Car Aerodynamics - A2 Wind Tunnel - Wind Camp Tech Theory Here, Eaker is examining the stock Z28-type side spats, which did not alter the Cd but added 40 pounds of rear downforce. He thought they may be more helpful in yaw but says testing 0 degrees to the airflow gives you 75 percent of the data you'd need at slight yaw. We also tested the Moon discs, which reduced drag by two to four counts. The farther the wheels are from the outer edge of the fender, the less the discs matter.