So I've had three flats on the rear of the Derbi the past year. Each time the inner tube is still good, no leaks. I've filled the tube and checked it in a tub of water, left it sit overnight, no leakdown. Three times. What's causing the tube to lose air? A leaking schrader valve in the stem only occurring when riding?
This time I put on a 3" tube ($14.50!) which seems beefy enough to ride on without a tire.