This is the long winded reply from Bridgestone
Hi Adam, apologies again, after reviewing the images, I can say that the tyres condition is very minor and will not pose any detriment to safety or performance.
The reason for the cracking can be contributed to a variety of factors such as:
• The application of ‘Tyre Dressing’. - Used by dealer forecourts and carwash companies mainly to make the sidewalls/shoulders of the tyres look ‘more black’. These can be quite strong and some are even mildly corrosive if used consistently over time.
• Poor pressure maintenance during the tyre’s life. This causes over flexing in the tyre casing eventually causing tyres to prematurely age. The tyres crazing in the images provided in the shoulder area typical wear characteristics and suggest that at some point in the tyres service life the tyres may have not been kept to the vehicles recommended tyre pressure setting for the loads carried.
• Exposure to contaminates. - Petrol, anti-freeze, motor oil, tyre ‘dressing’ (used on the sidewalls to appear more “black”).
• Regular, but short frequent trips in the vehicle (lack of use), where the tyres are heating up, then cooling down over a short period of time. (essentially preventing the aforementioned oils from percolating/migrating throughout the tyre fully). We add anti-aging oils into the rubber compound, however for these oils to be released the tyres need to get to an operating temperature. If the tyres are not used regularly then the releasing agents in the anti-aging oils are unable to percolate through the casing and ensure the tyre stays subtle.
The upside to this phenomenon is that ‘crazing’ only affects the top cap layer of rubber, so is only cosmetic in nature. - This type of appearance does not have any effect on the tyres handling or performance, and based on the appearance in the image provided we would advise that these tyres are ok to continue running.
Kind regards Bridgestone UK