CA1051331A - Pneumatic tyre and wheel rim assemblies - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A pneumatic tyre having a tread reinforced over substantial the whole of its width by a breaker assembly, a pair of curved sidewalls each terminating in a tyre bead and a radially extending reinforcing carcass ply. The tyre has a shape factor of less than + 0.20 when it is inflated to its normal working pressure.

Description

`: iO51331 '~ The present invention relates to pneumatic tyres.
It is standard practice in the vehicle industry to fit a larger tyre to carry a greater load. This is necessary because attempts to increase the load on a tyre of a particular ,~ 5 size result in excessive heat generation. Where necessary heat `' generation can be minimized by increasing inflation pressure . but this leads to loss of comfort.
It is the aim of the present invention to increase the total load-carrying capacity of a pneumatic tyre without loss of comfort or excessive heat generation in the tyre.
Accordingly the present invention provides a pneumatic tyre having a tread reinforced over substantially the whole of its width by a breaker assembly, a pair of curved sidewalls ~ each t~rminating in a tyre bead, and a radially extending ,~ 15 reinforcing carcass ply, the tyre having a shape factor Q
(as hereinafter defined), of less than + 0.85 when it is inflated to its normal working pressure.
The shape factor Q, as used herein is equal to the tangent of half the angle subtended by the idealized sidewall at its centre of curvature.
Preferably the tyre has an aspect ratio of 30 - 75i~.
`, The invention will now be described in more detail with reference to the accompanying drawings.
Figure la is a diagrammatic cross-section of an ' 25 inflated pneumatic tyre and wheel rim '! assembly showing the forces produced by inflation;
Figure lb is a diagrammatic elevation of the assembly illustrated in Figure la;

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Figure 2a is a diagrammatic cross-section of an Ti inflated pneumatic tyre and wheel rim :
:, assembly showing the deflection of the :., tyre in the contact patch when the assembly is under load and showing how the forces are modified: :
.,; Figure 2b is a diagrammatic elevation of the 1 assembly illustrated in Figure 2a;
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Figures 3a and 3b are diagrammatic cross-sections ` 10 of the inflated pneumatic tyre and wheel rim assemblies each having different , ,~ . .
,~ shape factors;
Figure 4 shows the results of comfort tests carried out on the two assemblies illustrated in ~ 15 Figures 3a and 3b;
,l~ Figures 5a and 5b are diagrammatic cross-sections of , .
. part of an inflated radial ply tyre and , wheel rim assembly showing how the dimensions ., of a real tyre may be related to the idealized tyres discussed above;
Figure 6 is a diagrammatic cross-section of part of an , inflated radial ply pneumatic tyre and wheel ~:i rim a~sembly showing a difference between the effective rim and breaker width.
The load carried by a tyre varies approximately according to the equation: L = (A + BP) d L = Load A = Structural stiffness ' ..

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:: BP = Pneumatic stiffness ; ' ; P ~ = Inflation pressure B = Pneumatic constant d = Deflection at the working load The assembly illustrated in Figures la, lb, 2a and 2b comprises a tyre 11 having a tread 12 and sidewalls 13 ~, terminating in tyre beads 14 mounted on a wheel rim 15.
~'. In general attempts to increase either structural ,: stiffness A or deflection d lead to excessive heat generation ,, .
whilst increases in the inflation pressure P lead to 1089 of comfort. The aim of this invention is to increase the pneumatic , constant B.
,i. Referring to Figures l(a) and l(b), inflation of the :
~' tyre 11 generates tensions as shown where to = PrO
,i o o - o U~6 fo is the radial component of to pulling outwards against the bead 14.
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~ Referring to Figures 2(a) and 2(b) application of a load ,.~i 20 causes the sidewalls near the contact patch to deflect 90 that fO
is reduced to f where . f = Pr cos e:-. .
This unbalances the forces round the bead by an amount exactly equal and opposite in total to the applied load.
The unbalance is proportional to:-(fO ~ ~) = P (rOco8 eO ~ r C08 e) ~
Thus the pneumatic constant B is related to:-. (fO - f) = (rO cos eO ~ r cos e) P

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' 105i331 ,~ and can be increased by increasing:-" o/p o o ... . . .
In Figure l(a) it is clear that half the height is given by ;~ ho = rO sin eO

;- Comparing tyres of the same height this invention seeks to improve load by varying rO and eO so as to maintain a constant ho while increasing rO C08 eO.
That is by reducing the ratio . ~} Q ~ 8in eO = tan e rO COS eO o ` Where Q is the shape factor.
Thus for a fixed height of cross-section and a fixed pressure the pneumatic stiffness can be increased by reducing Q, i.e. by straightening the sidewalls.
;~ Normally a tyre with re~atively straight sidewalls would return to a more circular cross-sectional shape when inflated and ~i 2~ it is therefore neceasary to change the tyre design in order to prevent this. The tyre reverts to the more circular shape bscause the increased sidewall tension tends to pull the edges of the breaker assembly inwards towards the tyre beads while the inflation pressure pushes the centre of the breaker outwards.
This tendency can be minimised by increasing the resistance of the breaker assembly to bending moments about the circumfer-ential centre line of the breaker assembly.
This can be achieved by inserting high angle plies above ' . , ' ~ :. ' "' ': . ' '' - .. ' ' :

` ~ lOS1331 ;- and below the low angle plies used in the breaker assembly, ~; increasing the thickness and stiffness of the materials uged ~, in the breaker asgembly, using high angle plies only in the ~` breaker assembly except for strips of low angle plies at the ,~ ., edges of the breaker assembly or applying any of the well-known engineering principles which determine the rigidity of reinforced structures.
~ It is also necessary to ensure that the effect of ; initially straight sidewalls is not lost by stretch due to inadequate sidewall tensile modulus. Thus relatively inextensible reinforcing cords are preferred.
The "comfort" of the tyre, or ability to ,.. : . 0.
. the effects of road irregularities, depends to some extent on the breaker tension. This is given by:-_ = PW - 2M fO

.i where T is the breaker circumferential tension .:., '~'3 R is the main breaker radius . W is the breaker width M is the rim or bead radius PW is the force per unit circumference pushing outwards.
2M fO is the combined force per unit circumference of two sidewalls pulling inwards. The breaker tension is proportional to the difference.
'4 125 Thus increasing fO or reducing Q increaseg load but tends to reduce breaker tension which leads to small but definite improve-ments in comfort.
.~ The equation may be rewritten T = 1 _ 2hoM x ' 6.

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lOSi331 Thus reducing Q leads eventually to tyre designs having zero tension when:-Qc = 2hoM
WR
and having compressive forces when c ~' Firstly it is useful to recognise that large values of W, the :¦ width of breaker, lead to small values of Qc' i.e. the possible reductions in Q are much greater for tyres in which W
is large compared with 2ho.
J Thus the scope for improved load-carrying is greater for i~ Low Aspect Ratio radial ply pneumatic tyres.
:~ Secondly it has been found that, for values of Q very much less than Qc the breaker collapses and buckles under the compressive stress. Nevertheless obvious means to support the ~`; breaker against this collapse are foreseen and will be used where the improved properties are sufficiently valuable to justify the extra cost.
Referring now to Figures 3(a) and 3(b) these show the actual inflated shapes of two tyre and wheel rim assemblies.
Figure 3(a) represents a conventional radial ply tyre having a shape factor Q = 0.90. Figure 3(b) represents a modified tyre ~! having the same height and breaker width but with much straighter sidewalls and extra high angle layers of breaker reinforcing material above and below the conventional layers used for Figure 3(a).
The shape factor for this modified tyre was Q = 0.70.
The expected improvement in performance was therefore:-l(lSi3;3~
(0.90 - 1) x 100% = 29%
(0.70 In fact, due to inaccuracies and to non linearities the figure achieved was less. About 24% increase in pneumatic stiffness and, since the structural stiffness was less, about 2~/~ increase in total stiffness.
These tyres were, in turn, pressed against the rough surface of a rotating drum and frequency spectra of the ; vertical acceleration of the axle averaged over a range of ....
speeds, were recorded.
; 10 Figure 4 compares the results for the two tyres. It is clear that over most of the frequency range 10 to 100 Hz the stiffer tyre generated less axle vibration. Thus extra pneumatic stiffness, reduced heat generation and slightly . improved comfort were achieved by reducing Q from 0.90 to 0.70.
It is emphasized that Q factor refers to the inflated shape and that most tyres show Q factors much larger than this lying most often in the range 1.10 - 1.50 and that they cannot be reduced below 0.85 without novel modifications to the shape and structure.
Since in practice tyres do not have simple ideal shapes it is necessary to define what is meant by the "breaker assembly ' width", the "centre of curvature of the sidewall flex zone" and such factors as "ho" and "rO cos eO"
In general tyre shapes can be drawn to a very close degree of approximation using three radii to reconstruct the shape of the inner sidewall as shown in Figure 5a. The transition region 16 which is sharply curved ha~ a radius of 8.
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curvature rl, the flex region 17 has a radius of curvature r2 and the bead region 18 a radius of curvature r3. The breaker assembly 19 usually shows a curvature as well but this is irrelevant. In Figure 5a the breaker assembly 19 is shown as straight.
Normally the edge 20 of the breaker assembly 19 is separated from the sidewall by a transition region 16 which ; is thicker than the sidewall 13. If the thinnest part 21 of '; the sidewall 13 has a thickness "t" then it is possible to find a position in the transition region 16 where the thickness . is "2t". A line drawn axially across the sidewall section at this position cuts the centre line of the plies at the point "C". The first such point moving radially inwards round the section from the edge of the breaker is the relevant choice.
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The effective breaker assembly width W, is the axial distance CC across the tyre between these points.
~, Moving radially inwardly around the tyre section in the direction CA there is a region of roughly constant radius "r2". In practice "r2" is not exactly constant and it is the minimum value of "r2" which is relevant. The centre for this radius is D.
Given the vertical centre line of the section, the vertical line through C, and an appropriate starting point D, as in Figure 5b, the idealized cross-section is constructed by drawing an arc of radius rO and centre D to cut the ` vertical line through C at the point C . The relevant dimensions of the idealized tyre are marked on the diagram.
The effective sidewall radius rO, is close to r2 in value and :

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10~1331 can be found by adding the distance from the centre ply line to the inner surface of the tyre onto the value r2. The result rO, i8 usually very close to the length CD in '., Figure 5(a).
Figure 6 shows the idealized cross-section of a tyre - mounted on a rim which is narrower than the effective breaker .~ width 'W'. The parameters chosen are exactly the same as in .~:
Figure 5 with the exception that the effective height of the ;,~,. 0 tyre is changed from 2ho to 2ho C08 ~. This is only relevant when:the actual magnitude of the radial force for a given tyre deflection is considered.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows :

1. A pneumatic tyre having a tread reinforced over substantially the whole of its width by a breaker assembly of high transverse bending resistance, a pair of curved sidewalls each including a flex zone and each terminating in a tyre bead, and a radially extending reinforcing carcass ply of inextensible cords, the tyre having a shape when fully inflated wherein the tangent of half the angle subtended by the flex zone of the sidewall at its centre of curvature is less than + 0.85 when it is inflated to its normal working pressure.