Tyre ageing is not a function of time alone but rather the cumulative exposure to adverse environmental and operational factors.
Prolonged exposure to ozone or ultra-violet light can lead to degradation of the natural and synthetic rubbers used in tyres. Tyres incorporate additives to reduce this phenomenon.
The flexing of the tyre in normal use helps the diffusion of these additives to the surface of the tyre where they act against the attack from ozone and ultra-violet light. This diffusion process is greatly reduced in tyres fitted to vehicles that are used infrequently: such tyres are more susceptible to the effects of adverse environmental factors. Caravan and trailer tyres are a case in point.
Ageing-type damage causes localised hardening of the rubber leading to visible surface crazing. Similar damage can also be caused by overheating resulting from under-inflation or overloading. In severe cases, this stiffening can lead to delamination of the inner components of the tyre. If a tyre is showing signs of ageing it should be examined by a competent tyre technician.
Since tyre operating conditions and environmental exposure vary so widely, it is not possible to establish a universal age limit beyond which tyres are not suitable for further use. However, from 1st February 2021 it will be illegal to use tyres more than 10 years old on the front steered axles of heavy goods vehicles, buses and coaches. The ban will also apply to any wheel in a single configuration on a minibus. Some tyre manufacturers recommend that car or van tyres over ten years old should also be withdrawn from service.