My new motorcycle has Bridgestone Battlax T30 Sport Touring tyres. I noticed a weird dark line/mark running around the front tyre, slightly off-centre and able to be felt as well as seen. This is what it looks like. Wondering what it was I asked Professor Google. I found out it is common to tyres with a high silica content in the compound. "Silica rich compounds build up a static electrical charge as you ride. On occasion this can be enough to give you a small electrical shock as your foot hits the ground, giving the electrical charge a route to earth. The thin dark line is a silica free band of “antenna tread” which helps to dissipate the static electricity as you ride, rather than letting it build up to give you a nasty shock. It’s common in motorcycle, car and truck tyres – although it’s much easier to see on a motorcycle tyre." Who knew? Mystery solved and mind put at ease.