MOTORCYCLE TYRES

Abstract: The invention concerns a tyre, especially suited for motor­ cycles, that is capable of high performances during racing competitions, due to the high rigidity of the tyre sidewall which permits considerable increases in speed in cornering. The said tyre presents, in correspondence of the radially inner portion of the sidewall and in a straight section, the profile of the carcass plies and of the axially inner surface, concave inwardly, together with the profile of the axially outer surface, concave outwardly.

Description: The present invention concerns motor-cycle tyres. In particular, it concerns a tyre which is capable of high performances, such as those required during competitive racing. These tyres must have a very special structure for supporting the very critical conditions during use; it is sufficient to consider that they must run with high values of the camber angle, that can reach and even exceed the value of 55°. It must be remembered that the camber angle is the angle that exists between the meridian plane of the tyre inclined on the road (for example, when cornering) and the above-said meridian plane in a vertical position. Under these working conditions, the tyre sidewall has to be very rigid. In fact, when the vehicle rounds a curve at a given speed, the value of the lateral thrust that balances the centrifugal force acting upon the vehicle, depends upon said rigidity of the sidewall. Therefore, the rigidity of the sidewall determines the maximum velocity that can be developed by the motor-cycle, during cornering. Many provisions have been adopted for reinforcing and for stiffening the tyre sidewall, in particular by using radially very extended special fabric reinforcements on the tyre sidewall with starting from the bead-zone, and/or with inserting lunette-shaped elastomeric material between the carcass plies or adjacent to them, or under the belt extremities. Another example of strengthening of the sidewall of the motorcycle tyres which corresponds to the preamble of claim 1 is obtained, as disclosed in JP-A-5953204, by a tyre carcass whose sidewalls are substantially rectilinear starting from the vicinity of the end part of a breaker arranged outside to the contact point with the bead core. The sidewalls of the carcass must also be diverging towards the central axis of the tyre. It is also known, from the document DE-A-3218315, which relates to a tyre for heavy vehicles as lorries and the like, to increase the rigidity of the tyre sidewalls in correspondence of the beads by increasing the thickness of the beads. In this way, however, in a small zone about the middle of the sidewall, the profile of the tyre presents a concavity outwardly directed and reduced thickness providing flexibility in the zone interested by deformations when the tyres are on load. If sidewall flexibility is desirable in lorry tyres, this feature must be avoided in the tyres for motorcycle running at high speed. Although achieving certain results, such provisions have always involved a considerable complexity in the carcass structure, with increasing the weight of the tyre, and hence, with having a greater rotating masses, and difficulty in getting rid of the heat produced, a loss of riding comfort, a negative influence on the tyre's driving characteristics and an increase in production costs. The Applicant has now surprisingly found an extremely simple mode, that is easily realizable and with being devoid of the above-cited defects, for reinforcing the tyre sidewall and without undergoing the above-cited negative consequences. Hence, the aim of the present invention is a tyre, especially suited for two-wheeled vehicles, provided with a structure that furnishes quite a high value of rigidity to the sidewall and consequently, which permits a greater development of speed during cornering, when compared to the performances had with other normal tyres. Therefore, what forms the object of the present invention, is a tyre for motorcycle wheels comprising: a textile carcass, a tread-band disposed in the crown zone of said carcass, sidewalls extending from the ends of the tread band to beads, for anchoring said tyre on the corresponding mounting rim, each bead comprising at least one annular, circumferentially inextensible, reinforcing core, said carcass comprising at least one pair of plies having reinforcing cords and ends turned-up around said reinforcing cores, said carcass plies presenting on cross-section a curvilinear profile, concave inwardly, the profile of the axially outer surface of each sidewall on said cross-section being concave outwardly when the tyre is deflated, the point of minimum thickness of the tyre, which divides the tyre sidewall into radially outer portion and radially inner portion, being at a height comprised between 40% and 60% of the section height, the maximum thickness of the sidewall being at the point of contact with the outer surface of the mounting-rim flange, characterized by: (a) the reinforcing cords of said plies being placed parallel to one another in each ply and crossed with those of the adjacent ply and symmetrically inclined, with respect to the circumferential direction, at an angle comprised between 22° and 26°, measured in the tyre crown, and (b) the minimum to maximum thickness ratio of each sidewall, measured in the direction perpendicular to the profile of the carcass plies, being comprised between 0.40 and 0.15, said minimum thickness having a value between 1.5 and 2 times the thickness of the carcass, the value of said inclination angle and the value of said minimum to maximum thickness ratio of each sidewall being selected to have said axially concave outer surface of the radially inner portion of said sidewall becoming rectilinear upon inflation of the tyre at its operative pressure. In any case, the present invention will now be explained in more detail in the following description and illustrated in the attached drawing table, both given solely by way of non-limiting example. Hence, the tyre of the invention comprises a carcass formed in a manner that is entirely traditional, with at least one pair of crossed-plies 1 and 2, and having its ends turned-up around an annular, circumferentially inextensible, metallic ring 3, usually known as a core. The reinforcing cords, of the plies, can be made of any whatsoever material known in tyre technology, in particular, rayon, as in the version of the tyre realized by the Applicant. Even as far as regards the angular disposition of the above-said cords, this can be according to any whatsoever known structure already known to the technicians of the field. In particular, referring to the measures taken in the crown in correspondence of the meridian plane m-m, the reinforcing cords which are parallel to one another in each ply and crossed with those in the adjacent ply, can be inclined with respect to the circumferential direction of the tyre, according to very high angular values, of not lesser than 75°, with thus realizing a substantially radial structure. Otherwise, they can be inclined according to lower values, however of not lesser than 20°, with realizing a structure having crossed-plies. The preferred solution chosen by the Applicant, was that having crossed-plies structure, with cords inclined at an angle comprised between 22° and 26° with respect to the outline of the meridian plane, but, in particular, limited to 24°. In the crown of said tyre, there is the normal tread-band 4, provided with a raised pattern for in this way guaranteeing the road-holding of the tyre even under wet-road conditions. Moreover, inserted between the carcass and the tread-band, there can be pre-disposed a breaker structure 5, substantially as wide as the above-said tread-band, and circumferentially and axially inextensible. Even the breaker structure is constructed according to well-known techniques, with comprising, in particular, a pair of layers that contain cords which are parallel to each other in each layer and inclined, preferably symmetrically to one another, with respect to the circumferential direction of the tyre, according to low values, comprised practically, between 15° and 30°. Moreover, concerning this breaker structure, the materials used for the reinforcings cords could be all those already known in tyre technology, in particular, nylon, metal, aramide fibres. Also regarding the structure, di per se, there can be adopted the geometrical configurations relative to the number and the disposition of the already known layers cited previously. The profile of the above-said carcass structure it is very arcuate, for the purpose of keeping the tread-band wound around, as in fact, is required for the tyres used to equip two-wheeled vehicles. Substantially speaking, in the tyre of the invention, we shall define as being the crown portion that comprised radially, between the ridge point of the tyre (intersection of the outer surface with the meridian plane m-m) and the chord C i.e. the axial straight line, in correspondence of which the tyre presents its maximum width, and as being the sidewall that comprised radially between the above-said chord C and the chord T i.e. the straight line axial and tangent to the vertex of the flanges 7 of the mounting-rim, and to end with, as being the bead portion that comprised radially between the said chord T and the mounting-rim. Obviously the radial height of the three successive portions corresponds with the section height i.e. to the total radial height H of the tyre. The radial distance f between the chord (C) with a maximum value and the radially inner surface of the tyre, in correspondence of the meridian plane m-m, is indicated as being the deflection: for convenience's sake, the tyre of the invention has a deflection value comprised preferably, between 20% and 40% of the section height H . When passing on to examining the sidewall of the tyre, we firstly notice the broken-line 6 representing the convex profile of the outer surface, in traditional tyres. The difference is to be seen at once, when compared with the tyre of the invention which instead, presents a concave profile that is turned outwardly and extended radially between the end of the tread-band and the extremity of the mounting-rim. Thanks to this profile, the thickness of the sidewall passes from a maximum value in correspondence of the end of the tread-band, to a minumum value on the section A-A (always lying in the sidewall's radially outer mid-plane), to thereafter, return to a maximum value, in correspondence of the radially outer surface of the mounting-rim flange. For the purpose of the present invention, the section A-A of a minimum thickness, divides the tyre sidewall into two portions, respectively the radially outer and the radially inner portions, of which the radially inner portion forms the innovatory part with respect to the 'known' tyres. In the three points indicated, said thicknesses are conveniently measured in the direction perpendicular to the profile of the carcass plies. The Applicant has found it convenient to keep the minimum thickness, of the tyre sidewall, comprised between 1.5 and 2 times the thickness of the carcass, with a value of the ratio between the said minimum thickness and the maximum thickness, measured in the direction perpendicular to the profile of the carcass plies comprised preferably, between 0.40 and 0.15. For preference, the point of the minimum thickness in the tyre sidewall, is to be found at a radial height h comprised between 40% and 60% of the total section height H . The Applicant has obtained particularly brilliant results with the tyres according to the invention that have their H/C ratio comprised between 0.70 and 0.80, in view of the fact that the invention results as being advantageously applicable in tyres having an H/C ratio comprised between 0.55 and 0.90. Moreover, it has proved convenient to keep the radial development of the sidewall i.e. of the portion comprised between the chords C and T , at a value comprised between 30% and 45% of the height H . All the above-said values, are meant to be taken on a non-inflated tyre. In fact, inflation pressure puts the tyre structure into a pre-loaded state, with a tension that is very advantageous for tyre behaviour during exercise. Nevertheless, the form of the originally moulded tyre is partially modified, in particular, by annulling the cavities on the axially outer surface of the radially inner portion of the sidewalls, that now acquire a substantially rectilinear profile. The tyre of the invention has proved to clearly exceed the limits of the performances as given by tyres of the state of the art. In particular, this signifies an unthought-of and unforeseen increase in the sidewall's rigidity with a consequent improved behaviour in respect of the camber, that has now allowed for tollerating rather higher speeds, when cornering, than those speeds possible for the 'known' tyres. At the same time, the tyre has not at all modified, either or its performance, during straight running, even at very high speed with improving rather, its behaviour directional stability, its road- holding capacity and its ready steering-response, at a parity of every other condition, as with respect to the known tyres. Without wanting in any way to bind the aims of the patent, the Applicant holds that the following theory, could explain the improvement obtained with the tyre of the invention. When a tyre is being driven around a curve, with its axis being inclined in respect of the horizontal plane of the road surface, on the crown surface in correspondence of the tyre's contact-area, there also acts a tangential thrust F , whose value depends on the camber angle i.e. upon the inclination of the tyre axis, that balances the centrifugal force acting on the vehicle. Through the effect of this thrust, the sidewall of the tyre flexes, with determining an axial shifting of the tyre-crown, in the direction of F with respect to the mounting-rim. For facilitating the comprehension of this phenomenon, said problem can be schematized by supposing the tyre sidewall to be like a cantilevered beam inserted into one extremity i.e. on the rim, and charged, at the other end i.e. at the radially outermost point having the minimum thickness, with a force F perpendicular to the beam-axis. In fact, owing to the greater rigidness of the crown portion in respect of the sidewall, the force F is transmitted substantially unaltered in its absolute value and direction, on the sidewall portion that has a minimum thickness. It is commonly known to the one skilled in the art that, under the said conditions, the beam is subjected to a flexing moment, that has a null value in the point where the force is applied, but which increases gradually up to a maximum value at the point of insertion. Owing to this flexing moment the beam, having a uniform section, undergoes the maximum flexion, in fact, just as occurs to the sidewall of the state of the art tyre, with a substantially constant thickness, that flexes due to the force F , resulting as being subjected to the maximum stress-strain, in correspondence of the mounting-rim flange. For preventing any flexion of the beam, at a parity of all the other conditions, the beam should have a section that progressively increases, by proceeding towards the inserted end, in such a way as to compensate for the increase in the flexing moment, with thus maintaining substantially constant the value of said stress/strain (force/straight section area) on the section of the beam. The tyre of the invention which presents its sidewall-thickness as increasing in the radial direction, inwardly towards the mounting-rim, stands for a good comparison with the above-said beam that has its section augmenting towards the inserted end, and which also behaves in a like manner i.e. flexing to a much lesser degree, at a parity of the applied force F value in known tyres. In other words, it results as being a sidewall that is absolutely very rigid and much more rigid than the sidewalls of the known tyres, in such a way as to tollerate, at a parity of the flexions, very high values of the thrust F and consequently, to allow for greater speeds, during cornering. Moreover, it will be obvious that the present description has been given solely by way of non-limiting example and that hence, anyone skilled in the art will have no difficulty in carrying out to it, all those alternative modifications and variations that, though not expressly described, are quite easily deducible from the present inventive idea without leaving the field of the present patent as defined by the appended claims.

Claims: Tyre for motorcycle wheels comprising a textile carcass, a tread-band (4) disposed in the crown zone of said carcass, sidewalls extending from the ends of the tread band to beads, for anchoring said tyre on the corresponding mounting rim, each bead comprising at least one annular, circumferentially inextensible, reinforcing core (3), said carcass comprising at least one pair of plies (1,2) having reinforcing cords and ends turned-up around said reinforcing cores, said carcass plies (1,2) presenting on cross-section a curvilinear profile, concave inwardly, the profile of the axially outer surface of each sidewall on said cross-section being concave outwardly when the tyre is deflated, the point of minimum thickness of the tyre, which divides the tyre sidewall into radially outer portion and radially inner portion, being at a height (h) comprised between 40% and 60% of the section height (H), the maximum thickness of the sidewall being at the point of contact with the outer surface of the mounting-rim flange, characterized by: (a) the reinforcing cords of said plies (1,2) being placed parallel to one another in each ply and crossed with those of the adjacent ply and symmetrically inclined, with respect to the circumferential direction, at an angle comprised between 22° and 26°, measured in the tyre crown, and (b) the minimum to maximum thickness ratio of each sidewall, measured in the direction perpendicular to the profile of the carcass plies, being comprised between 0.40 and 0.15, said minimum thickness having a value between 1.5 and 2 times the thickness of the carcass, the value of said inclination angle and the value of said minimum to maximum thickness ratio of each sidewall being selected to have said axially concave outer surface of the radially inner portion of each sidewall becoming rectilinear upon inflation of the tyre at its operative pressure. Tyre, according to claim 1, characterized by the fact that the value of the deflection (f) on the meridian plane is comprised between 20% and 40% of the section height (H). Tyre, according to any one of the preceding claims, characterized by the fact that between said tread-band (4) and said carcass there is inserted an annular reinforcing breaker structure (5), that is inextensible circumferentially and axially, and substantially as wide as the tread-band (4), that comprises at least one pair of rubberized layers, provided with reinforcing cords that are parallel to each other in each layer and crossed with the cords of the adjacent layer, said cords being symmetrically inclined with respect to the circumferential direction of the tyre, at an angle comprised between 15° and 30°, measured in the tyre crown. Tyre, according to claim 3, characterized by the fact that said breaker structure (5) comprises only one pair of crossed plies, the cords of said plies being disposed parallelly to one another in each ply, crossed with the cords of the adjacent ply, and inclined with respect to the circumferential direction, at an angle of not less than 75°. Tyre, according to claim 1, characterized by the fact that the H/C ratio value, is comprised between 0.55 and 0.90. Tyre, according to claim 1, characterized by the fact that the value of the ratio between the radial development of the sidewall and the section height H, is comprised between 0.30 and 0.45.

Inventors: ARMELLIN, GIANCARLO
Applicants: PIRELLI COORDINAMENTO PNEUMATICI SOCIETA PER AZIONI
Application year: 1987
Main classification: B60C3
IPC codes: B60C9, B60C15, B60C13, B60C3