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Enough tread…but what about the whole casing?
If you drive 10,000 km a year, by the time a tyre is five years old, it will have rotated 25 million (!) times, even if you drive gently enough to leave some tread.
Following up on your advice last week on slow punctures, is it possible for air to leak out through the casing material itself, without an actual “hole” anywhere? Dev.
Yes, especially if the casing is very old and/or perished. Natural rubber itself is potentially “porous”, and many decades ago all tyres used that material and pressures had to be regularly topped up to compensate – even when they were relatively new and undamaged.
But for many years now, tyres have been made with at least “semi” synthetic rubber, which is airtight…unless there is a hole or the material is perished.
Perishing, caused mainly by UV rays from sunlight, can lead to micro-cracks through which air can escape in quantities too small to cause bubbles in water testing, but - in large numbers and over time - these cracks are enough to liberate some tyre pressure.
This degrading of the casing skin also weakens the tyre –even if the tread is good, the risk of a blow-out increases. UV (and time) can also make the rubber harder and more brittle (which is why micro-cracks appear, and why a very old tyre has less grip and gives a slightly harsher ride.
Bear in mind that the average car tyre does one complete and continuously flexing rotation every two metres of travel. So, in 1 km, that’s 500 rotations.
If you drive 10,000 km a year, by the time a tyre is five years old, it will have rotated 25 million (!) times, even if you drive gently enough to leave some tread.
All these factors are why tyres have an age-limit as well as a wear-limit. In many markets tyres above 10 years old are legally unroadworthy, and over 7 years garages are not permitted to repair punctures – irrespective of the treat depth. Every tyre has an age stamp on its tyre-wall.
The level of risk of course depends on the vehicle the tyre is on, and how and where it is driven. Handcarts (and jalopies on rural roads) for example, can safely use a tyre that is well past its best-by date for a car, and a town runabout is in less jeopardy than a powerful limo.