No idea how this happened - maybe pothole damage looking at the accompanying graze on the alloy?? I now have a cut in the sidewall of the tyre on our RRS - it doesn't look too deep. Is the tyre still OK or is it dangerous?
That bit of the tyre is designed as a big fat lump of rubber so it doesn't destroy the tyre when you kerb it. It does look quite deep but it's difficult to say whether it's too deep or not.