprises a first part delimiting the tread surface and at least one second part radially on the inside of the first part and in which a base of each stud is anchored, the first part being formed from a first rubber composition and the said second part being formed from a second rubber compo- 5 sition different from the first rubber composition. Thus it is possible to provide a first rubber composition that has good properties of resistance to wear and to grip. The second rubber composition may for its part be chosen to encourage the obtain- ing of good mechanical retention of the studs in the tread. According to one optional feature, the complex dynamic shear modulus G*(-10°C) of the first rubber composition is comprised between 1 MPa and 2 MPa. The complex dynamic shear modulus of the said second rubber composition may itself change with temperature such that G*(5°C) is greater than or equal to 5 MPa and G*(20°C) is less than or equal to 0.5xG*(5°C). The “complex modulus” G* is defined by the following relation- — ship: G*= Je + 6"*) in which G’ represents the elastic modulus and G” represents the viscous modulus. The terms elastic viscous modulus refer to dynamic properties well known to those skilled in the art. These properties are measured on a viscosity analyser of Metravib VA4000 type on test specimens moulded from uncured compositions. Test specimens such as those described in the standard ASTM D 5992-96 (the ver- sion published in September 2006 but initially approved in 1996 is used) in Figure
X2.1 (circular test specimens) are used. The diameter “d” of the test specimen is 10 mm (it therefore has a circular cross section of 78.5 mm”), the thickness “L” of each of the portions of rubber composition is 2 mm, giving a *d/L” ratio of 5 (unlike in standard ISO 2856, mentioned in the ASTM standard at paragraph X2.4, which recommends a d/L value of 2). The response of a sample of vulcanized rubber composition subjected to a simple alternating sinusoidal shear stress at a frequency of 10 Hz is recorded. The — test specimen is subjected to sinusoidal shear stress at 10 Hz, with a stress (0.7 MPa) imposed symmetrically about its equilibrium position. The test specimen is made to undergo accommodation prior to measurement.
The test specimen is then subjected to sinusoidal shear stress loading at 10 Hz, at 100% full-scale defor- mation, at ambient temperature.
The measurements are taken on a temperature curve increasing by 1.5°C
— per minute, from a temperature Tmin below the glass transition temperature Tg of the material up to a temperature Tmax which may correspond to the rubber plateau of the material.
Before beginning the sweep, the test specimen is stabilized at the temperature Tmin for 20 minutes in order to have a uniform temperature throughout the test specimen.
The result exploited is the dynamic shear elastic modulus G' and
— the viscous shear modulus G” at the chosen temperatures (in this instance -10°, 5° and 20°C). The glass transition temperature Tg of the first rubber composition may be comprised between -50°C and -30°C.
In the foregoing, the endpoints indicated for a range of values are included within this range, partic-
ularly in the expressions « comprised between » and «ranging from. to .. ».
The present invention will be better understood from reading the detailed description of an embodiment considered by way of entirely non-limiting example and illustrated by the appended figures, in which:
- Figure 1 is a schematic perspective view of a studded tyre according to one embodiment of the invention,
- Figure 2 is a face-on view of a stud of the tyre of Figure 1, and
- Figure 3 is a schematic part section view of the tyre of Figure 1.
Figure 1 schematically depicts a tyre 10 comprising a tread 12 having a tread surface (unreferenced) intended to come into contact with a roadway during running.
The tread 12 comprises a plurality of transverse 14 and circumferential 16 grooves which delimit a plurality of tread blocks 18. Each block 18 comprises a contact face forming part of the tread surface of the tread 12.
The tyre 10 also comprises a plurality of studs 20 fixed into the tread 12
— of the tyre and arranged across the entire width of the tread surface in the rubber blocks 18. The arrangement of the studs 20 on the tread 12 as illustrated in Figure 1 is given solely by way of nonlimiting illustration.
It is for example possible to provide several studs 20 on the one same block 18 of rubber.
The tread 12 of the tyre here comprises a central rib 22 devoid of studs.
Alternatively, it is possible to provide a rib 22 that has studs.
The studs 20 are arranged in several positions around the periphery of the tread 12 so that at any moment some of the studs 20 are in contact with the roadway on which the tyre 10 is running.
As will be described in greater detail later, the total number of studs 20 projecting from the tread surface of the studded tyre and the maximum cross section of each stud are designed so that this studded tyre possesses excellent grip on ice while at the same time having a reduced impact on a roadway not covered in ice or not covered in snow.
As illustrated in Figure 2, each stud 20, of longitudinal axis X-X? com- prises a base 24 for anchoring it into the tread 12 of the tyre, a stud pin 26 intended to come into contact with the roadway (ice, snow or bare surface) when the tyre is running, and a jacket 28 connecting the stud pin and the base.
In the exemplary embodiment illustrated, the stud 20 has a cylindrical profile.
As an alternative, the stud 20 could have any other profile, for example polygonal.
In the exemplary em- bodiment illustrated, the stud pin 26 is centred on the axis X-X’. Alternatively, the stud pin 26 could be off-centred with respect to the said axis.
The pin 26 of the stud may advantageously be made from a material dif-
ferent from that of the rest of the stud 20. That makes it possible to use, for this part, a material that is harder than the material of the base 24 and of the jacket 28, in so far as the stud pin 26 is subjected to very high mechanical stresses.
That also makes it possible, for certain product families, to create a jacket 28 and a base 24 made from a moulded or injection-moulded material, to which the stud pin 21 is
— fixed The jacket 28 may be made from a metallic material, for example steel.
Al- ternatively, the jacket may be made from a plastic material.
The stud pin 26 may be made of tungsten carbide.
Alternatively, the stud 20 may be made from a single material.
The maximum cross section Smax of the stud 20 is at most equal to 35 mm”,
this cross section corresponding to the largest cross section through the stud 20 in any plane perpendicular to the axis X-X' of the stud, whatever the geometric shape of this cross section (circular, polygonal, etc.). In the embodiment illustrated, this maximum cross section Smax of the stud corresponds to the maximum cross section of the base 24 of the stud. The maximum cross section of the pin 26 of the stud is comprised between 3 mm? and 3.5 mm”, and preferably equal to 3.14 mm?. This maximum cross sec- — tion corresponds to the largest cross section of the stud pin 26 in any plane perpen- dicular to the axis X-X?, whatever the geometric shape of this cross section (circu- lar, polygonal, etc.). The total height Hc of the stud 20 is comprised between 8 mm and 11 mm, and preferably egual to 10 mm. The total height Hc is defined as the cumulative height of the base 24, of the jacket 28 and of the stud pin 26. The mass of the stud may be comprised between 0.7 g and 1.2 g, and is preferably equal to
1.15 g. Figure 3 schematically depicts part of the tread 12 of the tyre which is provided with a pocket 30 in which a stud 20 is fitted. The pocket 30 opens onto the tread surface 32 of the tread 12. In a way known per se, in the unconstrained — state, i.e. before the stud 20 is inserted, the pocket 30 may exhibit a cylindrical shape of dimensions smaller than those of the stud so that after insertion, the stud is perfectly enveloped by the tread by elasticity and anchored therein. The stud 20 is arranged in the tread 12 in such a way that its axis X-X' is substantially parallel to a radial direction. The stud 20 projects out from the tread 20 — surface 32 of the tread 12 when it is not in contact with the roadway, as illustrated in Figure 3. The projecting height Hs of the stud 20 is at most equal to 1.6 mm. For preference, the projecting height Hs of the stud 20 is comprised between 0.8 mm and 1.2 mm, and advantageously equal to 0.9 mm. The projecting height Hs of the stud 20 is at most equal to 20% of the total height Hc of the said stud. In the em- bodiment illustrated, the upper end of the jacket 28 and the pin 26 of the stud project out from the tread surface 32. According to a preferred alternative form, only the pin 26 of the stud may project out from the tread 12. The anchored height Ha of the stud 20 within the tread 12 is at most equal to 9.4 mm. The Applicant Company has determined that a mean surface density of studs 20 on the tread surface of the tyre 10 at least equal to 6.7 studs per dm”, combined with a maximum stud 20 cross section at most equal to 35 mm”, allows for a significant improvement in the compromise between grip on ice and the nui- sance caused by the studs 20 in terms of roadway wear and noise inside the vehicle.
The increase in the number of studs 20 on the tread 12 in comparison with conventional studded tyres makes it possible to increase the effectiveness of the tyre 10 on an icy roadway, while limiting the maximum cross section Smax of each stud makes it possible to avoid excessive degradation of the condition of the road- way surface when this surface is not covered in ice or snow.
This particular com- bination of the mean surface density of studs 20 and of the maximum cross section of the studs thus makes it possible to obtain a good compromise between improved
— grip of the studded tyre 10 on ice and the limited abrasiveness of the studs 20 on the roadway.
For preference, the static striking force for each stud 20 is comprised in a range ranging from 120 N to 170 N.
This makes it possible to further favour the obtaining of a good compromise between improved grip on ice and the limited nuisance caused by the studs 20 in terms of roadway wear and noise inside the vehicle.
For preference, the mean linear density of studs 20 on the tread surface part of the tyre 10 is at least equal to 115 studs per metre.
By way of indication, for a tyre of size 205/55 R16, the mean surface density of studs 20 may be equal to 6.7 studs per dm”, the static striking force of each stud 20 may be equal to 152 N and the mean linear density of studs 20 may be equal to 115 studs per metre.
In the exemplary embodiment illustrated, the tread 12 comprises a first part 34 delimiting the tread surface 32 and a second part 36 positioned radially on the inside of the first part 201. The first part 34 of the tread 12 is formed from a
— first rubber composition and the second part 36 is formed from a second rubber composition different from the first rubber composition.
The jacket 28 of the stud is at least partially in contact with the first part 34, whereas the base 24 is wholly anchored in the second part 36 of the tread 20. The second part 36 completely en- velops the base 24 of the stud.
Creating the tread 12 with at least first and second parts 34, 36 is particu- larly advantageous in so far as it makes it possible to provide a first rubber composition suited to obtaining good properties of wear resistance and grip on ice, and a second rubber composition that favours mechanical anchorage of the studs
20. Alternatively or in combination, it is also possible to choose the second — rubber composition of the second part in order to obtain a tyre 10 the mechanical behaviour of which changes according to the temperature of the roadway on which it is running. If a second rubber composition chosen is stiff at low temperature and softer at high temperature, then the stud 20 will have a tendency to remain project- ing out from the tread 12 when the roadway is cold (covered in ice or snow), and to angle itself, deforming the second rubber composition which surrounds it, when the roadway is hotter (not covered in ice or snow). This effect is optimized when the complex dynamic shear modulus G*(-10°C) of the first rubber composition is comprised between 1 MPa and 2 MPa and the complex dynamic shear modulus of the second rubber composition evolves as a function of temperature such that G*(5°C) is greater than or equal to 5 MPa and G*(20°C) is less than or equal to
0.5xG*(5°C). The glass transition temperature Tg of the first rubber composition may be comprised between -50°C and -30°C. The invention has been illustrated on the basis of a tyre fitted with studs exhibiting a particular geometry. It would not constitute a departure from the scope of the present invention if the studs of the tyre had a different geometry.