The most useful aero testing is done on-track. Because we drive through still air as opposed to moving air flowing over our cars, wind tunnels might not tell us all there is to know about aero efficiency. Moving air has energy whereas still air does not. That makes a huge difference in how the air reacts to the surfaces on our race cars. Here, we see pre-prepared lines of yarn strips attached to a car for evaluation. On our Sprint Car, we could attach these onto the wing and body panels and take video using a GoPro camera or similar video source to see how the air is flowing over those surfaces. We could learn a lot about how everything works aero wise.