Almost all my flats have been from riding up sidewalk curbs too hard, the spoke breaks at the elbow then punctures the tube from the inside. I just had another flat yesterday just like that. I've been using a couple layers of electrical tape as tire liner to help prevent that but I ran out and had nothing there to prevent my most recent flat. I can only remember one or two flats being caused by debris passing through the tire in my case. I never have flats or broken spokes on the front wheel, only the rear with the heavy motor.