Tyre age is one of the most common topics of speculation in the tyre market. Below are the official positions of the world's two largest manufacturers — Michelin and Bridgestone — on tyre ageing, together with comments from their representatives in Lithuania and the Baltic states. For what independent German research has shown, see Germans debunk the myths about tyre age.
Michelin's position: ageing is driven by use, not by the calendar
Tomasz Młodawski, product manager for Michelin's passenger-car, commercial-vehicle and off-road tyre groups. Warsaw, January 2010.
Tyres are made of a variety of materials and rubber compounds whose properties are essential for a tyre to function properly. Those properties change over time. How each tyre ages depends on many factors — storage and operating conditions (weather and temperature, load, driving speed, inflation pressure, maintenance and so on) that the tyre encounters throughout its life. Because storage and usage conditions can vary so widely, the service life of any particular tyre cannot be predicted in advance.
Ageing processes do occur during storage, but they are minute compared with the changes that begin once tyres are mounted and put into service. Over many years Michelin has perfected storage techniques which guarantee that tyres reach dealers in proper condition, ready to serve properly throughout their working life. Michelin group tyres are kept in factory warehouses for up to 3 years under scrupulously controlled conditions (temperature, humidity, light…). Michelin has also established tyre-storage requirements and standards that its dealers must observe.
To assess how the performance of passenger-car tyres changes, Michelin, working with authoritative organisations such as ADAC, has carried out several tests over the past decade comparing new tyres with older ones (1 to 4 years old). In terms of durability and endurance, no essential differences were found between newer and older tyres. Nor were any significant differences recorded in the results on wet or snow-covered surfaces.
When considering tyre ageing it is very important to distinguish the effect of a tyre's calendar age (its manufacturing date) from the changes that arise during use — the physical and chemical ageing of its components caused by the loads and temperature swings of driving. In service, a tyre endures enormous "stress": road irregularities, changing temperatures, friction forces and other conditions that simply do not exist in storage.
Two main factors govern the course of the ageing process: the internal temperature of a working tyre's components, and the mechanical forces and loads that act on a tyre in use — neither of which affects tyres in storage. Anything that raises a tyre's temperature accelerates the chemical reactions inside it: the higher the temperature, the faster the reactions and the ageing process. Based on the effect of temperature on chemical reactions, it can be said that a tyre used for one year and driven 19,000 km has aged as much as a tyre stored for 17 years at 21 °C. Or take another example: a tyre in use ages as much every three weeks as a stored tyre does in a whole year, and a tyre under-inflated by 35 per cent wears more in a week than a tyre stored for a year. Drivers should therefore bear in mind that their safety depends far more on how a tyre is maintained and used throughout its life than on its storage age.
Michelin recommends having tyres checked at specialised tyre centres at least once a year, and replacing them with new ones if the tyres in use were manufactured more than 10 years ago. Over its working life a tyre endures countless impacts, forces and adverse conditions. After ten years it is practically impossible to establish everything the tyre has been through over its entire service life, so for safety's sake Michelin recommends taking such tyres out of service, as their fitness for use can no longer be assessed.
Michelin's representatives in Lithuania: operating conditions matter most
Paulius Gaurilčikas, manager at Michelin's representative office in Lithuania, likewise confirmed that the greatest influence on tyre ageing comes from how the tyre is used — driving speed, inflation pressure, load, maintenance and so on. If brand-new tyres "work" in an aggressive environment — fitted to a car driven fast on poor-quality roads — their properties deteriorate more within a couple of weeks, or the first thousand kilometres, than those of tyres kept in a warehouse for several years.
"These things should not be confused: 'old age' is caused by intense physical stress and the associated temperature changes within the tyre's internal structure — not by anything else," stresses Gaurilčikas.
Bridgestone's position: maintenance is the user's responsibility
Bridgestone corporate information on the service life of passenger-car and light-truck tyres (marked with C, CP or LT size designations), including spares.
Responsibility for tyre maintenance always rests with the user, and it is the user who must decide when tyres are due for replacement. Consumers should be aware that tyres are made of various materials and rubber compounds whose properties are essential for proper performance, and that those properties change over time. How each tyre changes depends on many factors: weather, storage and usage conditions (load, speed, inflation pressure, maintenance and so on) that act on the tyre in service. These use-dependent changes vary so widely that the service life of a particular tyre cannot be predicted in advance.
That is why consumers should not only inspect their passenger-car and light-truck tyres — spares included — regularly, but also have them examined periodically by a qualified specialist, such as a tyre dealer, who will assess their condition and their fitness for further use. Tyres in service for five years or more should be inspected by a specialist at least once a year. Consumers are urged to pay attention not only to a tyre's visual condition and pressure, but also to changes in its behaviour — increased air loss, noise or vibration — which would indicate that the tyre needs replacing before it fails completely.
Calendar age alone cannot tell you when tyres should be replaced. On the other hand, the older a tyre, the greater the likelihood that it will need replacing owing to changes that arise in service, or to other defects noticed during inspection or use. Most tyres need replacing within 10 years, and it is recommended that all tyres in use for more than 10 years from the manufacturing date indicated by the maker be replaced with new ones — purely as a precaution, even if they still look fit for use or are not yet worn out. This recommendation does not diminish the user's responsibility for replacing tyres on time. When replacing the tyres your vehicle came with, follow the vehicle manufacturer's recommendations (and do not exceed the 10-year limit).
Bridgestone's representatives in the Baltics: the "5+5" rule
Eugenijus Marcinkevičius, head of Bridgestone's representative office for the Baltic states, in turn pointed to the so-called "5+5" rule. Its essence: the manufacturer allows that if a tyre enters service within 5 years of its manufacturing date, it can then be used fully for another 5 years. Only once tyres reach 10 years from the manufacturing date marked by the maker are they recommended for replacement — even if they still look impeccable. It is simply a precaution.
"If the minimum permitted tread depth has not been reached within that time, the tyres may be used further on condition that specialists inspect them at least once a year. Users themselves are also urged to watch not only the tyre's visual condition and pressure, but also changes in its behaviour — increased air loss, noise or vibration — which would mean the tyre needs replacing," emphasised Marcinkevičius.
How to determine a tyre's manufacturing date
The manufacturing date is easy to read from the serial number on the tyre's sidewall. It is usually (though not always) preceded by the acronym DOT, and its last four digits encode the date: the first two give the week of manufacture, the last two the year. For details — and a tyre age calculator — see DOT code: how to check a tyre's age.
Sources: official Michelin and Bridgestone information.
