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Tyres and Crime
Tyres and Crime R. J. GROGAN Technical Department, Dunlop Company Limited, Fort Dunlop, Birmingham 24, England T. R. WATSON Home Ojice Forensic Science Laboralory, Birmingham, England.
T h e need for accurate, comprehensive and speedy permanent records of tyre marks i s emphasised. T h e signiJicance of various features found in tyre marks i s discussed, and t h means by which the make of vehicle m a y be deduced from tyre marks are szlggested. Features which enable a tyre mark to be related to a specijc tyre are detailed. Reference i s also made to the double transfer of tyre marks onto clothing and cloth marks onto tyres.
Introduction Ever since the days of Dick Turpin, criminals have placed a premium on mobility when plying their trade. The modern criminal has all the resources of modern transport a t his disposal and naturally he takes advantage of them. He uses a wide range of motor vehicles, from the fast high powered saloon for a brisk get away after a hold up, t o the 'down a t heel' and inconspicuous van used to transport safes t o a quiet spot for leisurely attention. Often the only indication of how and whence the criminal departed is a set of tyre prints, but much can be learned from these, as we hope to show. We cannot, of course, pretend that tyre prints are as individual as finger prints or even foot prints, but neither are they characterless uniform objects which, from the lack of attention they receive, one might conclude. How often a t the scene of crime is one and only one conclusion drawn from tyre prints that 'They came and went by wheeled vehicle'? How often are the public, Police, or (dare we say it) other experts allowed to obliterate these potentially useful marks simply because they are not recognised as important clues? The first requirement, therefore, is an awareness by everyone likely to visit the scene of crime, that tyre prints can repay careful and detailed examination. Once this lesson is learnt we shall not see tyre prints allowed to deteriorate by exposure to the elements, to be obscured by the tracks of other vehicles or by foot prints (Figure 1). Instead appreciation of their value will lead either t o quick and detailed permanent records being made, or precautions taken to protect the marks until they can be recorded. I t must be emphasised a t the outset that the complete length of the tyre tracks are important if their full value is to be realised. Small areas or isolated prints are better than nothing, but less than half the story will probably come to light under these circumstances. An accurate and fairly detailed plan of the relationship of the tyre marks to each other and surrounding objects is essential. The plan should show the distances between marks accurately, ideally t o within &in. Attention should be paid to areas where multiple tracks are observed, because these probably suggest the course of the vehicle and allow the wheel positions to be identified. The distance between the wheel tracks is most important, as are the widths of individual tyre marks. Useful information can also be gained from the radii of turns and the position of small irregularities in the tyre marks. Once the information for the accurate and detailed plan has been 3
Figure 1. A common experience. Foot prints partially obliterating tyre marks, fortunately still discernible as Avon Super Safety and Dunlop C41.
Figure 2. An example of family differences. Both tyres 145 section, both SP41 pattern. The five rib version is for 10" wheels, the six rib is for 13" wheels. 4
Figure 3. Top : Pirelli Cariload with worn centre but still identifiable by characteristic pattern groove. Centre: Unknown remould. Bottom : Firestone Transport. Identified by shoulder pattern.
Figure 4. A typical tyre print and three tyres with similar characteristics. Left : Henley Super Miler. Centre : Avon Super Safety. Right : Avon Turbospeed.
5
compiled, photographs should be taken of the details of individual tyre marks. Side lighting is useful when the marks are poorly defined and often details not immediately apparent to the naked eye are subsequently revealed. Photographs should, of course, include a scale. A careful inspection along the length of the tyre marks will sometimes show that the tyre has traversed soft ground or brushed against an earthen bank and left the impression of the sidewall showing details of the pattern of the tyre shoulders or even lettering. All such marks are extremely helpful and should be carefully recorded. Perhaps a t this stage it would be useful to summarise the points we consider important :Tyre mark widths. Tyre pattern. Tyre irregularities. Tyre wear. Position of marks related to other objects. Position of marks related t o each other. Separation of prints. Radii of turns. Depth of pattern. When these details have been recorded photographically and on the plan, plaster casts may be made t o show such details as the height of pattern ridges and the relationship of sidewall details (mentioned earlier) to the tyre pattern. What information can the tyre prints be made to reveal? Ideally, one should be able to deduce the type of vehicle, the direction in which it travelled, the makes of the tyres used and their positions and given a little luck, later on be able to say whether a given tyre made the marks found. Many of these factors depend on each other to give a balanced picture and so it is difficult to separate the points for individual consideration, but taking them in order :-
Tyre Mark Width Tyre width is the separation of the outermost marks of the tread. Normally, a single measurement across the tyre mark is sufficient but some tyres, notably winter or on-off the road types, have shoulder patterns comprising two different pattern elements and these are often disposed so that a long element on one side of the tyre is opposite a short one on the other side. In such cases, careful measurements are called for from the centre line. The width of the tyre print will give an indication of the sectional size of the tyre and this in itself may narrow the field by suggesting vehicles which commonly fit tyres of that width. For example, a single completely plain tyre mark 78in. wide would suggest a Commercial vehicle with 9.00 tyres, such a section not being commonly fitted to anything but the largest cars, usually of American origin. Tyre Pattern The tread pattern will indicate the maker of the tyre and often the type of vehicle, i.e., car, van or lorry, although there are a number of traps for the unwary. Certain makers tailor their patterns to suit the requirements of specific vehicles and, therefore, one should not be too surprised to find quite large detail differences within the family pattern (see figure 2). Another point often observed is that certain features, such as 'knife cuts' (or sipes), holes, decorative lines, or in some cases complete tread grooves, disappear with wear, as in figure 3. Some tyre makers pass on moulds to retreaders or they may assume the brand name of another tyre made within the same organisation, sometimes with only very small changes to the pattern, e.g., figure 4. Such details will be known only to the tyre manufacturer and, 6
Figure 5. Uniroyal tyre mark showing top of sidewall letters 'ROY'
Figure 6. Relationship of sidewall lettering to tread and shoulder pattern. All tyres same size.
7
roll
Figure 8. Camber wear. Note the absence of sharply defined edge on the worn side. Inset-.the tyre responsible. 8
therefore, when a pattern is t o be identified positively, it is wise to enlist the aid of the local tyre depots or the manufacturer. Help of this nature is invariably freely and speedily given. Having determined the make of tyre and found it t o be a common one, it is still possible to determine the make of vehicle likely to be involved from a consideration of the make of tyres various manufacturers fit as original equipment. Even replacement tyres can conform to this because of brand loyalty, i.e., the tendency of a satisfied customer to buy the same again. Given the tyre make and the section size there is a fair chance of being able to deduce the likely vehicles involved, and a knowledge of the popularity of various makes of vehicle helps still further. T y r e Irregularities Features such as trapped stones, cuts or repairs, variations in tread pattern or moulding blemishes, give a good indication of the rolling circumference of the tyre when the recurrence of two or more such features can be located and the distance between them measured. Most manufacturers issue tables giving the rolling radii of their tyres which enable the circumference to be calculated. Further use may be made of such irregularities by relating tyre marks to a given tyre. A second type of tyre irregularity is sidewall numbering or lettering. I t is not uncommon to find the imprint of sidewalls on soft ground and these can be most useful (see figure 5). Firstly, the imprints may immediately give the make or size and so save time, but secondly, the relationship of the lettering to the tread pattern can help to relate the tyre mark to a particular tyre. The tread pattern is not a constant and fixed unit, the pitch of elements of the pattern are usually 'scrambled' to prevent noise generation, and the sidewall lettering is engraved a t random around the tyre mould. Thus the sidewall letteringltread pattern relationship is nearly always different for each mould, as shown in figure 6. A note of caution, however : some retreaders leave the original sidewall lettering and numbering in place and it is possible in these cases to have an apparent contradiction between the sidewall details and the tread pattern. T y r e Wear Tyre wear is often a characteristic of a particular vehicle. I t is fairly wellknown that cars with divided rear axles produce tyres with the inner or outer shoulders worn depending on whether the car normally travels heavily or lightly laden a t the rear. Other cars produce similar results by having a camber angle of the front wheels to improve steering. Cars with sophisticated suspensions which give a very level platform on curves, produce a feathering of the outer shoulders of the front tyres (see figure 7). Very heavy cars often exhibit camber wear on the front tyres, i.e., a rounded wearing away of the outer shoulder, produced when the wheel leans on cornering (see figure 8). Misalignment wear is a symptom of a mechanical defect and, therefore, not related to types of vehicle but when observed, it will serve to identify the front tyre prints with certainty. I t may also be useful later as a further identifying feature if the vehicle is found. Other such features are the scolloped wear produced by worn king pins, rapid centre wear due to over inflation or conversely rapid wear to both shoulders when the tyre is used under-inflated or overloaded. Pronounced 'heel and toe' wear is a manifestation of hard driving. Position of T y r e M a r k s i n Relation t o O t h e r Objects Under this heading come such events as the dumping of bodies, safes, tools, explosives and other incriminating objects from vehicles. A careful study will often show that by examining the tyre prints in conjunction with the position of such objects, one can deduce where in the car the object was carried. 9
Figure 9. Four tyre tracks in soft ground. Rear track width 4' 3#", tyre width 4". Front track width 4' 2 r , tyre width 4f". B.M.C. 1100.
Figure 10. The double transfer of tyre and clothing marks. (Crown Copyright Reserved).
10
I t is worth mentioning here that there is a very common tendency to believe that criminals reverse into the gateways to fields before discarding their wares, but examination of tyre prints in relation to the gateway usually shows that the vehicle has been driven in forwards and has either reversed when out of sight or, more commonly, been driven round in a circle inside the field. Oil spots will indicate the position of the engine and when taken in conjunction with tyre marks show whether a front or rear engined car has been used.
The Position of Marks Related to Each Other Relationstlip of tyre marks to each other will often serve to show which tyres are front and which rear. For example, front tyres take larger radii on curves than the rears. A pair of narrowly separated tyre marks indicate rear tyres of a commercial vehicle. Winter tyres on the front with normal tyres on the rear would strongly suggest a front wheel drive car. Deeper impressions or wider marks indicate the more heavy laden wheels. The Separation of Prints Separation of tyre prints give a good measure of vehicle track width (figure 9), one can often narrow the field to certain vehicles by referring t o a table of track widths and other dimensions such as that published by the Autocar or Motor. Some vehicles have a feature known as a 'crabtrack', that is a different track width, front and rear. Pronounced examples of this are the Citroen DS or ID19 and some older Jaguars, but several cars exhibit this feature t o a limited extent. This feature would also be revealed by the dimension lists. The Radii of Turning Circles Radii of turn is not quite as useful as an identifying feature as track width but, in conjunction with other factors, it, too, can help to suggest vehicles worthy of study. Here again a list of vehicle dimensions with turning circles will be helpful. One might be lucky in making a positive identification from turning circle alone. A very small turn, for example, would suggest either a Triumph Herald or a London taxi. A large turning circle coupled with a suggestion that a tighter turn could not be made (for example, the need t o reverse) might indicate a front wheel drive car. Depth of Pattern Again this is a difficult feature to use as a positive identification. Kevertheless, four deep tyre prints of the same make would indicate that some consideration should be given t o the probability of a new vehicle being involved. Conversely, tyres which are badly worn, or of various makes, make it likely that the vehicle is an old one. Although direct measurement of height of the tyre pattern impression is unquestionably the best method of determining the pattern depth, indirect methods can be used. As we mentioned earlier, some patterns change their appearance later on in life. I t must be admitted that only where tyre impressions are found in soft clay, mud or snow, can accurate measurements be made, sand or earth quickly dry out and the heights of the ridges diminish. Other Aspects of Tyre Examination In the foregoing, we have considered tyre marks on the ground, but there is a further important type of tyre impression which is worthy of note, that is tyre marks on clothing. Such marks are usually the result of a 'hit and run' incident, and much of what has been described above is also relevant to marks on clothing. Sidewall details are very commonly found on clothing, indeed on occasions they are the only tyre marks found. When such marks are found it is often the 11
1 tyre of make on car 2 tyres same make
3 tyres same rnakcl
all tyres same make
1
a
Figure 11. Car tyre fitments ; number of tyres against the make.
Date of
Registration of Car
Figurc 12. Tyre makc combinations against agc of car (Combined London and Rirminghan Survey).
case that there are corresponding cloth marks on the tyre. This double transfer of marks is a powerful identifying feature when the vehicle in question is located (see figure 10). We are continuing our collaboration and one facet which we are currently working on is the collection of data to use as a basis for predicting probability levels of certain combinations of tyre types. We have already had two surveys made of large car parks, one in London and one in the Midlands, and we now have a t least the beginning of a method for evaluating such features as vehicle popularity and hence tyre size popularity. We have also investigated the probability of one, two, three or four makes of tyres appearing on a vehicle (figure ll),and the age of the vehicles to which the various combinations of tyres are fitted (see figure 12). Clearly, this information must be continuously collected and processed to be of value. I t is also obvious that the collection of such information can be expensive and we are looking into ways to overcome this problem. We hope ultimately to be able to determine such features as regional preferences for tyres and perhaps even seasonal variations in tyre buying habits, all of which will enable us to make firmer and quicker deductions when confronted by a series of tyre tracks at the scene of the crime.
Acknowledgements The Authors jointly wish to thank the Directors of the Dunlop Company and the Home Office for permission to publish this paper and the Director of the West Midlands Forensic Science Laboratory, for his help and encouragement.
T h e need for accurate, comprehensive and speedy permanent records of tyre marks i s emphasised. T h e signiJicance of various features found in tyre marks i s discussed, and t h means by which the make of vehicle m a y be deduced from tyre marks are szlggested. Features which enable a tyre mark to be related to a specijc tyre are detailed. Reference i s also made to the double transfer of tyre marks onto clothing and cloth marks onto tyres.
Introduction Ever since the days of Dick Turpin, criminals have placed a premium on mobility when plying their trade. The modern criminal has all the resources of modern transport a t his disposal and naturally he takes advantage of them. He uses a wide range of motor vehicles, from the fast high powered saloon for a brisk get away after a hold up, t o the 'down a t heel' and inconspicuous van used to transport safes t o a quiet spot for leisurely attention. Often the only indication of how and whence the criminal departed is a set of tyre prints, but much can be learned from these, as we hope to show. We cannot, of course, pretend that tyre prints are as individual as finger prints or even foot prints, but neither are they characterless uniform objects which, from the lack of attention they receive, one might conclude. How often a t the scene of crime is one and only one conclusion drawn from tyre prints that 'They came and went by wheeled vehicle'? How often are the public, Police, or (dare we say it) other experts allowed to obliterate these potentially useful marks simply because they are not recognised as important clues? The first requirement, therefore, is an awareness by everyone likely to visit the scene of crime, that tyre prints can repay careful and detailed examination. Once this lesson is learnt we shall not see tyre prints allowed to deteriorate by exposure to the elements, to be obscured by the tracks of other vehicles or by foot prints (Figure 1). Instead appreciation of their value will lead either t o quick and detailed permanent records being made, or precautions taken to protect the marks until they can be recorded. I t must be emphasised a t the outset that the complete length of the tyre tracks are important if their full value is to be realised. Small areas or isolated prints are better than nothing, but less than half the story will probably come to light under these circumstances. An accurate and fairly detailed plan of the relationship of the tyre marks to each other and surrounding objects is essential. The plan should show the distances between marks accurately, ideally t o within &in. Attention should be paid to areas where multiple tracks are observed, because these probably suggest the course of the vehicle and allow the wheel positions to be identified. The distance between the wheel tracks is most important, as are the widths of individual tyre marks. Useful information can also be gained from the radii of turns and the position of small irregularities in the tyre marks. Once the information for the accurate and detailed plan has been 3
Figure 1. A common experience. Foot prints partially obliterating tyre marks, fortunately still discernible as Avon Super Safety and Dunlop C41.
Figure 2. An example of family differences. Both tyres 145 section, both SP41 pattern. The five rib version is for 10" wheels, the six rib is for 13" wheels. 4
Figure 3. Top : Pirelli Cariload with worn centre but still identifiable by characteristic pattern groove. Centre: Unknown remould. Bottom : Firestone Transport. Identified by shoulder pattern.
Figure 4. A typical tyre print and three tyres with similar characteristics. Left : Henley Super Miler. Centre : Avon Super Safety. Right : Avon Turbospeed.
5
compiled, photographs should be taken of the details of individual tyre marks. Side lighting is useful when the marks are poorly defined and often details not immediately apparent to the naked eye are subsequently revealed. Photographs should, of course, include a scale. A careful inspection along the length of the tyre marks will sometimes show that the tyre has traversed soft ground or brushed against an earthen bank and left the impression of the sidewall showing details of the pattern of the tyre shoulders or even lettering. All such marks are extremely helpful and should be carefully recorded. Perhaps a t this stage it would be useful to summarise the points we consider important :Tyre mark widths. Tyre pattern. Tyre irregularities. Tyre wear. Position of marks related to other objects. Position of marks related t o each other. Separation of prints. Radii of turns. Depth of pattern. When these details have been recorded photographically and on the plan, plaster casts may be made t o show such details as the height of pattern ridges and the relationship of sidewall details (mentioned earlier) to the tyre pattern. What information can the tyre prints be made to reveal? Ideally, one should be able to deduce the type of vehicle, the direction in which it travelled, the makes of the tyres used and their positions and given a little luck, later on be able to say whether a given tyre made the marks found. Many of these factors depend on each other to give a balanced picture and so it is difficult to separate the points for individual consideration, but taking them in order :-
Tyre Mark Width Tyre width is the separation of the outermost marks of the tread. Normally, a single measurement across the tyre mark is sufficient but some tyres, notably winter or on-off the road types, have shoulder patterns comprising two different pattern elements and these are often disposed so that a long element on one side of the tyre is opposite a short one on the other side. In such cases, careful measurements are called for from the centre line. The width of the tyre print will give an indication of the sectional size of the tyre and this in itself may narrow the field by suggesting vehicles which commonly fit tyres of that width. For example, a single completely plain tyre mark 78in. wide would suggest a Commercial vehicle with 9.00 tyres, such a section not being commonly fitted to anything but the largest cars, usually of American origin. Tyre Pattern The tread pattern will indicate the maker of the tyre and often the type of vehicle, i.e., car, van or lorry, although there are a number of traps for the unwary. Certain makers tailor their patterns to suit the requirements of specific vehicles and, therefore, one should not be too surprised to find quite large detail differences within the family pattern (see figure 2). Another point often observed is that certain features, such as 'knife cuts' (or sipes), holes, decorative lines, or in some cases complete tread grooves, disappear with wear, as in figure 3. Some tyre makers pass on moulds to retreaders or they may assume the brand name of another tyre made within the same organisation, sometimes with only very small changes to the pattern, e.g., figure 4. Such details will be known only to the tyre manufacturer and, 6
Figure 5. Uniroyal tyre mark showing top of sidewall letters 'ROY'
Figure 6. Relationship of sidewall lettering to tread and shoulder pattern. All tyres same size.
7
roll
Figure 8. Camber wear. Note the absence of sharply defined edge on the worn side. Inset-.the tyre responsible. 8
therefore, when a pattern is t o be identified positively, it is wise to enlist the aid of the local tyre depots or the manufacturer. Help of this nature is invariably freely and speedily given. Having determined the make of tyre and found it t o be a common one, it is still possible to determine the make of vehicle likely to be involved from a consideration of the make of tyres various manufacturers fit as original equipment. Even replacement tyres can conform to this because of brand loyalty, i.e., the tendency of a satisfied customer to buy the same again. Given the tyre make and the section size there is a fair chance of being able to deduce the likely vehicles involved, and a knowledge of the popularity of various makes of vehicle helps still further. T y r e Irregularities Features such as trapped stones, cuts or repairs, variations in tread pattern or moulding blemishes, give a good indication of the rolling circumference of the tyre when the recurrence of two or more such features can be located and the distance between them measured. Most manufacturers issue tables giving the rolling radii of their tyres which enable the circumference to be calculated. Further use may be made of such irregularities by relating tyre marks to a given tyre. A second type of tyre irregularity is sidewall numbering or lettering. I t is not uncommon to find the imprint of sidewalls on soft ground and these can be most useful (see figure 5). Firstly, the imprints may immediately give the make or size and so save time, but secondly, the relationship of the lettering to the tread pattern can help to relate the tyre mark to a particular tyre. The tread pattern is not a constant and fixed unit, the pitch of elements of the pattern are usually 'scrambled' to prevent noise generation, and the sidewall lettering is engraved a t random around the tyre mould. Thus the sidewall letteringltread pattern relationship is nearly always different for each mould, as shown in figure 6. A note of caution, however : some retreaders leave the original sidewall lettering and numbering in place and it is possible in these cases to have an apparent contradiction between the sidewall details and the tread pattern. T y r e Wear Tyre wear is often a characteristic of a particular vehicle. I t is fairly wellknown that cars with divided rear axles produce tyres with the inner or outer shoulders worn depending on whether the car normally travels heavily or lightly laden a t the rear. Other cars produce similar results by having a camber angle of the front wheels to improve steering. Cars with sophisticated suspensions which give a very level platform on curves, produce a feathering of the outer shoulders of the front tyres (see figure 7). Very heavy cars often exhibit camber wear on the front tyres, i.e., a rounded wearing away of the outer shoulder, produced when the wheel leans on cornering (see figure 8). Misalignment wear is a symptom of a mechanical defect and, therefore, not related to types of vehicle but when observed, it will serve to identify the front tyre prints with certainty. I t may also be useful later as a further identifying feature if the vehicle is found. Other such features are the scolloped wear produced by worn king pins, rapid centre wear due to over inflation or conversely rapid wear to both shoulders when the tyre is used under-inflated or overloaded. Pronounced 'heel and toe' wear is a manifestation of hard driving. Position of T y r e M a r k s i n Relation t o O t h e r Objects Under this heading come such events as the dumping of bodies, safes, tools, explosives and other incriminating objects from vehicles. A careful study will often show that by examining the tyre prints in conjunction with the position of such objects, one can deduce where in the car the object was carried. 9
Figure 9. Four tyre tracks in soft ground. Rear track width 4' 3#", tyre width 4". Front track width 4' 2 r , tyre width 4f". B.M.C. 1100.
Figure 10. The double transfer of tyre and clothing marks. (Crown Copyright Reserved).
10
I t is worth mentioning here that there is a very common tendency to believe that criminals reverse into the gateways to fields before discarding their wares, but examination of tyre prints in relation to the gateway usually shows that the vehicle has been driven in forwards and has either reversed when out of sight or, more commonly, been driven round in a circle inside the field. Oil spots will indicate the position of the engine and when taken in conjunction with tyre marks show whether a front or rear engined car has been used.
The Position of Marks Related to Each Other Relationstlip of tyre marks to each other will often serve to show which tyres are front and which rear. For example, front tyres take larger radii on curves than the rears. A pair of narrowly separated tyre marks indicate rear tyres of a commercial vehicle. Winter tyres on the front with normal tyres on the rear would strongly suggest a front wheel drive car. Deeper impressions or wider marks indicate the more heavy laden wheels. The Separation of Prints Separation of tyre prints give a good measure of vehicle track width (figure 9), one can often narrow the field to certain vehicles by referring t o a table of track widths and other dimensions such as that published by the Autocar or Motor. Some vehicles have a feature known as a 'crabtrack', that is a different track width, front and rear. Pronounced examples of this are the Citroen DS or ID19 and some older Jaguars, but several cars exhibit this feature t o a limited extent. This feature would also be revealed by the dimension lists. The Radii of Turning Circles Radii of turn is not quite as useful as an identifying feature as track width but, in conjunction with other factors, it, too, can help to suggest vehicles worthy of study. Here again a list of vehicle dimensions with turning circles will be helpful. One might be lucky in making a positive identification from turning circle alone. A very small turn, for example, would suggest either a Triumph Herald or a London taxi. A large turning circle coupled with a suggestion that a tighter turn could not be made (for example, the need t o reverse) might indicate a front wheel drive car. Depth of Pattern Again this is a difficult feature to use as a positive identification. Kevertheless, four deep tyre prints of the same make would indicate that some consideration should be given t o the probability of a new vehicle being involved. Conversely, tyres which are badly worn, or of various makes, make it likely that the vehicle is an old one. Although direct measurement of height of the tyre pattern impression is unquestionably the best method of determining the pattern depth, indirect methods can be used. As we mentioned earlier, some patterns change their appearance later on in life. I t must be admitted that only where tyre impressions are found in soft clay, mud or snow, can accurate measurements be made, sand or earth quickly dry out and the heights of the ridges diminish. Other Aspects of Tyre Examination In the foregoing, we have considered tyre marks on the ground, but there is a further important type of tyre impression which is worthy of note, that is tyre marks on clothing. Such marks are usually the result of a 'hit and run' incident, and much of what has been described above is also relevant to marks on clothing. Sidewall details are very commonly found on clothing, indeed on occasions they are the only tyre marks found. When such marks are found it is often the 11
1 tyre of make on car 2 tyres same make
3 tyres same rnakcl
all tyres same make
1
a
Figure 11. Car tyre fitments ; number of tyres against the make.
Date of
Registration of Car
Figurc 12. Tyre makc combinations against agc of car (Combined London and Rirminghan Survey).
case that there are corresponding cloth marks on the tyre. This double transfer of marks is a powerful identifying feature when the vehicle in question is located (see figure 10). We are continuing our collaboration and one facet which we are currently working on is the collection of data to use as a basis for predicting probability levels of certain combinations of tyre types. We have already had two surveys made of large car parks, one in London and one in the Midlands, and we now have a t least the beginning of a method for evaluating such features as vehicle popularity and hence tyre size popularity. We have also investigated the probability of one, two, three or four makes of tyres appearing on a vehicle (figure ll),and the age of the vehicles to which the various combinations of tyres are fitted (see figure 12). Clearly, this information must be continuously collected and processed to be of value. I t is also obvious that the collection of such information can be expensive and we are looking into ways to overcome this problem. We hope ultimately to be able to determine such features as regional preferences for tyres and perhaps even seasonal variations in tyre buying habits, all of which will enable us to make firmer and quicker deductions when confronted by a series of tyre tracks at the scene of the crime.
Acknowledgements The Authors jointly wish to thank the Directors of the Dunlop Company and the Home Office for permission to publish this paper and the Director of the West Midlands Forensic Science Laboratory, for his help and encouragement.