Spotting cracks in a tyre sidewall is unnerving, but not every crack means the tyre is finished. Sidewall cracking ranges from harmless surface weathering to serious structural damage, and knowing which is which can save you either an unnecessary purchase or a dangerous gamble. Here’s how to read the cracks the way a fitter does.

What causes sidewall cracking?

Most sidewall cracking is ageing and weathering — the rubber drying out and losing flexibility. The main drivers are UV, heat and ozone, plus time, low use and incorrect pressure. In Australia these conditions are common, which is why cracking often appears before the tread wears out. It’s the same process behind UV damage and dry rot, and it’s tied to overall tyre age. Impact damage and chronic underinflation can also crack or crease a sidewall.

When is sidewall cracking just cosmetic?

Fine surface crazing — a network of tiny, shallow lines in the outer rubber, sometimes called weather checking — is generally harmless if it doesn’t penetrate deep into the tyre. This is the rubber’s protective outer layer showing its age. If the cracks are hairline, don’t expose any different-coloured material underneath, and the tyre is otherwise sound, you can keep driving while monitoring them closely. Note the date code and re-check every few weeks; if they spread or deepen, get the tyre assessed.

When is sidewall cracking dangerous?

Cracking becomes a safety problem when it reaches the tyre’s internal structure. As a general rule, cracks deeper than about 1.5mm (1/16 inch), or any crack deep, wide or numerous enough to expose the cords or steel belts, mean the tyre should be replaced immediately. Deep cracks weaken the carcass and make the tyre susceptible to air leaks, belt separation and blowout — a real risk at highway speed. A sidewall can’t be repaired the way a tread puncture sometimes can, because it flexes constantly and carries the load.

How do I check the cracks properly?

Clean the sidewall, then inspect in good light with the wheel turned so you can see the whole circumference — including the inner sidewall. Gently flex or press near a crack: if it opens up and you can see into it, or spot a different material, that’s deep. Check both sidewalls, as the inner one often ages unseen. If you also see a rounded lump rather than a crack, that’s a different and more urgent problem — see is a sidewall bubble repairable (the answer is no).

Does cracking affect a roadworthy inspection?

It can. Deep cracking, cord exposure or perishing can fail a roadworthy or safety inspection, and it’s separate from the 1.5mm legal minimum tread. Rules and assessments vary between states, so confirm with your state authority or an inspector if you’re unsure. Cracking is one of several signs it’s time to replace tyres beyond tread depth.

What about cracking on the spare or a low-use car?

Sidewall cracking shows up first on tyres that sit still. A spare wheel, a caravan or a car driven only on weekends can develop cracks while the tread stays almost new, because the rubber isn’t flexing to keep its protective oils working. Don’t assume a barely-used tyre is a safe tyre — inspect the sidewalls and check the age. If a spare is cracked, replace it before you need it, not after a puncture leaves you stranded on the roadside with no usable backup.

Frequently asked questions

Can I seal or fix a cracked sidewall?
No. Sidewall damage isn’t repairable because the area flexes and bears load. Dressings only mask the look. Replace a structurally cracked tyre.

Are small cracks an MOT/roadworthy fail?
Fine surface crazing usually isn’t, but deep cracking or exposed cords can be. It depends on depth and the inspector — check your state’s rules.

How fast do cracks get worse?
It depends on sun, heat and use. Once started, cracking tends to accelerate, so monitor and act early.

My tyres have tread but are cracking — replace them?
Likely yes if the cracks are more than surface deep. Age and cracking matter as much as tread. Read our tyre wear pattern guide for the full picture.

General guidance for 2026; have any sidewall cracking assessed by a qualified tyre fitter and confirm roadworthy rules with your state authority. See also TireGrades on tyre cracking and Bridgestone Australia on tyre age.