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Tyre Punctures—How, Why and Where
J. firens. Sci. Soc. (1974), 14, 165
Tyre Punctures-How,
Why and Where
R. 1. GROGAN Technical Department, 5unlop Ltd, Birmingham, England and T. R. WATSON Home Ofice West Midlands Forensic Science Laboratory, Priory House, Gooch Street, Birmingham 5, England An analysis of over 500 punctures is made. The position, nature and cause of the punctures is related to tread pattern, depth, and type of tyre. Puncturing objeGts are analysed. It is shown that nails are the most common cause of punctures and that in many cases they have been present for some time prior to discovery. Tests are described which enable the wear on the nail to be related to distance travelled. The results are compared with datafrom the West Mercia Police and shown to be relevant. Introduction Although history does not record the fact, the first puncture probably followed soon after the use of the first pneumatic tyre. They have been with us ever since and, despite recent developments and improvements, are likely to remain so for some time to come. We were therefore somewhat surprised to find that, although some work on the effect of punctures has been conducted by Baker and McIlrath (1g68), Mackay et a1 ( I969), and Grogan ( I972), and on their incidence by Kemp et a1 (1g72), Godley (1g72), and Lowne (1g73), little has been done on the cause and nature of punctures themselves. We all know that a puncture is a well worn excuse for a road accident, indeed the frequency with which this alibi occurs rivals that of the "man in the pub" who sells stolen goods to the innocent citizen. Over the years we have been asked many questions relating to the puncture itself:
Are radial tyres more resistant to punctures than cross-ply? Is a worn tyre more prone to puncture than a new one? What is the commonest cause of a puncture? How long had the puncturing object been in the tyre prior to actual deflation? Does a front tyre have more punctures than a rear? What are the chances of having two punctures in the same tyre? Workers in this field can only quote their own experiences and we know experiences can conflict. Accordingly we decided to investigate punctures more closely. The following work cannot cover the whole subject of punctures but we trust that at the very least we shed a little light into some dark corners. Collection of Data Two main sources of information were used : I . Replies to questionnaires from a large tyre distribution organization in the South of England (National Tyre Services). 2. A tyre survey conducted by the West Mercia Police on the Mg Motorway during September, October and November 1971. National Tyre Service Survey National Tyre Service is a large tyre retail chain operating in the United Kingdom. It was decided that the more densely populated South East of England would give the greatest information and accordingly, questionnaires were sent out via the N.T.S. Service Organization to their Depots in December 1971, and again in June 1972. Obviously there is a limit to the amount of 165
infbrmation which can be expected. For this reason, no specific details of wheel position or make of vehicle were requested. No attempt was made to define a puncture and inevitably a few forms were returned which did not relate strictly to the subject. Such features as major structural damage, leaking valves, and failed repairs were therefore deleted. I n this way 57 of 367 examples were deleted from the first survey and 21 of the 247 in the second survey. Of the 536 cases left, not all the forms were complete. This is not surprising when one considers that the repair man is having to fit this extra chore into his already busy day. Nevertheless, we feel that the information is sufficient to enable valid deductions to be made. A rather surprising feature was the similarity of the results from both the December and June surveys, so similar in fact that, with one exception referred to later, the results could be safely combined. TABLEI WHEEL R I M SIZE AND T H E PERCENTAGE DISTRIBUTION O F PUNCTURES 10" 12" I 3" 14" 15~ 16 20" 6.5% 24% 36% 16% 9.5% 3.5% < 1% I n addition there were a few punctures involving g", 17 and 22.5" rims
National Tyre Service Survey Results The majority of the tyres were from cars (76.5%), 14.5% were fitted to vans and only 9% were commercial vehicle tyres. This bias is obviously due to the source of the information. Table I classifies the data on a basis of wheel rim size. What causes punctures? Figure I shows most of the debris which the 526 tyres retained. No less than 246 of the punctures were caused by nails. As they formed such a large group, they warranted closer attention. I t was immediately obvious that nails fall into 2 categories: nails with heads (Fig. 2) and nails where the head had been worn away (Fig. 3). Measurement of the length of the nails from both groups (Fig. 4) shows such a similarity, that we conclude that the nail becomes fully embedded in the tyre leaving only the head to wear away. As a further check on this, we obtained a few dozen tyres, which had been sent in for puncture repairs, and measured the angle of penetration. All the holes were found to be between 70" and go0 to
Fig.
I.
Puncturing objects removed from tyres (National Tyre Service Survey).
Fig.
2.
Fig. 3.
Nails with heads: selected from the collection of puncturing objects shown in Figure I.
Nails with worn heads: selected from the collection of puncturing objects shown in Figure I .
167
W o r n Nails
Unworn Nails
LENGTH Fig. 4.
(ins.)
Histograms of the lengths of worn and unworn nails from the collection shown in Figure I .
Fig. 5. Worn and unworn screws selected from the collection of puncturing objects shown in Figure I.
168
the circumference so we can say with confidence that in most cases the nail goes "straight" in. Screws present a very similar situation (Fig. 5). Nearly 40% had significant head wear and although in this case we have conducted no tests to establish the duration of service, it seems reasonable to conclude that screws are retained in the tyre for about the same period as nails. Figure 6 shows the diameters of the nails, screws and bolts. Bolts are shown in Figure 7. In view of the size of a bolt it is perhaps surprising to find that here too, many of the bolts had been present for a long enough time to wear the head down. We have long marvelled at the insensitivity of some drivers to the noise and vibration which large foreign objects cause and this group only serves to underline that situation. Miscellaneous pieces of metal are shown in Figure 8. Notice that size and shape are no guide to puncturing ability. The last group found (Fig. 9) is road flints and glass. Many people will be surprised to find this such a small porportion of the total. Glass, that favourite bogy of the motorists, is not a very frequent cause of punctures. Indeed, Dobie (1969) has shown in his famous broken bottle experiment that it is almost impossible to induce glass to puncture a tyre in the dry. In the wet, of course, the picture is quite different; a wet tyre being comparatively easily penetrated by almost any object. Not illustrated separately, as a few thorns and pieces of wood, including a match-stick. The moral here is that given the right circumstances, even soft or brittle objects will penetrate a tyre. Now we know what punctures a tyre let us look a t where. Figure 10 shows the location of punctures from the June survey. Notice that there is little difference between the two sides, neither is there any difference between the grooves and the ribs. The latter is somewhat surprising as it seems logical to assume that the pattern grooves will acquire the object more readily and retain it, allowing more time for it to be driven in, but such is obviously not the case. Nearly half the tyres were found to be inflated to some degree and of these, twice as many were tubeless as tubed. A fairly predictable result. When asked how far the vehicle had travelled before the discovery of the puncture, more than half of the motorists reported that the vehicle was stationary and a further 14% had driven only up to 2 miles. After this there was a fairly even spread of distances up to 30 miles. An almost unbelievable discovery from the survey was that g of the tyres contained 2 or, in some cases, even 3 penetrations. Given that nearly 2 % of tyres had more than one cause of deflation, we were not too surprised to find that a significant number of tyres had secondary damage caused by the nail point gouging the inside of the tyre lower down. A noticeable difference between the December and June surveys manifests itself here. More than 4 times as many tyres suffered secondary damage in the winter (33%), which suggests that in the cold, the motorist is less inclined to investigate the cause of the puncture and change the wheel but tries instead to limp to his nearest repair man and, in doing so, damages the tyre. About a quarter of the motorists are unwilling (or unable) to change a wheel when they have a puncture and leave this chore to the tyre repairer. As a result of the survey, we knew that nails formed by far the biggest cause of punctures. Of these, 50% or more had wear on the nail head. How long had they been in the tyre to get to that state? We conducted a few simple tests to find out.
Nail W e a r Tests Method Two of the most common nails encountered in tyres are the ordinary carpenter's round nail of about 0.09" diameter, $ - I " in length, and the short
1
Screws & Bolts
0,
iI Fig. 6.
I
DIAMETER (ins.)
The diameters of nails (both worn and unworn) and screws from the collection of puncturing objects shown in Figure I .
Fig. 7. Bolts from the collection of puncturing objects shown in Figure
I.
Fig. 8. Miscellaneous pieces of metal taken from the collection of puncturing objects shown in Figure I
Fig. 9.
Flint and glass particles from the collection of puncturing objects shown in Figure
I.
Fig.
10.
Number of penetrations related to position on tyres.
galvanized roofing-felt nail. Accordingly our tests were made with representatives of these two types, I & x o.103" carpenter's round nails and $ X O . I 16" galvanized felt nails. Eight equally spaced points round the circumference were marked out and 3 nails driven into the tread ribs of inflated 5.20-13 tyres at each point, one at the crown and one at each shoulder giving 24 nails in each tyre. Two tyres with carpenter's nails inserted were fitted to front and rear of the off side of a Cortina estate car. The near side tyres contained the felt nails. Standard 4" rims were used and normal pressures were maintained 24 p.s.i. front and 28 p.s.i. rear. The vehicle was driven on a test track with a surface corresponding to a little used but well maintained road, i.e. good condition with little traffic polishing. The 2-mile course was chosen to present a n equal number of right- and left-hand bends and an average speed of 40 m.p.h. was selected. Nails were removed a t distances of 3, I , 2, 4, 8, 16, 32 and 64 miles. The nail holes were plugged and pressures adjusted before proceeding to the next distance.
Results At 3 mile and I mile theie was no appreciable wear. Some abrasion was observed at 2 miles especially on the front inner positions. Wear progressed steadily until between 16 and 32 miles the heads tended to buckle under cornering (indicating that they were becoming very thin). At 32-64 miles all the heads had worn away (Fig. I 1-14) in a few instances nails were ejected and lost. The drivers were alert to this possibility and due to the frequent bends the deflating tyre was quickly detected, plugged and reinflated. The test was admittedly a severe one, oegmm of tread pattern was worn away so that the distances must represent minimum values. Comparing the condition of the worn nails with nails obtained from the December N.T.S. survey we concluded that of the 83 cases, I I (13%) had been present for a t least 16 miles, 15 (18%) had been present for at least 32 miles, and rg (23%) had been present for a t least 64 miles. Bearing in mind the severity of the test it seems reasonable to conclude that 200 miles is not uncommon for a nail to remain undetected. I t could be argued that by placing the nails in the tread rib and not the groove the abrasion was at its most severe, particularly since Figure 10 shows that nearly half the punctures occur in the tread groove. However, we feel that the results are quite
Fig.
I I.
Rate of wear of carpenter's nails (rear tyre).
Fig.
12.
Rate o r wear of carpenter's nails (front tyre).
Mileage
0
f
Fig. 13.
1
2
4
8
16
32
Rate of wear of galvanized roofing-felt nails (front tyre).
173
64
Mileage 0
1
2
4
B
16
32
64
Fig. 14. Rate of wear of galvanized roofing-felt nails (rear tyre).
valid, firstly because as we shall show later, punctured tyres are most frequently worn tyres (where the effect of rib and grooves is small) and secondly, at the road surface the grooves pinch together so that the nail tends to be gripped with the head at the tread pattern surface until it has worn away.
West Mercia Police Survey Here we gained several additional facts. This survey was made by Motorway patrol officers who investigated vehicles stopped on the Mg. The vehicle make-up is quite different from the N.T.S. survey. Of the 309 vehicles examined, 38% were cars, 37% were articulated lorries, 15 -5% were rigid lorries and 8% were vans; the remainder were coaches, trailers and caravans. The higher proportion of commercial vehicles probably reflects the high density of this type of vehicle on the Mg. Of the 69 reported causes, nails or metal were responsible in 23 cases and tread shedding in 2 1 . This group is not really a cause of punctures and will not be considered further here. Of the remaining reports and causes of puncture, these were 5 holes or splits, 2 plug failures and one thorn penetration. This is admittedly a very small sample, but the results are not too dissimilar from the other survey reported earlier. Thirty-four per cent of the deflations occurred on the front and 66% on the rear, with a small but significant bias toward more near side deflations. A feature of this survey was the number of double deflations observed, 16 in all. These occurred in the rear tyres of commercial vehicles and there is little doubt that the cause of this is the deflation of one tyre on a twinned pair which throws a double load onto its companion which subsequently fails from overloading in some form or other. The ratios of various brands of tyres were sufficiently close to the N.T.S. surveys to lead us to conclude that they are drawn from a similar population. Effect of Pattern Depth Similarity between two surveys is shown in Figure 15 which illustrates the pattern depth of cross-ply car tyres with punctures compared with a normal cross section of the population. The skewed distribution of the punctured tyres from both sources is most noticeable and we are forced to conclude that as the tyre becomes thinner, it becomes more prone to punctures. If this is the case, then radial tyres which are slightly heavier at the crown because of their construction, should be a little more resistant to punctures and Figure 16 shows this to be the case. (The West Mercia figures for radial tyres were too few to permit accurate analysis.)
1 CROSS PLY CAR N R E S
* -*punctured tyres
.-.-. non-punctured tyres -4 West Mercia Police
puncture survey
2015% TYRES
I0-
\
5-
x
\
.'
9.
--...a
x '0..
-0-
0
1
I
2
I
3
4
I
5
I
6
1
7
8
1
9
PATrERN DEPTH (rnrn.)
Fig. 15. Frequency of punckres v. tread pattern depth (cross-ply car tyres).
RADIAL PLY CAR N R E S
r--r punctured tyres non-punctured tyres
25 20.
.-0-.-,,
-. - I
\
'\
i
1 0-
5-
PAlTERN DEPTH (mm.)
Fig. 16. Frequency of punctures v. tread pattern depth (radial ply car tyres).
Looking at both graphs of tread pattern of punctured tyres, we see a heartening feature. I t is the definite indication that most drivers obey the law and remove their tyres before they become illegal in Great Britain, i.e. whilst a t least Imm of tread pattern depth remains. However, we cannot be entirely happy to see that a few tyres are still to be found with less than Imm of pattern. Our calculations show that the likelihood of a puncture is increased at least six- and perhaps, ten-fold, when the tyre is used at Imm or below comparedwith the first half of the tread life. Conclusions From our interpretation of the accumulated data we draw the following conclusions based upon the questions posed at the beginning of this paper. Radial tyres are slightly more resistant to punctures than cross-ply but this is simply due to the effect of increased crown thickness. A worn tyre is more prone to puncture than a new one. in length and I / 10" The commonest causes of puncture are nails of about I i" diameter. Many nails found in tyres can be shown to have been present for 2 0 0 miles or more prior to detection. The risk of a puncture occurririg in a rear tyre is twice that of a puncture occurring in a front tyre. The chances of 2 punctures (or puncturing objects) occurring in the same tyre are nearly I in 50. We also concluded that whilst almost any object however brittle will penetrate a tyre, glass is of comparatively little danger. There is little difference in the puncture susceptibility of the tyre tread, grooves, or ribs. Motorists who in winter drive on a deflated or deflating tyre, rather than stopping to change it as they may do in the summer, greatly increase the risk of causing additional damage to the tyre. Acknowledgements This work, in a co-operative vencure between the tyre industry and the Home Office Forensic Science Service, has depended heavily on the two sources of information mentioned in the text. We should like to thank the service organization of National Tyre Service, in particular Mr. J. Hogan, for their part in the survey. We should also like to thank the West Mercia Police for the information they supplied, made available to us by Chief Superintendent Blackie. Finally our thanks to the Directors of the Dunlop Company, and the Home Oflice for their permission to publish this work. References BAKER, J. S., and MCILRAITH, G. D., 1968, Tyre Disablements Followed by Accidents, Traffic Institute N.W. University, Illinois. DOBIE,W. J., 1969, Materials Research and Standards, March. GODLEY, M. J., 1972, The Incidence of Burst Tyres to Injury Accidents on M I and Mq Motorways, T.R.R.L. Report LR498. GROGAN, R. J., 1972,J. Forens Sci. Soc., 12, 285. KEMP,R. N., NEILSON, I. D., STRAUGHTON, G. C., and WILKINS, H. A., 1972, A Preliminary Report on an On-the-spot Survey of Accidents, T.R.R.L. Report LR434. LOWNE,R. W., 1973, Tyre Failures on Part of M5 Motorway, T.R.R.L. Report LR585. MACKAY, G. M., I 969, The Vehicle, in : Causes and Efects of Road Accidents, Vol. 3, by Kolbuszewski, J., Mackay, G. M., Fonseka, C. P., Blair, I., and Clayton, A. B., Department of Transportation and Environmental Planning, University of Birmingham. 176
Tyre Punctures-How,
Why and Where
R. 1. GROGAN Technical Department, 5unlop Ltd, Birmingham, England and T. R. WATSON Home Ofice West Midlands Forensic Science Laboratory, Priory House, Gooch Street, Birmingham 5, England An analysis of over 500 punctures is made. The position, nature and cause of the punctures is related to tread pattern, depth, and type of tyre. Puncturing objeGts are analysed. It is shown that nails are the most common cause of punctures and that in many cases they have been present for some time prior to discovery. Tests are described which enable the wear on the nail to be related to distance travelled. The results are compared with datafrom the West Mercia Police and shown to be relevant. Introduction Although history does not record the fact, the first puncture probably followed soon after the use of the first pneumatic tyre. They have been with us ever since and, despite recent developments and improvements, are likely to remain so for some time to come. We were therefore somewhat surprised to find that, although some work on the effect of punctures has been conducted by Baker and McIlrath (1g68), Mackay et a1 ( I969), and Grogan ( I972), and on their incidence by Kemp et a1 (1g72), Godley (1g72), and Lowne (1g73), little has been done on the cause and nature of punctures themselves. We all know that a puncture is a well worn excuse for a road accident, indeed the frequency with which this alibi occurs rivals that of the "man in the pub" who sells stolen goods to the innocent citizen. Over the years we have been asked many questions relating to the puncture itself:
Are radial tyres more resistant to punctures than cross-ply? Is a worn tyre more prone to puncture than a new one? What is the commonest cause of a puncture? How long had the puncturing object been in the tyre prior to actual deflation? Does a front tyre have more punctures than a rear? What are the chances of having two punctures in the same tyre? Workers in this field can only quote their own experiences and we know experiences can conflict. Accordingly we decided to investigate punctures more closely. The following work cannot cover the whole subject of punctures but we trust that at the very least we shed a little light into some dark corners. Collection of Data Two main sources of information were used : I . Replies to questionnaires from a large tyre distribution organization in the South of England (National Tyre Services). 2. A tyre survey conducted by the West Mercia Police on the Mg Motorway during September, October and November 1971. National Tyre Service Survey National Tyre Service is a large tyre retail chain operating in the United Kingdom. It was decided that the more densely populated South East of England would give the greatest information and accordingly, questionnaires were sent out via the N.T.S. Service Organization to their Depots in December 1971, and again in June 1972. Obviously there is a limit to the amount of 165
infbrmation which can be expected. For this reason, no specific details of wheel position or make of vehicle were requested. No attempt was made to define a puncture and inevitably a few forms were returned which did not relate strictly to the subject. Such features as major structural damage, leaking valves, and failed repairs were therefore deleted. I n this way 57 of 367 examples were deleted from the first survey and 21 of the 247 in the second survey. Of the 536 cases left, not all the forms were complete. This is not surprising when one considers that the repair man is having to fit this extra chore into his already busy day. Nevertheless, we feel that the information is sufficient to enable valid deductions to be made. A rather surprising feature was the similarity of the results from both the December and June surveys, so similar in fact that, with one exception referred to later, the results could be safely combined. TABLEI WHEEL R I M SIZE AND T H E PERCENTAGE DISTRIBUTION O F PUNCTURES 10" 12" I 3" 14" 15~ 16 20" 6.5% 24% 36% 16% 9.5% 3.5% < 1% I n addition there were a few punctures involving g", 17 and 22.5" rims
National Tyre Service Survey Results The majority of the tyres were from cars (76.5%), 14.5% were fitted to vans and only 9% were commercial vehicle tyres. This bias is obviously due to the source of the information. Table I classifies the data on a basis of wheel rim size. What causes punctures? Figure I shows most of the debris which the 526 tyres retained. No less than 246 of the punctures were caused by nails. As they formed such a large group, they warranted closer attention. I t was immediately obvious that nails fall into 2 categories: nails with heads (Fig. 2) and nails where the head had been worn away (Fig. 3). Measurement of the length of the nails from both groups (Fig. 4) shows such a similarity, that we conclude that the nail becomes fully embedded in the tyre leaving only the head to wear away. As a further check on this, we obtained a few dozen tyres, which had been sent in for puncture repairs, and measured the angle of penetration. All the holes were found to be between 70" and go0 to
Fig.
I.
Puncturing objects removed from tyres (National Tyre Service Survey).
Fig.
2.
Fig. 3.
Nails with heads: selected from the collection of puncturing objects shown in Figure I.
Nails with worn heads: selected from the collection of puncturing objects shown in Figure I .
167
W o r n Nails
Unworn Nails
LENGTH Fig. 4.
(ins.)
Histograms of the lengths of worn and unworn nails from the collection shown in Figure I .
Fig. 5. Worn and unworn screws selected from the collection of puncturing objects shown in Figure I.
168
the circumference so we can say with confidence that in most cases the nail goes "straight" in. Screws present a very similar situation (Fig. 5). Nearly 40% had significant head wear and although in this case we have conducted no tests to establish the duration of service, it seems reasonable to conclude that screws are retained in the tyre for about the same period as nails. Figure 6 shows the diameters of the nails, screws and bolts. Bolts are shown in Figure 7. In view of the size of a bolt it is perhaps surprising to find that here too, many of the bolts had been present for a long enough time to wear the head down. We have long marvelled at the insensitivity of some drivers to the noise and vibration which large foreign objects cause and this group only serves to underline that situation. Miscellaneous pieces of metal are shown in Figure 8. Notice that size and shape are no guide to puncturing ability. The last group found (Fig. 9) is road flints and glass. Many people will be surprised to find this such a small porportion of the total. Glass, that favourite bogy of the motorists, is not a very frequent cause of punctures. Indeed, Dobie (1969) has shown in his famous broken bottle experiment that it is almost impossible to induce glass to puncture a tyre in the dry. In the wet, of course, the picture is quite different; a wet tyre being comparatively easily penetrated by almost any object. Not illustrated separately, as a few thorns and pieces of wood, including a match-stick. The moral here is that given the right circumstances, even soft or brittle objects will penetrate a tyre. Now we know what punctures a tyre let us look a t where. Figure 10 shows the location of punctures from the June survey. Notice that there is little difference between the two sides, neither is there any difference between the grooves and the ribs. The latter is somewhat surprising as it seems logical to assume that the pattern grooves will acquire the object more readily and retain it, allowing more time for it to be driven in, but such is obviously not the case. Nearly half the tyres were found to be inflated to some degree and of these, twice as many were tubeless as tubed. A fairly predictable result. When asked how far the vehicle had travelled before the discovery of the puncture, more than half of the motorists reported that the vehicle was stationary and a further 14% had driven only up to 2 miles. After this there was a fairly even spread of distances up to 30 miles. An almost unbelievable discovery from the survey was that g of the tyres contained 2 or, in some cases, even 3 penetrations. Given that nearly 2 % of tyres had more than one cause of deflation, we were not too surprised to find that a significant number of tyres had secondary damage caused by the nail point gouging the inside of the tyre lower down. A noticeable difference between the December and June surveys manifests itself here. More than 4 times as many tyres suffered secondary damage in the winter (33%), which suggests that in the cold, the motorist is less inclined to investigate the cause of the puncture and change the wheel but tries instead to limp to his nearest repair man and, in doing so, damages the tyre. About a quarter of the motorists are unwilling (or unable) to change a wheel when they have a puncture and leave this chore to the tyre repairer. As a result of the survey, we knew that nails formed by far the biggest cause of punctures. Of these, 50% or more had wear on the nail head. How long had they been in the tyre to get to that state? We conducted a few simple tests to find out.
Nail W e a r Tests Method Two of the most common nails encountered in tyres are the ordinary carpenter's round nail of about 0.09" diameter, $ - I " in length, and the short
1
Screws & Bolts
0,
iI Fig. 6.
I
DIAMETER (ins.)
The diameters of nails (both worn and unworn) and screws from the collection of puncturing objects shown in Figure I .
Fig. 7. Bolts from the collection of puncturing objects shown in Figure
I.
Fig. 8. Miscellaneous pieces of metal taken from the collection of puncturing objects shown in Figure I
Fig. 9.
Flint and glass particles from the collection of puncturing objects shown in Figure
I.
Fig.
10.
Number of penetrations related to position on tyres.
galvanized roofing-felt nail. Accordingly our tests were made with representatives of these two types, I & x o.103" carpenter's round nails and $ X O . I 16" galvanized felt nails. Eight equally spaced points round the circumference were marked out and 3 nails driven into the tread ribs of inflated 5.20-13 tyres at each point, one at the crown and one at each shoulder giving 24 nails in each tyre. Two tyres with carpenter's nails inserted were fitted to front and rear of the off side of a Cortina estate car. The near side tyres contained the felt nails. Standard 4" rims were used and normal pressures were maintained 24 p.s.i. front and 28 p.s.i. rear. The vehicle was driven on a test track with a surface corresponding to a little used but well maintained road, i.e. good condition with little traffic polishing. The 2-mile course was chosen to present a n equal number of right- and left-hand bends and an average speed of 40 m.p.h. was selected. Nails were removed a t distances of 3, I , 2, 4, 8, 16, 32 and 64 miles. The nail holes were plugged and pressures adjusted before proceeding to the next distance.
Results At 3 mile and I mile theie was no appreciable wear. Some abrasion was observed at 2 miles especially on the front inner positions. Wear progressed steadily until between 16 and 32 miles the heads tended to buckle under cornering (indicating that they were becoming very thin). At 32-64 miles all the heads had worn away (Fig. I 1-14) in a few instances nails were ejected and lost. The drivers were alert to this possibility and due to the frequent bends the deflating tyre was quickly detected, plugged and reinflated. The test was admittedly a severe one, oegmm of tread pattern was worn away so that the distances must represent minimum values. Comparing the condition of the worn nails with nails obtained from the December N.T.S. survey we concluded that of the 83 cases, I I (13%) had been present for a t least 16 miles, 15 (18%) had been present for at least 32 miles, and rg (23%) had been present for a t least 64 miles. Bearing in mind the severity of the test it seems reasonable to conclude that 200 miles is not uncommon for a nail to remain undetected. I t could be argued that by placing the nails in the tread rib and not the groove the abrasion was at its most severe, particularly since Figure 10 shows that nearly half the punctures occur in the tread groove. However, we feel that the results are quite
Fig.
I I.
Rate of wear of carpenter's nails (rear tyre).
Fig.
12.
Rate o r wear of carpenter's nails (front tyre).
Mileage
0
f
Fig. 13.
1
2
4
8
16
32
Rate of wear of galvanized roofing-felt nails (front tyre).
173
64
Mileage 0
1
2
4
B
16
32
64
Fig. 14. Rate of wear of galvanized roofing-felt nails (rear tyre).
valid, firstly because as we shall show later, punctured tyres are most frequently worn tyres (where the effect of rib and grooves is small) and secondly, at the road surface the grooves pinch together so that the nail tends to be gripped with the head at the tread pattern surface until it has worn away.
West Mercia Police Survey Here we gained several additional facts. This survey was made by Motorway patrol officers who investigated vehicles stopped on the Mg. The vehicle make-up is quite different from the N.T.S. survey. Of the 309 vehicles examined, 38% were cars, 37% were articulated lorries, 15 -5% were rigid lorries and 8% were vans; the remainder were coaches, trailers and caravans. The higher proportion of commercial vehicles probably reflects the high density of this type of vehicle on the Mg. Of the 69 reported causes, nails or metal were responsible in 23 cases and tread shedding in 2 1 . This group is not really a cause of punctures and will not be considered further here. Of the remaining reports and causes of puncture, these were 5 holes or splits, 2 plug failures and one thorn penetration. This is admittedly a very small sample, but the results are not too dissimilar from the other survey reported earlier. Thirty-four per cent of the deflations occurred on the front and 66% on the rear, with a small but significant bias toward more near side deflations. A feature of this survey was the number of double deflations observed, 16 in all. These occurred in the rear tyres of commercial vehicles and there is little doubt that the cause of this is the deflation of one tyre on a twinned pair which throws a double load onto its companion which subsequently fails from overloading in some form or other. The ratios of various brands of tyres were sufficiently close to the N.T.S. surveys to lead us to conclude that they are drawn from a similar population. Effect of Pattern Depth Similarity between two surveys is shown in Figure 15 which illustrates the pattern depth of cross-ply car tyres with punctures compared with a normal cross section of the population. The skewed distribution of the punctured tyres from both sources is most noticeable and we are forced to conclude that as the tyre becomes thinner, it becomes more prone to punctures. If this is the case, then radial tyres which are slightly heavier at the crown because of their construction, should be a little more resistant to punctures and Figure 16 shows this to be the case. (The West Mercia figures for radial tyres were too few to permit accurate analysis.)
1 CROSS PLY CAR N R E S
* -*punctured tyres
.-.-. non-punctured tyres -4 West Mercia Police
puncture survey
2015% TYRES
I0-
\
5-
x
\
.'
9.
--...a
x '0..
-0-
0
1
I
2
I
3
4
I
5
I
6
1
7
8
1
9
PATrERN DEPTH (rnrn.)
Fig. 15. Frequency of punckres v. tread pattern depth (cross-ply car tyres).
RADIAL PLY CAR N R E S
r--r punctured tyres non-punctured tyres
25 20.
.-0-.-,,
-. - I
\
'\
i
1 0-
5-
PAlTERN DEPTH (mm.)
Fig. 16. Frequency of punctures v. tread pattern depth (radial ply car tyres).
Looking at both graphs of tread pattern of punctured tyres, we see a heartening feature. I t is the definite indication that most drivers obey the law and remove their tyres before they become illegal in Great Britain, i.e. whilst a t least Imm of tread pattern depth remains. However, we cannot be entirely happy to see that a few tyres are still to be found with less than Imm of pattern. Our calculations show that the likelihood of a puncture is increased at least six- and perhaps, ten-fold, when the tyre is used at Imm or below comparedwith the first half of the tread life. Conclusions From our interpretation of the accumulated data we draw the following conclusions based upon the questions posed at the beginning of this paper. Radial tyres are slightly more resistant to punctures than cross-ply but this is simply due to the effect of increased crown thickness. A worn tyre is more prone to puncture than a new one. in length and I / 10" The commonest causes of puncture are nails of about I i" diameter. Many nails found in tyres can be shown to have been present for 2 0 0 miles or more prior to detection. The risk of a puncture occurririg in a rear tyre is twice that of a puncture occurring in a front tyre. The chances of 2 punctures (or puncturing objects) occurring in the same tyre are nearly I in 50. We also concluded that whilst almost any object however brittle will penetrate a tyre, glass is of comparatively little danger. There is little difference in the puncture susceptibility of the tyre tread, grooves, or ribs. Motorists who in winter drive on a deflated or deflating tyre, rather than stopping to change it as they may do in the summer, greatly increase the risk of causing additional damage to the tyre. Acknowledgements This work, in a co-operative vencure between the tyre industry and the Home Office Forensic Science Service, has depended heavily on the two sources of information mentioned in the text. We should like to thank the service organization of National Tyre Service, in particular Mr. J. Hogan, for their part in the survey. We should also like to thank the West Mercia Police for the information they supplied, made available to us by Chief Superintendent Blackie. Finally our thanks to the Directors of the Dunlop Company, and the Home Oflice for their permission to publish this work. References BAKER, J. S., and MCILRAITH, G. D., 1968, Tyre Disablements Followed by Accidents, Traffic Institute N.W. University, Illinois. DOBIE,W. J., 1969, Materials Research and Standards, March. GODLEY, M. J., 1972, The Incidence of Burst Tyres to Injury Accidents on M I and Mq Motorways, T.R.R.L. Report LR498. GROGAN, R. J., 1972,J. Forens Sci. Soc., 12, 285. KEMP,R. N., NEILSON, I. D., STRAUGHTON, G. C., and WILKINS, H. A., 1972, A Preliminary Report on an On-the-spot Survey of Accidents, T.R.R.L. Report LR434. LOWNE,R. W., 1973, Tyre Failures on Part of M5 Motorway, T.R.R.L. Report LR585. MACKAY, G. M., I 969, The Vehicle, in : Causes and Efects of Road Accidents, Vol. 3, by Kolbuszewski, J., Mackay, G. M., Fonseka, C. P., Blair, I., and Clayton, A. B., Department of Transportation and Environmental Planning, University of Birmingham. 176