I have had a flat on both the front and the back tires. I took it to my lbs - they checked the tires (and helped me change them - I'm still not real skilled at that - but learning-quickly) - both were on the outside of the tube - not the inside rim - although they did show me how to check the tape on the inside to make sure it wasn't because of some problem with rim/tape/tube. We found nothing on the tire. I hope I just had two bad tubes. Do tire liners go under the tire between tire and tube? Sorry for asking such a basic question . Thanks.