Method of selecting a tyre tread pattern for preventing vehicle skew

Abstract: The present invention relates to a method of setting a tyre tread pattern that is provided with grooves at angles with respect to a vehicle's travel direction that enables the vehicle to run straight on a sidewise sloping road with good manoeuvrability. The invention also relates to a method for selecting a vehicle of a given weight that is equipped with a tyre having a set tread pattern for preventing skewing.

Description: To ensure good water drainage, many roads on which vehicles travel are constructed with a small inclination from the centre to either edge of the road. Vehicles running on such roads are subjected to a "skew force" which causes them to drift from the centre of the road towards either edge following the slope. If a driver removes his hands from the steering wheel, the vehicle will deviate in a sideways direction from the initial straight line of travel. This often causes "skewing" of the vehicle. To solve this problem, attempts have been made to control the direction of breaker reinforcements and/or the stiffness of the breaker in such a way that the residual cornering force (RCF) acts in a direction to cancel the effect of the slope of the road surface. However with tyre structural designs that can be utilised on conventional vehicles, marked changes in RCF are not possible and the approach described has often proved to be unsatisfactory. The problem is further complicated by the fact that the skew force varies greatly with the vehicle weight. It is thus an object of the present invention to provide a method for selecting, according to the vehicle weight, a suitable tyre tread pattern which enables a vehicle to run in a straight direction on a sidewise sloping road with better manoeuvrability. The method comprises equipping a vehicle with a tyre that is suited for the weight of the vehicle and which produces an inherent residual cornering force to counteract skewing. Another object of the present invention is to provide a vehicle whose weight is appropriate for a tyre having a tread pattern selected to compensate for the skew force. In accordance with a first aspect of the present invention there is provided a method of selecting a tyre tread pattern according to vehicle weight characterised by equipping a vehicle weighing M kg with a tyre that has grooves and sipes at angles of no more than 90 degrees with respect to the rotational axis of the tyre and in which the region from the centre of either edge of the road contacting surface is divided into three equal portions, a crown portion, a middle portion and a shoulder portion, the centre being defined in association with the direction in which the road contacting surface of the tyre rotates and the edge being defined as the axial edge of the road contacting surface where the tyre has a tread pattern with a pattern index PI that satisfies the relationship with respect to the residual cornering force (RCF) inherent in the tread pattern: 0.9 ≦ (60·PI+900)M ≦ 1.1 where PI = (ΔRCF CR +ΔRCF SH ) + (ΔRCF CR1 +ΔRCF SH1 ); wherein (ΔRCF CR + ΔRCF SH ) represents the residual cornering force developed by grooves in the tread pattern, which is determined by the equations: Crown portion: Δ RCF CR = Σ(-0.020·m·α·) Shoulder portion: Δ RCF SH = Σ(0.047·n·β·δ) and the term (Δ RCF CR1 + Δ RCF 1 ) represents the residual cornering force developed by sipes in the tread pattern and is determined by the equations: Crown portion: Δ RCF CR1 = Σ(0.020·m 1 ·α 1 · 1 ) Shoulder portion: Δ RCF SH1 = Σ(0.021·n 1 ·β 1 ·δ 1 ) where the symbols used in the four equations have the following meanings: m is the number of grooves in the crown portion of road contacting surface of the tyre; n is the number of grooves in the shoulder portion of the road contacting surface of the tyre; α is the ratio of the axial length of a groove in the crown portion of the road contacting surface of the tyre to the length of the crown portion;  is the angle a groove in the crown portion of the road contacting surface of the tyre forms with the axial direction of the tyre; β is the ratio of the axial length of a groove in the shoulder portion of the road contacting surface of the tyre to the length of the shoulder portion; δ is the angle a groove in the shoulder portion of the road contacting surface of the tyre forms with the axial direction of the tyre; m 1 is the number of sipes in the crown portion of the road contacting surface of the tyre; n 1 is the number of sipes in the shoulder portion of the road contacting surface of the tyre; α 1 is the ratio of the axial length of a sipe in the crown portion of the road contacting surface of the tyre to the length of the crown portion;  1 is the angle a sipe in the crown portion of the road contacting surface of the tyre forms with the axial direction of the tyre; β 1 is the ratio of the axial length of a sipe in the shoulder portion of the road contacting surface of the tyre to the length of the shoulder portion; and δ 1 is the angle a sipe in the shoulder portion of the road contacting surface of the tyre forms with the axial direction of the tyre. Thus in the present invention, a tyre is selected in accordance with the vehicle weight M kg such that it has a tread pattern with a pattern index PI that provides a residual cornering force (RCF) by means of the appropriate tread pattern. The vehicle equipped with this tyre has improved manoeuvrability in running straight on a sidewise sloping road while experiencing only limited "skew". In accordance with the second aspect of the present invention, a method if provided for selecting a vehicle having a weight M kg and which is equipped with a tyre having a tread pattern whose pattern index PI as defined hereinabove satisfies the following relationship with vehicle weight M: 0.9 ≦ (60·PI+900)/M ≦ 1.1 In the present invention, a selected vehicle weighing M kg is equipped with a tyre having a tread pattern whose pattern index PI satisfies the specified relationship with the vehicle weight M kg and this also ensures improved manoeuvrability in running straight so that the vehicle can travel on a sidewise sloping road in a stable manner while experiencing only limited "skew". The following detailed description of the invention will be more readily understood when considered together with the accompanying drawings in which:- Figure 1 is an illustration of a tyre tread pattern designed in accordance with one embodiment of the present invention; and Figure 2 is a graph illustrating the relationship between residual cornering force (RCF) and pattern index (PI). The tread pattern 1 of Figure 1 comprises a first pair of circumferentially extending, zigzag main grooves 2 located in the central portion of the tread with respect to the tyre's width (indicated by arrow F). The first pair of main grooves 2 are generally parallel to each other and are comparatively close together. A second pair of circumferentially extending, zigzag main grooves 3 are spaced outwards from the first main grooves 2 by a predetermined distance either side of the tyre. A first pair of zigzag subgrooves 4 lie on each of the first main grooves 2 and the second main grooves 3. The zigzag subgrooves 4 are comparatively narrower than the main grooves 2 and 3. A second pair of straight, circumferentially extending subgrooves 5 and 51 are positioned near the edges of the tread 1. First transverse grooves 6 extend across each first subgroove 4 and connect at opposite ends to a first main groove 2 and a second main groove 3. The transverse grooves 6 extend at an angle of  with respect to the axial direction. Second transverse grooves 7 extend between the second main groove 3 and the second subgroove 5 and lie at an angle of δ with respect to the axial direction of the tyre. First sipes 8 extend from the sidewalls of both first main grooves 2 outwards in the axial direction of the tyre at an angle of  1 . Second sipes 9 extend from the sidewalls of both second main grooves 3 and outwards in the axial direction of the tyre are at an angle of δ 1 . Auxiliary sipes 10 and 11 extend from the sidewalls of the first and second main grooves 2 and 3, respectively, towards the centre of the tyre tread. The centre of the road contacting surface of the tread 1 which in most cases coincides with the centre line of the tread is designated by CL in Figure 1. The area between CL and each edge OL of the road contacting surface in the axial direction of the tyre is divided into three equal portions by lines FL and SL spaced apart by equal distances w. The three equal portions are defined (in order from the centre of the road contacting surface CL) as a crown portion CR, a middle portion MID and a shoulder portion SH. Next will be described the residual cornering force RCF developed by the tread pattern in the road contacting surface of the tyre. As far as the road contacting surface of the tyre is concerned, those parts of the patterns which contribute to increases in residual cornering force RCF are not the first main grooves 2, the second main grooves 3 or the first subgrooves 4 which extend circumferentially around the tyre. Instead it is the grooves and sipes that extend axially. Stated more specifically, the contributing parts are the grooves and sipes that are provided in the crown portion CR and the shoulder portion SH which lie axially outwards. The grooves and sipes located in the middle portion and the MID portion do not significantly contribute to adjustment of the RCF. As shown in Figure 1, the first transverse grooves 6 in the crown portion CR which slope upwards to the right of the centre line CL have a negative (-) effect on the increase of the residual cornering force RCF. The amount of contribution Δ RCF CR is calculated by the following equation Δ RCF CR = Σ(-0.020·m·α·) where m is the number of first transverse grooves 6 in the crown portion CR α is the ratio of the axial length s of a groove 6 to the width w of the crown portion CR; and  is the angle formed by a groove 6 with respect to the axial direction of the tyre, with the counterclockwise direction taken as positive. The second transverse grooves 7 in the shoulder portion SH which slope upwards to the right have a positive (+) effect on the increase of the residual cornering force RCF. The amount of contribution ΔRCF SH is calculated by the following equation: ΔRCF SH = Σ(0.047·n·β·δ) where n is the number of second transverse grooves 7 in the shoulder portion SH; β is the ratio of the axial length d of a groove 7 to the width w of the shoulder portion SH; and δ is the angle formed by a groove 7 with respect to the axial direction of the tyre, with the counterclockwise direction taken as positive. The first sipes 8 in the crown portion CR slope upwards to the right and have a negative (1) effect on the increase of the residual cornering force RCF. The amount of contribution ΔRCF CR1 is calculated by the following equation: ΔRCF CR1 = Σ(-0.020·m 1 ·α 1 · 1 ) where m 1 is the number of sipes 8 in the crown portion CR α 1 is the ratio of the axial length s 1 of a sipe 6 to the width w of the crown portion CR; and  1 is the angle formed by a sipe 8 with respect to the axial direction of the tyre, with the counterclockwise direction taken as positive. The second sipes 9 in the shoulder portion SH which slope upwards to the right have a positive (+) effect on the increase of the residual cornering force RCF. The amount of contribution ΔRCF SH1 is calculated by the following equation: ΔRCF SH1 = Σ(0.021·n 1 ·β 1 ·δ 1 ) where n 1 is the amount of sipes 9 in the shoulder portion SH; β 1 is the ratio of the axial length d 1 of a sipe 9 to the width w of the shoulder portion SH; and δ 1 is the angle formed by a sipe 9 with respect to the axial direction of the tyre, with the counterclockwise direction taken as positive. On the basis of the thus calculated amounts of contribution to the residual cornering force RCF, the pattern index PI of the tread pattern is calculated by the following equation: PI = (ΔRCF CR + ΔRCF SH ) + (ΔRCF CR1 + ΔRCF SH1 ) where (ΔRCF CR + ΔRCF SH ) represents the amount of contribution made by the grooves and (ΔRCF CR1 + ΔRCF SH1 ) the amount of contribution made by the sipes. Figure 2 is a graph plotting the residual cornering force RCF against various values of the pattern index PI. Straight line A in the graph refers to a tyre sample in which the breaker cords embedded in the tyre tread are arranged in the normal direction whereas straight line B refers to a tyre sample in which the breaker cords are arranged in the reverse direction which is symmetric to the normal direction with respect to the direction of tyre revolution. As seen from the graph, the change in the direction of arrangement of breaker cords causes a difference of between 14 and 15 kg in the residual cornering force RCF. In accordance with the present invention, the pattern index PI of the tyre to be mounted on a vehicle weighing M (kg) is set to satisfy the following condition: 0.9 ≦ (60·PI+900)/M ≦ 1.1 If M is 800 kg, PI should be within the range of between -3 (inclusive) to 0.3 (inclusive) in order to satisfy this condition. If M is 1200 kg, 3≦PI≦7. Four tyre samples were prepared in accordance with the present invention and at the same time four comparative samples were prepared adopting a conventional structure. These samples were mounted on a vehicle that was subject to a "skew test", in which the vehicle ran on a flat road at a speed of 80 km/h over a distance of 100m where the amount by which the vehicle deviated either to the right or left from a straight line was measured. The results are shown below in Table 1 (for the samples of the present invention) and Table 2 (for comparative samples). Sample No 1 2 3 4 Tyre size 155/70R13 155/70R13 185/70R13 185/70R13 Vehicle weight 820 820 1160 1160 PI -2.0 0.0 3.0 6.0 (60·PI+900)/M 0.95 1.10 0.93 1.09 Skew (R: right L: left L 0.2 R 0.3 L 0.3 R 0.3 Sample No 1 2 3 4 Tyre size 155/70R13 155/70R13 185/70R13 185/70R13 Vehicle weight 820 820 1160 1160 PI -4.0 2.0 0.0 9.0 (60·PI+900)/M 0.80 1.24 0.78 1.24 Skew (R: right L: left L 1.8 R 2.0 L 2.2 R 2.1 As seen from Tables 1 and 2, the possible skew of a vehicle can be reduced by selecting tyres that have PI values satisfying the condition 0.9 ≦ (60·PI+900)/M ≦ 1.1 . Thus, the vehicle can travel straight in a stable manner not only on a flat road but also on a non-flat road that slopes towards either side in a direction that crosses the direction in which the vehicle travels. In its second aspect, the present invention provides a method for selecting a vehicle that weighs M (kg) which is equipped with a tyre having a tread pattern whose pattern index PI as defined hereinabove satisfies the following relationship with respect to the vehicle weight M: 0.9 ≦ (60·PI+900)M ≦ 1.1. In accordance with this second aspect, a vehicle is selected in such a way that its weight M satisfies the specified relationship with the pattern index PI of the tread pattern of the tyre which is to be mounted on the vehicle; this ensures improved manoeuvrability in running straight and the vehicle can travel on a sidewise sloping road in a stable manner while experiencing only limited "skew". Having the structural features described hereinabove, the present invention offers the following advantages. First, a tyre may be selected for a vehicle in accordance with its weight such that the tyre has a tread pattern with a pattern index PI that produces an inherent residual cornering force (RCF) to improve the vehicle's manoeuvrability in straight running along a sidewise sloping road while experiencing only limited "skew". Alternatively, a vehicle may be selected for receiving a tyre in such a way that its weight satisfies the specified relationship with the pattern index of the tread pattern that ensures improved manoeuvrability in straight running on a sidewise sloping road in a stable manner while experiencing only limited "skew".

Claims: A method of selecting a tyre tread pattern according to vehicle weight comprising equipping a vehicle weighing M (kg) with a tyre (1) having a tread with grooves and sipes in which the width from the centre CL of the tread (1) to either edge of the road contacting surface is divided into three equal portions, a crown portion (CR), a middle portion (MID) and a shoulder portion (SH), characterised in that the tread has a pattern index PI that satisfies the following conditions: 0.9 ≦ (60·PI+900)M ≦ 1.1 PI = (ΔRCF CR + ΔRCF SH ) + (ΔRCF CR1 + ΔRCF SH1 ) ; ΔRCF CR = Σ(-0.20·m·α·) ....the sum of (-0.020·m·α·) on the road contacting surface; ΔRCF SH = Σ(0.047·n·β·δ) ....the sum of (0.047·n·β·δ) on the road contacting surface; ΔRCF CR1 = Σ(0.021·m 1 ·α 1 · 1 ) ....the sum of (-0.020·m 1 ·α 1 · 1 ) on the road contacting surface; ΔRCF SH1 = Σ(0.021·n 1 ·β 1 ·δ 1 ) ....the sum of (0.021·n 1 ·β 1 ·δ 1 ) on the road contacting surface; where m is the number of grooves in the crown portion of road contacting surface of the tyre; n is the number of grooves in the shoulder portion (SH) of the road contacting surface the tyre; α is the ratio of the axial length of a groove in the crown portion (CR) of the road contacting surface of the tyre to the length of the crown portion (CR);  is the angle a groove in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β is the ratio of the axial length of a groove in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion; δ is the angle a groove in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre; m 1 is the number of sipes (8) in the crown portion of the road contacting surface of the tyre; n 1 is the number of sipes (9) in the shoulder portion of the road contacting surface of the tyre; α 1 is the ratio of the axial length of a sipe (8) in the crown portion (CH) of the road contacting surface of the tyre to the length of the crown portion(CR);  1 is the angle a sipe (8) in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β 1 is the ratio of the axial length of a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion; and δ 1 is the angle a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre. A method of selecting a vehicle weighing M kg for a tyre to counteract skew, characterised by selecting a tyre having a tread (1) with grooves and sipes in which the area from the centre of the tread to either edge of the road contacting surface is divided into three equal portions, a crown portion (CR), a middle portion (MID) and a shoulder portion (SH), said tread (1) having a pattern index PI that satisfies the following conditions: PI = (ΔRCF CR + ΔRCF SH ) + (ΔRCF CR1 + ΔRCF SH1 ); ΔRCF CR = Σ(-0.020·m·α·) ....the sum of (-0.020·m·α·) on the road contacting surface; ΔRCF SH = Σ(0.047·n·β·δ) ....the sum of (0.047·n·β·δ) on the road contacting surface; ΔRCF CR1 = Σ(0.020·m 1 ·α 1 · 1 ) ....the sum of (0.020·m 1 ·α 1 · 1 ) on the road contacting surface; ΔRCF SH1 = Σ(0.021·n 1 ·β 1 ·δ 1 ) ....the sum of (0.021·n 1 ·β 1 ·δ 1 ) on the road contacting surface; where m is the number of grooves in the crown portion (CR) of road contacting surface of the tyre; n is the number of grooves in the shoulder portion (SH) of the road contacting surface of the tyre; α is the ratio of the axial length of a groove in the crown portion (CR) of the road contacting surface of the tyre to the length of the crown portion (CR);  is the angle a groove in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β is the ratio of the axial length of a groove in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion (SH); δ is the angle a groove in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre; m 1 is the number of sipes (8) in the crown portion (SH) of the road contacting surface of the tyre; n 1 is the number of sipes (9) in the shoulder portion (SH) of the road contacting surface of the tyre; α 1 is the ratio of the axial length of a sipe (8) in the crown portion (CR) of the road contacting surface of the tyre to the length of the crown portion (CR);  1 is the angle a sipe (8) in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β 1 is the ratio of the axial length of a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion; and δ 1 is the angle a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre; and selecting the vehicle for said tyre having the tread pattern with pattern index PI such that: 0.9 ≦ (60·PI+900)/M ≦ 1.1. A pneumatic tyre for a vehicle weighing M kg to counteract skewing, comprising a tread with grooves and sipes in which the area from the centre of the tread to either edge of the round contacting surface is divided into three equal portions, a crown portion (CR), a middle portion (MID) and a shoulder portion (SH), characterised in that said tread has a pattern index PI that satisfies the following conditions: 0.9 ≦ (60·PI+900)/M ≦ 1.1; PI = (ΔRCF CR + ΔRCF SH ) + (ΔRCF CR1 +ΔRCF SH1 ); where ΔRCF CR = Σ(-0.020·m·α·) ....the sum of (-0.020·m·α·) on the road contacting surface; ΔRCF SH = Σ(0.047·n·β·δ) ....the sum of (0.047·n·β·δ) on the road contacting surface; ΔRCF CR1 = Σ(0.020·m 1 ·α 1 · 1 ) ....the sum of (0.020·m 1 ·α 1 · 1 ) on the road contacting surface; ΔRCF SH1 = Σ(0.021·n 1 ·β 1 ·δ 1 ) ....the sum of (0.021·n 1 ·β 1 ·δ 1 ) on the road contacting surface; where m is the number of grooves in the crown portion (CR) of road contacting surface of the tyre; n is the number of grooves in the shoulder portion (SH) of the road contacting surface of the tyre; α is the ratio of the axial length of a groove in the crown portion (CR) of the road contacting surface of the tyre to the length of the crown portion (CR);  is the angle a groove in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β is the ratio of the axial length of a groove in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion (SH); δ is the angle a groove in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre; m 1 is the number of sipes (8) in the crown portion (SH) of the road contacting surface of the tyre; n 1 is the number of sipes (9) in the shoulder portion (SH) of the road contacting surface of the tyre; α 1 is the ratio of the axial length of a sipe (8) in the crown portion (CR) of the road contacting surface of the tyre to the length of the crown portion (CR);  1 is the angle a sipe (8) in the crown portion (CR) of the road contacting surface of the tyre forms with the axial direction of the tyre; β 1 is the ratio of the axial length of a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre to the length of the shoulder portion; and δ 1 is the angle a sipe (9) in the shoulder portion (SH) of the road contacting surface of the tyre forms with the axial direction of the tyre.

Inventors: WAKABAYASHI NOBORU, WAKABAYASHI, NOBORU, WAKABAYASHI, NOBORU, KARUMU GAKUEN MIDORIGAOKA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1993
Main classification: B60C11
IPC codes: B60C11