The vehicle rolls down a constant, precisely known downslope the terminal velocity it finally achieves is recorded. Among the methods used ( 20, 21, 24) only terminal-velocity rollout is relatively popular. The floor can be much more favorable than normal road surfaces, and also the walls of the measuring section can result in large differences from reality outdoors. The rollout method simulates the real bicycle almost perfectly, but because of the absolute prerequisite of zero wind, measurements are usually taken in enclosed spaces, and are valid only for each particular space. Some of the published measurements which have been obtained with this method are difficult to rank with confidence, as they do not indicate how clean the work was. There are many possibilities for unrecognized errors with this method. Data measured at widely varying speeds allows the rolling friction and air resistance to be calculated. The drop in speed over the measuring section is measured. The bicycle rolls without pedaling over a measuring section with as smooth a surface and as little wind as possible. There are other, usually less expensive methods to determine the air resistance of a bicycle. Without specifying speed and atmospheric pressure, only data taken using the exact same measuring method can be compared. The researchers do not specify a value of c wA, but instead instead give only the aerodynamic drag force (in lbf or 'grams'). Some researchers seem to be aware that this problem exists - although it is never openly addressed. The differences among individual test riders are not sufficient to cause these differences. As the results for very similar bicycles (conventional racing cycles) vary by approximately 20% among the different wind tunnels' measurements, an absolute specification of the effective frontal area c wA is largely avoided in reviews of the literature. It is also problematic that many of the wind tunnels used probably have enclosed measuring sections, requiring corrections to adjust the results to the reality of the cyclist on the road or the track. With very few exceptions (4), the wheels do not turn during measurement in the wind tunnel.

The energy consumption of a wind tunnel is enormous and is absurd in relation to the small power requirements of a bicycle. The measurements are not much fun for the test rider: he is blasted with hot air at 50 km/h and the noise is over 100 dB(A). Up to now, almost exclusively racing cycles, not everyday cycles, have been measured in the wind tunnel. The extremely high cost of a good wind tunnel and its instrumentation makes its use expensive. Bicycles also have been measured in the wind tunnel, usually in connection with a record attempt or Olympics project. The most well known way to measure air resistance - as used above all in automotive engineering - is in the wind tunnel. Rinard and John Allen from the original German Measuring Aerodynamic Drag Measuring Aerodynamic Drag by Rainer

Air drag