What should I verify before using this information?

Use TireFitLab values as a sizing reference, then verify the vehicle handbook, tire placard, rim compatibility, load rating, and physical clearance before fitting.

Tyre cracking and ageing guide

Why do tyres crack and when are cracks dangerous?

Tyre rubber cracks when the antiozonant and antioxidant compounds built into the tyre compound are depleted by exposure to ozone, UV radiation, heat cycling, and time. Surface crazing — shallow cracks across the sidewall or tread grooves — is common in tyres over 5 years old and is often cosmetic. Deep cracks (over 1 mm), cracks at the tread base exposing cord, or cracks that run continuously around the bead area are structural safety concerns requiring immediate replacement. Age is a primary driver: most tyre manufacturers and motoring organisations recommend replacement at 6–10 years from manufacture (DOT date code), regardless of tread depth remaining. Ozone cracking is accelerated by parking near electric motors and ozone-generating equipment, and by leaving tyres stationary for extended periods (which prevents the tyre from flexing and redistributing antiozonant).

Tyre rubber cracks when the antiozonant and antioxidant compounds built into the tyre compound are depleted by exposure to ozone, UV radiation, heat cycling, and time.
Surface crazing — shallow cracks across the sidewall or tread grooves — is common in tyres over 5 years old and is often cosmetic.
Deep cracks (over 1 mm), cracks at the tread base exposing cord, or cracks that run continuously around the bead area are structural safety concerns requiring immediate replacement.

FAQ

Why do tyres crack and when are cracks dangerous?: Tyre rubber cracks when the antiozonant and antioxidant compounds built into the tyre compound are depleted by exposure to ozone, UV radiation, heat cycling, and time. Surface crazing — shallow cracks across the sidewall or tread grooves — is common in tyres over 5 years old and is often cosmetic. Deep cracks (over 1 mm), cracks at the tread base exposing cord, or cracks that run continuously around the bead area are structural safety concerns requiring immediate replacement. Age is a primary driver: most tyre manufacturers and motoring organisations recommend replacement at 6–10 years from manufacture (DOT date code), regardless of tread depth remaining. Ozone cracking is accelerated by parking near electric motors and ozone-generating equipment, and by leaving tyres stationary for extended periods (which prevents the tyre from flexing and redistributing antiozonant).
What should I verify before using this information?: Use TireFitLab values as a sizing reference, then verify the vehicle handbook, tire placard, rim compatibility, load rating, and physical clearance before fitting.

The chemistry: why rubber cracks with age

Tyre rubber is a complex compound containing natural rubber (NR), styrene-butadiene rubber (SBR), carbon black, silica, sulphur (from vulcanisation), and a range of stabilising chemicals — including antiozonants and antioxidants. Antiozonants are the critical cracking defence: they are formulated to migrate slowly to the tyre surface and create a thin protective film that reacts with ozone before it can attack the underlying polymer chains.

This migration mechanism is driven by flexing. Every rotation of a tyre in use stresses the rubber, which promotes antiozonant migration. Tyres that are in regular use maintain a fresher surface antiozonant layer. Tyres that sit stationary for months — whether fitted to a rarely used vehicle or stored as a seasonal set — do not flex, and the surface film is not replenished. Ozone and UV then attack the unprotected surface.

Once the polymer chains begin to degrade, the cracks form perpendicular to the direction of tensile stress — which is why sidewall cracks typically run radially (top-to-bottom) and groove cracks run across the groove face. This is called ozone crazing.

Types of tyre cracking

Type	Location	Appearance	Cause	Typical depth	Risk level
Ozone crazing (surface micro-cracking)	Sidewall, tread groove walls, tread shoulders	Fine network of shallow cracks, sometimes visible only at full sidewall flex. Perpendicular to stress direction.	Ozone in the atmosphere attacks the carbon-carbon double bonds in natural rubber (NR) and SBR. Antiozonants in the compound migrate to the surface to neutralise ozone — they deplete over time.	Typically < 0.5 mm in early stages	Low–medium (early stage); monitor. High if cracks deepen or multiply rapidly.
UV / surface oxidation cracking	Outer sidewall (exposed face), tread surface in unused grooves	Flat, dry, often slightly discoloured surface. Shallow fine cracks, tyre may look faded or chalky.	UV radiation degrades rubber polymer chains. Antiozonants offer some protection but UV breaks them down too.	< 1 mm in most cases	Low cosmetically; medium if combined with age and ozone damage.
Thermal cycling cracks	Tread groove bases, shoulder edges	Small radial cracks at the base of tread grooves, often first noticed at the groove corners.	Repeated heating and cooling causes differential expansion and contraction of rubber layers. Tread groove bases are a stress concentration point.	0.5–2 mm typical	Medium — groove base cracks near the critical depth can expose cord. Measure depth carefully.
Bead area cracks (dry rot)	Inner and outer bead area (where tyre seats on the rim)	Radial or circumferential cracking in the rubber bead filler area. Sometimes only visible on dismounting.	Age and low-flex zone — the bead area receives less flexing than the sidewall, so antiozonant migration is lower. Combined with moisture and corrosion at the rim-bead interface.	Variable — can be deep (> 2 mm) at bead filler	High. Bead area integrity is critical for airtight seal. Cracks here require immediate replacement.
Sidewall flexion cracks (fatigue cracking)	Lower sidewall, near rim flange	Larger cracks, often circumferential, appearing at the flex zone boundary.	Low inflation pressure combined with age allows excessive sidewall flexion, fatiguing the rubber. Common in infrequently moved vehicles with chronically underinflated tyres.	1–3 mm typical. Can reach cord.	High if deep. May indicate internal cord fatigue not visible externally.
Chemical contamination cracking	Any area exposed to the contaminating agent	Localised cracking in a specific area, often with surface softening or swelling near the crack.	Contact with petroleum-based products (chain lube, brake fluid, tyre "dressing" containing petroleum), ozone-generating machinery nearby, or industrial chemicals.	Variable	Medium–high depending on contamination depth and tyre age.

Crack severity assessment guide

To assess crack severity, use a tread depth gauge probe (or fingernail as a rough guide) to estimate depth. Inspect in good light, with the tyre fully inflated to its normal operating pressure (so the sidewall is in its normal shape), and again with the tyre deflected under the vehicle weight (which opens cracks that are compressed when inflated). Check all four tyres, including the inner sidewall if accessible.

Grade	Visible depth	What you see	Action	Replace?
Grade 1 — Cosmetic	< 0.5 mm	Fine surface network, not measurable with a standard depth gauge. Visible on close inspection at a flex angle.	Monitor at every inspection. Not cause for immediate concern but indicates ageing has begun.	No — but start tracking and note the tyre age (DOT date code).
Grade 2 — Moderate	0.5–1.0 mm	Clearly visible cracks in normal light. Cracks visible in tread groove bases or across sidewall.	Inspect by a tyre professional. Consider replacement if tyre is over 5 years old or if cracks are in the tread groove base.	Consider. If tyre is 5+ years old and Grade 2 cracks are present, professional advice strongly recommended.
Grade 3 — Serious	1–2 mm	Deep, open cracks. Cord (white or yellow fibres) may be visible at the crack base in severe cases. Cracks may be continuous or form a connected network.	Replace immediately or as soon as safely possible. Do not ignore.	Yes — immediate replacement required.
Grade 4 — Critical	> 2 mm or cord visible	Cord visible, structural integrity compromised. Cracks in bead area. Tyre may have lost stiffness.	Do not drive. Replace immediately. If a blowout risk exists, call for recovery.	Yes — do not drive.

How to read the DOT date code

Every tyre manufactured since 2000 carries a DOT code on the sidewall. The last 4 digits give the manufacture date: first two digits = week of year, last two digits = year. Example: 3521 = week 35 of 2021 = manufactured approximately late August 2021.

Find the DOT marking on the outer sidewall. It reads: DOT XXXXXX YYYY 3521 (where XXXXXX is the plant code, YYYY is the size code, and 3521 is the date). On some tyres the full DOT code is only on one sidewall — check both. For a more detailed decoder guide, see our Tire age guide.

Age replacement guidelines by organisation

Organisation	Recommendation	Source	Legal status
ETRTO (European Tyre and Rim Technical Organisation)	Replace by 10 years from manufacture date. Visual inspection recommended from 5 years.	ETRTO guidelines	Industry guideline, not a legal requirement in most EU countries for private vehicles.
UK Highway Code / DVSA	Tyres over 10 years not permitted on PSV buses and coaches (legal). Personal vehicle recommendation: 10 years max.	DVSA guidance 2021	PSV age limit is law; passenger car guidance is advisory.
NHTSA (US National Highway Traffic Safety Administration)	No formal federal limit. Many manufacturers (Michelin, Continental, Pirelli) recommend 10 years. Ford, Chrysler: 6 years.	NHTSA consumer guidance	State-by-state variation. No US federal law for passenger vehicles.
Bridgestone / Michelin / Goodyear	10 years from manufacture date as absolute maximum. Visual inspection and professional check from 5 years.	Individual manufacturer websites	Manufacturer recommendations create a practical standard even where no law exists.
BMW / Mercedes-Benz / VW Group	6 years from manufacture date for OEM-equivalent safety (internal OEM guidance for dealer service teams).	OEM service manuals	More conservative than ETRTO. Reflected in some dealer inspection criteria.

Practical rule: replace any tyre over 6 years old that shows Grade 2 or higher cracking, or any tyre unconditionally at 10 years from manufacture, regardless of visible condition. Tread depth is not a reliable guide to rubber ageing — a tyre with 6 mm of tread can have degraded rubber internally and be less safe than a 3 mm tyre that has been in regular use and maintained correctly.

Crack vs bulge: an important distinction

Surface cracks in the rubber compound are a different failure mode from sidewall bulges. A bulge indicates internal structural damage — a broken or separated ply cord — and is an immediate replacement criterion regardless of crack status. Cracks may not co-exist with a bulge, but both should be assessed separately. For bulge assessment, see our Sidewall damage guide.

How to slow rubber ageing

Measure	Why it works	Impact
Garage storage (out of UV)	UV is a primary degrader of antiozonants and rubber polymer chains. Indoor storage away from windows eliminates UV exposure.	Significant — can double effective tyre life for stored seasonal sets.
Correct inflation pressure (including for stored seasonal tyres)	Slightly deflated tyres deform under their own weight, creating stress concentrations that accelerate cracking in the sidewall flex zone.	Moderate — inflate seasonal sets to 0.3–0.5 bar above normal before storage.
Avoid petroleum-based tyre dressings	Many "tyre shine" products contain petroleum solvents that leach antiozonants from the rubber surface, accelerating depletion.	Moderate — use water-based tyre dressings only, or none at all.
Store away from ozone-generating equipment	Electric motors (including power tools, compressors, generators), fluorescent lighting with aging ballasts, and welding equipment generate ozone. Ozone attacks rubber immediately.	Significant — do not store tyres near compressors, electric vehicles during charging, or workshop equipment.
Wrap in opaque plastic bags for long-term seasonal storage	Reduces both UV and ozone exposure during storage periods. Standard practice for professional seasonal storage.	Moderate — useful for 4–8 month seasonal storage periods.
Keep tyres moving (avoid prolonged stationary storage with vehicle weight on tyres)	Antiozonants migrate to the surface during flexing, continuously renewing the protective layer. Stationary tyres under load cannot flex, depleting the surface layer.	Significant — move the vehicle periodically if stored for months. For unmounted seasonal tyres, horizontal stacking eliminates this issue.

For storage position, humidity and temperature ranges, see our Tyre storage guide.

Common misconceptions

"A tyre with lots of tread is fine regardless of age." — False. A 9-year-old tyre with 5 mm of tread can have internally degraded cord and rubber that provides significantly less protection than its appearance suggests. Age-related degradation is a separate axis from tread wear.
"Surface cracks are always serious." — Not always. Grade 1 (very shallow, fine) surface crazing is expected in outdoor-stored tyres over 4–5 years. The concern is depth and progression, not mere presence.
"Tyre dressing prevents cracking." — Petroleum-based tyre dressings (the most common type at car washes) can actually accelerate cracking by extracting antiozonants. Use water-based products only, or leave the tyre natural.
"If the tyre holds air, it is safe." — Airtightness is not the only safety criterion. A degraded rubber structure can fail catastrophically under the combined stress of load, heat, and cornering forces — even while holding pressure at rest.

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Last reviewed: 2026-06-22

Last reviewed: 2026-06-28

What changed

Reviewed deterministic geometry, load/speed references, sitemap inclusion and localized page shell.