Une première conclusion révèle que l’âge du pneu a plus d’effet sur la dégradation des propriétés mécaniques que le kilométrage, c'est-à-dire que la fatigue mécanique a une faible influence sur les propriétés mécaniques des pneus par rapport au vieillissement chimique.

Research and regulation

Older tires often are put into use after having served as a spare. Others may have been stored in a garage or warehouse, or simply used on a vehicle that was infrequently driven. Unfortunately, the owner of the tire may not know this history.Used-tire sales make up a huge market in the United States; an estimated 30 million tires are resold each year. Many consumers buy used tires through tire dealers, swap meets, flea markets, and online forums. Tires with adequate tread and without any visible defect are likely to find their way onto a vehicle regardless of their age, which consumers can determine only by decoding a 10- or 11-digit U.S. Department of Transportation (DOT) number molded on one side of the tire.

Although tire and vehicle manufacturers have been aware of the age degradation problem for years, they continue to keep warnings hidden, leaving service shops, dealers, and consumers dangerously uninformed.

German vehicle manufacturers (including Audi, Volkswagen, BMW, and Mercedes) and Toyota began adding warnings about tire age in their owner’s manuals beginning in the early 1990s. They differed a bit, but the messages were consistent: Tires more than six years old present an increased risk.

For instance, a Volkswagen manual says: “WARNING: Old tires can fail in use, causing loss of vehicle control and personal injury. Replace tires after six years regardless of tread wear. Always reduce speed and drive cautiously if you must use an old tire in an emergency. Replace the tire as soon as possible.” A Toyota warning reads: “Any tires which are over six years old must be checked by a qualified technician even if damage is not obvious. Tires deteriorate with age even if they have never or seldom been used. This also applies to the spare tire and tires stored for future use.”

European manufacturers developed these warnings based on research results. For example, the German testing and scientific research firm DEKRA issued a “special topic” report in 1986 examining tire defects that resulted in crashes. The study found an increase in tread separations after two years and a continuous increase from the fifth through the eighth years, with a dramatic increase in tires more than six years old. These researchers concluded that consumers should not drive on tires that are six or more years old, regardless of tread depth, particularly tires stored for an extended period of time.

The results of another German study concluded that failure frequency rose disproportionately with increasing tire age . It estimated that a breakdown of a nine year old tire was eight times as likely as a breakdown of a two year old tire. The author noted that one reason “over aged” tires were being sold was the “consumer unfriendly” way the date of manufacture was coded in the DOT number. The author shared the results with vehicle and tire manufacturers.

These reports are only two examples of the type of research that has been undertaken. Other more comprehensive and detailed findings exist. One of the most important disclosures about the tire-age hazard came from the United Kingdom based Tyre Industry Council (TIC), which issued an unprecedented warning to consumers about the dangers of old tires in September 2003.

The TIC is a nonprofit organization funded by UK tire manufacturers and retailers whose principal objective is to improve public awareness of tire safety.  While conducting roadside inspections in conjunction with the police, the TIC observed that numerous vehicles were fitted with tires that were 10 to 15 years old.  In response, the TIC recommended that motorists check the date codes on their tires and replace any that were 10 years old or older, and that tires 6 years old or older that have not been used should not be placed into service.

The TIC pointed out that tire components dry with age and can separate, and that anti aging chemicals added to tires are “only active when the tyre is in use; therefore tyres fitted to spare wheels, caravans, and trailers are particularly at risk of premature aging brought on by ozone degradation and static ‘sitting’ for lengthy periods.”

According to a TIC spokesperson, the organization based its warning on a practice recommended in 2001 by the British Rubber Manufacturers Association (BRMA) that was previously unknown outside that organization. BRMA members include manufacturers of tires, tire related products, and other rubber and polymer based products in the United Kingdom. The BRMA “strongly recommends” that previously unused tires not be used if they are more than 6 years old and that all tires be replaced 10 years from the date of their manufacture.

The BRMA also noted that environmental conditions such as exposure to sunlight and coastal climates, as well as poor storage and infrequent use, accelerate the aging process. A tire may have a life expectancy of more than 10 years in ideal conditions, but such conditions are rarely met, according to the recommendation.

The BRMA noted that aging may be identified by small cracks in the tire sidewall, but that aging “may not exhibit any external indications and, since there is no nondestructive test to assess the serviceability of a tyre, even an inspection carried out by a tyre expert may not reveal the extent of any deterioration.” The BRMA also noted that spare tires and tires that are not frequently used are particularly vulnerable to aging.

Sean Kane, president of Safety Research and Strategies, obtained the BRMA recommendation-which was never published-and submitted it to NHTSA in September 2003, along with a list of aged-tire cases in the United States and a request to consider requiring expiration dates on tires.

In late 2003, the U.S. Rubber Manufacturers Association (RMA) Tire Engineering Policy Committee held meetings chaired by a Bridgestone representative and attended by representatives of Continental, Cooper, Goodyear, Michelin, and Pirelli. The committee drafted this recommendation for maximum service life for light-truck and passenger car tires:

While most tires will need replacement before 10 years, it is recommended that any tires in service older than 10 years from the date of manufacture be replaced with new tires as a simple precaution (including spare tires), even if such tires appear serviceable (or even if such tires have not reached the legal wear limit).

The RMA never disclosed this draft recommendation to the public.

Les pneus neufs ou les pneus âgés, usés ou endommagés peuvent compromettre le bon contrôle du véhicule et l'efficacité de freinage.

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Même s'ils n'ont pas été utilisés, les pneus âgés peuvent soudainement se dégonfler ou éclater, surtout lorsque vous roulez à grande vitesse, et ainsi provoquer des accidents et des blessures graves. Les pneus de plus de six ans ne doivent être utilisés qu'en cas d'urgence et avec la plus grande prudence, en conduisant avec la vigilance qui s'impose.

Pneus de plus de 6 ans
En raison de processus physiques et chimiques, les pneus vieillissent, ce qui peut entraver leur bon fonctionnement. Les pneus stockés longuement sans être utilisés, durcissent et se fragilisent plus vite que ceux utilisés en permanence sur le véhicule.

Volkswagen recommande de remplacer les pneus de plus de six ans par des pneus neufs. Ceci est également valable pour les pneus – y compris la roue de secours – qui, de l'extérieur, paraissent encore utilisables et dont la profondeur des sculptures n'a pas encore atteint la valeur minimum prescrite par la loi.

L'âge de chaque pneu peut être déterminé en consultant la date de fabrication qui fait partie intégrante du numéro d'identification de pneu (TIN).

Même s'ils n'ont pas été utilisés, les pneus âgés peuvent soudainement se dégonfler ou éclater, surtout lorsque vous roulez à grande vitesse, ce qui peut entraîner des accidents et des blessures graves.

Les pneus de plus de six ans ne doivent être utilisés qu'en cas d'urgence et avec la plus grande prudence, en conduisant avec la vigilance qui s'impose