Cracking and perishing are signs that a tyre's rubber is ageing. They show up as a network of fine lines on the sidewall or between the tread blocks, and the condition is often called dry rot or weather checking. Whether it matters depends entirely on how deep the cracks run.
Why tyres crack
Rubber does not last forever. The compounds contain oils and protective agents that keep it flexible, and over time those break down. Several things speed it up:
- Age: the rubber hardens whether or not the tyre is used
- UV light and ozone, sunlight and the atmosphere degrade the surface
- Heat cycling: repeated expansion and contraction works the rubber
- Standing unused: a tyre that rarely flexes can perish faster
- Under-inflation: extra sidewall flexing stresses the rubber
- Chemicals: some tyre dressings and cleaning products strip the protective oils, bringing cracking on sooner
That last point surprises people: shining tyres with the wrong product can actively shorten their life.
Cosmetic crazing or a real problem?
The key question is depth.
Fine surface crazing, with no cords showing, is largely cosmetic. It is worth keeping an eye on, checking weekly for any sign it is deepening or spreading, but it does not by itself make a tyre unsafe.
Deep cracks are a different matter. Once cracks run far enough to reach or expose the internal cords, the structure is compromised and the tyre is no longer safe to drive on. Perished rubber is brittle, and a brittle tyre can crack or separate under the stress of cornering, braking or motorway heat, sometimes with plenty of tread still on it. There are well-documented cases of tyres with healthy tread suffering a motorway blowout purely because the rubber had perished while the vehicle sat unused.
The lesson is that tread depth tells you nothing about ageing. A tyre can pass every tread check and still be unfit because of its condition.
Are they legal?
Cracked tyres are not automatically illegal. They become a problem when the cracking is severe, deep cracks, exposed cords, or any tread separation make the tyre unsafe and can mean an MOT failure or an illegal tyre. When the rubber has clearly degraded, replacement is the safe and sensible answer, and a fresh set is easy enough to order online from the likes of Tyres.co.uk.
Slowing it down
Perishing cannot be reversed, but it can be delayed: keeping tyres at the correct pressure, parking out of strong sun where possible, avoiding harsh dressings, and not leaving a vehicle standing for months all help. Checking the age of a tyre from its date code is a useful habit, since rubber that is several years old deserves a closer look regardless of mileage.
From the workshop: the classic is a low-mileage car or a caravan that has barely moved in years. Owners are proud of the tread, and the tread is fine, but the sidewalls are crazed all over and the rubber is hard as a board. That is a tyre living on borrowed time.
Sources and accuracy. The causes, the cosmetic-versus-dangerous distinction and the legal position here reflect tyre-maker guidance and UK rules at the time of writing, which can change. Anything safety-critical should be confirmed against current official guidance. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Are cracked tyres dangerous?+
It depends on depth. Fine surface crazing with no cords showing is mostly cosmetic and can be monitored. Deep cracks that reach the internal cords weaken the tyre and can lead to a blowout, so those mean replacement.
What causes tyres to crack and perish?+
Ageing of the rubber, helped along by UV sunlight, ozone, heat, long periods unused, under-inflation, and harsh cleaning chemicals or tyre dressings that strip the rubber's protective oils. It is often called dry rot or weather checking.
Can a perished tyre fail even with good tread?+
Yes. Perished rubber goes brittle and loses flexibility, so a tyre with plenty of tread can still crack or come apart, sometimes at low speed. Tread depth says nothing about the condition of the rubber itself.
Are cracked tyres illegal in the UK?+
Not in themselves. But cracks deep enough to expose the cords, or that come with tread separation, make the tyre unsafe and can fail an MOT or count as an illegal tyre, so severity is what matters.