TIRE FOR AN AGRICULTURAL VEHICLE

Abstract

The tread pattern of a tire for a vehicle for agricultural use comprises two rows of lugs (R1, R2), a first row being made up of a circumferential distribution of at least two families of lugs, over one revolution of the wheel; a first family is reproduced with a shortest spacing S, and a second family is reproduced with a longest spacing L; the tread pattern of the tread is designed such that the distance between the axially outer end of a following lug and the axially inner end at the centre of the tread is referred to as the overlap distance R. The ratio of the shortest spacing S of the family of lugs to the longest spacing L, S/L, lies in the range [0.6; 0.8]. The circumferential overlap distance R between the first and second circumferential rows of lugs is constant around the entire circumference of the tire.

Claims

1.-9. (canceled)

10. A tire for a vehicle for agricultural use, comprising a tread (10) intended to come into contact with a ground via a tread surface, the tread (10) comprising: lugs (20) that are separated from one another at least in part by grooves (30) and extend radially toward an outside from a bottom surface as far as the tread surface over a radial height H at least equal to 30 mm and at most equal to a radial thickness H.sub.max of the tread (10), the lugs (20) being distributed in two circumferential rows (R1, R2); a first circumferential row (R1) being made up of a circumferential distribution of at least two families of lugs distributed periodically around a circumference of the tire, the lugs of a first family being distributed circumferentially with a shortest spacing S, and the lugs of a second family being distributed circumferentially with a longest spacing L, each lug (20) of the first circumferential row (R1) being inclined at an angle ? with respect to a circumferential direction (XX) of the tire; the second circumferential row (R2) being obtained through symmetry of the first circumferential row (R1) with respect to an equatorial plane (100), passing through the center of the tread and perpendicular to an axis of rotation (YY) of the tire, followed by a rotational offset of angle ? about the axis of rotation (YY), the rotational offset being characterized by an overlap distance (R) between the lugs of the first circumferential row and the lugs of the second circumferential row, defined as being the circumferential distance between an axially outer end of each lug (20) of the first circumferential row (R1) and an axially inner end of the lug (20) of the second circumferential row (R2), wherein a ratio S/L of the shortest spacing S of the first family of lugs to the longest spacing L of the second family of lugs lies within the range [0.6; 0.8], and wherein the circumferential overlap distance R between the first and second circumferential rows of lugs is constant around an entire circumference of the tire.

11. The tire according to claim 10, wherein the overlap distance R between the first and second circumferential rows of lugs lies within the range [L/4; L/2].

12. The tire according to claim 10, wherein the rotational angle ? of the second circumferential row (R2) with respect to the first circumferential row (R1) lies within the range [5?; 15? ].

13. The tire according to claim 10, with the lugs of the first family having a mean thickness E1 and the lugs of the second family having a mean thickness E2, wherein the relative variation in thickness (E2?E1/E2) is less than or equal to 25%.

14. The tire according to claim 10, wherein the mean inclination angle ? of each lug with respect to the circumferential direction (XX) is less than or equal to 50?.

15. The tire according to claim 10, with the tread comprising at least two rows of lugs with three families (A, B, C) associated respectively with the shortest spacing S, the intermediate space M, and the longest spacing L, wherein the intermediate spacing M is equal to an arithmetic mean of the shortest spacing S and the longest spacing L.

16. The tire according to claim 10, with the tread having a development at the center that lies in the range [6300; 6850] mm, and having two rows of N lugs, N ranging from 20 to 25, each of the rows having three families of lugs (A, B, C), wherein the arrangement of the families of lugs follows the following sequences for each row: N=20, the sequence is: BCCCBBAAAAACBBCABBAA; N=21, the sequence is: AABCBCAACBAABCCCBBBAA; N=22, the sequence is: CCABBBBBAAACCCCBAAAAAB; N=23, the sequence is: ABBCCBAABAABBCCBCCBAABC; N=24, the sequence is: BBCCCBBAAAABCCCAAABBBCAA; and N=25, the sequence is: BBCBBAAABCCCBBBAAAAACCCAA.

17. The tire according to claim 10, with the tread having a development at the center that lies in the range [5500; 6000] mm, and having two rows of N lugs, N ranging from 20 to 25, each of the rows having three families of lugs (A, B, C), wherein the arrangement of the families of lugs follows the following sequences for each row: N=20, the sequence is: BBACBCAAAAABBCCCBAAB; N=21, the sequence is: CCAACBBAABCCCBBBAAAAB; N=22, the sequence is: CCAABABBBBBCCAAAAAABCC; N=23, the sequence is: BAAAAAABCCCBBAAACBBBBCC; N=24, the sequence is: BCCAAAAACCCCBBAABBAACBBB; and N=25, the sequence is: BBAACCCAAAAABBBCCCBAAABBC.

18. The tire according to claim 10, with the tread having a volumetric void ratio TEV equal to a ratio between a total volume of the grooves (30) separating the lugs (20) and a total volume radially contained between the bottom surface and the tread surface, wherein the volumetric void ratio TEV is between 50% and 80%.

Description

[0074] FIG. 1-A: illustration of the tread pattern of the tread of a tyre according to the invention.

[0075] FIG. 1-B: elements of the tread pattern of the tread of a tyre according to the invention, illustrating the design principle of the tread pattern.

[0076] FIG. 1-C: sequence of the tread of a tyre according to the invention with the tread pattern elements.

[0077] FIG. 2: spectrum of the signal associated with the sequence arrangement: ABBCCBAABAABBCCBCCBAABC of the elements of the tread pattern of the tread.

[0078] FIG. 3: spectrum of the measured acoustic response of the tyre according to the invention when running on a road with an asphalt pavement at more than 40 km/h.

[0079] More specifically, FIG. 1-A shows a tyre 1 for a vehicle for agricultural use according to the invention, comprising a tread 10 intended to come into contact with the ground via a tread surface: [0080] the tread 10 comprising raised elements, referred to below as lugs (40, 50, 60), that are separated from one another at least in part by grooves 30 and extend radially towards the outside from a bottom surface as far as the tread surface over a radial height H at least equal to 30 mm and at most equal to the radial thickness H.sub.max of the tread 10; [0081] the tread 10 comprising two rows of lugs (R1, R2), a first row R1 being made up of a circumferential distribution of three families of lugs (A, 60), (B, 50) and (C, 40) over one revolution of the tyre; the family (A, 40), which is reproduced with a shortest spacing S, has a length L1, the family (C, 40), which is reproduced with a longest spacing L, has a length L2, and the family (B, 50) is associated with an intermediate spacing M defined as being the arithmetic mean of the spacings S and L; [0082] each lug (A, B, C) therefore being associated respectively with a repetition spacing (S, M, L) as depicted at 90, which illustrates an excerpt of the arrangement of the elements of the tread with the concatenation of the successive spacings.

[0083] FIG. 1-B shows tread pattern elements which extend across the entire width of the tread. Each element is formed of two symmetric lugs. The axial ends of a lug, I for the row R1 and I for the row R2, are positioned with respect to the axial ends of the lug at the centre of the tread (M and M). The design principle of the tread pattern of the tread necessitates starting from a lug of row R1, then positioning the same lug in row R2 by symmetry with respect to the equatorial plane 100 such that the axial end I is at a given distance R from the end M of the initial lug of row R1.

[0084] FIG. 1-B illustrates the above design principle with a concatenation of several tread pattern elements formed of three families of lug A, B, C.

[0085] FIG. 1-C shows the sequence of the tread of a tyre according to the invention with three families of lugs (A, B, C). The lug C has the greatest length L2 and a mean thickness E2, and the lug A has the shortest length L1 and a mean thickness E1. The angle ? represents the mean inclination of a lug with respect to the circumferential direction.

[0086] The invention was studied more particularly in the case of an agricultural tyre of size 710/70R42. For this size, two versions were tested: a first with a single-spacing tread pattern according to the prior art, and a second version according to the invention with three families of lugs A, B and C at variable spacings.

[0087] Each lug A, B and C is the basic element of three tread pattern elements of the tread. The respective spacings (S, M, L) are associated respectively with the lugs (A, B, C) such that S<M<L.

[0088] Such an example of a tread pattern element is shown in FIG. 1-B with the two basic lugs which are symmetric with respect to the equatorial plane and offset in the circumferential direction. The noses of the lugs converge towards the centre of the tread so as to define the direction of rotation of the tyre, taking the shape of a chevron V.

[0089] There are two major types of feature which are caused by the impact of the lugs on the roadway when they are not arranged optimally: whining and beating. These are features, the acoustic power of which is much greater than the mean power of the spectrum and to which the human ear is particularly sensitive.

[0090] The timing of the impacts of the tread pattern on the ground on entering the contact patch is given its pattern by the order of succession of the elements. If the elements are all the same size, they follow one another with a perfectly regular rhythm. A single frequency will then be brought about, and this will produce a whine-like sound. Having several sizes of element makes it possible to scramble the sound signal emitted by the tread pattern of the tyre, that is to say to reduce the features, so as to tend towards white noise.

[0091] Beating, also known as amplitude modulation, occurs when two sounds which have very similar frequencies are emitted simultaneously. If these sounds have comparable amplitudes, they cancel each other out each time they are in phase opposition, and then add together when they are in phase. This forms an amplitude modulation phenomenon.

[0092] To optimize the arrangement of the elements so as to reduce the whining and beating noise, each tread pattern element is associated with an elementary, for example sinusoidal, signal. For one complete revolution of the wheel, the associated signal is periodic and results from the sum of the elementary signals.

[0093] With the aid of a digital tool, the initial arrangement is optimized with respect to the whining and beating noise by carrying out simulations on different arrangements. Using a Fourier transform on the signal associated with the arrangement, the spectrum of the signal is analysed in the frequency domain. The criteria for stopping the optimization process are linked to the amplitude of the whining and beating features, and to their spread along the frequency axis.

[0094] At the end of this iterative approach for the tyre size studied, the total number of tread pattern elements of the tread is established at 23 pairs of lugs arranged in the sequence: ABBCCBAABAABBCCBCCBAABC, around the circumference of the tyre equal to 6465.4 mm. The tread pattern of the tread of the tyre that is created therefore comprises 7 A elements, 9 B elements and lastly 6 C elements. The following table recaps the features of the lugs in at least one row R1 or R2:

TABLE-US-00001 TABLE 1 Mean lug Circumferential Num- Circumferential thick- spacing of an Incli- ber length of a lug ness element nation A elements 7 307.6 48.7 234.3 47.6? B elements 9 332.7 52 275.7 47.6? C elements 6 357.8 60.3 334.8 47.6?

[0095] FIG. 2 shows the harmonic spectrum of the arrangement of the tread pattern elements. It reveals two frequency clusters, H1 and H2. The cluster H1 is centred on the harmonic no. 23 linked with the number of pairs of lugs, and the cluster H2, for its part, is linked with each single lug, i.e. 46 lugs. The appearance of the spectrum shows the absence of features and the spread of the radiated-power amplitudes of the clusters H1 and H2, which confirm the optimization of the spectrum linked with the adopted arrangement.

[0096] In this example, the maximum height of the lugs H.sub.max is 65 mm, and the inclination angle ? with respect to the circumferential direction is 47?.

[0097] With the volumetric void ratio TEV being equal to the ratio between the total volume of the grooves separating the lugs and the total volume radially contained between the bottom surface and the tread surface, the volumetric void ratio TEV is equal to 80%.

[0098] The tyre produced in the size 710/70 R42 was subjected to a test evaluating the noise in the passenger compartment. The test consisted in measuring the noise in the passenger compartment with the aid of two microphones positioned by the driver's ears, on driving at constant speeds of 55/50/45/40 km/h over a road with an asphalt pavement.

[0099] The tyre tested was mounted on the rear axle, inflated to a pressure of 1.6 bar, and subjected to a load of 5350 kg. On the front axle, the tyres are smooth following planing.

[0100] FIG. 3 shows the result of the test, which is the harmonic spectrum of the signal measured in the passenger compartment of the vehicle. The broken-line curve C2 represents the tyre according to the invention and the solid-line curve C1 represents the reference tyre. In the range of frequencies 100 Hz-130 Hz covered by the resonance of a harmonic of the tread pattern and the natural mode of the cab, the amplitude of the acoustic power is significantly reduced, thereby eliminating the nuisance perceived by the user.

[0101] The invention can be easily extrapolated to tyres, for example and non-exhaustively, which equip utility vehicles for mixed use on a road with an asphalt pavement, and/or on an all-terrain road.