Q: I have Bridgestone BT023 tires on my bike. Recently, I discovered something that worries me. I was adjusting the chain when I noticed a thin line around the circumference of the rear tire, right in the middle of the tread. The line was a little bit darker than the rubber on either side of it. I felt it with my thumb, but it didn’t feel like a groove. I checked the front of the swingarm and under the fender in case something was rubbing the tire, but I couldn’t find anything. Then I checked the front tire and found another line just like the one on the rear tire. Both tires appear to be working fine. They hold pressure, and the handling of my bike is the same as it was on the last set of tires. But I can’t stop wondering about that dark line. Do you have any idea what might have caused it?
Mike O’Kelly, Berkeley, CA
A:We have a bike in our road-test fleet with the same line around the rear tire. It’s nothing to worry about. In fact, you should be glad it’s there unless you enjoy electrical shocks. Motorcycle tires have a lot of different materials in the compound. One in particular, silica, helps the rubber heat up quickly and improves grip in cold and wet conditions. One drawback of silica is that it’s a good electrical isolator. Tires with rubber compounds that use a lot of silica build up a static-electricity charge as you ride, sometimes enough to give you a light shock when you put your foot on the pavement, becoming the ground in the circuit.
The line around your tires is called antenna tread. Unlike the rest of the compound that makes up the tire, this line doesn’t have any silica in it, so it dissipates the static electricity as you ride rather than letting it accumulate. Antenna tread isn’t new or peculiar to any one brand. All motorcycle tire manufacturers build it into their tires—even car and truck tires have it—though it’s much harder to see there because of the dense tread pattern. The visible line of antenna tread might fade or disappear over time, but meanwhile it’s nothing to worry about.