Fine cracks in a tyre's sidewall are the rubber perishing, ageing and drying out under sun, heat and time. Some of it is cosmetic and some of it is a tyre on its way out, and the difference is mostly about how deep the cracks go.
What causes it
Rubber is not permanent. It hardens and cracks from:
- Age, as the oils that keep it supple dry out over years
- UV light and ozone, which attack the surface, worst on a car left outside
- Heat and repeated flexing in normal use
- Long periods unused, which is why caravan, trailer and classic-car tyres often perish before they wear out
Cosmetic crazing vs dangerous dry rot
The judgement comes down to depth and spread:
- Shallow surface crazing: fine lines across the sidewall on a tyre that is otherwise sound is often cosmetic, worth watching but not an immediate failure
- Deep or wide cracks, especially any reaching down toward the cords, weaken the casing and mean the tyre is replaced
- Cracking with a bulge is a casing failure and a replace-now fault, as covered under a sidewall bulge
Cracking in the tread grooves is read the same way: shallow is usually fine, deep is not.
Age is the real measure
Because perishing is about time, not mileage, the tyre's age matters as much as its looks. Most makers suggest reviewing tyres at around five to six years and replacing by ten regardless of tread, and the date code on the sidewall gives the week and year it was made. A tyre that is both old and cracking has reached the end whatever the tread shows, the fuller picture under tyre age limits.
When it is replacement
A perished tyre is replaced like for like in the same size, load index and speed rating, and because age tends to affect a whole set together, it is often the moment to look at more than one. Ordering the correct size from a tyre retailer such as Tyres.co.uk and booking a local fitting keeps it simple, with the repair-or-replace decision settling any tyre that is borderline.
Do not hide the evidence with dressing
Clean the area gently if needed, but avoid applying a coating to make cracks less visible. Record the tyre age, storage history and whether all tyres show similar changes. A competent inspection can assess depth and associated damage; a surface appearance alone cannot certify the casing. If the tyre has a bulge, exposed cords or significant loss of pressure, treat that as an immediate condition issue rather than waiting for an age-based replacement interval.
Common questions
Are cracks in tyre sidewalls dangerous?+
It depends how deep they are. Fine surface crazing on an otherwise sound tyre is often cosmetic, but cracks that are deep, wide, or reaching down to the cords weaken the casing and mean the tyre should be replaced. Any cracking combined with a bulge is a replace-now fault.
What causes dry rot in tyres?+
Age and exposure. Rubber hardens and cracks over the years under UV light, ozone, heat and repeated flexing, and tyres that sit unused for long periods, like those on caravans and classics, suffer most because the protective oils are not worked through the rubber.
How long do tyres last before they perish?+
Most makers suggest considering replacement at around five to six years and replacing by ten regardless of tread, because the rubber ages whether the tyre is used or not. The date code on the sidewall gives the week and year of manufacture so you can tell a tyre's true age.
Can cracked tyres pass an MOT?+
Light surface cracking may pass, but cracking deep enough to expose the cords or ply is a failure, as is any bulge or lump. An MOT is a snapshot, though, so a perishing tyre that scrapes through is still one to plan on replacing.