The simple answer? In certain situations – yes. The rubbers used in tyre manufacture can be natural, synthetic or a combination of both. After the tyre has been manufactured it still goes through changes to its physical properties.
Rubber curing continues for a while and then the aging process sets in. Tyre rubber is continually hardening, a very slow but nevertheless always ongoing process.
How does this affect the tyre?
Speaking in broad general terms, the hardening tread rubber will probably increase in durability – i.e. more kilometres, but the tyre will most likely suffer a downturn in handling, particularly in wet conditions.
Harder rubber is more likely to crack so small cracks can appear in older tyres, both in the tread and sidewall. It should be noted here that sunlight and proximity to electrical discharge (welding equipment, generators, etc) can also initiate fine cracking in rubber. The harder rubber can also cause complications with other non-rubber components in the tyre due to the lessened flexibility of the rubber – in the extreme worst case, separation of these components can occur.
Tyres used infrequently – such as on caravans and trailers – are generally stored in sunlight and this will tend to accelerate the hardening and cracking process. As these tyres are not being warmed up very often, the ability of the rubber to flex is impaired which can add to the potential for tyre related issues. A classic example of this is the spare tyre mounted to the rear door of a 4WD vehicle.
In most cases the tyre is never used and is exposed to the weather at all times. When the original tyres wear out the general practise is to purchase only three new tyres, keep the least worn of the tyres being removed as the new spare, and to use the existing spare as the fourth new tyre.
In many instances, tyre related issues occur with the ex-spare tyre after it has been in use for a while. Some of these issues can be quite serious. In these circumstances you should ask your tyre-man to carefully check the spare before fitting. Fortunately the trend is now to have fancy spare wheel covers so this obviates some of the potential for future tyre issues.
Any tyre over five years old should be checked regularly by a tyre expert. In fact some vehicle manufacturers state in the Owner’s Manual that tyres over a certain age should be replaced regardless of the amount of tread remaining.