The hardening of tyres is caused primarily by the ultra-violet light in daylight, which breaks down the plasticiser in the rubber. Tyres kept in cool and above all DARK conditions, will last almost indefinitely. The man who rebuilds wheels for me, Steve Lomas, at ‘Five-One Wheelbuilding’ (near Cadwell Park), has a gizmo that he can press onto tyres to measure how hard/soft they are, a very good guide to the ageing of tyres that may have plenty of tread and no visible surface cracking. I suppose that as most of our machines are exempt from MOT testing, it is a matter of judgement based on what speeds you may ride at, how exuberant your cornering may be, and how much you value your life. Hardened but otherwise sound tyres will not grip properly on wet roads. I think that I am saying “If in any doubt at all, renew the tyre”
Brian
