ROAD RESEARCH LABORATORY

Ministry of Transport

RRL REPORT LR 172

CONTACT AREAS OF COMMERCIAL

VEHICLE TYRES

by

N. W. Lister and D. E. Nunn

Road Research laboratory ~ Crowthorne, Berkshire

1968

CONTENTS

ABSTRACT

1. Introduction

2. Manufacturers' data relating to tyres

3. Laboratory measurements of tyre contact areas

4. Discussion of the measurements of contact areas

5. Comparison of rolling and static tyre prints

6. Tyre operating conditions

7. Tyres in current use

8. Pressure distribution within the contact envelope

9. Conclusions

10. Acknowledgements

11. References

Page

1

1

O CROWN COPYRIGHT 1968 Extracts from the text may be reproduced

provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

C O N T A C T AREAS OF C O M M E R C I A L

VEHICLE TYRES

ABSTRACT

Static measurements have been made of the dimensions and area of

the contact envelope between commercial tyres and the road surface, for

a wide range of wheel-loads and inflation pressures.

The contact envelope area for each tyre size is found to be very

nearly constant over the manufacturer's recommended working range for

the tyre. T e s t s on moving'wheels indicate a small increase in contact

area, but this factor is unlikely to be of importance in calculations of

induced stresses in road pavements and bridge decks.

I. INTRODUCTION

The shape and area of the contact between the tyre and the road surface is important in theoretical

calculations of the stresses in road pavements and bridge decks, and in connection with the laboratory

simulation of traffic loading. When a smooth tyre of low wall stiffness is in contact with a smooth

road the ratio of the load to the tyre pressure gives a reasonable approximation to the area of contact.

Where the tyre is heavily treaded and is of high wall stiffness, as in the case with commercial

vehicles, actual areas enclosed within the contact perimeter may be only 60 per cent of the area calcu-

lated by dividing the wheel-load by the tyre pressure. Under such circumstances it is more realistic

to obtain the contact pressure by dividing the load by the measured contact area. s

To make practical use of this assumption it is necessary to know:- l

(1) the contact areas for typical commercial tyres for various loads and tyre pressures,

(2) the tyre manufacturers' recommended operating conditions

(3) the actual conditions (in relati0n to loading and inflation pressure) under which the

tyres are operated in practice.

This report is chiefly concerned with the first two of these factors.

2. MANUFACTURERS' DATA RELATING TO TYRES

Tyre manufacturers' catalogues include information on the recommended loads to be carried by

tyres at the appropriate inflation pressures. These data are also available in a British Standard (1).

In the manufacturers' catalogues the loaded and unloaded 'radii' are given for each si~e of tyre

and from this information the approximate length of the contact area can be deduced, but not the width

or shape. Examination of manufacturers' data on tyre deflections under load shows that for any

particular tyre the same value of loaded 'radius' is given for each combination of wheel-loading and

inflation pressure. This implies constant deflection and tyre manufacturers have confirmed that it is

the practice todesigu tyres intended for highway use to have a constant deflection for the range of

working loads, the load/inflat ion pressure tables being compiled on this basis. The deflection is

usually about 15 per cent of the free-section height for the tyre.

3. LABORATORY MEASUREMENTS OF TYRE CONTACT AREAS

The contact areas of the following tyres, all in unworn condition, have been measured:-

7.50 x 20 12 ply

8.25 x 20 14 ply

9.00 x 20 12 ply

9.00 x 20 radial

11.00 x 20 14 ply

18.00 x 19.5 16 ply

In all cases the tyre print was obtained by lightly painting the tyre tread with aluminium paint

and loading it onto a sheet of white cardboard placed on a regid steel plate. With care a clear print

of the tyre tread was obtained. The perimeter of the print is irregular due to the tread pattern, but the

area enclosed within the contact perimeter can generally be measured with an accuracy of about 5 per

cent. A photograph of a tyre print obtained in this way is shown in Plate 1. In Plate 2 a 9.00 x 20

tyre is shown in side view loaded to its rated load and to ~ and 1~ times the rated load at the same inflation pressure.

The results of the measurements made over a wide range of loads and inflation pressures on the

first five of the l isted tyres are given in Figs. 1-5. Included on the graphs for the 7.50, 8.25 and two

9.00 tyres are the results of tes ts carried out specif ical ly,of the manufacturers recommended loads and tyre pressures, the upper and lower limits of the recommended ranges also being indicated. For the 11.00 tyre the corresponding range of contact areas has been obtained by interpolating the data shown on the curves.

A summary of the tyre contact areas and dimensions for the manufacturers maximum recommended

conditions of loading for highway operation is given in Table 1.

2

R.R.L.. Report LR 172 (Addendumto Table I)

Tyre Size

I0.00 x 20 (16 ply)

12.00 x 20 • (14 ply)

14.00 x 20 (18 ply)

16.00 x 20 (24 ply)

Application

Single

Single

Single

See note below

Inflation pressure

Ib/in 2 (bar)

100 (6.88)

80 (5.51)

85 (5.86)

12o (8.27)

Load Ibs

(Newtons)

6380 (28378)

6395 (28445)

8730 (38831)

25200 "(112100)

Area in 2 (mm 2)

72.4 (46.7 x 103)

83.2 (53.6 x 103)

113.0 (72.9 x 103)

254 (163.8 x 103)

Measured. contact envelo )e

Length in (ram)

10.7 (271)

12.8 (325)

13.0 (287)

19.4 (493)

Width in (ram)

7.4 (188)

8.0 (203)

9.9 (251)

14.9 (378)

Note: The 16.00 x 20 tyre was tested to provide data about the tyres fitted to

the Ministry of Transport Bridge Testing Trailer.• This trailer has four

16.00 x 20 tyres on a single axle which can be loaded to 45 tons and

represents one axle of the HB loading specified in BS 153 Part 3A.

Current tyre regulations do not permit the use of a 16.00 x 20 tyre for highway use at 11~ tons load and a 18.00 x 24 (28 ply) tyre would

probably be the minimum size suitable (at I2 m.p.h, rating). The con-

tact print for an 18.00 x 24 tyre has not been measured but is estimated to be approximately 23 in (584 ram) long x 16 in (406 ram) wide.

J

TABLE I

Details of tyre contact envelopes at the

manufacturers' maximum rated loadings

Tyre size

7.50 x 20 12 pl y

8 . 2 5 x 20 14 pl y

9.00 x 20 12 ply

9.00 x 20 radial

11.00 x 20 .14 ply

18.00 x 19.5 16 p l y

Application

Sin gle

• Twin

Single

Twin

Single

Twin

Single

Single

Single.

Inflation

pressure lb/in 2

(bar)

90 (6.21)

9O (6.21)

85 (5.86)

85 ( 5~ 86)

85 (5.86)

85 (5.86)

9O (6.21)

9O (6.21)

75"- (5.17)

Load

lbs

(Newtons)

375O • (16650)

336O (14o10)

4135 (18350),

3750 (16650)

49O0 (21750)

4480 (19900)

4368 (194Q0)

5750 (25550Y

9040 (40200)

Measured Contact envelope

Area

in2(mm 2)

48.1

(31 X 103)

48.1 ( 28.5 x 103)

54.0 ( 34.8 x 103)

48.5 (31.2 x 103)

65.0 ( 4 1 . 9 x 103)

62.3 (40.3 x 103)

57.0 (38.6 x 103)

"71.5 (46.1 x 103)

116.0 (75.0 x 103)

Length

in (mm)

9.4 (238)

8.8 (224)

0.0 (228)

8 .4 (214)

10.3 (262)

9.8 (249)

10.5 (267)

10.5 (267)

10.8 (274)

Width

in (mm)

5.9 (150)

5.8 (148)

6.8 (172)

6.4 (163)

7.3 (185)

7.3 (185)

6.4 (163)

7.8 (198)

11.5 (300)

* interpolated

4. DISCUSSION OF THE MEASUREHENTS OF CONTACT AREA

The alteration in contact area with applied load is clearly shown in Plate 1. Only when the tyre is

considerably underloaded, for the inflation pressure used, does the shape of the contact area become

approximately elliptical. Over a wide range of loadings and inflation pressures the tyre tread width defines the width of the contac t envelope for most of the print length. In the range of working load, the length of the tyre print is thus the only variable with load at fixed inflation pressure, and the contact area then increases approximately linearly with load. For the range of loading and tyre pres- suresconditions recommended by the manufacturers, the results confirm that the contact area is almost constant for each size of tyre.

Fig. 6 compares the measured contact areas given in Fig. 1 for the 7.50 x 20, 12-ply tyre with

corresponding "contact areas" calculated from the ratio of applied load to inflation pressure. The

considerable error involved in the latter method of deducing contact areas is apparent. Errors such

as these would lead to large inaccuracies in stress and deformation calculations for the upper layers

of the pavement or in the orthotropie deck plate for a bridge. The lower elements of the pavement and

the main components of a bridge are less sensitive to the geometry of the applied load. The calcula-

tions for pavement design are discussed in (2).

Fig. 7 shows the results of measurements on those tyres which are in common use.

The expanded scale for contact envelope areas is u sed to separate the area/load curves for each

tyre size .but it is to be noted that this also exaggerates the scatter between measurements on

individual tyres.

It will be noted that there is a nearly linear relationship between the contact areas at maximum

load over the range of tyres tested. A similar relationship is shown in Fig. 8 where the widths of

the contact areas are plotted against load it must be understood that these linear relationships can-

not be used to derive contact areas or widths for given loads since changes of tyre size would be

required in the derivation of the data.

5. COHPARISON OF ROLLING AND STATIC TYRE PRINTS

Recent work (3), in which a car tyre was photographed from below while passing over a glass sheet,

indicated that the length of the moving contact area for the ear tyre was less than the static length by

about 20 per cent, and that most of this reduction of length took place at speeds less than 5 m.p.h.

It was not possible to use this experimental technique commercial wheels carrying much heavier

loads, so, to determine whether a similar effect was present for commercial tyres, a piezo-eleetric

load cell was built into an experimental road surface. The cell was installed to give a measuring

face flush with the road surface and was then covered by a thin skin of piaster of Paris to ensure

smooth passage of the loaded wheel. The length of the tyre contact area was deduced by measuring

the duration of the stress pulse applied by the wheel moving at a known speed. The finite diameter

of the load cell introduced some uncertainty as to the true length of the contact envelope derived from

these measurements. The figures given in table 2 were based on the assumption that the gauge began

to register when its surface was half covered by the approaching wheel and ceased when the retreating

wheel reached the corresponding position. If the alternative limiting assumption is made that registra-

tion commenced when the wheel reached the edge of the cell, the figures should be increased by ~ in

(19 mm), the diameter of the gauge. However, other work with the load cell suggests that the first

assumption is most likely to be justified, :and this assumption is used in the comparisons.

Results obtained for the three centre ribs of a 10.00 x 20 tyre inflated to 100 p.s.i, are given in

Table 2. Compared with the static values also quoted in the Table, the results show a small increase

in the contact length for a moving wheel compared with the static value.

4

TABLE 2

Comparison of commercial tyre print lengths for s ta t ic ' and

Moving wheels

Rib on tyre

tread

1. inner

2. centre

3. outer

Print length - in.(mm)

Static 1.5 mile/h (2.4 Kin/h) 10 mile/h ' (16 Km/h)

9.7 (24.6)

10.1 (25.7)

8.8 (22.4)

10.5 (267)

10.9 (267)

9.0 (229)

10.3 (262)

10.7 (272)

9.1 (231)

As the width of the contact envelope is the same under stat ic and roiling conditions, it i s

reasonable to conclude that the s ta t ical ly measured contact areas given in this report are a close

approximation to those applied by moving wheel loads.

6. TYRE OPERATING CONDITIONS

It has not proved possible to obtain any rel iable data on how closely vehicle operators follow tyre

manufacturers ' recommendations. However, a large number of observat ions have been made on normal

traffic as a part of the experimental studies for pavement andbridge design and in many eases photo-

graphs were also taken of the vehicles .

The observat ions suggest that for laden vehicles , tyres are usual ly def lected by the amount

expected from the manufacturers data. It seems reasonable to assume for design purposes that tyres

~are more or less correctly inflated and that the contact envelope areas given in this report can be used

without introducing significant errors.

7. TYRES IN CURRENT USE

Although the curves given in Fig. 7 apply to the wide range of tyres tes ted, it is relevant to consider

how representat ive those tyres are of the types in common u s e . Limited figures of sa les of com-

mercial vehicle tyres are available. They are currently as f o l l ows : -

• 9.00 x 20 . . . . . 50 per cent

7.50 x 16 . . . . . 25 per cent

8 .25x 16) . . . . . . 10 per cent

8.25 x20) . . . . .

10.00 x 20 . . . . . 10 per cent

others . . . . . 5 per cent

These figures include all "ply" ratings of the various sizes.

The tyres tested in this investigation thus represent about 60 per cent of current sales. At

least 05 per cent of current sizes fall between the limits of size embraced by the tests. The results

of the investigation should therefore be generally applicable to commercial vehicles operating within the present Construction and Use regulations;

In the United States of America (and more recently in the United Kingdom) tyre manufacturers

are offering a wide tread tyre to replace existing twin-tyre configurations. Savings in tyre manufac-

ture and maintenance costs are claimed as a justification for this change. The 18.00 x 19.5 tyre

included in Table 1 and Fig 8 was tested as a representative of this new type of tyre.

8. PRESSURE DISTRIBUTION WITHIN THE CONTACT ENVELOPE

This has not been considered in this report but it will be a factor of increasing importance as

experimental techniques and theoretical models are refined. A summary of the available experimental

data and their implications in pavement design are discussed in another paper (2)

9. CONCLUSIONS

The contact envelopes of a number of tyres have been examined and the areas deduced over a range

of loads and inflation pressures. The results show that for a correctly loaded and inflated tyre, the

contact area is very nearly constant over the working range recommended by the tyre manufacturers.

The tests have shown that the results are valid to a sufficiently close degree of accuracy for moving wheels.

A linear though not proportional relationship exists between contact area and maximum recom-

mended wheel load for all the tyres tested, and the corresponding relationship between the width of contact area and wheel load for the tyres also appears to be linear.

I0. ACKNOWLEDGEMENTS

The authors wish to acknowledge the help of D. Kingston, A. P. Mayo, and A~ H. Wilson who made

the measurements given in this report. The report was prepared in the Pavement Design and Bridges Section of the Design Division.

I I , REFERENCES

. British Standards Institution: Tyre and wheel data. Automobile Series, BS.AU 50: Part 1, 1964. Part 1, Tyres. London, 1964.

. Lister, N. W. and R. Jones. The behaviour of pavements under moving wheel-loads. Proc. 2nd

Internl. Conf. on the Struct. Des. of Asphalt Pavements. University of Michigan, Ann Arbor, 1967.

. Sabey, B. E. and C. N. Lupton. Photography of the real contact area of tyres during motion.

Ministry of Transport. Road Research Laboratory Report No. 57 Road Research Laboratory, Crowthorne. 1967.

6

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20 " I I I I 0 10 20 30 N x 103 50

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Load on wheet

Fig. 1. CONTACT ENVELOPE AREAS OF 7.50 X 20 12PLY TYRE FOR VARIOUS LOADS AND PRESSURES

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Fig. 2. CONTACT ENVELOPE AREAS OF 8.25 x20 1&.PLY TYRE FOR VARIOUS LOADS AND PRESSURES

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Load on wheet I, bf x 103

Fig. 3. CONTACT ENVELOPE AREAS OF 9.00 x20 12 PLY TYRE FOR VARIOUS LOADS AHO PRESSURES

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20 I I I I 0 10 20 30 N x 103

I 0

50

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Loo.d on wheet

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Fig. i,. CONTACT ENVELOPE AREAS OF 9.00 x 20 12 PLY TYRE (RADIAL PLY CONSTRUCTION)

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Load on wheet

Fig. 5. CONTACT ENVELOPE AREAS OF 11'00 x20 l& PLY TYRE

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I I I I 0 10 20 30 N x 103 50

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Load on wheet

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Fig. 6. COMPARISON OF MEASURED CONTACT AREA WITH AREA COMPUTED FROM RATIO OF WHEEL LOAD TO TYRE PRESSURE

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J O Singte wheel operation • Dual wheel operation

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I I I I I I 0 2 & 6 8 10 Ibf x 103

Load on wheel

Fig. 8. CONTACT ENVELOPE WIDTHS OF COMMERCIAL TYRES AT MANUFACTURERS RATINGS

16

i ABSTRACT

= Contact areas of commercial vehicle tryes: N. W. Lister and g. D.E. Nunn: Ministry of Transport, RRL Report No. 172: Crow- m " • . ,

: thorne, 1968 (Road Research Laboratory). Statm measure- r. ments have been made of the dimensions and area of th e con-

tact envelope between commercial tyres and the road surface, me • • . = for a wzde range of wheel-loads and lnflanon pressures. = -" The contact envelope area for each tyre size is found

to be very nearly constant over the manufacturer's recommen- .~ ded working range for the tyre. -Tests on moving wheels indi-

. , , . • .% o ,~ cate a small increase m contact area, but this factor is un- i , . . , ° , = likely to be of zmportance in calculanons of znduced stres- i~ . - . .

ses m road pavements and bridge decks.

i | | | | | I I I I 1 1 1 | I I I I I | l | I I I l I I | | | | I l l [ l l ] l ] ~ | l ] l l l l I l l | l ] l l l l | l l R l l | | | | | l I 1 H I | | | I n l | | i | i | | H i | l N I H ! i | 1 ] | | | | | | I i D ~ | I | H |

i ABSTRACT

Contact areas of commercial vehicle tryes: N. W. Lister and .~ D: E. Nunn: Ministry of Transport, RRL Report No. 172: Crow-

thorne, 1968 (Road Research Laboratory). Static measure- ments have been made of the dimenMons and area of the con- tact envelope between commercial tyres and the road surface, for a wide range of wheel-loads and inflation pressures.

The contac t envelope area for each tyre size is found to be very nearly constant over the manufacturer's recommen- ded working range for the tlre. Tests on moving wheels indi- cate a small increase in contact area, but this factor is un- likely to be of importance in calculations of induced stres- ses in road pavements and bridge decks.

l °°

Contact areas of commercial vehicle tryes: N. W. Lister and D. E. Nunn: Ministry of Transport, RRL Report No. 172: Crow- thorne, 1968 (Road Research Laboratory). Static measure- ments have been made of the dimensions and area of th e con- tact envelope between commercial t i res and the road surface, for a wide range of wheel-loads and inflation pressures.

The contact envelope area for each tyre size is found to be very nearly constant over the manufacturer, s recommen- .. ded working range for the tyre. Tests on moving wheels indi- cate a small increase in contact area, but this factor is un- likely to be of importance in calculations of induced stres- Ses in road pavements and bridge decks.

Printed at the Road Research Laboratory, Crowthorne, England.