Monday 7 March 2011: one more direct hit on the back quarter
I wasn't too concerned to discover a large patch of missing tread on my rear tyre this morning when I went to pump it up; as the brake's been locking up the wheel I knew there were likely to be some worn patches but hadn't expected quite such a large chunk to be missing, nor for the tread to be flapping distinct from the fabric underlayer, though this is merely borne of experience with bald tyres from childhood where the rubber tread was continuous with the fabric beneath. My lack of concern stemmed from the general lack of vulnerability of this type of tyre to puncturing; whilst I would lose a bit of grip the conditons were good and if they ceased to be good I could slow down. I was already later than I wanted to be so even if the tyre flattened at some point on the journey in I would still have saved some time.
A side-effect of the tyre's longevity (it hasn't had to have been removed for anything since it was fitted new in June) was that I hadn't yet inspected the rim from the inside after noticing the concavity of the outside when fiddling with brakes. It was immediately apparent from looking at the inside that the exterior concavity was not so much due directly to wear from the brakes but was a result of the outer edge of one side of the rim bending through about thirty degrees along what it presumably a line of weakening from brake-wear, forced outwards by tyre and tube pressure. I'd read of tyres blowing out when rims gave way but had from the contexts surmised it to be a consequence of fancy high-end lightweight rims combined with evil modern abrasive brake compound. Fortunately mine had not bent sufficiently to be folded enough to crack and fortunately the tyre had retained sufficient grip on the re-angled rim to not detach suddenly, poppingly and potentially injuriously.