GB2093777A

GB2093777A - Tire with tread for snow traction

Info

Publication number: GB2093777A
Application number: GB8136842A
Authority: GB
Original assignee: General Tire and Rubber Co
Current assignee: Aerojet Rocketdyne Holdings Inc
Priority date: 1981-02-23
Filing date: 1981-12-07
Publication date: 1982-09-08
Also published as: JPS57147902A; DE3146362A1; PT74460A; LU83930A1; PT74460B; IT1139834B; FR2500375A1; IT8125258A0

Abstract

A plurality of transverse grooves 8 are provided in the tread of a tire 2, each groove 8 having both sidewalls 10, 11 slanted in the same direction as one another with respect to the normal to the tread surface 4 (Fig. 2), with the sidewalls of some grooves 8 being slanted in one direction r and the sidewalls of the remaining grooves 8 being slanted in the opposite direction, whereby the tire 2 has snow traction regardless of which direction it is rotating. The angle a at which the sidewalls 10, 11 are slanted may be 15 DEG , and the grooves 8 may be disposed at 75 DEG to the circumferential. The groove sidewalls may be parallel to one another as shown in Fig. 2 or such that each groove is of V-section. The grooves 8 may be located in ribs bounded by circumferential grooves 6. <IMAGE>

Description

SPECIFICATION Tire with tread surface for snow traction This invention relates to a tire having a tread surface that provides improved traction in snow.

The grooves and sipes in the tread surfaces of most tires have walls that are either substantially perpendicular to the tread surface, as shown in U.S. Patents 2,962,072; 3,658,108; 2,501,828 and 3,945,41 7, or converge toward the base of the groove to form a V-shape as shown in Figure 3a of U.S.

Patent 4,122,879. Grooves such as these, however, do not provide the best traction in snow, because their edges tend to ride on top of the snow rather than bite down into it. The shouider grooves of U.S.

Patent 4,214,618 and the grooves of U.S. Patent 3,586,086 have walls that are disposed at acute angles to the tread surface. However, these grooves extend circumferentially around the tire, and consequently they do not function in the same way transverse grooves would in digging into snow.

A tire with transverse grooves that does provide better snow traction is shown in U.S. Patent 2,891,594 to E. Ford. The tread of the tire of that patent has extremely large lugs of almost square cross section. Between the lugs are equally large grooves. The lugs and the transverse grooves are slanted with respect to the normal to the tread surface inwardly and away from the direction the tire is intended to rotate. With this slant, the lugs dig into soft earth or snow in much the same manner as the teeth of a back-hoe shovel scoop up dirt. However, the extremely large lugs make the tire very heavy, and where vehicle fuel economy is an important consideration, the added weight of any kind of enlarged lug or rib is a major drawback in attempting to improve snow traction in this manner.Furthermore, the large grooves between the lugs tend to pick up rocks and other debris that can damage the tire. Another disadvantage of this is that the slanted lugs shown in U.S. Patent 2891594 will improve traction only when the tire is rotating in one direction. Thus, it is necessary that such a tire be mounted on the wheel with the lugs pointing in the correct direction, and, in cases where such tires have white sidewall designs or lettering on one side (only), they have to be purchased in matched sets. Furthermore, the snow traction characteristics of such a tire are substantially reduced when the vehicle is driven in reverse.

In another approach to improving traction, U.S. Patent 3,000,421 to N. Hack et al. teaches the use of rounded, inclined transverse ribs that buckle under the pressure of contact with the road. The rounded shapes of the tips of these ribs causes them to be bent radially inwardly, thus closing the grooves between them, regardless of which direction the tire is rotating. According to the Hack patent, the bending of these ribs increases their area of contact with the road and in that manner improves traction. However, such round-tipped ribs would not improve traction in substances such as snow where a biting action is necessary, because by bending radially inwardly at their point of contact with the ground, the ribs present a relatively smooth surface that tends to slide over snow rather than bite into it.

Still another approach to increasing traction is the groove design shown in U.S. Patent 3,104,693 to Bolenbach. In this design, transverse grooves are slanted inwardly and in the same direction as the tire is intended to rotate. Thus, as the tire rotates, the grooves tend to close as their edges hit the ground. The patent states that this improves traction because there is more tread surface in direct contact with the road and because water and slush are squeezed out of the grooves as they close.

However, the inventors in the present case have found that such grooves decrease rather than increase traction in snow, and have found that open groove edges that dig into the snow are most desirable.

A groove design that has been successful in improving snow traction is one tried by Societe Pneumatic Pirelli S.p.A. several years ago on tires used in road rallies. The walls of these grooves have a reverse taper, converging toward each other as they approached the tread surface, instead of diverging as with conventionally tapered grooves. Near the bases of the grooves the walls merge into rounded fillets, thus giving the cross-section of the groove a tear-drop shape, with the narrow end of the tear-drop at the tread surface. The theory behind Pirelli's groove is that the groove traps snow inside its tear-drop shape and this snow tends to adhere to the snow on the ground each time the groove opening passes through the footprint of the rolling tire. This adherence between the trapped snow and the snow on the ground thus improves traction.One problem with this groove is that it also tends to trap other things besides snow, such as rocks, mud and other types of debris. The harder substances such as rocks can cause damage to the belt layers of the tire, while the softer substances such as mud can give the surface of the tire less traction rather than more traction.

An object of the present invention is to provide a snow tire that has transverse groove edges in its tread surface that dig into the snow to provide better traction regardless of which direction the tire is rotating.

Since a slanting walled groove improves traction on snow for one direction of rotation and reduce it for the other, it would be expected that a tire having some grooves slanting one way and some slanting the other would have no overall advantage over a normal tire. We have discovered that this is not the case.

The tire of the present invention has a tread surface having a series of transverse grooves, both sidewalls of which are slanted in the same circumferential direction with respect to the normal to said tread surface. One of the sidewalls of each of the grooves meets the tread surface at an acute angle in an unrounded, snow-grabbing edge extending transversely with respect to said tread surface. The tire is characterized by the sidewalls of a portion of the transverse grooves being slanted in one direction with respect to the normal to said tread surface, and the sidewalls of another portion of the transverse grooves being slanted in the other direction with respect to the normal to the tread surface. Preferably, the sidewalls of the grooves are slanted at an angle of approximately 1 50 with respect to the normal to said tread surface.The grooves may have walls that are substantially parallel to one another, or they may be V-shaped in cross section.

The grooves can thus be a shape which does not trap stones and other debris.

Embodiments of the present invention given by way of example only, will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of a section of a tire embodying the present invention; Figure 2 is a sectional view of the tire of Figure 1, taken along line Il-Il of Figure 1; Figure 3 is a plan view of the tread of the tire shown in Figure 1; and Figure 4 is a sectional view of a tire similar to the tire of Figure 1, but illustrating another embodiment of the present invention and taken in the same position on the tire as the section of Figure 2 was taken for the tire of Figure 1.

In Figure 1 , the tire 2 has a tread surface 4 with circumferentially extending grooves 6, separating the tread into five circumferential ribs 7a, 7b, 7c, 7d and 7e. The ribs 7a to 7e are each interrupted by narrow, transversely extending grooves 8. In addition the ribs 7b and 7d are divided periodically by wider transverse grooves 9 (Fig. 3). Preferably, the grooves 8 and 9 are biased at an approximately 1 50 angle to the fully transverse (axial) direction, or put another way, 750 to the circumferential direction.

As shown in the section of Figure 2, the grooves 8 are preferably very narrow slits, commonly called sipes. The sidewalls 10 and 11 of the grooves are substantially parallel to one another and meet in filleted portions 12 that form the bases of the grooves. As Figure 2 shows, the grooves 8 are disposed at an angle a (preferably 1 50) to the normal to the tread surface 4, so that when the tire is rotating in the direction of the arrow r, the edges 1 6 bite into the snow in which the tire is rolling in much the same manner as a back-hoe shovel bites into dirt as it is scooping up. This action causes the walls 10 of the grooves to bend backwards slightly, creating a further lateral force between tire and the snow that prevents slipping.

Tests were made in snow using both tires having grooves slanted as shown in Figure 2, and tires having grooves with walls perpendicular to the tread surface.

The dimensions of the tread designs of both the tires with slanted grooves and those with nonslanted grooves were the following, using the dimension reference letters indicated in Figures 1 and 3: Tread width A: 5.5 inches Narrow transverse groove spacing B: 0.4 to 0.6 inch Narrow transverse groove width C: 0.04 inch Narrow transverse groove depth D: 0.36 inch Inclination of narrow transverse grooves a; 150 for tires with slanted grooves 0 for tires with perpendicular grooves Bias angle b of narrow transverse grooves: 150 Circumferential groove width E: 0.40 inch Circumferential groove depth F: 0.36 inch Shoulder rib width G: 1.00 inch Intermediate rib width H: 0.75 inch Center rib width S: 0.40 inch Wide transverse groove width K: 0.20 inch Wide transverse groove depth L: 0.36 inch The tires were tested under standard snow traction test conditions using a layer of packed snow on top of which a layer of between 1 and 4 inches of new, unpacked snow had fallen.

The test results showed that the tires with the 1 50 inwardly slanted grooves had coefficients of friction averaging 48% more than the coefficients of friction of the tires with grooves perpendicular to the tread surface. This, of course, was with the tires rotating against the snow in the direction of the arrow r in Figure 2. Surprisingly, however, when the tires with the slanted grooves were rotated in the opposite direction from arrow r, the decrease in coefficient of friction was not nearly as great as the increase in the opposite direction.The coefficient of friction was only 13% less than that of the tires having grooves perpendicular to the tread surface, in spite of the pressure forcing the slanted groove walls 10 (Figure 2) against the walls 11, instead of spreading the walls away from each other as when the tires were rolling in their preferred directions.

Because of the surprising phenomenon observed in the foregoing tests, the tire embodying the present invention has been constructed with greatly improved traction in both directions of rotation, by approximately half of its transverse grooves 8 being slanted in the direction shown in Figure 2, while an equal number of grooves are slanted in the opposite direction. The grooves 8 in ribs 7a and 7d are slanted in the direction shown in Figure 2 while the grooves 8 in ribs 7b and 7e are slanted in the opposite direction. The grooves in the narrow centre rib 7c are either perpendicular to the tread surface, or have alternate grooves slanted in opposite directions.Since the portions of the tire having grooves that are slanted away from whatever direction the tire is rotating provide an improved snow coefficient of friction that is almost four times the decrease in coefficient experienced by the portions having grooves slanted toward the direction of tire rotation, the result is a tire that has a greatly improved coefficient of friction for snow traction in both directions of tire rotation.

In an alternative embodiment shown in Figure 4, a tire 102 has a tread surface 104 with grooves 108 whose walls 110 and 111 are not parallel, but converge toward each other at the groove bases 11 2 to form a V-shaped cross-section. However, it should be noted that both of the groove walls 110 and 111 are slanted in the same direction, specifically a direction which gives greatly improved snow traction when the tire tread moves from right to left as seen in Figure 4. As with the tire 2 of Figures 1 to 3, the tire 102 has grooves 108 slanted in both directions for improved snow traction regardless of the direction of tire rotation.

It will be appreciated that the important feature of the present invention is that the tread contains groove portions angled in both directions, and that the exact pattern of the distribution of the two respective types of groove portion (angled one way and angled the other way) is not important.

Claims

1. A tire having a tread surface with transverse grooves in it, both sidewalls of each of said grooves being slanted in the same circumferential direction with respect to the normal to said tread surface and one of said sidewalls of each of said grooves meeting said tread surface at an acute angle in an unrounded edge extending transversely with respect to said tread surface, the sidewalls of some of said grooves being slanted in one direction with respect to the normal to said tread surface, and the sidewalls of the other of said transverse grooves being slanted in the other direction with respect to the normal to the tread surface.

2. A tire according to claim 1 wherein the sidewalls of said transverse grooves are slanted an angle of approximately 1 50 with respect to the normal to said tread surface.

3. A tire according to claim 1 or claim 2 wherein said transverse grooves have sidewalls of which are substantially parallel to one another.

4. A tire according to claim 1 or claim 2 wherein said transverse grooves are V-shaped in crosssection.

5. A tire according to any one of the preceding claims in which the said transverse grooves extend at approximately 1 50 to the axial direction of the tire.

6. A tire according to any one of the preceding claims in which the tread surface is divided into circumferential ribs by one or more circumferentially extending grooves.

7. A tire according to claim 6 in which all the transverse grooves in a first said rib have sidewalls slanted in a first direction and all the transverse grooves in a second said rib have sidewalls slanted in a second direction opposed to the first.

8. A tire according to claim 7 in which there is a third said rib between the first and second ribs, and in which all the transverse grooves in the third said rib have sidewalls which are substantially parallel to the normal to the tread surface.

9. A tire according to claim 7 in which there is a third said rib between the first and second ribs, and in which some of the transverse grooves in the third said rib have sidewalls slanted in the first direction and others of the transverse grooves in the third said rib have sidewalls slanted in the first direction and others of the transverse grooves in the third said rib have sidewalls slanted in the second direction.

10. A tire having a tread surface substantially as herein described with reference to and as illustrated in Figures 1 to 3 or Figure 4 of the accompanying drawings.