Note: Descriptions are shown in the official language in which they were submitted.
1050869
BACKGRO~D OF THE I~ENTION
-- . :
`
The present invention relates generally to
pneumatic tires~ and more particularly to pnewma-tic
tires equipped with a specially designed ~read to reduce
~ 5 the amount of noise generated by the tire as it rolls
-~ upon a g~ven surface~
Among the many ~actors ~rhich relate to the
generation of noise by pneumatic tires, there are four
factors which are most paramount. These factors are
~.
(1) resonant lug vibration, (2) slip/stick vi~ration, ~ ;~
(3) air pumping and (4) resonant rein~orcement
"Lug vibration" results ~rom stressing the lugs
...
in the contact patch (the footprint or inter~ace between
t~e road and the tread~ in mutually perpendicul~r direc~
tions within the pl&ne of the contact ~atch as we'l -
as in a direction normally of the contact patch. The
~ore and aft stresses of the lugs in the~contact patch
play the greatest roll in generating noise. As each~ ;
lug leaves the contact patch, the stresses are suddenly
relieved and the lug thereby "pops out" of the contact ; ~-
patch, undergoes severe vibrations and generates noise.
"Slip/stick vibration" occurs ~s various portions
o~ the tread ~t least partially slide ("slip'7) over the
.
- road in dif~erent directions a~ various spots in the
contact patch. In the course of such sliding, at one
or more other spots ln the contact patch the horizontal
shear stresses are usually sufficiently low to allow ;
,. : ~- ,:
~ interfacial (contact patch) friction to instantaneously ~ `
.,
~ ~ "anchor" one or more portions (for exa~ple, one or more ~ ;
-
,
' ' ~ ' ~ ' ' ,
, ' ' '', ' ' ' ' '` ' ~' . ', .' ~' . , ;' I ,
` ~ ' - ' '~' -'-' . ' ' ' ' ' , ',
--2-- .
` 1~5~869 :
lu~s~ of the tread to the road at the spots of low
stress level. Continued partial slidin~ of the tread,
ho~rever, results in a build-up of stress ln the
"anchored" lugs ~hich releases them from their anchored
condition and causes them to undergo an instantaneous
slip. ~ ~ :
If the tire is one which is prone to permit the
vibration of its lugs to resonate, the "slipping" and
"stic~ing" of the tread lugs, ~hen in sync.lronism ~rith
the vibratory resonances of the tire, are subjec-t to re-
- inforcement causing the lugs to undergo more se~ere
vibrations and generate noise of increased intensit~
"Air pumping" is the action o~ forcing air in
and out of the voids and sipes in and between the tread
lugs. As a given void or sipe enters the contact patch,
its ~olume is suddenly co~pressed ~hereby expe~ling or
pumping air out. As each void or sipe leaves the
contact patch, its volume suddenly e~p~nds, thereby
pumping air back in. This rh~hic pumping of air in
and out of the voids or sipes generates conti~uous
~ . : .
pressure ~Javes or sound energy and, if excessive, nois~.
"Resonant reinforcement" involves certain mass
- dis~ribut~ons and elastomeric propertles o~ a tire ~hich
result in the vibration of parts of the tire that respond
2~ to vibration-inducing energy i~parted to the tire in a
repetitively timed sequence by reacting to create sh~rply
increased ~ibrations at various speeds of the tire. If
.
the tread lugs are spaced from one another such that
~ tbeir excit~tion peak frequencles coincide with the
resonant frequencies of the tire, resonant reinforcement
is said to occur with the result o~ sound bulld-up.
.
-2
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Summary of the ~nvention
Accordingly~ it ls an ob~ect of the present ~:
lnvention to provide a tread design whlch takes into
account -the aforementioned four factors of noise gener-
: 5 ation (as well as others) ln order to minlmize the ef~ect
of each
In order to minirnize the extent o~ lug vibration,
the tread of the present invention has been designed such
: that each presents an inclination (defined generally by `~::
, . . .
10 the major void or groo~e between adjacent ones o~ the -~ ~.
lugs) which forms an acute angle with -the circumferential
center-line o~ the tread. This is in accordance witn a ~ -
;~ determination that lugs extending at an angle o~ sub-
stantially 90 relative to the circumferential center- ~
15 line o~ the tread generate substantially more noise than .~; :
lugs incllned at an acute angle relative to such center- .
~ .
,, lineO
Moreover, in order to reduce lug vibra-tion, the
lugs have been designed to present a thickness o~ approxi- -
mately two inches between the grooves which flank the
. respective lugs, it having been determined that lugs
which are too thin (on the order, for example, of an
inch and a half) or lugs ~hich are too thic~ (on the
order, for exarnple~ of in excess of three inches)
generate greater noise than lugs which are approxirnately ~:
~wo inches in thic~ness.
The present invention also contemplates anchor-
lng of each of the lugs of the tread to a central c~.r-
: ' .
cum~erential rib in order to re~uce the ~lexibility o~
the lugs and~ thereby~ interfere with the slip/stick
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. .
~L~I 369
vibrations.
With respec-t to "resonant reinforcement", the tread
of the present invention includes lugs wh:ich are generally
similar to one another in configuration, each of the lugs
and its corresponding groove adjacent thereto in a given
circumferential direc~ion together defining a pitch of specified
length which circum~erentially is dimensionally identical
to the pitch of certain others of the lugs and their
corresponding grooves and is yet dimensionally different
from the pitch of still others of the lugs and their
corresponding grooves.
The lugs and their corresponding grooves are
circumferentially arranged such that certain (though not
necessarily all) of the lugs and their corresponding grooves
of the same pitch adjoin each other in succession in the form
of a pitch-identifiable group differing in pitch from an ~
adjoining pi ch-identifiable group of still others of the lugs
and their corresponding grooves. A selected arrangement of
- the groups which a~join each other in circumferential succession
~ 20 define a pitch sequence which over its circumferential extent
.
has excitation peak fre~uencies that do not coincide with the
xesonant frequencies of the tire.
The sound generated as a result of "air pumping" is
reduced substantially by venting all of the sipes to a major
void or groove which in turn itself is vented to the outside to
prevent air entrapment.
In accordance with one broad aspect, the invention
relates to an Lmproved pneumatic tire having a substantially
~` toroidally shaped carcass provided with a pair of bead
reinforced sidewall regions which are bridged by a
circum~erential crown region and a tread overlying and secured
to said crown xegion, the improvement comprising a plurality of
~ _4_
lOSO~
circumferentially spac~d lugs 1n said tread, each of said lugs
being similar in confi~uration, said lugs being arranged in
said tread on OppQsite sides of the circumferential centerline
of said pneumatic tire, and a continuous circumferential rib
means disposed along said circumferential centerline of said
pneumatic tire and integral with said lugs to anchor said luys `.
on either side of said circumferential centerline, each of said ~-
lugs having a groove associated therewith which groove ~-
determines the physical boundary of said lug, each of said lugs ~:
and its associated groove having a predetermined p.itch length ~ ~.
measured circumferentially of said pneumatic tire, all of
said lugs and their associated grooves being arranged such -
; that a number of said lugs and their associated grooves of
the same pitch length join each other in succession to form a
pitch identifiable group which pitch identifiable group differs
in at least its pitch length, from adjoining pitch identifiable
groups formed from others of said lugs and their associated .~ :
grooves, a selected arrangement of said pitch identifiable
groups which adjoin each other in circumferent.lal succession
: 20 defining a pitch se~uence which over its circumferential ~
extent has excitation peak fre~uencies that do not coincide
with the xesonant frequencies of said pneumatic tire. -~
BRIEF DESCRIPTION OF THE DRAWIN~S
With the abo~e and additional objects and
,, ~'.
3~ ~ :
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- 4a - :
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5- ~sal~36~ ~
advantages in v:Le~r as will hcreinafter appear, the
instant invention conlprises the dev:lces, comb:inations
and arrangements of parts hereinafter described and
illustrated in the accompanyln~r drawi.ngs of a preferred
embodiment, in which:
; Fig. l is a perspective view o~ the pneumatic
tire pursuant to the present invention; ;
Flg. 2 is a schematic view -lllustrating the
general deckline profile of the ~ajor grooves in -the
tread of the present invenkion;
Fig. 3 is a schematic plan development of the
lug pattern of the present invention, though not
necessarily in a preferred sequence; and
Fig. 4 is a schematic representation o~ the
preferred "pltch sequence" of the present in~ention.
Dekailed Descri~tion of the Invention
; Referring now to the drawings, and more partic-
ularly to Fig~ 1, the present invention relates generally
to a pneumatic tire denoted by the reference character
lO. The tire 10 includes a pair of opposite bead-
rein~orced sidewalls 12 which are bridged by a circum-
ferentially extendlng tread 14. The tread 14 includes
. j
what may be characterized as a road-contacting portion
- 16 and a pair of opposite non-road-contacting portions
18 (only one of which is sho~n) which annularly adjoin
corresponding ones of the sidewalls 12.
The tread 14 is provided with a circumferenkially
extending rib 20 ko which are anchored on ei~her side
there~f a plurality of tread lugs 22. The lugs 22
derine therebetween respective grooves 24 Each of the
;'~ 1, ~ .
, ... . .
-6- `
~ 8 ~9
grooves 24 has a ~irst extent 26 ~hich is inclined with
respect to the rib 20~ and a second ex-tent 28 formed ln ~ ~;
; the non-road-contacting portion 18 of the tread 14 and
which extends beyond the depth of its corresponding
flrst extent 26 formed in the road-contac~ing portion 16
of the tread 14. Each of the aforementioned second `~
extents 28 of the grooves 24 is directed generally
: radially of the tire 10 and closes proximate to its
corresponding one of the sidewalls 12 ~ ~
As illustrated in Fig. 2, the first extent 26 : ~ :
of each of` the grooves 24, over the majority of its
~ .
length, has a substantially uniform depth, whereas
each of the aforementioned second extents 28 of each
of the grooves 24 has a varying depth which decreases
,i . , ~.
in the aforementioned second extents 28 in the ~ ~.
direction toward its corresponding one of the sidewalls
12. Accordingly~ each of the grooves 24 has a deckline
,~
~ ~ profile defined by the inner but exposed surface 30 of ~
.1: .
each of the a~orementioned first extents 26 and ~y the ~`
inner but exposed surface 32 of each o~ the a~ore-
; mentioned second extents 28. Pursuant to the present
.,
invention, the inner but e~posed surfaces 30 and 32 ;~
merge with each other ~t a radius o~ curvature in
excess o~ the radius of curvature with whIch the outer~
l~ 25 most surfaces 22a and 22b of each of the lugs adjoin
I each other.
.l As illustrated in Fig. l~ each of the lugs 22
in the non-road-contacting portions 18 is formed with
`! a substantially V-shaped cut-out 34 to reduce the amount
.j
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- . ~ ,. ~ ..... . .. . .
7 :~5(~
of tread s~ock used in the tlre 10 and to enhance
slightly the degre~ o~ flexlbllity of each o~ the lugs
22 at -the s~dewalls 120
Referrin~ now to Fig. 3~ there is schematically
illustrated a plan development o~ a portion of the
tread 1l~ to show ~le relationship of the lugs 22 ~nd the -
grooves 24 defined therebetween ~ith one another. It ~-
will be understood, however, that the illustrated
juxtaposition of certain of the lugs 22 ~ith one another
may not necessarily be preferred. The preferred juxta-
position o~ the lugs 22 with one another will~ however,
be discussed below.
~; Each of the lugs 22 is substantially ~dentical to
one another in con~iguration though, as well be dlscussed
below, their relative sizes may be different. Each of
the grooves 24 is provided with three sipes; n~nely~ a
first sipe 36 which is inclined with respect to the
circlLmPerential center-line ~ o~ the tread 143 a second
~ipe 38 which ext~ends substantially parallel to the
circumferential center-line L~ and a third sipe 40
which is inclined only sli.ghtly with respect to the cir-
cumferential center -line L.
Moreover, the aforementioned first extent 26
of the grooves 24 is provided with a first wall 26a
which is inclined, preferably~ with respect to the
circumferential center-line L at approximately 40~,
and a second wall 26b which is inclined with respect to
the circumferentlal center-line L at approxlmately 43.
Thus~ each of the lugs 22 may be said to be inclined
with respect to the circumferential center-line L or
~7-
- ~8- 1 ~
1~)50869 `:
the r~b 20 at approximately l~o-43. As illustrated~ each
of the sipes 36, 38 and 40 opens into its corresponding .
one of its ~rooves 24 and may~ thus, be characterl~ed ! ~
; as "vented" sipesO . '.~ :
5 Preferably, the lugs 22 are arr~ged on either ' ~ 'side of the circum~erential center-line L as two arrays, ' ~::
' the lugs 22 o~ one of the arrays on one side o~ the '.~
, ,. ~ .,
rib 20 having an orientation relative to rib 20 which
ts opposite of, ~nd out of phase ~ith (or staggered
. .
. lO relative to), the orientation of the lugs 22 of the
'~ ` other of the arra.ys on the other side of the rib 20.
' Moreover, at a juncture ~ bet~leen the road-contacting
: portion 16 of the tread 14.and the non-road-contacting
portion 18 o~ the tread ~, each of the ~ugs 22 ha.s
a~circu.~ferential span S of a.~proximately two inches.
" ~ Ho~rever, as will be seen below, ertain of the
~ lUg9 22 hæve a circuraferential span S which may be ~ J:~
; slightly grea~er or less than two inches, this dif~erence
: in circ~mferential sp~n o~ the lugg~22 at the ~uncture ~ . '.
~ J being the principal basis for the difference in size
~' : of the lugs 22. The difference in siz~e o~ the lugs 22
and the relative juxtaposition of the differently sized
lugs 22, in combination with certain~of the structural
relationships aforeme~tioned~ give~rise~to a tread
~: 25 ~ pattern of lugs;~ hich prevents.the~resonance of~'sound'~
that may be o~her~ise generated by the tire when the
latter is put into veh~cular use and~rolle.d on a~'surface, :~
for example, in a range encomp~ssine~approximately fifty
'miles per hour.
30 : ~ : With respect to l'resonant rein*orcementl!, in order to ~
appreclate thé manner by ~h~ch the~present ~vèntion achieves ~ ~ -
lts ol~ective ~o dl~inish the amount o~ nois~generated by
9-- ::
~ 0S~69 ~ ~
a rollin~ tlre,certain terms will now be defined. Each
of the lu~s 2~ is associated with a corresponding
(ad~acent) one of the grooves 24 in a given circum- ;
ferential directionO The circumferentLal span S of ; ;~.
- 5 a lug 22 when added to the circumferential span G o~
its adjacent correspondin~ groove 24, at the ~ncture J
between the road-contacting por~ion 16 and the non-road~
contactlng portion 18, gives rise to the term "p~tch"
hav~ng units of length
It will be understoodg that for purposes herein
; - the term "pitch" re~ers to a circu~erentially ex~ending
dimension o~ a portion of the tread 14, and does not
. re~r to a Farameter or characteris~ic o~ sound. Thus,
as illustrated in Fig. 3~ the combined extents S~
resul~ in a speci~ied pitch, ~or~example, the pi~ch B. `~
: . Similarly3 the circum~erentiaI span Sc when added to
. clrcumferentia~ span GC gives rise~to the pitch C. The
ame ls true with respec~ to the pitch A.
As illustrated in Fig. 4, the lugs 22 (with ~ ~;
~: 20 their corresponding grooves) o~ the present invention
are divlded, for example, into three catagories each
of ~he categories being defined by. a specified pitch;~
: namely, the pitch ~, the pitch B and~the pitch C~ Not~
all of the lugs 22 o~ the same category or p~tch are
~;~ 25 ~ Juxtaposed adjacen~ to one a~other~in series.` 0nly a ;~
cer~ain predetermined number of~thè lugs 22 o~ the same
~ pitch are juxta~osed adjacent to orè~another, for example,
; ~ two, three or four o~ such lugs 22... Each group of lugs
:: : 22 of the same pitch wh~ch are ad~acent to one another ~ ~ -
30 ~ ~s characterized herein as a "pltch-identifiable" group :
' ~ ~
9..
: ,
. ~ . . . .
. : .
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~L~5~ 69
~ident~fiable by pitch and not necessarily by the number
Or the lugs 22 of a particular group).
Pursuant to the prese~t invent-Lon~ each pitch~
ldentifiable group separates two other pitch-identlfiable
groups ~hose respective pitches dlffer from the group
~eparating them but may or may not dif~er .rom each
otherO For example, as illustrated in Fig~ 4, there
exists a p~tch-identifiable group denoted by the refer- :
ence character A' 3 each o~ the lugs of which (with ~s
corresponding groove) has a pitch A. To the le~t o:~ the ;~
group ~', there exists a group o~ thrce lugs 22 each
ith its corresponding groove) having a respective
pitch B, that group being denoted by reference character
B'. Similarl~J, to the right o~ the group A', there
exists a group o~ two lugs 22 each (with its correspond-
- ing groove~ having a respective pitch B~ that group
being denoted by the re~erence character B". Thus~ in
this instance~ the pitch-identi:~iable group A' separates
: the two groups B' and B'~ having the same pitch but : .
; 20 d~fferen~ numbers of lugs. A similar situation exists
with respect to the group C'which separates the group
B' of three lugs aforementioned ~on the right of the ;
. group C') from another group B" i ~on the left of the
: ~ group C'), It will be seen that the ~roups B', B" and
B" ' are each comprised o~ lugs;having the-same pitch B~
A dif~erent situation-exists with respect- to the
group B' of three lugs aforementioned which separates the~
~ a~orementioned group~A' ~hose lugs have:a pitch of A3
`~ ~from the group C' aforementionéd (~Jhose lugs have a -
~30 ~ p~tch of C)g
Xt is the rel~tive ~uxtaposition o~ the various
., ~
86~ , "
groups A', B' and ~' in series or circumferential
succession that gi~-es rise to ~rhat is c'haracterized
herein as a "~itch sequence"O The pitc'h sequence of
' pitch-ldentifiable groups is designed so that the
resonant frequencies of the tire ~re not excited in a
tlmed, re~e~itively coinciding3vibratory sequence when
the tire is rolled on a surf~ce. In other wor~s, the
lugs of the tread design of the present invention are
arranged so that their peak excitation frequencles do
not coincide repeti~ively ~Tith the resonant frequencies
of the tire. As a result, the vibrations of the lugs
do not rein~orce one another or other means o~ tire
vibration, and are allowed to be-dissipated without
generating excessive sound.
1$ ~at must be emphasized with respect to the
pitch sequence of the present invention is that it is
not a rPndomly or arbitrarily selected pattern. It is ~ -~
an empirically determined arrangement established as a
result of evaluatin~ the physical characteristics o~ a
par~icular type of lug pattern of a'particular type of
tread and predetermining the various resonant frequencies
o~ tha~ lug pattern w'hich must not be excited at various
. .
speeds o~ the tire.
For example, each type o~ tire, like any article~
capable of vibrating, has one or more resonant frequencies. ~'~
m e ~arious resonant frequencies can be determined by
: .
rolling the tire at ~arious speeds to~excite the tire
into ~ibrating at its different resonant frequencies.
, ~ - - . .
'~ ~ ' By gradually increasing the rolling speed of a ~est
-specimen tire having a design wherein the lugs cire o~
~ the same size and equidistantly spaced ~rom each ot'her,
i :
~ the sound level gradually increases. However, at clertain
, - -11
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~ -12- 1~50~69
. ~:
speeds of the specimen tire9 the sound levcl (usually
measured in decibels) suddenly rises rapidly and
graphic~lly peaksc The frequency of each "peak" sound
ln such a tire occurs at a dlfferent reson2nt frequency
~ 5 o~ the tireO Each "peak" (or resonant) frequency is
; easily determinable by conventiona~ audio equip!.~ent.
A11 of the so-called "peak" or resonant fre
quencies o~ the specimen tire over a preferred range
o~ speeds that the tire is expected to be used can be
similarly deter~ined. A graph can then be established
in which sound intensity, in decibels, is ~lotted alon~
: .. ..
the vertical axis ænd sound frequency, in cycles per ~ - `
- - ~
- second, is plotted along the horiæontal axis. The re-
sultant curve ~Jill present varioUs~pea~s ~hich, in this
~nstance, constitute the resonant frequencies of the
. ~ ~
specimen tire. It is these resonænt ~requencies that
must not be overly excited by a~lug~pattern~of a;tir~e~
that is to radiate a low level of~noise and is onè~
si~ilar to the one from which the~aforementioned ~raph
- 20 is derived, but altered slightly with respect to the
distance between, or pitch o~, the lugs of the lug~
pattern.
In this respect, a ~irst modi~ied lug pattern
design can be selected (for example, arbitrarily, ~ri~hout; 25 ~ building same) such that the spacing~between the lugs is
altered ~rom the a~orementionea specimen tire. Such
alteration involves a slight;chan~e in pit~h between
ad~acent ones of the lug5 such that the leading edges
~-~ o~-~he lugs are not equidistantiy~spaced uniformly over
the`circumference o~ the tread. It will be understood,
that the resonant ~requcncies o~ the aforemenfloned specimen
tire are substantially the same as the~r~sonant ~requencies
.
~ ~ -12-
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~,; . : : . : . . ,
-13- ~ ~ 5~ 8 ~ 9
o~ the firs-t modif`ied lug pat-tern design because of
the only sli~ht alteration ln pitch of the modified
lug pattern desi~n ~rom the specimen ti.re.
~ith a first modified lug pattern design as so
selected (not built), one c~n "calculate1' a graohic
spectrum of e~citation frequencies that; wou d occur
o~er a pre~erred range o~ speeds that a tire encompassing
such a pattern ~ould be e.xpected to be used. Because
of the lack of uniformity in the modi~ied '.ug pattern
design, the various calculated pe~cs presented by the
graphic spectrum of excitation frequencies are not
necessarily the various resonant frequencies o~ such ~
tread deslgn. Ho~Jever, such various "calculated" peaks
can be used as a basis of comparison with the "empiricall~
: ~
~15 determined" peak (and, in fact, the resonant) ~requencies ; ;~
. . .
; o~ the a~oremon~ned spe~imen tire appearing in the afore~
mentioned gr~ph derived lro~ the specimen tire. If ~he
calculated" peæ~s coincide with ~he "empirically deter~
; mined" peaks, such a modified lug pattern design ~Jould
generate an unacceptable high level of noise. I~ the
"calculated" peaks do not coincide with the "empi~ically
determined" peaks, such a modified lug pattern design
- w~uId generate only a 10~J~ ànd thus tolerable, level of ~ -~
noise. In such inst~nce, it- is said ~hat the excitation
- 25 peak ~requencies o~ a tire having the~modified lug
; ~ pattern design do not coincide iJith the resonant ~re- ~-
- quencies o~ such a tireO Such a tire is, there~ore, a
;quleter r~ding tire.
It will be understood ~hat second ~d stll~ other
modiried lug pattern desi~ns may have to be selected and
evaluated in order to maximize the number o~ excitation
pe~k ~requenc~ 'ocations that do not~coincide w~th the
.
~ 3~
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4_
1~5~ 9
reson~nt frequency locations of each such design. By
ef~ecting "non-coincidence" at certain local;ions~ other ~-
locations origlnally non-coincident may be caused to
become coincident. ~ach such cause-and-effect must be
accounted for before achieving the most desir~ble quiet~
riding tread pattern designO ;
The procedure by which the graphic spectrum ol ~ -
excitation ~requencies is calculated is a standard Fourier
Harmonic Analysis in which each leading edge of each lug
is characterized as producing a unit pulse o~ excitation,
and in ~,rhich the ti~.ing of repetition of the ~ulses of
excitation is equal to the repetitive dist2nces between
~he leading edges divided by the rotary speed o~ the
tire. Such an analysis is described in a text entitled ~ -
;~ 15 "Mathematical Handbook for Scienti~st~s and Engineers" ~
published by iIcGraN-Hill ~1961) and~authored by Xorn and
Korn. Section 4~ 4 particularly involves l'Fourier
Analysis (Har~onic Analysis) of Periodic Functions".
It is by the above process~that the pitch se~
quence of the present invention lS determined. With
the test information so established, the lugs 22 can be
sized with selected circumferential dimensions ~pitch) ;~
`~ ~ and arr~nged in~selected groups. ~The groups can then be
arranged in a p~referred series or ~itch sequence~which
25~ most ~effectively allows th vibrations to dis6ipate~uith
out reinrorcing one another.
With respect to a preferred embodiment of the~
present invention, the tire 10 of thè present invention
may be one ~hich, for example, presents a bead diam6ter
~;30 of substantially t~Jenty-two inches~and a maximum in~lated
wldth, from sidewall-to-sidew-~ll, of~approximatèly tcn
~! inches~ rith such dimensions, it ha~ been-determin~d~
14
-15- ~.050869 . ~ ~:
that the tread of the tire 10 preferabl~ include su~
~tantially thirty-slx lugs in ~ircumferential succession
on each side o~ the central rib 20. As such5 those lugs
22 havin~ a pitch A have a circum~erentLal span o~, for
example, 4.03 inches~ whereas the lugs 22 having a pitch
B have a circum~erential sp~n of, for example, 3.71
inches, and whereas the lugs 22 having a pitch C have
sircumferential span of, ~or examp~e,3.34 inches. The
lu~s 22 having a pltch A are twelve in number3 whereas
the lugs ~2 having a pitch B are sixteen in nu~ber, and
; whereas the lugs 22 having a pitch C are ei~ht in number.
Generall~, it is pr~rred that the lugs 22
having a pitch C be approximately gc$ ~plus or minus
2-1/2~) of the circumferential span of the lugs 22 hav~
:, ,, . , ~ , ..
ing a pitch 3. Similarly, it is pre~erred that the lugs
~` 2? having a pitch A be appro~imately 110%~(plus~or minus; ;~
2-1/2~) of the circumferential span of the lugs 22 hav-
ing a pitch~B. The pre~erred~pitch sequence (as-illus-
`~ trated in Figo 4) of such a tire has been determined to
~i 20 be:
A-A-A-B~B-B-C-C-C-C-B-B-B-A-A-A-B-B
This pitch sequence lnvolves~the~use of eighteen
of the lugs 22 and, thus, at least inso~ar as the tire
10 o~ the~dimensions~aforementioned is concerned, -the
a~orementioned pitch sequence is repeated a second time.
mis pitch sequence represents the preferred
sequence for a tlre of the eforem~Dned dimensions
. .
which has excitation peak ~requencies that do not colncide
with the resonant frequencies thereo~. It is just such
30~ ~ a pitch~sequence~that, although alone~will substantially
reduce ~read noise, will also, ln cornbination ~ith ~a)
the deckline pro~lle o~ the ~ented grooves 24, and
-:
5~
.~ . . . ., - . .. .
` ~16-
6 9
~.
(b) the inclination of the extent 26 of each of the
grooves 24 with respect to the central rib 20, and ~c)
the anchoring of tread lugs of appro~imately t~o inches
in circ~mferential span to a central rib, opti~ize a
reduction in lug vibrations, slip/stick vibrations and
air pumping ænd~ thereby, optimize a reduction in the
a~ount of noise generated by the ~ire lhen it is rolled
on a surf~ceO
In short, when a conventional tire is rolled on
~ given surface, for example, in a range encompassing
approximately fi1ty miles per hour~ as each tread element
sr.ar~ circu~ferentially when exiting fro~ the contact
patch, it creates an impulse of vibration. ~nen the
timing of these impulses are such as to coincide with
the timing of the resonances o~ the tire, large resonant
`; - vibrations are induced, and this in tllrn causes large
amounts of noise. In addition, the large levels o
resonant vibrations then promote slip/stick vibrations
at the sarne frequency, ~Jhich slipjstick vibrations thQm~
selves create vibra~ory impulses ~!hich assist in main~
taining the large resonant vibrations
However, when the spacing and timing o~ the pitch
sequence is properly irre~ular (as is so in the present
invention)~ the impulses of snappin& due to the tread
.
~ 2~ lugs both partiælly rein~orce and partially interfere
~ ~ .
~ with the resonant vibrations o~ the tire. The net re- ~
: ~ , .
sult ls primarily a diminished de~reè of resonant vibrations.
Correspondingly, there ~s secondarily effected a reduction
in the amount of slip/stick excitation resonances of the
tire and, there~ore, there is a double basis for a re-
duced le~el o~ noise that accrues just frorn the pi-tch
:~ :
sequence o~ the present lnvention~ The other des1~n
~16-
~. . ., . ... : ... ~
17- ~05t)~369 ` ~
`,~
features of the tlre o~ the present invention complement :
the pitch sequence in still further reducing noise
- generationO
It will be understood thzt the foregoing des-
cription of a preferred embodiment of the present in-
vention is for purposes of illustration only and the
various structural and operational fea.tures and re~
lationships herein disclosed are susceptible to a n~umber
o~ modific~tions and changes none of ~hich entails any
departure fro~ the spirit and scope of the present in-~
vention as defined in the hereto appended claims.
!
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