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Examination of Damaged Motor Car Wheels and Tyres
Examination of Damaged Motor Car Wheels and Tyres W. H. D. MORGAN AND R. A. HALL Department of Industrial and Forensic Science, Belfast, Northern Irelalzd
A description i s given of a laboratory procedure used for examining damaged wheels and tyres removed from motor vehicles involved in accidents. These examinationshave been helpful in establishing the sequence of events prior to the accident. Introduction When the police are investigating accidents involving motor vehicles an explanation which is very often given by the drivers of the vehicles concerned is that "the tyre burst", or "the tyre blew out", or "the tyre punctured and suddenly went flat" and this affected the steering causing the driver to lose control of the vehicle. If such deflation does occur then the resulting collision is truly accidental, but in many cases the accident is the result of careless driving and the wheels are damaged as a result of the impact of the ensuing collision and thereby become deflated. The driver readily blames a burst tyre for the accident and in some cases the driver has been known to release the air from the tyre through the valve so that the tyre will be flat when the police arrive at the scene. Over the past number of years experience in Northern Ireland has shown that a laboratory examination of the damaged wheels of motor vehicles can provide useful factual information regarding the sequence of events in which these have become damaged. Table 1 shows the number of such cases referred to the Laboratory for examination since 1956 from which it will be seen that these investigations have increased markedly in the past few years thereby suggesting that they have been helpful to the courts. TABLE 1
In the majority of these cases it has been established that the wheels and tyres have been damaged as a result of the impact of collision. I t was considered that it may be helpful to others confronted with this problem to describe the manner in which these examinations are carried out in this laboratory. No attempt has been made to deal with the technicalities of tyre manufacture. Morton (1965) has published an informative account of tyre technology and French (1964) deals with some aspects of tyres and road safety. In another paper Tompkins (1965) discusses the mechanism of tyre grip in the wet, and includes an informative account of the phenomenon referred to as aquaplaning. An interesting pamphlet has been issued by the British Tyre Industry giving recommendations on the use and care of tyres.
Equipment and terminology Whilst these examinations can be carried out with a pair of tyre levers as minimum equipment it does assist greatly to have some additional tools. These include bead breakers, tyre spreaders, leather hammer, air pump, pressure gauge, probing tool and needle. These are available from the shops selling motor car accessories or through the normal trade channels (Figure 1). I t is desirable to define the terms used in our examination to describe the various parts of a 192
AIR RETAINING LINER
Figure 2
Figure 3
ALE0 VALVE
Structure of Tubeless Tyre
Structure of Tubed Tyre
TREAD
INNER LiNER
EAD
ING tt
Figure 4
r RAP ' c
MIRES
Structure of a Cross Ply (Conventional) Tyre
.AVERS
AL PLIES
Figure 5
Structure of a Radial Ply Tyre
Figure 6
Damage to Wheel corresponds to damage in Cover and Tube
Figure 7
Deflation Damage
wheel. When the wheel is removed from a vehicle this is referred to as a wheel unit complete and consists of a metal wheel on which the tyre is mounted. A tyre consists of a cover and tube, when these are incorporated into the same structural unit by the manufacturer it is called a tubless tyre. Sections of the two types of tyre are illustrated in Figures 2 and 3. Figure 4 shows the structure of a conventional cross ply tyre and Figure 5 shows that of a radial ply tyre. The cords refer to the reinforcing material on which the rubber is moulded and gives the tyre its necessary rigidity. The two edges of the tyre are called beads, and enclosed in these are bead wires. When the beads are fitted to the ledges of the wheel rim flanges they retain their seating under suitable air pressure in the tyre. With tubeless tyres this seating also performs the function of an air seal. The side wall of the tyre which can be seen when the wheel is attached to the motor vehicle is called the outer wall, and the other surface which faces inwards towards the chassis is the inner wall. The portion of the tyre designed to contact the road is called the tread and the section of the tread which bears on the road surface at any given time is the contact area. The inside surface of the tyre, as the term suggests, can only be examined after the tyre is removed from the wheel.
Laboratory examination All wheels which are to be examined in a laboratory should be removed from the vehicle by undoing the retaining nuts and dispatched as a wheel unit complete after appropriate labels have been attached for identification purposes. Particular care should be taken to ensure that all parts are sent for examination, as often small portions of the tread or tube become detached and are lost and these may provide information of vital importance. The type of vehicle, its registration number, and the position of the wheel unit to be examined should be carefully recorded. No attempt whatsover should be made by the police or persons requesting the examination to remove the tyre from the wheel otherwise much valuable information will be lost. At the commencement of the laboratory examination a detailed overall examination of the complete wheel unit is carried out and these observations recorded in a permanent manner. These include the residual pressure in the tyre (if any), the condition of the valve, whether or not a valve cap is present and securely attached, and also the condition (broken or not) of the beads or seals at the two lips of the rim of the metal wheel. Details of make, size, and number of the tyre are noted, also whether or not it is a tubed, tubeless or remoulded tyre. The number of the tyre is also recorded. The metal wheel is then examined for the presence of foreign matter (e.g. cement, soil, wood) adhering to it and also for any signs of recent abrasion or damage. In the majority of cases received the tyre is completely deflated and the lips of the metal wheel are damaged. If the tyre can be inflated, this is done immediately and the pressure increased to the value recommended for normal use. The inflated tyre is then tested for air leaks-a large water tank would be useful for this purpose but is not a t present used by us. If no air leaks are detected the air pressure is recorded and the wheel set aside and tested after a lapse of time. No further work is carried out on the wheel if the pressure is retained for an indefinite period and it is produced in court in this condition. No satisfactory method has yet been evolved for testing the valves of tubeless tyres which have been mounted on a metal wheel the circumference of which has been damaged. The valve insert, however, may be worn having been used in more than one unit and by fitting it in another unit may be shown to be leaking. The tyre is now ready for removal from the wheel so that the cover, tube and wheel can be examined separately. At this stage it is important that the metal wheel and tyre are marked carefully to indicate unambiguously their relative positions. Marks are made on the tyre with a coloured wax crayon 193
to indicate the position of the valve and also which is the outer wall of the tyre. These markings should be such, that it is possible to demonstrate exactly how the wheel unit had been assembled when received. The cover and the tube (if present) are carefully removed from the wheel using the bead breakers and the tyre levers noting particularly whether or not, either or both of the beads are broken. At this stage particular note is made if the damage to the cover, tube and metal wheel correspond as this indicates damage by impact (Figure6).
The cover or tyre The outer and inner walls of the tyre are examined for general condition and any gross tearing will be very obvious. A detailed inspection will show if this has been caused by penetration from the outside and also whether or not the damage is fresh or old. Fresh damage shows a clean unchafed break. This should be contrasted with old damage, cuts, looseness or other breakdown which has subsequently chafed over a long period before finally giving way. These walls are examined particularly for any indication of casing strain illustrated in Figure 7 which is an indication of the tyre having been run in a deflated condition. The bead is examined for possible bead wire fracture and this can be done, small lengths at a time, by flexing the bead circumference by hand. If an X-ray machine is available this can be used for the inspection, but in our experience this is rarely necessary. The tread is then examined and the condition of the tread pattern noted particularly for wear and any signs of tracking-a characteristic type of wear indicating misalignment of the wheels. The tread is then examined for cuts and the presence of any foreign objects such as nails, glass and stones which may have penetrated the rubber. I t has been found that the most satisfactory method of doing this is visually, flexing the tyre by hand as it is moved slowly round. Any cuts or defects are marked with coloured crayon and probed with the probing tool and needle to determine if the rubber has been penetrated completely. The inside surface of the tyre is now examined using the tyre spreaders for signs of cord damage, strain or fracture. Figures 8 and 9 show typical impact fractures and cord damage. Casing strain is more readily seen on this surface and indicates that the tyre has been run in a deflated condition. The damage to the cords or the presence of impact fractures suggests that the tyre has had a violent impact in this area. Casing separation, Figure 10, is readily detected as a flabbiness or bubbly appearance caused by lack of adhesion of the inner rubber to the outside layers or alternatively between the casing plies. The tube The make, size, number of the tube are noted and any patches present are examined for their effectiveness. Where possible, the tube is tested by inflation and immersion in water and it is sometimes possible to test the valve of a damaged tube by nipping the fracture between the fingers to enable an air pressure to be formed inside the tube. Figures 11,12, 13 show typical blow-outs in tubesthis type of damage is characterised by a starred pattern radiating from a central point. The position of any cuts or nips in the tube are noted and can usually be correlated with the damage to the cover or metal wheel. The metal wheel Finally the metal wheel is examined and the damage to the lips of the rims noted in particular. The edge of the rim is also examined for signs of recent abrasions. 194
Figure 8
Impact fracture showing Cord Damage
Figure 9
Impact fracture showing Cord Damage
Figure 10 Casing Separation
Figure 11 Blow out in Tube on Tread
Figure 12
Figure 13
Blow out in Tube on Wall
Blow out in Tube in well of Wheel
Having carried out these individual examinations they are then considered collectively with a view to establishing if there is any correlation between the damage to the tyre or cover, tube, and metal wheel. When this is done it is usually possible to suggest how the deflation of the tyre occurred and the sequence of events prior to the accident.
Acknowledgements The authors wish to thank the Technical Department of the Dunlop Rubber Company Limited for providing and giving permission to use the diagrams in Figures 2, 3, 4 and 5, Mr. A. G. Wilson, Assoc. Inst. M.I. and Mr. R. Stewart of the Dunlop Rubber Company, Belfast for their helpful advice and continued interest in these investigations, Mr. G. E. Fairclough who prepared the photographs and the Director of the Department of Industrial and Forensic Science for permission to publish this paper. References BRITISHTYREINDUSTRY, Pamphlet, Tyres and Your Safety. FRENCH T., 1964, Tyres and Road Safety-Dunlop Publication. MORTON G. F., 1965, Chemistry and Industry., 636. Recent Advances in Tyre Technology. TOMPKINS E. S., 1965 I.R.I. Trans Proc. 12, 69. Tyres for Modem Roads.
A description i s given of a laboratory procedure used for examining damaged wheels and tyres removed from motor vehicles involved in accidents. These examinationshave been helpful in establishing the sequence of events prior to the accident. Introduction When the police are investigating accidents involving motor vehicles an explanation which is very often given by the drivers of the vehicles concerned is that "the tyre burst", or "the tyre blew out", or "the tyre punctured and suddenly went flat" and this affected the steering causing the driver to lose control of the vehicle. If such deflation does occur then the resulting collision is truly accidental, but in many cases the accident is the result of careless driving and the wheels are damaged as a result of the impact of the ensuing collision and thereby become deflated. The driver readily blames a burst tyre for the accident and in some cases the driver has been known to release the air from the tyre through the valve so that the tyre will be flat when the police arrive at the scene. Over the past number of years experience in Northern Ireland has shown that a laboratory examination of the damaged wheels of motor vehicles can provide useful factual information regarding the sequence of events in which these have become damaged. Table 1 shows the number of such cases referred to the Laboratory for examination since 1956 from which it will be seen that these investigations have increased markedly in the past few years thereby suggesting that they have been helpful to the courts. TABLE 1
In the majority of these cases it has been established that the wheels and tyres have been damaged as a result of the impact of collision. I t was considered that it may be helpful to others confronted with this problem to describe the manner in which these examinations are carried out in this laboratory. No attempt has been made to deal with the technicalities of tyre manufacture. Morton (1965) has published an informative account of tyre technology and French (1964) deals with some aspects of tyres and road safety. In another paper Tompkins (1965) discusses the mechanism of tyre grip in the wet, and includes an informative account of the phenomenon referred to as aquaplaning. An interesting pamphlet has been issued by the British Tyre Industry giving recommendations on the use and care of tyres.
Equipment and terminology Whilst these examinations can be carried out with a pair of tyre levers as minimum equipment it does assist greatly to have some additional tools. These include bead breakers, tyre spreaders, leather hammer, air pump, pressure gauge, probing tool and needle. These are available from the shops selling motor car accessories or through the normal trade channels (Figure 1). I t is desirable to define the terms used in our examination to describe the various parts of a 192
AIR RETAINING LINER
Figure 2
Figure 3
ALE0 VALVE
Structure of Tubeless Tyre
Structure of Tubed Tyre
TREAD
INNER LiNER
EAD
ING tt
Figure 4
r RAP ' c
MIRES
Structure of a Cross Ply (Conventional) Tyre
.AVERS
AL PLIES
Figure 5
Structure of a Radial Ply Tyre
Figure 6
Damage to Wheel corresponds to damage in Cover and Tube
Figure 7
Deflation Damage
wheel. When the wheel is removed from a vehicle this is referred to as a wheel unit complete and consists of a metal wheel on which the tyre is mounted. A tyre consists of a cover and tube, when these are incorporated into the same structural unit by the manufacturer it is called a tubless tyre. Sections of the two types of tyre are illustrated in Figures 2 and 3. Figure 4 shows the structure of a conventional cross ply tyre and Figure 5 shows that of a radial ply tyre. The cords refer to the reinforcing material on which the rubber is moulded and gives the tyre its necessary rigidity. The two edges of the tyre are called beads, and enclosed in these are bead wires. When the beads are fitted to the ledges of the wheel rim flanges they retain their seating under suitable air pressure in the tyre. With tubeless tyres this seating also performs the function of an air seal. The side wall of the tyre which can be seen when the wheel is attached to the motor vehicle is called the outer wall, and the other surface which faces inwards towards the chassis is the inner wall. The portion of the tyre designed to contact the road is called the tread and the section of the tread which bears on the road surface at any given time is the contact area. The inside surface of the tyre, as the term suggests, can only be examined after the tyre is removed from the wheel.
Laboratory examination All wheels which are to be examined in a laboratory should be removed from the vehicle by undoing the retaining nuts and dispatched as a wheel unit complete after appropriate labels have been attached for identification purposes. Particular care should be taken to ensure that all parts are sent for examination, as often small portions of the tread or tube become detached and are lost and these may provide information of vital importance. The type of vehicle, its registration number, and the position of the wheel unit to be examined should be carefully recorded. No attempt whatsover should be made by the police or persons requesting the examination to remove the tyre from the wheel otherwise much valuable information will be lost. At the commencement of the laboratory examination a detailed overall examination of the complete wheel unit is carried out and these observations recorded in a permanent manner. These include the residual pressure in the tyre (if any), the condition of the valve, whether or not a valve cap is present and securely attached, and also the condition (broken or not) of the beads or seals at the two lips of the rim of the metal wheel. Details of make, size, and number of the tyre are noted, also whether or not it is a tubed, tubeless or remoulded tyre. The number of the tyre is also recorded. The metal wheel is then examined for the presence of foreign matter (e.g. cement, soil, wood) adhering to it and also for any signs of recent abrasion or damage. In the majority of cases received the tyre is completely deflated and the lips of the metal wheel are damaged. If the tyre can be inflated, this is done immediately and the pressure increased to the value recommended for normal use. The inflated tyre is then tested for air leaks-a large water tank would be useful for this purpose but is not a t present used by us. If no air leaks are detected the air pressure is recorded and the wheel set aside and tested after a lapse of time. No further work is carried out on the wheel if the pressure is retained for an indefinite period and it is produced in court in this condition. No satisfactory method has yet been evolved for testing the valves of tubeless tyres which have been mounted on a metal wheel the circumference of which has been damaged. The valve insert, however, may be worn having been used in more than one unit and by fitting it in another unit may be shown to be leaking. The tyre is now ready for removal from the wheel so that the cover, tube and wheel can be examined separately. At this stage it is important that the metal wheel and tyre are marked carefully to indicate unambiguously their relative positions. Marks are made on the tyre with a coloured wax crayon 193
to indicate the position of the valve and also which is the outer wall of the tyre. These markings should be such, that it is possible to demonstrate exactly how the wheel unit had been assembled when received. The cover and the tube (if present) are carefully removed from the wheel using the bead breakers and the tyre levers noting particularly whether or not, either or both of the beads are broken. At this stage particular note is made if the damage to the cover, tube and metal wheel correspond as this indicates damage by impact (Figure6).
The cover or tyre The outer and inner walls of the tyre are examined for general condition and any gross tearing will be very obvious. A detailed inspection will show if this has been caused by penetration from the outside and also whether or not the damage is fresh or old. Fresh damage shows a clean unchafed break. This should be contrasted with old damage, cuts, looseness or other breakdown which has subsequently chafed over a long period before finally giving way. These walls are examined particularly for any indication of casing strain illustrated in Figure 7 which is an indication of the tyre having been run in a deflated condition. The bead is examined for possible bead wire fracture and this can be done, small lengths at a time, by flexing the bead circumference by hand. If an X-ray machine is available this can be used for the inspection, but in our experience this is rarely necessary. The tread is then examined and the condition of the tread pattern noted particularly for wear and any signs of tracking-a characteristic type of wear indicating misalignment of the wheels. The tread is then examined for cuts and the presence of any foreign objects such as nails, glass and stones which may have penetrated the rubber. I t has been found that the most satisfactory method of doing this is visually, flexing the tyre by hand as it is moved slowly round. Any cuts or defects are marked with coloured crayon and probed with the probing tool and needle to determine if the rubber has been penetrated completely. The inside surface of the tyre is now examined using the tyre spreaders for signs of cord damage, strain or fracture. Figures 8 and 9 show typical impact fractures and cord damage. Casing strain is more readily seen on this surface and indicates that the tyre has been run in a deflated condition. The damage to the cords or the presence of impact fractures suggests that the tyre has had a violent impact in this area. Casing separation, Figure 10, is readily detected as a flabbiness or bubbly appearance caused by lack of adhesion of the inner rubber to the outside layers or alternatively between the casing plies. The tube The make, size, number of the tube are noted and any patches present are examined for their effectiveness. Where possible, the tube is tested by inflation and immersion in water and it is sometimes possible to test the valve of a damaged tube by nipping the fracture between the fingers to enable an air pressure to be formed inside the tube. Figures 11,12, 13 show typical blow-outs in tubesthis type of damage is characterised by a starred pattern radiating from a central point. The position of any cuts or nips in the tube are noted and can usually be correlated with the damage to the cover or metal wheel. The metal wheel Finally the metal wheel is examined and the damage to the lips of the rims noted in particular. The edge of the rim is also examined for signs of recent abrasions. 194
Figure 8
Impact fracture showing Cord Damage
Figure 9
Impact fracture showing Cord Damage
Figure 10 Casing Separation
Figure 11 Blow out in Tube on Tread
Figure 12
Figure 13
Blow out in Tube on Wall
Blow out in Tube in well of Wheel
Having carried out these individual examinations they are then considered collectively with a view to establishing if there is any correlation between the damage to the tyre or cover, tube, and metal wheel. When this is done it is usually possible to suggest how the deflation of the tyre occurred and the sequence of events prior to the accident.
Acknowledgements The authors wish to thank the Technical Department of the Dunlop Rubber Company Limited for providing and giving permission to use the diagrams in Figures 2, 3, 4 and 5, Mr. A. G. Wilson, Assoc. Inst. M.I. and Mr. R. Stewart of the Dunlop Rubber Company, Belfast for their helpful advice and continued interest in these investigations, Mr. G. E. Fairclough who prepared the photographs and the Director of the Department of Industrial and Forensic Science for permission to publish this paper. References BRITISHTYREINDUSTRY, Pamphlet, Tyres and Your Safety. FRENCH T., 1964, Tyres and Road Safety-Dunlop Publication. MORTON G. F., 1965, Chemistry and Industry., 636. Recent Advances in Tyre Technology. TOMPKINS E. S., 1965 I.R.I. Trans Proc. 12, 69. Tyres for Modem Roads.