Front and rear tyre combination for vehicle having abs and vehicle

Abstract: A vehicle having an Anti Lock Brake System (ABS) and front tyres (2R,2L) and rear tyres (3R,3L) which are capable of providing maximum braking effect when the ABS is in operation, each of the tyres comprising a plurality of tread elements (6) divided by tread grooves (5) , and the tread pattern rigidity Pr of the rear tyres (3R,3L) is more than the tread pattern rigidity Pf of the front tyres (2R,2L), wherein the tread pattern rigidity P (Pr, Pf) is defined as a quotient of the total of circumferential rigidity of all the tread elements (6) existing in a ground contacting region of the tyre divided by the area of the ground contacting region, and the circumferential rigidity K of each tread element is defined as a quotient of a tangential force F in kgf received at the ground contacting region which is divided by a displacement y in mm at the ground contacting region.

Description: The present invention relates to a combination of a front tyre and rear tyre for a vehicle having ABS and a vehicle including the combination of the tyres. In recent years, the number of automobiles having an Anti Lock Brake System (ABS) is increasing. In such Anti Lock Brake Systems, as shown in Fig.14, the slip rate of tyres when the ABS is operated is usually adjusted to be in a range of from about 5 % to about 10 %, and in order to prevent unstable motions such as spin, a difference is provided between the rear wheels and the front wheels such that the slip rate of the rear tyres is smaller than that of the front tyres. Document DE 39 01 624 A discloses a vehicle according to the pre-characterizing portion of claim 1. The breaking effect of this known vehicle is not yet optimal. Document EP 0 913 274 and WO 98/25776 A each disclose a pneumatic tyre set comprising two left hand and right hand front wheel pneumatic tyres and two left hand and right hand rear wheel pneumatic tyres in which the tread pattern of the front tyres is substantially different from the tread pattern of the rear tyres. Also with this tyre set the breaking effect is not yet optimal. An object of the present invention is to provide a combination of a front tyre and rear tyre having different characteristics which can provide maximum braking effect when used with the above-mentioned ABS. Another object of the present invention is to provide a vehicle having such ABS and equipped with the front tyres and rear tyres which can provide maximum braking effect. These objects are achieved by the features of claim 1 and the features of claim 12 respectively. Here, the tread pattern rigidity P (Pr, Pf) is defined as a quotient of the total of circumferential rigidity of all the tread elements existing in a ground contacting region of the tyre which is divided by the area of the ground contacting region. The circumferential rigidity K of each tread element 6 is defined as the quotient of a tangential force F in kgf received at the ground contacting region divided by the displacement y in mm in the ground contacting region. K = F y In order to obtain maximum braking effect during operation of the general ABS, the present inventor studied relationships between frictional coefficient and various factors, and discovered that it is most important that the tyre displays a maximum frictional coefficient under the above-mentioned relatively small slip rate of from 5 % to 10 %. As explained above, because a certain difference in slip rate is provided between the front and rear wheels, when the maximum frictional coefficient of the rear tyres is less than the maximum frictional coefficient of the front tyres, a larger braking force can be obtained. Fig.10 shows an exemplary µ-s curve, wherein "µ" on the vertical axis is the frictional coefficient between the tyre tread and road surface, and "s" on the horizontal axis is the slip rate therebetween. In this case, the maximum frictional coefficient occurs at a slip rate of about 7 %. In general, however, the maximum frictional coefficient occurs at a relatively higher slip rate. To decrease the slip rate at which the maximum frictional coefficient occurs, it is preferable to increase the rising angle or inclination angle of a substantially straight portion of the µ-s curve rising from the origin of co-ordinates. Hereinafter, this inclination is called "µ-s stiffness". As a result of the inventor's study, it was also found that the µ-s stiffness and the slip rate at the maximum frictional coefficient have a close relationship with tread pattern rigidity. Fig.15 shows an exemplary relationship between the pattern rigidity (index) and the slip rate, and an exemplary relationship between the pattern rigidity (index) and the µ-s stiffness (index). As shown in this figure, as the pattern rigidity increases, the µ-s stiffness increases, and the slip rate at the maximum frictional coefficient decreases. This shows that when the tread pattern rigidity of the rear tyre is more than the tread pattern rigidity of the front tyre, the slip rate at a maximum frictional coefficient becomes smaller on the rear tyre than the front tyre, which meets the condition of the ABS. Further, when the µ-s stiffness of the rear tyre is more than the µ-s stiffness of the front tyre, the tread pattern rigidity becomes larger on the rear tyre than the front tyre. This also meets the condition of the ABS. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings in which:- Fig.1 is a schematic perspective view of a vehicle as an embodiment of the present invention; Figs.2 to 5 each show an example of the tread pattern of a pneumatic tyre according to the present invention; Fig.6(A) is a perspective view of a tread element for explaining the rigidity thereof; Fig.6(B) is a perspective view of a tread element with sipes; Fig.7 shows a ground contacting patch (shaded area) for the tread pattern shown in Fig.2; Figs.8(A), 8(B) and 8(C) are graphs showing the vertical load, forward and backward force, braking force, test car speed, and wheel speed as a function of the elapsed time which were measured to obtain the µ-s curve; Fig.9 shows a ground contacting patch for explaining the length of the axial component of the groove edge; Fig.10 shows the µ-s curve of test tyre A; Fig.11 shows the µ-s curve of test tyre B; Fig.12 shows the µ-s curve of test tyre C; Fig.13 shows the µ-s curve of test tyre D; Fig.14 shows relative frequency of the slip rate of ABSs in operation; Fig.15 is a graph showing relationships between the pattern rigidity, µ-s stiffness and slip rate at the maximum frictional coefficient; and Fig.16 is a graph showing a relationship between the maximum frictional coefficient and the edge length (index). In Fig.1, a vehicle 1 according to the present invention is an automobile (passenger car) and has an Anti Lock Brake System (ABS). The ABS is adjusted such that, when the ABS is in operation, the slip rate is about 5 % to about 10 % and the slip rate is lower on the rear wheels RR and RL than the front wheels FR and FL as shown in Fig.14. On the front wheels FR and FL, pneumatic tyres 2R and 2L are mounted, and on the rear wheels RR and RL, pneumatic tyres 3R and 3L are mounted. As shown in Figs. 2, 3, 4 and 5 each of the pneumatic tyres 2L, 2R, 3L and 3R comprises a tread portion 4 provided with a plurality of tread elements 6 divided by tread grooves 5. The tread groove 5 is a groove having a certain width enough for water drainage. The tread grooves 5 may be circumferential grooves 5a and axial grooves 5b. As to the configurations of the circumferential grooves 5a and axial grooves 5b, various grooves, e.g. straight grooves, curved grooves or zigzag grooves may be used. The tread elements 6 may include a circumferentially continuous rib such as 6a shown in Fig.2 and/or circumferentially discontinuous blocks such as 6b shown in Figs.3 to 5. Thus, the tread pattern in this invention includes so called "rib pattern", "block pattern", "rib-and-block pattern", "rib-and-lug pattern", "lug pattern" and the like. The tread patterns can be symmetrical about the tyre equator C as shown in Figs.2 to 5, but it is also possible to make the tread patterns not symmetrical. Further, it is also possible to make the tread pattern bi-directional or unidirectional. Furthermore, as shown in Figs.4 and 5, it is possible that the tread elements 6s are provided with sipes 7 having no substantial width (for example under 1.0 mm). Fig.2 shows an example of the tread pattern comprising four straight circumferential grooves 5a. Fig.3 shows an example of the tread pattern comprising four straight circumferential grooves 5a and axial grooves 5b extending straight from one tread edge to the other tread edge in parallel with the tyre axial direction. Fig.4 shows an example of the tread pattern which is similar to Fig.3, but with sipes 7 extending straight from one tread edge to the other tread edge in parallel with the tyre axial direction which are disposed alternately with the axial grooves 5b. Fig.5 shows an example of the tread pattern which is similar to Fig.4, but the number of sipes 7 is doubled. Therefore, in the example shown in Fig.2, all the tread elements 6 are a circumferentially continuous rib. Thus, the pattern is a rib pattern. In the examples shown in Figs.3-5, all the tread elements 6 are a rectangular block. Thus, the patterns are block patters. The difference between the pattern in Fig.2 and that in Fig.3 is the number of axial grooves 5b. The difference between the patterns in Figs.3-5 is the number of sipes 7. The tread pattern rigidity Pr of each rear tyre 3L and 3R is set to be more than the tread pattern rigidity Pf of each front tyre 2L and 2R. Preferably, the tread pattern rigidity Pr is more than 1.1 times, more preferably more than 1.25 times, still more preferably more 1.50 times the pattern rigidity Pf. It is also preferable that the pattern rigidity P (Pf, Pr) is set in the range of from 70 to 200 kgf/mm cm 2 to achieve both steering stability and wet performance. Here, the tread pattern rigidity P (Pr, Pf) is defined as the quotient of the total of circumferential rigidity of all the tread elements 6 existing partially or wholly in the ground contacting region around the tyre which is divided by the area in cm 2 of this ground contacting region. The ground contacting region is defined as the region between the tread edges E. The tread edges E are the axially outmost edges (e) of the ground contacting area 9 of the tread portion 4 when the tyre is under its standard loaded condition mounted on a standard rim and inflated to a standard inner pressure and loaded with a standard load. The standard rim is the "standard rim" specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The standard pressure is the "maximum air pressure" in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tyre Load Limits at Various Cold Inflation Pressures" table in TRA or the like. The standard load is the "maximum load capacity" in JATMA, the "Load Capacity" in ETRTO, the maximum value given in the above-mentioned table in TRA or the like. The circumferential rigidity K of a tread element 6 is defined by the following equation K = F y wherein F = tangential force in kgf received at the ground contacting region y = displacement in mm at the ground contacting region, (shown in Fig.6(A)) On the basis that a tread element 6 is regarded as a cantilever beam fixed at its radially inner end as shown in Fig.6(A), the circumferential rigidity K (kgf/mm) of the tread element 6 can be obtained by the following equation. wherein h = height in mm of the tread element E = Youngs modulus in kgf/mm 2 of the tread rubber I = Second moment of area in mm 4 of the tread element A = sectional area in mm 2 of the tread element G = Shearing stiffness in kgf/mm 2 of the tread rubber (=E/3). When the tread element 6 is a circumferentially continuous rib 6a as shown in Fig.2, the circumferential rigidity K is calculated on the assumption that the tread element 6 is a block having a height equal to 50% of the actual height (h) and a circumferential length equal to that of a portion of the rib actually contacting with the ground under the standard loaded condition. When the tread element 6 protrudes partially from the tread edge E as shown in Fig.7 as the blocks 6b1 for example, the width W2 to the tread edge E is used instead of the actual block width W3. If it is difficult to use the equation 1 because the element has a complex shape, the rigidity may be obtained by applying a quadrature and/or co-ordinate conversion. In this embodiment, the front tyres 2 have a µ-s stiffness αf, and the rear tyres 3 have a µ-s stiffness αr which is larger than the µ-s stiffness αf. The µ-s stiffness α (αr, αf) is, as explained above, the inclination angle of the substantially straight portion of the µ-s curve of the tyre which rises from the origin of rectangular co-ordinates, wherein the horizontal axis is the slip rate (s) and the vertical axis is the frictional coefficient µ. The rear tyre's µ-s stiffness αr is preferably more than 1.1 times, more preferably more than 1.15 times, still more preferably more than 1.30 times the front tyre's µ-s stiffness αf. In this application, the µ-s curve is obtained as follows: the test tyre is mounted on a standard rim and inflated to a standard inner pressure; the tyre and rim assembly is mounted on a trailer which is attached to a motorcar; during running straight at a constant speed of 64 km/hr on a wet asphalt road covered with a 1-2 mm thickness water film, a controlled brake is applied to the test tyre such that the time from a free state to wheel-locked state becomes one second; and at regular intervals of four seconds, the vertical load (Fig.8 (A)), forward-and-backward force and braking force (Fig.8 (B)) are measured on the tyre axle, and the running speed and wheel speed (Fig.8 (C)) are measured. From the measured data, the frictional coefficient µ is computed as follows. µ = forward-and-backward force/vertical load The slip rate (s) between the tyre and road is wherein Vt = wheel speed Vc = vehicle running speed. The µ-s curve is obtained by plotting data of the slip rate (s) and frictional coefficient µ computed as above. In this invention, it is possible to provide sipes 7 in order to improve braking performance under wet conditions. When the tread element 6 is provided with sipe(s) as shown in Fig.6 (B), the value K should be rectified by the following equation because the actual circumferential rigidity K' of the tread element 6 is decreased from the above-mentioned computed value K. wherein W = Width in mm of the tread element L = Length in mm of axial component of the sipe(s) hs = Depth in mm of the sipe(s) n = Number of sipe(s). This equation was empirically formulated. If the sipes 7 are formed too much, the above-mentioned µ-s stiffness becomes insufficient. Thus, the number of sipes per one block is preferably one to three. In this embodiment, further, the total axial edge length Sf of the front tyre 2 is set to be more than the total axial edge length Sr of the rear tyre 3. The total axial edge length S (Sf, Sr) is the total length in mm of the axial component of groove edges existing in the ground contacting region under the above-mentioned standard loaded condition. The groove edges include edges of the sipes 7 as well as edges of the tread grooves 5 such as circumferential grooves 5a and axial grooves 5b and the like. In the case of a tread groove 5 having a certain width, two edges one on each side thereof are counted. However, in the case of a sipe having no substantial width, two edges should be regard as one edge. For example, in a tread pattern shown in Fig.9, a right edge 5e and a left edge 5e of each of two zigzag grooves 5 are counted. If only the ground contacting patch shown in this figure is considered and the zigzag amplitude is supposed to be 5 mm, the axial edge component per one zigzag groove is 5 X 4 X 2 = 40 mm. Thus, the total axial edge length is 40 X 2 = 80 mm. Preferably, the total axial edge length Sf of the front tyre is more than 1.5 times, more preferably more than 1.8 times, still more preferably more than 2.0 times the total axial edge length Sr of the rear tyre. As to the internal structures of the front and rear tyres 2 and 3, it is not necessary to limit to a specific structure, but a belted radial tyre having a radial ply carcass and a stiff belt may be suitably employed. Comparative Test Table 1 shows the results of a comparison test conducted using two types of 1500cc FF-type Japanese passenger cars (A and B) both equipped with ABS. In the test, the test car was run on a wet asphalt road and sharply braked at a speed of 100 km/hr so that the ABS worked. Such sharp braking was repeated five times to obtain the average braking distance. The specifications of the test tyres A, B, C and D used are shown in Table 2. Ref.1 Ref.2 Ex.1 Ref.3 Ex.2 Ex.3 Ref.4 Ex.4 Ex.5 Front tyre A B B C C C D D D Pattern rigidity Pf 195 152 152 128 128 128 100 100 100 µ-s stiffness αf 159 146 146 138 138 138 100 100 100 Total axial edge length Sf 0 25 25 50 50 50 100 100 100 Rear tyre A B A C B A D C B Pattern rigidity Pr 195 152 195 128 152 195 100 128 152 µ-s stiffness αr 159 146 159 138 146 159 100 138 146 Total axial edge length Sr 0 25 0 50 25 0 100 50 25 Pr/Pf 1 1 1.28 1 1.18 1.51 1 1.29 1.52 αr/αf 1 1 1.09 1 1.06 1.15 1 1.38 1.46 Sf/Sr 1 1 - 1 2 - 1 2 4 Braking distance (m) test car A 57.3 53.1 52.2 51.3 51.1 49.8 50.7 48 47.4 test car B 52.4 51.8 51.2 52.6 52.2 50.9 55.2 53 52.6 When the braking distance of Ex.1 is compared with that of Ref.2, those of the test car A and test car B were shortened 0.9 m and 0.7 m, respectively. When the braking distance of Ex.3 is compared with that of Ref.3, the braking distances of the test car A and test car B were shortened 1.5 m and 1.7 m, respectively. When the braking distance of Ex.5 is compared with that of Ref.4, those of the test car A and test car B were shortened 3.3 m and 2.6 m, respectively. Tyre A B C D Tread pattern Fig.2 Fig.3 Fig.4 Fig.5 Tread groove depth (mm) 8.2 8.2 8.2 8.2 Positive ratio (%) 77 70 70 70 Number of sipe per tread element 0 0 1 2 Sipe depth (mm) - - 8.2 8.2 Pitch number - 60 60 80 µ-s curve Fig.10 Fig.11 Fig.12 Fig.13 Max. frictional coefficient 0.845 0.858 0.884 0.908 Slip rate at max. frictional coefficient 0.09 0.11 0.13 0.21 Pattern rigidity P (N/mm cm 2 ) 14.07 10.96 9.27 7.22 (index) 195 152 128 100 µ-s stiffness α 30.47 28.04 26.61 19.21 (index) 159 146 138 100 Total edge component (index) 0 25 50 100 Tyre lateral rigidity (kgf/mm) 11.97 10.82 10.74 10.54 Tyre size: 195/60R14 Inner pressure: 200 kPa In the tread patterns shown in Figs.3-5, the pitch number corresponds to the number of axial grooves. The measuring conditions for µ-s curves shown in Figs.10 to 13 were as follows: Tyre Vertical load: 4.75 kN Running speed: 64km/h Tyre inner pressure: 200 kPa Road surface: wet asphalt with 1 to 2 mm depth water pool

Claims: Vehicle comprising front tyres (2R, 2L) and rear tyres (3R, 3L), each of the tyres comprising a plurality of tread elements (6) divided by tread grooves (5), the rear tyres (3R, 3L) each having a tread pattern rigidity Pr, the front tyres (2R, 2L) each having a tread pattern rigidity Pf, wherein the tread pattern rigidity Pr is more than the tread pattern rigidity Pf, wherein the tread pattern rigidity P (Pr, Pf) is defined as a quotient of the total of circumferential rigidity of all the tread elements (6) existing in a ground contacting region of the tyre which is divided by the area of the ground contacting region, and the circumferential rigidity K of each tread element (6) is defined as a quotient of a tangential force F in kgf received at the ground contacting region which is divided by a displacement y in mm at the ground contacting region, characterised in that the tread pattern of the front tyres (2R, 2L) is substantially the same as the tread pattern of the rear tyres (3R, 3L) except that the number of sipes (7) in the front tyres (2R, 2L) is larger than the number of sipes (7) in the rear tyres (3R, 3L). Vehicle according to claim 1, characterized in that the front tyres (2R, 2L) have substantially the same tyre size as the rear tyres (3R, 3L). Vehicle according to claim 1 or 2, characterized in that each of the tyres (2R, 2L, 3R, 3L) has a tread pattern rigidity (P) of from 70 to 200 kgf/mm cm 2 . Vehicle according to any of the preceding claims, characterized in that the tread pattern rigidity (Pr) of the rear tyres (3R, 3L) is more than 1.1 times the tread pattern rigidity (Pf) of the front tyres (2R, 2L). Vehicle according to any of the preceding claims, characterized in that the tread pattern rigidity (Pr) of the rear tyres (3R, 3L) is more than 1.25 times the tread pattern rigidity (Pf) of the front tyres (2R, 2L). Vehicle according to any of the preceding claims, characterized in that the tread pattern rigidity (Pr) of the rear tyres (3R, 3L) is more than 1.5 times the tread pattern rigidity (Pf) of the front tyres (2R, 2L). A vehicle according to any of the preceding claims, characterised in that the front tyres (2R, 2L) have a µ-s stiffness αf and the rear tyres (3R, 3L) have a µ-s stiffness αr, and the µ-s stiffness αr is larger than the µ-s stiffness αf. Vehicle according to claim 7, characterized in that the µ-s-stiffness α-r of the rear tyres (3R, 3L) is more than 1.1 times the µ-s-stiffness α f of the front tyres (2R, 2L). Vehicle according to any of the preceding claims, characterized in that each of the rear tyres (3R, 3L) has no sipe and each of the front tyres (2R, 2L) has sipes (7). Vehicle according to any of the preceding claims, characterized in that the tread pattern of the front tyres (2R, 2L) and the tread pattern of the rear tyres (3R, 3L) are block patterns. Vehicle according to any of the preceding claims comprising an antilock brake system for controlling slip rates of the front tyres (2R, 2L) and rear tyres (3R, 3L) characterized in that said slip rates are within a range of from 5 % to 10 % and the slip rate of the rear tyres (3R, 3L) is smaller than the slip rate of the front tyres (2R, 2L). A combination of front and rear tyres, characterised in that the tyres have the characteristics of any one of claims 1 to 11.

Inventors: WADA YASUO, WADA, YASUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C11
IPC codes: B60C19, B60C11, B62D61, B60C5