Just curious how often some of you have found it necessary to replace tires. I had had no flats for about 6 months, then suddenly the last 2 weeks I have been getting several flats from small punctures in the tube. Sometimes I can't find what has made the puncture, and sometimes I find a sharp fiber inside the tire. These fibers seem to be coming from the inside of the tire itself rather than a foreign object. The tires are nothing special, a nylon hybrid type tire, Cheng Shin brand, that came with the bike. I figure I have put about 3,000 miles on them, and the tread still looks fine. Time to get new tires?