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Bikes: 748S, HM1100S, V4SP, Was: DD-A #111
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Join Date: Feb 2002
Location: Back in Essex
Mood: Passion Killer
As Twinfan says, the blue is the silicon working it's way out of the tyre after too many heat cycles which gives the blue tinge to the tyre, thus it needs replacing.
They get very slippery at this point, so it's best to get them changed asap.