Note: Descriptions are shown in the official language in which they were submitted.
~59899~
PNEUMA~IC TIRE ~OR HEAYY VEHICLES
BACKG~OUND OP THE INVENTION
1. Field of the invention
This invention relates to heavy duty pneumatic tire for use in heavy
vehicles such as trucks a~d buses, having tread patterns incorporating
anti-skid grooves of a shape to minimize stone pick-up and stone retention
without reducing wet-grip performance of tires.
2. Description of the prior Art
Pneumatic tires are conventionally provided with a tread surface
having an anti-skid pattern defined by circumferential grooves or, and
transverse grooves which are molded in the surface of the tread during
manufacture.
In general, a pneumatic tire has a tread patterns of the lug, rib,
rib-lug, block, lug-block or rib-block pattern type which is selected in
accordance with performances required for the tire depending on the use of
the tire, seasonal factors, road conditions, etc. especially a pattern of tread
grooves for heavy duty tires for use in heavy vehicles such as trucks and buses
are mainly used with a tread pattern of the rib, rib-lug or rib-block type
pattern with respect to the strong requirment for wet grip performance and
long wear life tread performance.
However, such a tread Pattern baving circumferentially extending long-
itudinal grooves has the following drawbacks when compared with the other tread
patterns such as a lug-type pattern.
That is, stone pick-up is apt to occur in the circumferential groove.
If this occurs and the tires is driven, it is difficult to discharge stones
from the groove due to the strong restraining force of the groove. As a result,
the stones strike the base rubber between the bottom of the circumferential
grooves and the reinforcement for the tread portion on the belt for every
~25~394
rotation of the tire and tend to cause the breakage of the base
rubber and hence damage the belt layers or carcass plies inside
the tread rubber. Particularly, when the belt is comprised of
metal cords, rust is induced by penetrating water from the dam-
aged portion of the base rubber in-to the belt or the carcass
plies and as a result, separation failure of metal cords from the
rubber is caused. Finally there is the fatal possibi~ity of dam-
aging the durable life of the tire.
AS a countermeasure against such stone pick-up, lt has
hitherto been proposed to arrange a groove with a step ~
shaped groove) continuously extending toward the circumferential
direction of the tire on the groove walls of the longitudinal
groove and to arrange a protruded stripe continuously extending
toward the circumferential direction of the tire on the groove
bottom. In the former case, the stone pick-up is somewhat pro-
tected in the early skage of -the tire use by the wider groove and
the narrower groove. However, when the wider grooves disappeared
due to tread wear, only the narrower groove remalns, and as a
result, there is problem to reduce the wet grip performance in
the worn tire saving only the narrower groove. In the latter
case, the base rubber on the groove bottom is somewhat protected,
but it cannot practically and satisfactorily be expected to
facilitate the discharge of picked up stones.
The present invention advantageously solves the afore-
said problem without reducing the wet grip performance in pneu-
matic tire for heavy vehicles having a rib-type tread pattern
irrespective of having a carcass of bias or radial construction.
According to the present invention a heavy duty pneu-
matic tire for heavy vehicles having a tread comprising at least
one circurnferential groove provided on the surface of the tread,
and extending zigzag in the circumferential direction of the tire
to form alternating pro~ecting portions and re-entrant portions
on the opposed groove walls thereof; a plurality of pro~ectlng
-- 2 --
1~59~39a~
blocks within said circumferential groove and projecting oppo-
sitely from the opposed groove walls under the tread surface to
form a narrow groove between the axially ad~acent projecting
blocks in the circumferential groove, and arranged circumferen-
tially discon-tinuously to form a space between circumferentially
adjoining projecting blocks at the projecting portion of the
groove wall; the space between circumferentially adjoining pro-
jecting blocks being in a range of 10% to 50% of the circumferen-
tial length of the projecting block; and at least one cut in the
lo pro;ecting block, and extending radially to the circumEerential
groove. Suitably the open tip width of the narrow groove is in
the range of 5% to 50% of the open top width of the circumferen-
tial groove. Desirably the narrow groove is of zigzag form whose
angle to the circumferential direction of the tire is smaller
than the angle of the zigzag of the circumferential groove to the
circumferential direction of the tire. Suitably the width of the
cut is under 2 mm. Preferably the depth of the cut is almost the
same as the depth of the narrow groove.
Thus, according to the present invention a heavy duty
pneumatic tire having a tread provided on its outer surface with
at least one circumferential groove, the blocks thereof are
intermittently arranged on both groove walls of the aforemen-
tioned circumferential groove from the groove bottom and the sur-
face of the projecting blocks is under the outer surface of the
tire, and between the projecting blocks on the opposite groove
wall to each other, the circumferential narrower groove is to be
arranged to divide both opposite projecting blocks in the axial
direction.
; 30
The present invention will be explained in more detail
referred to the accompanying drawings, in which:
Fig. l is a developed plan view of an embodiment of the
tread portion in the tire according to this invention;
-- 3 --
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Fig. 2 is an enlarged view of a circumferential groove
embodying this invention;
Fig.s 3, 4 and 5 are sectional views taken along the
line A-A of Fig. 2; and
Fig. 6 shows the results of the tests conducted to com-
pare th~ conventional rib pattarn and the pattern embodying this
invention in respect of the relation between the number of picked
up stones and the travelling distance.
~ eferring to Fig. l, a heavy duty pneumatic tire 1 has
at least one circumferentially extending longitudinal grooves 2,
and number of the projecting blocks 5 to be defined by the nar-
rower groove 4 are intermittently arranged extending toward thecircumferential direction of the tire on the groove walls 3 of
the circumferential longitudinal grooves 2. In a preferred
embodiment of the pneumatic tire of the present invention a lon-
gitudinal groove with a narrower longitudinal groove has a zigzag
groove form and projecting blocks 5
- 3a -
~ Z59~39~
have a number of knife cut 6 spaced at a preferable pitch.
On the groove wall 3 of the cicumferential groove 2 are disposed
plural projecting block 5 each extending from on the groove base 7 of the
longitudinal groove 2 and b0ing defined by a split part 8 arranged at the
corners of respective groove wall 3, and the interval ~2 of between the adjoin-ing projecting blocks 5-5 is approximatelY 10 to 50 % of the circumferential
length el of each projecting block. The upper surface 5a of the Projecting
block 5 is at a distance Hl the tread surface G and having a step 9, where height
~1 is arranged to be 0.2 to 0.8 time of the height H of longitudinal groove 2.
The upper sarface 5a of block 5 is generally parallel to the
tread surface G when it leans toward the axial direction or the circumferential
direction of the tire can be used as another embodiment of the present invention,
forexample, as shown in ~IG.4, its upper surface 5a leans to be higher toward
the narrow groove 4 than at the groove wall 3 or as shown in FIG.5 its upper
surface 5a leans to be lower toward the narrow groove 4 than at the groove
wall 3.
The groove width Wl of the aforesaid narrower groove 4 which defines
the projectin~ blocks in the axial direction is preferably to be 5 to 50% of
the groove width W of the longitudinal groove 2 to drainage performance.
the lowering of wet grip performance by wearing down of the longitudinal wider
groove 2 can be prevented by adopting such arrangement as the angle K2 of
the zig-zag form of thr narrower groove 4 extending in the circumferential
direction of the tire should be smaller than the angle Kl of the zig-zag
form of the longitudinal groove 2.
The knife cuts 6 arranged to have about 2 mm width on the projecting
block 5 extend toward the axial direction of the tire and its depth is the sameas the depth of the narrower groove. The knife cuts 6 work to increase the
drainage efficiency, when the surface 5~ of the projecting block 5 beglm to
contact the road surface after the longitudinal wider groove 2 disappeared
due to wear down.
85~
As aforementioned, when the tread rubber wear down to the extent where
the wider groove of the longitudinal groove 2 disappears, the narrower groove 4appears on the tread surface and the groove width contacting with road suddenlybecome narrower to Wl from W in the case of ~IG.3.
However, after the longitudinal wider groove 2 disappears, the split
parts 8 having a beforesaid spaces ~ 2 which are arranged between projecting
blocks 5 assume the function to work as a traverse groove to maintain the wet
grip performance. Accordingly, if the beforesaid space Q 2 is less than 10 %
of the circumferential length ~ 1, of the block 5,the wet grip performance
become to be insufficient, on th~ other hand, if e2 is more than 50% of the
circumferential length ~ 1 of the block 5, there is a tendency toword non-
uniform or the uneven wear of the edges 5b of the block 5, after the continuoustravel of the tires over a long distance for a long period of time.
According to the present invention, if obstructions scattered on a
road surface such as ground stones and the like are picked up by the
circumferential grooves, the block 5 prevent bitten stones from reaching to thebase rubber between the botto~ of the circumferential grooves and the
reinforcement or the belt for the tread portion and causing the breakage of
base rubber and damaging the reinforcement or the belt inside the base rubber
and the bottom of the circumferential groove has such a step that the dischargeof picke up stones is facilitated.
~ urthermore the projecting blocks have a function to control the
movement of the rib portion 10 at the ground contact area for every rotation ofthe tires and to prevent rib edges from sufferin~ the uneven wear.
As stated hereinbefore, the invention effectively and adequately
solves the harmful influence of stone pick-up at the tread groove by arranging
a pluality of projectingt blocks on both groove walls of longitudinal grooves ofa so-called rib-type tread pattern irrespective where the carcass is of bias orradial construction without raising other serious defects particularly without
lowering the wet grip performance even if the circumferential grooves having
34
a wide open groove width due to wear.
The effectiveness of the present invention was confirmed by the
following tes$ with respect to two tires A and B having the same open groove
width of the circumferential groove, wherein the tire A is the above illustrated
tire having the tread pattern with a circumferential groove with the proj~ctingblocks intermittently disposed on the opposed groove walls thereof as sho~n in
FIG.l,~IG.2, FI6.3 and the tire B is a prior art tire having a U-shaped
circumferential groove without the projecting blocks S as shown in FIG.l.
The detailed specifications on test tires are shown in the following table 1.
Table 1
_ A B
Wider open groove width of the longitudinal
groove W ~mm) 13.0 13.0
Angle of the ~ig-zag form of a longitudinal
groove ~1 ( ) 17.0 17.0
Circumferential length of the projecting
block ~ 1 (mm3 25.0
Space between the projecting brocks
e 2 (mm) 5.0
Depth of the longitudinal groove
N (mm) 14.5 14.5
. _
Height of the step on the projecting blocks
Hl ~mm) 5.U
_
Narrower groove width between projecting
block Wl ~mm~ 3.0 _
Angle of the zig-~ag form of the oarrower
groove ~2 ( ) 15.0
_
Degree of damages by stone pick-up on the
groove bottom 0.2 2.4
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Test conditions are as follows :
~ adial tires of 10.00 R20,14PR. having steel cord layers for
reinforcing belts and rims of 7.50V x 20 were tested with an inner pressure of
7.25~g / c~ and subjected to a load of 2,700kg on a tire and traveling speed was
about 50 to 70 km / h.
The degree of damages by stone pick-up on the hottom of the
circumferential groove and the number of stones picked up in the circumferential
groove of each tire were measured respectively with respect to the traveling
distance.
The relation between the traveling distance and the number of picked up
sto~es was shown in FIG.6, from which'"it can be recongnized that the tire A
according to the,present invention hardly produces the stone pick-up, and any
degree of damages in the table 1 means the rate of suffering damages on the
groove bottom by the stone picked up and the great improvement of the tire A isshown.
