Note: Descriptions are shown in the official language in which they were submitted.
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FOOTPRINTS FOR NO'~TROTATABLE AUTOMOBILE
AND LIGHT TRUCK TIRES
Technical Field
This invention relates to automobile and light truck tire combinations
designed specifically
for either the front wheel position or rear wheel position of front engine
four wheeled mounted
vehicles.
to Background Art
Automobile and light track vehicles that have front engines and front steering
suspensions
have a vehicle weight distribution that is heavily loaded on the front
position tires and lightly
loaded on the rear position tires.
Light truck tires are routinely driven with no weight in the bed of the
vehicle causing the
rear tire position to typically operate at SO % of the tires rated load. When
the truck is carrying
weight the load can be increased up to 100 % of the tires rated load on the
rear tuns.
Mini-vans and sport utility vehicles in addition to being weight distribution
sensitive have
higher centers of gravity than automobiles.
These multi purpose vehicles (1V~V) put greater demands on tires due to their
higher center
of gravity and non-uniform weight distribution. This combination causes the
vehicle to undergo
greater amounts of vehicle roll putting higher loads on the outside shoulders
of the front tires with
resulting increases in wear rates. Goodyear developed the Wrangler GS-A and
Wrangler Aquatred
to meet these demands by engineering a tread design with distinct tread zones
for specific
performance demands. The outside shoulder was solidified to resist the
tendency for fast front tire
shoulder wear while providing traction through tread blocks in the other zones
of the tread. This
design approach provided an improved level of tread-wear and traction for
MPV's in all wheel
positions.
Tires for the front wheel positions of MPV's are subjected to special demands
because of
the higher center of gravity of the vehicle and the greater tendency for the
vehicle to roll onto the
outside shoulders of the tire. Looking at the footprint patch of the prior art
tire in the front right
wheel position (as depicted in Fig. 1) one notes the higher outside contact
area of the shoulder 2.
The tire geometry has been designed by increasing the tread mass distribution
in this portion of the
tread to resist the higher pressure and abrasion. The remainder of the tread
area is optimized for
traction and hydroplaning resistance. This state of the art design approach is
embodied in the
Wrangler GS-A and Wrangler Aquatred tires and represents the present state of
the art in tread
designs for MPV's.
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2
The rear wheel position of MPV's creates special demands on the tires because
of the
lighter and variable loading of the tire. The footprint of Fig. 2 depicts the
same prior art tire of
Fig. 1 when placed on the rear position at 50% load. At this position the
centerline pressure of the
footprint is highest and needs to resist abrasion. Larger tread elements are
needed at the centerline
to resist this higher abrasion tendency while still providing traction and
hydroplaning resistance by
providing open shoulders a design that is in direct conflict with the needs of
the front position tire.
The requirements for an all wheel position tire require a balance of design
characteristics to
meet the performance needs of both front and rear tire applications.
Inevitably, certain design
tradeoffs or balances must be made in order to achieve performance levels for
both positions. This
to results in a balanced design that cannot be fully optimized for either
position. The Wrangler GS-A
and Wrangler Aquatred represents the state of the art approach in balanced
performance while still
meeting the special needs of the front tire positions on MPV's.
To push tire performance to the next level, new approaches to design and
materials must be
discovered. One attempt to push design towards the next level of tire
performance for MPV's light
t5 trucks and automobiles focuses on optimizing the functional design based on
wheel position
requirements. The front tire requirements have been addressed by the prior art
but were not fully
exploited because of all wheel position demands. The demands of the rear
position have been
studied and understood but until recently have not been fully optimized due to
the requirements for
all wheel position capability. This invention focuses on optimizing both front
and rear positions by
2o designing a combination of tires having a unique tire for each position
front and rear. The front
tires can be more fully optimized to resist fast outside shoulder wear while
providing outstanding
wet traction and handling. The rear position can be more fully optimized to
resist the fast
centerline wear associated with rear wheel drive and light loads while
providing high levels of
driving traction.
25 To optimize the front tire position, the invention employs multiple tread
radii contouring
the tread to achieve improved tread pressur$ distribution. Additionally, tread
pattern mass is
adjusted to enhance anti-hydroplaning performance while still providing
resistance to outside
shoulder wear. Since front tires encounter water on the read first, they must
be more capable of
anti-hydroplaning performance than the rear tires that run in their trough or
wake.
3o To optimize the rear tires position, the invention also employs multiple
tread radii
contouring the tread to achieve full contact of the tire footprint from
shoulder to shoulder. This
allows for more even tread-wear across the tread with secondary benefits of
traction improvement
through full tread pattern contact. This has been previously difficult to
achieve because of the light
loading of the rear position and the use of all wheel position design balances
resulting in high
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centerline pressure and wear. Additionally, the tread mass distribution can
now be optimized to
resist fast centerline abrasion with tmad design pattern details specifically
suited for rear wheel
drive traction.
Optimizing both front and rear designs based on functional requirements can
result in tires
that require no rotation and perform to higher levels of performance than
tires that are designed for
all wheel position use.
While it has been known in the art that specific tires for a particular wheel
position can
improve performance by the selection of certain distinct tread patterns that
are wheel specific as is
taught in German patent DE 3901624A1. These concepts have been limited to
exotic racing type
to vehicles or high performance vehicles which may even employee different
sized tires.
The invention disclosed below employs not only a distinct tread mass
distribution but also
teaches a specific footprint shape factor at normal pressure for variable
vehicle loading conditions
which can be achieved by unique tread arc curvatures for the front position
tires and the rear
position tires.
I5 Description of the Invention
Summary of the Invention. A pneumatic radial tire combination for four-wheeled
automobile or light track vehicles has a pair of front steer position tires
and a pair of rear position
tires. Each front steer position tine or rear position tire has a footprint,
each footprint has an axial
width W, as measured at the lateral extremes of the footprint, a centerplane
CP midway between
2o the lateral extremes of the footprint.
The tire combination has a pair of front steer position tires and a pair of
rear position tires.
The front steer position tire has a footprint when the tire is normally
inflated for normal load that
has a footprint shape factor of greater than 1.00 at a 50 % load, and about
1.00 at both 85 % load
and 100 % load. The footprint shape factor is defined as the maximum
circumferential extent of the
z5 tire's footprint at the centerplane of the tile's footprint width divided
by the average of the
cir~cumferential extent of the tire's footprint width as measured at 40% of
the footprint from both
sides of the central plane of the footprint.
The rear position tire footprint, when the tire is normally inflated for
normal load, has a
footprint shape factor of about 1.00 at 50 % load, and 1.00 or less than 1.00
at 85 % load and 100
30 load when measured similar to the method described for the front position
tires.
The footprints of each tire is divided into a central region, and a first
shoulder and a second
shoulder region. The central region extends 20% of the footprint width W on
either side of the
centerplane CP. Each first and second shoulder region extends from a lateral
edge of the footprint
to the central portion. The tire combination has the footprint of the steer
position tires having a
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tread contact area at normal inflation and load wherein the central region
contact area is less than
the contact area of a first or second shoulder area, while the footprint of
the rear position tires have
a contact area at normal inflation and load in the central region greater than
the first or second
shoulder portions.
Preferably, the central portion tread contact area of the front steer position
tires is less than
the first shoulder and the second shoulder contact areas respectively and the
central portion of the
tc~ead contact area of the rear position tires is greater than the first and
the second shoulder contact
In a preferred embodiment, the central portion of the front steer position
tires has a wide
to circumferential groove having a groove width of about 10% of the footprint
width W at normal
load and inflation while the rear position tire has two wide circumferential
grooves, one wide
groove being located between the contact area of the central portion and the
contact area of each
first and second shoulder area.
The inventive pneumatic radial tire combination for four-wheeled automobile or
light track
15 vehicles has a pair of front steer position tires and a pair of rear
position tires each tire having an
axis of rotation and a casing. The casing has a carcass and a belt
reinforcement radially outward of
the carcass. A tread is radially outward of the belt stnlcture of the casing.
The tread has a pair of
lateral edges, a tread arc extending between the lateral edges and a
centerplane passing midway
between the lateral edges and perpendicular to the axis of rotation.
2o The tire combination has a first tread arc for the front position tires and
a second tread arc
for the rear position tires, the tread arcs defined by a radially outer
surface of the treads when the
tires are normally inflated but unloaded.
The first tread arc has a curvature of maximum radius at the centerplane of
the tire and a
curvature of decreasing radius extending toward the lateral edges, at an
intersection of the lateral
~s edge and the tread are a straight line drawn between the intersection of
the tread arc curvature and
the centerplane is inclined at an angle eF of greater than 5 ° relative
to a tangent line L, L being
tangent to the tread arc at the centerplane and parallel to the axis of
rotation.
The second tread arc has a curvature of maximum radius at the centerplane of
the tire and
when measured similar to the tires for the front position has an angle gR of
less than 5° relative to
~o the tangent line L, L being tangent to the second tread arc curvature at
the centerplane and parallel
to the axis of rotation.
In the preferred tire combination, the front tire position first tread arc
curvature extending
from the centerplane to a lateral edge has at least three radii of curvature
R,, R,, R3 decreasing in
size as the curvature extends from the centerplane to each lateral edge and
the rear tire position
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second tread arc of curvature has at least two radii R,, RZ of curvature
decreasing in size as the
curvature extends from the centerplane to each lateral edge.
The front tire position radius R, is the radius of curvature at the
centerplane and R3 is the
radius of curvature at the lateral edge. Most preferably R, is greater than
twice R3. R, of the front
5 tire position is preferably about 600 mm and the tread radius Rj is greater
than 200 mm.
The radii of curvature R,, RZ, R, of the front position tires have the
following ratio of
lateral width to arc width on at least one-half of the tread of less than 50 %
for R,, and greater than
20 % for R, and R, respectively.
The rear tire position is radius R, preferably greater than 800 mm and the
radius of
io curvature Rz of the rear position tire is less than 200 mm.
The radii of curvature R, and R, of the rear position tires have the following
ratio of lateral
width to arc width on at least one tread-half of greater than 50 % for R, and
between 30 % to 50
for R,.
Definitions
is "Aspect ratio" of the tire means the ratio of its section height (SIB to
its section width
(SV~ multiplied by 100 % for expression as a percentage.
"Asymmetric tread" means a tread that has a tread pattern not symmetrical
about the
centerplane or equatorial plane EP of the tire.
"Circumferential" means lines or directions extending along the perimeter of
the surface of
2o the annular tread perpendicular to the axial direction.
"Equatorial plane (EP)" means the plane passing midway between the width of
the tread
and perpendicular to the tire's axis of rotation.
"Groove" means an elongated void area in a tread that may extend
circurnferentially or
laterally about the tread in a straight, curved, or zigzag manner.
Cirrrumferentially and laterally
~s extending grooves sometimes have common portions. The "ghoove width" is
equal to tread
surface area occupied by a groove or groove portion, the width of which is in
question, divided by
the length of such groove or groove portion; thus, the groove width is its
average width over its
length. Grooves may be of varying depths in a tire. The depth of a groove may
vary around the
circumference of the tread, or the depth of one groove may be constant but
vary from the depth of
3o another groove in the tire. If such narrow or wide grooves are of
substantially reduced depth as
compared to wide circumferential grooves that they interconnect, they are
regarded as forming "tie
bars" tending to maintain a rib-like character in the tread region involved.
"Inboard side" means the side of the tire nearest the vehicle when the tire is
mounted on a
wheel and the wheel is mounted on the vehicle.
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"lateral" means an axial direction.
"Net contact area" means the total area of ground contacting elements between
defined
boundary edges divided by the gross ama between the boundary edges as measured
around the
entire circumference of the tread.
"Net-to-gross ratio" means the total area of ground contacting tread elements
between the
lateral edges around the entire circumference of the tread divided by the
gross area of the entire
tread between the lateral edges.
"Non-directional tread" means a tread that has no preferred direction of
foward travel and
is not required to be positioned on a vehicle in a specific wheel position or
positions to ensure that
io the tread pattern is aligned with the preferred direction of travel.
Conversely, a directional tread
pattern has a preferred direction of travel requiring specific wheel
positioning. "Axial" and
"axially" means lines or directions that are parallel to the axis of rotation
of the tire.
"Outboard side" means the side of the tire farthest away from the vehicle when
the tire is
mounted on a wheel and the wheel is mounted on the vehicle.
i 5 "Radial" and "radially" means directions radially toward or away from the
axis of rotation
of the tire.
"Rib" means a circumferentially extending strip of rubber on the tread which
is defined by
at least one circumferential groove and either a second such groove or a
lateral edge, the strip being
laterally undivided by full-depth grooves.
~o "Sipe" means small slots molded into the tread elements of the tire that
subdivide the tread
surface and improve traction.
"Tread element" or "traction element" means a rib or a block element.
Brief Description of Drawings. Fig. 1 is a plan view of a footprint of a prior
art tire
made according to U.S. patent 5,415,215, the tire being normally inflated and
loaded representing
a front steer position mounting on a light truck or automobile.
Fig. 2 is a plan view of a footprint of a prior art tire made according to
Fig. 1 the tire
inflated for a 100 % load as in Fig. 1 but being loaded to 50 % representing a
lightly loaded rear
position mounting on a light truck or automobile.
Figs. 3A, 3B and 3C are plan views of the footprints of the front steer
position inventive
3o tire, depicting the tire inflated to the 100 % load but loaded to 50 % , 85
% and 100 % loads
respectively .
Figs. 4A, 4B, and 4C are plan views of the footprints of the inventive rear
position tire,
depicting the tire inflated to 100 % load but loaded to 50 % , 85 % and 100 %
loads respectively.
Figs. 5 and 6 are cross-sectional views of the preferable embodiments
inventive front steer
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position tire and the rear position tire respectively.
Figs. 7 and 8 are plan views of portions of the preferred treads for the
inventive front steer
position tire and the rear position tire respectively.
Detailed Description of the Invention. With reference to Figs. 1 and 2 a prior
art tire's
s footprint or contact patch is shown. The tire shown is the Goodyear Wrangler
Aquatred has tread
and features disclosed in U.S. Patents 5,415,215 and 5,658,404. The tire
footprints were taken at
a constant inflation pressure of 35 psi ( 240KPa) which is the inflation at
100 % standard maximum
or maximum load for the tire shown, a P235/75R15 as specified in the Tire &
Rim Association
Handbook. For comparative purposes each test tire 10, 20 and the prior art
tire used this size tire.
to After the tire is broken in using the ASTM break-in procedure for the tire
the footprint
shape can be determined.
To measure the footprint shape a tire is either inked and pressed against a
paper or
cardboard sheet which is laid on a flat hard surface at a fixed load and with
the tire inflated at a
fixed pressure leaving the impression of the tread on the paper or cardboard
surface. This
t s technique of footprinting is old in the tire art and is commonly
understood. Alternatively, inkless
procedures are also available which include carbonless paper, pressure sensing
pads and the like.
In all cases, one of the objectives is to get the tmad contacting surfaces
within the footprint defined.
Once the tire engineer has the footprint shape he or she can make several
observations or
predictions about the tire and its tread.
~o Historically, the butterfly shaped footprint was determined to be
undesirable.
Alternatively, the footprints having a shape similar to the bow of a boat were
considered desirable
for pushing water away from the center of the tread. As shown in Figs. 1 and 2
the prior art tire
exhibits this bow shape of footprint.
Inherently, when the leading and trailing edges of the footprint are not
axially extending,
~s that is if they are curved or bowed, this means that as the tire rolls a
portion of the tread contacts
the ground first and laterally adjacent tread elements follow. This can cause
a phenomena known
as tread element squirm. As the tread elements leave the treads footprint the
elements snap out of
the contact patch as the pressure holding the element against the road is
released. The elements
lightly contacting the read are slid across the roadway wearing the element
similar to sliding rubber
3o eraser across a sheet of paper. The inventors of the present invention
believe ideally the tread
elements should have a uniform pressure distribution laterally across the
tread and more preferably
the leading and trailing edges of the footprint should be axially extending in
a straight line path
under all operating conditions.
To better understand this ideal relationship, the inventors have developed a
concept and
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methodology to define the footprint shape factor.
First, the maximum axial width W of the footprint is measured. Then, the
distance
halfway between the maximum axial width W is defined as the tire's centerplane
CP. A distance
40 % of the tread width (R~ on each side of the centerplane is located as
shown as reference
numerals 2, 4. A circumferential line S, 6 is drawn through points 2-2 and 4-4
respectively and the
length of line Ls, and Is= is calculated, summed and divided by 2 to arrive at
an average shoulder
length Ls. The footprint length L~ at the centerplane is measured. The
footprint shape factor F is
the ratio of h~JL.s.
As shown the footprint shape factor F of the prior art tire was 1.12 at normal
inflation and
to normal load, at the same pressure and at 50% load the footprint shape
factor F is 1.50. As can be
easily appreciated the footprint's shape is very different at these different
loads.
This prior art tire employed a large single radius of curvature R, and
attempted to optimize
the tread mass distribution for a "balanced tire" with very good wear
characteristics. Nevertheless,
the prior art tire had to be rotated after a period of use to optimize the
tread wear rates. The
problem is that these tires for light weight tracks face such a wide range of
loads particularly on the
rear position tires.
One partial solution to this variable load problem would be to reduce or
increase the tire's
inflation pressure to match each load condition. A central inflation system
could be mounted on
the vehicle which could automatically adjust the tire pressure to match the
load. In such a case the
?o footprint shape would most likely remain almost constant. The drawback to
such a system is cost.
The consumer would appreciate receiving this neutcaiizing of load variances at
no cost. To achieve
this result the tires must become somewhat load insensitive.
Figs. 3A, 3B and 3C show footprints of a tire 10 for a front steer position
four-wheeled
vehicle. Figs. 4A, 4B and 4C show footprints for a tire 20 for a rear position
four-wheeled
'S vehicle. In the figures each tire is inflated to a constant pressure and
then the tire is broken in
using ASTM tire break-in procedures. The inflated tire is then loaded to 50 %
, 85 % , and 100
loads and a footprint is taken at each load. The footprint shape factor F is
then calculated as
previously described.
Ideally the length Ls.' of the left side of the tire footprint is very close
to the same length Ls,
so as the right side, that is the distance between the points 2-2 and the
distance between points 4-4 are
about the same. Since one of the objectives is to have the leading and
trailing edges being almost
axially extending at or during the most typical vehicle load condition for
each wheel position it is
considered important that the difference in length from point 2-2 relative to
point 4-4 does not
exceed 10 % , more preferably about 5 % or less. This means that the left side
2-2 could be greater
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in length than the length L~ while the right side is smaller than Lc with the
resultant effect being
L~JL,S equaling 1.00 however, in such a case the distance 2-2 would be 105 %
and the distance 4-4
would be 95 % of the length L~. This asymmetry of the footprint shape can be
tolerated as long as
the deviation from generally axially extending leading and trailing edges is
rrlinimized. The main
feature is that the tires 10, 20 when exposed to the most common vehicle load
for the wheel
position, should have the footprint shape factor within 5 percent or less of
the LzJIy atio of 1.0
most preferable within 2 % . For example, given the front tire position
achieves this result between
85 % and 100 % of vehicle load while the rear position tire sees this
condition near the more typical
50 % load condition.
to The inventive front steer position tine 10 has a footprint shape factor
greater than 1.00, at
the 50 % load, and about 1.00 at 85 % load and about 1.00 at 100 % load. In
the particular
embodiment tire 10 illustrated the footprint shape factor was 1.13, 0.99, and
0.98 respectively for
the 50 % , 85 % and 100 % loads.
In the rear position tire 20 the footprints shown in Figs. 4A, 4B and 4C have
footprint
shape factors of about 1.00 at 50 % load and 1.00 or less at both 85 % load
and 100 % load. In the
particular embodiment illustrated the footprint shape factors were 1.00, 0.91
and 0.90 for the 50%,
85 % and 100 % loads respectively when measured after being broken in. In the
front steer position
tires 10 the expected vehicle loads on the tire are typically in the 85 % to
100 % standard maximum
load range. The four-wheeled front engine mounted vehicles typically exhibit
that much weight on
2o the forward portion of the vehicle. Therefore, the footprint shape factor F
can very closely be
designed to the ideal 1.00 give or take a few percent for the 85 % and 100 %
load. In the
exemplary tire 10 this was achieved when the 50% loaded tire 10 had a broken
in footprint shape
factor F set greater than i .00, preferably about 10 % greater.
In the rear position tire 20 the vehicle load range varies from the 50 % load
typically to
2s 100 % less frequently. Therefore, the tire 20 has the footprint shape
factor set at about 1.00 for
50 % load and less than 1.00 for the 85 % and 100 % loaded conditions. The
initial 50 % loaded
footprint shape factor preferably is less than 1.05 with about a .10 drop from
the initial 50 % loaded
condition as the loads increase to 85 % and 100 % .
It can be observed that a similar .10 or more drop occurred in the front
position tire 10 as
so the loads increased from 50 % to 85 % and 100 % . The difference being at
the 50 % loads the front
tire footprint shape factor is about 0.10 or more greater than the rear
position tire at the inflation
pressures as tested.
Having described the novel footprint shape factors needed to produce a
combination of tires
for a four-wheel front engine mounted vehicle that are specifically designed
for the front steer
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wheel position and the rear wheel position the readers attention is called to
Figs. S and 6. Fig. 5
shows the cross section of the tire 10 for the front steer position whereas
Fig. 6 shows the cross
section of the tire 20 for the rear position.
As shown the tires 10 and 20 have very similar structures and are made of
similar
5 components. Alternatively, the tires may employ different tread compounds
and other component
differences more suited for the specific wheel position if so desired.
Each tire 10, 20 has an axis of rotation R, a casing 30, the casing 30 has a
carcass 32 and a
belt reinforcing structure 34 radially outward of the carcass 32. The carcass
32 preferably includes
at least one elastomeric ply 36 reinforced with radial cords and an inner
liner 35 and a pair of
to sidewalls 21. The ply 36 is wrapped about two annular bead cores 40 as
shown. The tires 10, 20
each have tread 22 located radially outward of the belt reinforcing structure
34 of the casing 30.
The tread has a pair of lateral edges 42, 44, a tread arc 46, 48 extending
between the lateral edges
and a centerplane (CP) passing midway between the lateral edges 42, 44 and
perpendicular to the
axis of rotation. The combination of tires 10, 20 exhibit two distinct tread
arcs 46 for the front tire
10 and 48 for the near tire 20. The first tread arc 46 for the front position
tire 10 and the second
tread arc 48 of the rear tire 20 are both defined by the radially outermost
surface 52 of the tread
elements when the tire is inflated but unloaded.
The first tread arc 46 has a curvature of maximum radius R, at the centerplane
CP of the
tire 10 and a curvature of decreasing radius R=, R, extending toward the
lateral edges. As shown at
2o an intersection 47 of the lateral edge and the tread arc a straight line 50
is drawn between the
intersection 49 of the arc and the centerplane CP. The line SO is inclined at
an angle eF greater
than 5 ° relative to a tangent line L, L being tangent to the tread arc
at the centerline CP and
parallel to the axis of rotation.
It is believed preferred that both tread halves extending from the centerplane
CP to the
lateral edges 42, 44 have similar curvatures, however, it is contemplated that
the curvature could
be asymmetric or distinct between mead halves but it is believed that the line
50 should be inclined
at an angle gF, gP being greater than S ° on either side.
What is a clearly unique feature is that the tire 20 for the rear position has
a line 50 inclined
at an angle gR, gR being less than 5°.
3o For simplicity of measurement, these features can easily be measured
directly from the tire
mold or the engineering drawings. The rear tire 20 like the front tire 10 is
preferably symmetrical
on each tread half in terms of tread arc curvatures although the inventive
concept is not limited to a
symmetry of curvature.
The inventors believe that the above-described curvatures of the tread arc
provide one very
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11
good if not the best way of way achieving the footprint shape factors F.
In the front tire 10 the preferred tire had at least three radii of curvature
R,, R2, R3
extending from the centerplane to a lateral edge 42 or 44. The radii R,, RZ,
R3 decrease in size as
the curvature extends from the centerplane to each lateral edge. R, is the
radius of curvature at the
centerplane, the radius R, extending laterally to less than 50 % of the are
width while the radii R
and R3 each have lateral width of greater than 20 % of the arc width as
measured between the
centerplane CP and the lateral edges 42, 44 or on one-half of the tread as
shown in Fig. 5.
The tire 10 of the present invention had the R, radius at least twice as large
as R3,
preferably about three times and RZ falling in between R, and R3 in size and
location. R, for the
io test tire 10 was about 600 mm, R: was about 300 mm, while R, is greater
than 200 mm, about 229
mm. The lateral width ratio of R, was 45.2 % , RZ was 24 % and R, was 30. 8 %
. The actual total
tread arc width being the sum of 38.12 mm for R,, 20.22 mm for R=and 25.9 mm
for R3 or 84.24
mm for each half of the tread as shown in Fig. 5. The resultant shoulder drop
at the intersection
47 relative to the line L is about 8 mm and the angle gF was 5.4°.
For the preferred rear tire 20 the tread arc curvature 46 had two radii, R,
and R=, Rl being
greater than 800, or about 834 mm while RZ was less than 200 mm, or about 175
mm. The lateral
width ratios of the curvatures R, and R~ were greater than SO % for R, and
between 30 % to 50 %
for RZ. The actual test tire R, width to arc width ratio was 61. 8 % while RZ
was 38.2 % on each
tread half. The actual widths 53.64 mm and 33.2 mm on each tread half for a
total tread arc width
2o per tread half of 88.84 mm. The tread shoulder drop off at the intersection
47 was about 7 mm.
While gR was less than 5°, measuring 4.6°.
For comparison the prior art Wrangler Aquatred had a single tread radius R, of
about 433
mm and a tread width ratio of 100% with a total tread arc width of about 80 mm
per tread half.
The resultant angle g was 5.3°. What this meant was that the prior art
tire was arguably closer to
satisfying the angular relationship for the front steer position, however,
when one studies the
footprint shape factor F as shown in Figs. 1 and 2, a bow shaped footprint
outside the desired
range is exhibited. This to the inventors means that the prior art tire
although a good "balanced
tire" failed to optimize the features needed for the combination of unique
steer and rear position
applications.
3o Having established the basis for distinct optimized footprint shape factors
F and unique
tread aro curvatures for each wheel position the final problem solved by the
inventors was the
placement of and type and shape of the tread elements.
Since a specific tire was needed for each wheel position the inventors have
the freedom to
select distinct front and near position tread patterns. In Figs. 7 and 8 are
plan views of the
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preferred tread patterns.
In tire 10 the tread 22 has a wide cincumferential groove 60 in the center of
the tread
bounded by closely spaced tread elements 70 adjacent the wide groove. If the
tread were divided
into three equal width zones 80, 81, 82 between the lateral edges 42, 44. The
central zone 81
would have a net contact area less than either shoulder zone 80 or 82. While
the zone 80 should be
equal to or greater in net contact area than the other shoulder zone 82.
The rear tire 20 tread as shown in Fig. 8 has two wide circumferential
grooves, one such
groove 61, 62 being bounded by tread elements 70 and being located with each
half of the tread
and being spaced laterally about midway of each tread half. The central zone
91 has a net contact
to area greater than the net contact area of shoulder zones 90 and 92. The
shoulder zones 90, 92 have
net contact areas about equal as shown with open laterally extending grooves
94 for traction. This
combination of tread patterns optimizes the tread wear rates for each wheel
position while
enhancing snow and wet traction performance.
The front tire i0 has a tread arc width slightly narrower than the rear tire
20. This
t5 complements the use of only one wide cireumferential greove 60 on the front
tire. While the wider
rear tire employs two such grooves 61, 62, this combination has very
pronounced deep-water anti-
hydroplaning benefits. The narrower front tire pushes the water out of the way
the wider rear tires
following in the wake of the front tire sees less water in the center of the
tread and a little more in
the shoulders zones when traveling at speed. This is a phenomena that is
likely to result when a
2o driver suddenly comes upon deep puddle water on a roadway and unexpectedly
has no time to
reduce speed. These tires greatly enhance the vehicle's ability to track
straight without
hydroplaning.
It is believed beneficial if the wide groove, 60, 61, 62 leave a width of at
least 7 % of the
total tread width, preferably about 10 % .
:.5 As can be seen from the above description, the inventive front tire 10 is
dissimilar to the
rear position tire 20 is a multiple of ways. The footprint shape factors, the
tread arc curvatures and
the tread mass distribution are distinct. These differences due to wheel
position sensitivity are
noticeable. In commercial tires for heavy-duty trucks specific tires for the
various wheel positions
is common practice. In the lightweight vehicles such a practice was heretofore
avoided. By
3o discovering a way in which vehicle tire rotation is not needed to achieve
acceptable tread wear, the
present invention greatly enhances the agencies ability to tune the tire for
optimal performance.
One such way is to provide a harder better wearing tread compound on the front
wheel
position and a softer more traction sensitive compound on the rear tire.
Similarly a lower rolling
resistant compound can be used on the front wheel position while a superior
traction compound is
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used on the rear. By tuning the tires in such a way, the performance of the
combination will be
superior to the performance of the balanced or all position tires of the past.
